LIBRARY UNIVERSITY OF CALIFORNIA IARTH SCIENCES UNARY GEOLOGICAL MAP OF IlRIEILAIf '**** INDEX AQUEOUS ROCKS P. Pliocene days. HBB Cr. frrt one whom all will allow to be facile princeps ; who, though amongst the early explorers of the geological treasures of his country, and the first to reduce to order and system its tangled skein of strata, has only passed away from amongst us within the last few years. Sir Richard Griffith will always be remembered amongst men of science as the first to construct a geological map of Ireland ; doing for this country what William Smith accomplished for England, and Macculloch and Boue for Scotland. Griffith's map, 1 constructed with the help of the late Mr. Kelly and others, but chiefly by his own personal exertions, must be considered as remarkable for the accuracy with which the boundaries of the geological formations are delineated, the broad views it evinces in the grouping of the formations, and the effectiveness of its colouring. It is a work which only a man of uncommon industry, of 1 Published by Hodges, Foster & Co., Dublin (1855). Jukes's ' Geological Map,' on a smaller scale (1867), is taken partly from Griffith's map, and from the maps, when published, of the Govern- ment Survey. A new edition was issued in 1878. 6 Geological Formations oj Irelana great physical powers, and of high intellectual attainments could have accomplished. The author of the first geological map of Ireland has thus established his claim to be known amongst geolo- gists in all time to come as 'the Father of Irish geology.' 1 Archaean Rocks. Recent investigations have conclusively established the existence, in several de- tached areas, of representatives of these the most ancient of known rock-groups. The tracts alluded to are, as might be inferred, situated in the western and northern parts of the country where the crystal- line and metamorphosed strata predominate. The most important tract is probably that which lies along the northern shore of Galway Bay, and is bounded along the north by the quartzite range called the f Twelve Bins of Connemara.' This wild tract of country, indented by numerous arms of the sea, full of lochlets, with rocky masses rising at intervals from beneath thick folds of peat and morass, bears a strong resemblance to some tracts in Sutherlandshire formed of rocks of the same age. In both districts, also, they are glaciated and boulder- strewn, and are formed of similar materials consist- ing of beds of gneiss, composed of red orthoclase, 1 As William Smith, who constructed the first geological map of ngland, is generally known as ' the Father of English geology.' Archczan Rocks 7 often porphyritically developed, greenish or yellowish oligoclase, quartz, black, green, or white mica with accessory minerals. These gneissose rocks pass into hornblendic and micaceous schists with veins of pegmatite ; and the whole series is bounded on the north by the newer quartzites, schists, and lime- stones, presumed to be of Lower Silurian age, but under highly metamorphosed conditions. 1 Other districts where Archaean rocks presumably occur are the Slieve Gamph and Ox Mountains on the borders of Mayo and Sligo; the western and northern parts of Erris Head, and Belmullet, and the extreme western coast of Achill Island. In all these places the rocks are of similar character, con- sisting chiefly of gneiss with masses of schist, and in Achill overlain uncomformably by the newer schists with a conglomerate base. Finally, through the investigations of the Geological Survey it has been determined that the great group of metamor- phosed sedimentary strata, forming nearly the whole area of Donegal and Derry, rests with its southern base on the flanks of a partially developed ridge of 1 The Archaean age of these rocks was originally pointed out by the Author in his paper, ' On the Laurentian Hocks of Donegal,' Scient. Trans. Roy. Dublin Soc. Feb. 1882; and this view has been confirmed by Dr. A. Geikie in his paper on ' Recent Researches into the Origin and Age of the Highlands of Scotland and Ireland,' Proc. Roy. Inst. p. 528, 1889. 8 Geological Formations of Irelana Archaean gneissose rocks stretching from the banks of the Erne at Ballyshannon in a north-easterly direc- tion towards Pomeroy. This mass is principally formed of hornblendic rocks and gneiss, and is overlain in its eastern part by a volcanic series con- sisting of amygdaloidal basic lavas, tuffs, and coarse volcanic agglomerates, which gives place in turn to the chloritic schists forming the mountainous tracts of Derry and South Donegal, and which are presum- ably Lower Silurian beds under altered conditions. 1 Cambrian Rocks. The beds which have been referred to this formation are only found along the eastern coast, and in detached localities ; namely, Howth Hill, on the north shore of Dublin Bay ; Bray Head, and the coast nearly as far as the town of Wick- low ; and along the coast of Co. Wexford, from Cahore Point southwards. They consist of green and purple grits, quartzites, and rough slates, which occur generally in a highly disturbed and broken position, and are of great, but unknown, thickness. Fine 1 Part of this area in Tyrone has been referred to by Dr. Hicks as one of his Pre-Cambrian areas, Proc. GeoL Assoc. vol. vii. No. i. p. 28. See Expl. Mem. sheet 34 of the Geol. Survey, by J. Nolan (1878) ; also Dr. Geikie's paper above referred to, p. 544. At the time I wrote the paper on 'The Laurentian Rocks of Donegal,' I had arrived at the conclusion that the granitic and gneissic rocks of North-west Donegal were of Laurentian (or Archaean) age. This view, for which there is much conflicting evidence, I have been obliged to abandon. Cambrian Rocks 9 sections in these beds are laid open along the Dublin and Wicklow Railway, where it has been carried along the face of the steep and lofty cliffs of Bray Head, sometimes spanning chasms descending below the restless waters, at other times cutting deep into the projecting rocks, or traversing them in tunnels. To the north of Dublin Bay, the old rocks are covered over on the land side by beds of Carbonifer- ous limestone, which appear to have been deposited FIG. 1. W. Howth Hill. E. A, Cambrian quartzite and slate. B, Conglomerate at base of Carboniferous limestone (shore beds), c, Carb. limestone. D, Drift deposits. against their flanks, at that point forming a steeply shelving coast, or possibly sunken rocks in the sea of the Carboniferous period (see fig. ] ) ; but to the southwards, in the districts of Wicklow and Wex- ford, they are overlaid unconformably by the Lower Silurian beds, which fold round to the eastward and occupy the coast-line southwards from Wicklow. The organic forms yielded by these beds are few, and of low types of animal life : they consist of the io Geological Formations of Ireland tracks of marine worms, and of a peculiar genus of zoophyte called after its discoverer, 1 Oldhamia. There are two species, 0. antiqua and 0. radiata. (Forbes.) The surface features of the Cambrian Eoc"ks of Wiclilow are generally bold and varied. The beds of quartzite run in rugged and high ridges, or form conical hills, such as the Sugar Loaf Mountains, which are essential features in the landscape of that beautiful county. Seen from the north as, for example, from Killiney Hill the quartzite ridge appears to trend from east to west, culminating in the stately cone of the Great Sugar Loaf, at an elevation of 1,659 feet, and abruptly terminated along the bold bluffs of Bray Head. From the base of this ridge the wide and richly wooded valley of the river Dargle slopes gently down to the shore of Killiney Bay, which sweeps inwards in a graceful curve, and terminates in low cliffs of gravel and clay belonging to the Post-pliocene period, with which the floor of the valley is thickly covered. The view from this point has been compared to that of the 1 Dr. Oldham, F.E.S., late Superintendent of the Geological Survey of India. Prof. Ferdinand Koemer has expressed his opinion, fortified by microscopic study, that both species are in- organic. Neu. Jalirb. Heft I. 1877. Professor Sollas has come to a similar conclusion, but Irish geologists will not readily admit this view of their origin. Lower Silurian Rocks 1 1 Bay of Naples, in which the cone of the Sugar Loaf represents that of Vesuvius. But the comparison is illusory or fanciful ; each possesses features peculiar to itself, and the comparison has probably had its origin in the popular notion that all conical moun- tains are volcanic. 1 Lower Silurian Rocks. In Wales the Lower Cambrian beds, whose representatives in Ireland have just been described, are overlaid by a series of fossiliferous strata now generally denominated Upper Cambrian, including the c Lingula Flags ' 2 and ' Tremadoc slates,' with a peculiar fauna consisting mainly of Brachiopods and Trilobites. These beds, however, have not been discovered in Ireland, and there is every reason to suppose that they are unconformably overlapped, or concealed, along the margin of the Lower Cambrian beds, or have not been deposited over the Irish area. 3 The Lower Silurian beds of this country are, therefore, the re- 1 Professor Geikie has shown, in his Scenery and Geology of Scotland, how frequently in Scotland formations of quartzite as- sume the conical form as in the case of Schehallion ; other ex- amples will occur when we come to speak of the western districts. 2 The Lingula beds, with their peculiar forms of Trilobites and a few Brachiopods, were considered by Barrande and Murchison pro- perly to form the base (zone primordial^ of the great Silurian series. Some authorities assume the overlying ' Arenig beds ' as the base > it is possible these are represented in Ireland. 3 Prof. Jukes, Expl. Mem., sheets 121 and 136, p. 11, and Ibid. sheets 102 and 112, pp. 22 and 42 (note), 2nd edit, 1875. 1 2 Geological Formations of Ireland presentatives of those in the British series known as the ' Landeilo ' and c Caradoc' (or Bala) beds, and they rest discordantly on the Lower Cambrians of the south-eastern coast, and then stretch away north- wards, westwards, and southwards, forming the general floor upon which the Carboniferous and Devonian formations have been laid down. Two Types. The Lower Silurian beds of Ireland occur under two types, petrologically different, though generally representative ; and as this is an essential feature in the structure of the country, and must always take an important place in any sketch of its physical history, it will be necessary to dwell upon it for a few moments, arid to consider the form- ation under its two aspects. If the reader will look at a map of the British Islands (a geological map is preferable), and draw a line from Galway Bay on the south-west to Belfast Lough on the north-east, and then continue it in the same direction through the entrance to the Firth of Clyde, and right across the central valley of Scotland to the Firth of Forth, he will find that (generally speaking) the mountainous regions of both countries lying to the north and west of this line consist of crystalline strata, while the less elevated tracts to the south and east of the line consist of Silurian strata in their ordinary condition, Types of Lower Silurian Beds i and which are frequently stored with organic remains. In the case of Ireland, the former class of rocks (or the crystalline) form the mountainous districts of West Gal way, North Mayo, the Highlands of Donegal and Deny ; and in that of Scotland, the central Highlands from the flanks of the Grampians north- wards ; while the latter class (the ordinary fos- siliferous beds) in the south of Ireland rise into groups of isolated mountains such as Slieve Bough ta, the Devil's Bit, Galtymore, and Slieve Bloom ; and further north into the rugged uplands of Monaghan, Armagh and Down, represented in the south of Scotland by the Southern Uplands. The phenomena of both countries are strictly analogous, and admit of a similar explanation. The geological age of the beds south and east of the line I have indicated has been clearly established, both by their fossil contents and geological position : it is recognised that they belong to the great Lower Silurian series of Murchison. But on passing north and west of the line, which is generally obscured by Carboniferous strata, they undergo a singular transformation. In Scotland these beds are concealed under those of Car- boniferous age of the central valley, and in Ireland they also pass under a tract of Carboniferous lime- stone and other beds newer than themselves ; but on emerging in both countries, and rising into the 14 Geological Formations of Ireland mountainous regions which characterise the northern tracts of the British Isles, they are found to occur in the form of crystalline schist, quartzite, gneiss, and granitic rocks, all of which are included by geologists under the general term of ' Metamorphic.' * (1) Unaltered Silurian Beds. The Lower Silurian rocks in their normal condition, to the south of the line where metaniorphism sets in, consist in the lower part of dark schists with Graptolites, cleaved and interstratified with lenticular bands of grit, O 5 felstone, porphyry, and volcanic ash, the whole re- presenting the ' Landeilo Beds ' of North Wales ; and in the upper, of grey and greenish slates, grits, and fine conglomerates, in some places calcareous and fossiliferous, and in a few localities containing beds of limestone, as on the coast near Donabate, and at the Chair of Kildare. These are the oldest fossiliferous limestones in Ireland, and may be con- sidered as the representatives of the celebrated ' Bala Limestone ' group of North Wales. 1 The metamorphism of strata is doubtless due to the action of intense heat, in the presence of aqueous vapour or superheated steam, accompanied by pressure and shearing. It has taken place at considerable depths below the surface ; and thus shales and slates have been converted into mica-schist, hornblende schist, &c. ; grits and shales into schist and gneiss; sandstone into quartz- schist and quartzite ; and beds of augitic lava into diorite and epi-< diorite. These are largely developed in Donegal and Mayo. See paper by Dr. J. S. Hyland on ' Epidiorites in N.W. Ireland,' Scion. Proc. Roy. Dubl. Soc. Feb. 1890. Unaltered Silurian Beds 15 Throughout the north-eastern district from Long- ford to Down these strata are thrown into numerous flexures, and often reversed foldings, the axes of which run in E.N.E. and W.S.W. directions, corre- sponding to the trend of these beds in the Southern Uplands of Scotland. 1 In the districts north of Dimdalk Bay, and flanking the shores of Carlingford Lough, they have been penetrated by granitic and plutonic rocks in great variety, forming the Mourne and Carlingford Mountains, while along a line of country extending from Slieve Gullion to Slieve Croob they have been invaded by granite and subjected to intense metamorphism. It is also through these rocks that the granite of the Dublin and Wicklow Mountains has been in- truded, accompanied by eruptive outbursts, giving origin to dykes and granitic protrusions into the surrounding schists. All along the margin of the granite, the Silurian rocks are highly altered, being converted into micaceous schists, with chiastolite and other minerals which are frequently found under similar conditions. To the question of the 1 Some of these beds in Co. Down e.g. Coalpit Bay, near Donaghadee, Tullygarvan, Carnalea, Bally grot, and Craigavad have yielded a large number of genera and species of Graptolites and other fossils, collected and described by Mr. W. Swanston, F.G.S., and Prof. C. Lapworth, F.G.S., in Proc. Belfast Nat. Field Club, Appendix, 1876-77. 16 Geological Formations of Ireland geological age and origin of the mountain ranges here described we shall return in a future page. Another point of analogy between the Lower Silurian beds of Ireland and those of Wales is the occurrence of extensive sheets of felspathic trap, ashes, and agglomerates, which were ejected from submarine volcanoes at intervals during the deposi- tion of the sedimentary strata. These beds of volcanic materials are to be found in the districts of Waterford, Wexford, Wicklow, and Louth. They consist of f elstones, diorites, lampophyres, and elvan- ites, 1 which are interbedded with the grits and slates of the formation ; and it is probable that the vents from which they were poured out were in active eruption at the time when the Old Silurian volcanoes of North Wales were evolving those great sheets of trap which rise into the grand ridges of Cader Idris, Aran Mowddwy, and the flanks of Snowdon. But while the original materials out of which the physical features of North Wales have been sculp- tured are somewhat similar in both countries, the scenery of the unaltered Silurian districts of Ireland is seldom so striking and bold as in the former country. Except in the case of the isolated mass of 1 For a detailed account of the remarkable series of igneotis rocks in this district, see Expl. Mem. sheets 138, 139 (Appendix, with petrographical notes by Dr. F. H. Hatch), 1888. Metamorphosed Sihirian Beds 17 G iltyinore, which rises grandly from the central plain to an elevation of 3,015 feet, the Silurian tracts now being described consist of rugged and undulating ground, of no great elevation, and desti- tute of such striking physical features as those of North Wales. It is in the region of metamorphism? in the Western and Northern Highlands of Ireland, that the Silurian beds show themseh es under thsir nobler aspect; but an aspecb representing rather that of the scenery of the Scottish Highlands than of North Wales. To this type of structure and outline we shall presently turn our attention. (2) Metamorphosed Silurian Beds. We now pro- ceed to consider these rocks as they occur under their metamorphosed condition in the west and north-west of Ireland. They occupy four distinct tracts, sepa- rated from each other by bays and arms of the sea which seem to have originally been depressions filled in with Old Red Sandstone and Carboniferous strata, but which have since been partially re- moved by denudation and their place occupied by the waters of the Atlantic Ocean. These tracts con- sist of first, the district of West Gal way and part of Mayo lying between Galway Bay and Clew Bay, bounded on the east by the great chain of lakes, Loughs Carra, Mask, and Corrib ; on the west by the Atlantic, which often penetrates deeply into the land c 1 8 Geological Formations of Ireland in long arms or fiords, such as that of Killary Harbour. Second, the district of North-West Mayo, lying to the north of Clew Bay and bounded inland by the Carboniferous district of Erris, and by the long tongue of Old Red Sandstone which stretches along the coast of Clew Bay to the base of Nephin. The third is the district of the Ox Mountains, which stretches in a long ridge flanked by Carboniferous rocks, in a N.E. direction from the shores of Lough Cong to those of Lough Gill ; while the fourth and most extensive is the mountainous region of Donegal, Derry, and Tyrone, presenting its northern and western face to the Atlantic, and bounded inland by tracts of Carboniferous rocks or of Old Eed Sand- stone, which sweep round in a broken semicircle from the shores of Donegal Bay to those of Lough Foyle. West Galway District. This district includes that well-known group of mountains known as the ' Twelve Bins of Connemara,' which, rising in bold relief, and culminating in Benbaun, 2,395 feet, are remarkable for their individualised conical or dome -shaped forms. They are composed of beds of quartzite, rising in great arches, or folds, from the margin of the Archaean tract already described, which stretches away to the southward, and are traversed by many faults, or fractures, hewing (as it were) the masses of quartzite into rude blocks, which Nature, Connemara District 19 the great sculptor, has moulded into the forms they now present. Some of these fractures traverse the group from north to south, and coincide with the wildly beautiful valley of Lough Inagh, flanked on one side by Bennabeola, and on the other by Benbaun and the isolated mass of Lissoughter. The sides of these hills are sometimes perfectly destitute of vege- tation, their dry gritty substance offering but little footing for even the saxifrage, lichen, or fern. They have also undergone considerable polishing from former glacial action ; so that it will be easily under- stood how, seen from certain directions and under favourable sunlight, the mountain sides glisten like glass, or rather with the rich yellowish hue of bur- nished gold. The quartzite beds of the Twelve Bins dip to the southward beneath a series of hornblendic and micaceous schists with bands of crystalline limestone and serpentine, which furnish the variegated green ornamental stone known as * Connemara marble.' l Further south and extending along the shores of Galway Bay is the Archsean tract of porphyritic reddish gneiss, and hornblendic schists, amongst which masses of greyish granite and other igneous 1 This is really an ophi-calcite, or mixture of serpentine and calcite in irregular bands of varying hues. The principal quarry is at Recess, near Glendalough. c 2 2O Geological Formations of Ireland rocks have been intruded at an epoch more recent than that of the metamorphism of this region. To the northward of the Twelve Bins the quartz- ites and schists pass imcomformably below beds of Upper Silurian age, which extend along both banks of Killary Harbour, forming the heights of Muilrea and Bengorm ; they again emerge to the north of this tract, stretching to the shores of Clew Bay, from FIG. 2. Croagh Patrick, a quartzite mountain 2,510 feet high, in the west of Ireland, as seen from the stuth-west (after a sketch by Mr. 8. B. Wilkinson). which rises the quartzite ridge of Croagh Patrick, 2,510 feet at its summit above the waters of the bay (fig. 2), and which when seen from the east or west, in the direction of its axis, has the appearance of a perfect cone, sharply pointed and perfectly symme- trical. From the summit of this cone a wonderfully diversified panorama may be observed. To the east, the eye wanders over the low-lying central plain of View from Croagh Patrick 21 Ireland, its northern margin well defined by uprising hills. Along the north the waters of Clew Bay wash the base of the ridge, and, looking across its surface, the eye rests on the rugged moorlands of the Ox Mountains giving place to the broken ridges of quartzite which range through Erris, amongst which rises, in solitary isolation, the quartzite dome of Nephin. To the left are the ridges of Curraun Achill on the mainland, the pointed crest of Slieve More in Achill Island, and the bold headlands of Achill Head and Croaghaun. To the west the Atlantic stretches away to the horizon, studded by several islands, bordering the coast ; Clare Island, at the entrance to Clew Bay, being the most conspicuous. Turning towards the south, a great expanse of broken moor- land stretching from the base of the ridge is bounded by the terraced slopes of the Upper Silurian moun- tains which extend from Muilrea (2,688 feet) on the right, by Bengorm, to the tableland of Slieve Partry ' on the left, intersected by the valley of Doo Lough, the cleft of Delphi and the Errif valley. Behind this ridge, the eye may catch the bright glisten- ing summits of some of the quartzite mountains of Connemara, which contrast strongly with the darker masses of the Muilrea Mountains, composed as they are of massive beds of grit, slate, and trap, which stretch across the nearer field of view. 22 Geological Formations of Ireland o J The district of the Ox Mountains does not require much description. It consists of alternating beds of granite or gneiss, schist (sometimes calcareous), and quartzite. The granite is often largely crystalline and porphyritic. In this district we have examples of the results of metamorphic action in the produc- tion of varieties of crystalline rocks depending on the characters of the original strata. Thus beds of foliated granite or gneiss, schist, quartzite, and cry- stalline limestone are to be found succeeding each other, their characters under their metamorphosed condition depending in all probability upon their original composition, whether as ordinary granite, sandstone, shale, grit, or limestone. Donegal and Derry District. The rocks of this district may be considered as a prolongation of those of the Highlands of Scotland. They are of Lower Silurian age, but without the basis of Cambrian sandstone and conglomerate, known as the ' Torridon Sandstone,' on which their representatives rest dis- cordantly in Sutherlandshire. Nowhere in the Donegal, Mayo, or Galway districts does the base of this great series of schistose rocks reach the surface. The lowest beds of the district we are now con- sidering occupy the mountainous tract which borders the Atlantic, and which breaks off in a series of grand headlands along the northern coast. The Donegal and Derry Rocks 23 ridges which form this district range in a north- easterly direction, parallel to the remarkable valley of the Gweebarra and Owenbarra which forms a straight depression in the granitic region, somewhat similar in direction, but 011 a smaller scale, to that of the Caledonian Canal, which traverses the Scot- tish Highlands. The rocks consist of granite, gene- rally foliated, gneiss, and chloritic or hornblendic schists, sheets and dykes of epi-diorite, with crystal- line limestones and quartzites; the latter being a prominent feature in the physical structure of the district, and culminating in the peerless cone of Errigal at an elevation of 2,466 feet. These beds cross Lough Swilly, and reappear in the promontory of Inishowen, dipping generally towards the S.E. in the direction of Lough Foyle, but subjected, ac- cording to Professor Harkness, to several reversed flexures which locally alter the dip of the beds. 1 There can be no doubt that these quartzites and limestones represent the beds of similar character which in Ross and Sutherlandshire rest directly upon the Cambrian conglomerates of Suilven and 1 Quart. Journ. Geol. Soo. vol. xvii. The geological survey of this remarkable region has now been worked out in detail by the Government surveyors, and is described in the Memoirs to accom- pany the one-inch maps, viz. : The Geology of Inishowen, or North Donegal ; the Geology of North- West Donegal ; and the Geology of South Donegal. 24 Geological Formations of Ireland Queenaig. They pass below a great series of horn- blendic and other schists, with occasional bands of crystalline limestone, which form the interior of Donegal and Derry, and are traversed by the river Foyle throughout the greater part of its course ; they are ultimately covered by the newer formations along the south and east. These higher beds rise into high tracts of moorland in Derry, called the * Sperrin Mountains,' which attain an elevation of 2,240 feet; but owing to their comparative uni- formity of composition, and the absence of such solid masses of quartzite as those which characterise the older beds of North-West Donegal, they are destitute of the strongly marked physical features which make the N.-W. Highlands of Ireland so attractive to the student of art, as well as to the physical geologist. Geological Age of the Donegal Rocks. The geo- logical age of the crystalline schists, limestones, and quartzites of Donegal and Derry has been the sub- ject of patient investigation by several geologists as well as by the Government surveyors. Nor have these investigations been without result. In the first place these rocks are clearly the prolongations of those in Mayo which underlie the Upper Silurian beds of Muilrea, and which are themselves, in a few localities, fossiliferous ; the geological age thus Geological Age of Donegal Rocks 25 indicated being Lower Silurian. 1 But besides the evidence derived from stratigraphical relations there is to be added the evidence of more or less definite fossil forms themselves. This I shall endeavour succinctly to lay before the reader as follows : 1. Annelids. Cylindrical bodies and tubes occur in the quartzite of Knockalla, the Bloody- Foreland and other places, identical in form with those of the ' pipe-quartzite ' of Sutherland. Specimens on being compared with those from Scotland, now in the Museum of Practical Geology, leave no doubt that they are of similar origin. 2 No question has ever been raised regarding the organic origin of the Scottish forms. 2. Graptolites. Amongst the slates at Scallan Bridge, near Fintown, Mr. McHenry of the Geo- logical Survey has detected longitudinal markings in iron-pyrites strongly suggestive of Graptolites. Specimens having been submitted to Professor Lapworth, he admitted their strong resemblance, but was unable to pronounce definitely as regards 1 Expl. Mem., sheets 73, 74 of the Geological Survey. The slaty beds containing Lower Silurian fossils described by Mr. Baily occur at Doo Lough, Aillemore, and Lettereeneen, in Co. Mayo, amongst the more metamorphosed strata ; similar slaty beds occur south of Londonderry, but have not hitherto yielded fossils. 2 Amongst others holding this view is Dr. A. Geikie ; letter to the author, dated London, Nov. 3, 1887. See note, p. 21, Memoir on North- West and Central Donegal. 26 Geological Formations of Ireland their organic origin, owing to absence or indistinct- ness of structural form. He says, however, ' they may have been Diplograptidce transformed into pyrites, and then sheared with the containing rock, but beyond the general similarity of form there is no proof of this.' 1 Corals (?) from Church Hill. From a small quarry- in calcareous slate Mr. Me Henry obtained certain Serpula-like forms weathering out from a lateral sur- face, apparently that of a joint plane. The late Mr. Salter considered these forms to be those of a coral allied to Syringopora, and has left a drawing of the specimen. Similar opinions have been expressed by several other geologists, 2 and the only question seems to be whether they are not some form of Serpula. Other indefinite coral- like forms have been obtained from the neighbourhood of Fintown. Coralline forms from Culdaff. But perhaps the most important structural forms hitherto obtained are those from the earthy limestone of Culdaff in Inishowen. These were first discovered by Mr. Patrick Ganley, who described them in 1856 as corals allied to Halycites and other genera. 3 1 Letter to the author, dated November 1886. Sir A. Geikie _says in a letter, dated November 2nd of the same year, ' I have my- self little doubt that they were originally Graptolites.' 2 See Expl. Mem. North- West and Central Donegal, p. 22. 3 Journ. Geol. Soc. Dull. vol. vii. p. 163. Culdaff Coralloid Forms 2 7 Numerous specimens have been obtained from this stratum, which occupies a somewhat central position amongst the metamorphic series of North Donegal. The rock, however, is itself but very slightly altered, and to this we probably owe the preservation of these forms. 1 As regards their organic origin, nearly all the officers of the Geolo- gical Survey of Ireland are unanimous in the opinion that they are coralline structures an opinion which was shared by the late Mr. W. H. Baily, who identi- fied them as probably forms of Haly cites catenulatus. Photographs of some of the forms having been sent to several American paleontologists, amongst others to Dr. J. S. Newberry, Mr. C. D. Walcott, Professors J. D. Dana and James Hall, these eminent author- ities were unanimous in regarding them as corals of the genus Favistella or some allied form, such as Favosites or Columnaria. A similar opinion is also expressed by Dr. Ferdinand Roemer from an exami- nation of the photographic figures ; and although, oil the other hand, several British geologists have only given a hesitating assent, I can myself entertain no doubt as to their organic origin. This view is based both on the circumstances of their occurrence, namely, 1 Figures with description of the specimens will be found in the Expl. Memoir of Inishowen, Appendix B (1889), together with the opinions of geologists consulted. 28 Geological Formations of Ireland in a mass of limestone, scarcely at all transformed in structure, and bearing every resemblance to an ancient coral-reef, and also on a careful examination of many specimens themselves. The general form is that of a compound coral, consisting of polygonal tubes radiating from a centre, sometimes branching. These are intersected at short intervals by apparent tabuloe, generally perpendicular to the walls of the tubes, but sometimes oblique, owing probably to strain or pressure. Except the septa, which are rudimentary in some of the Astrwidce, it can be shown that in the specimens from Culdaif we have all the structural parts of the genus Favistella from the Trenton limestone of America. 1 Whatever might be the weight to be attached to the evidence afforded by any single specimen of those described above, it is impossible (as it seems to me) to ignore the cumulative weight of all taken to- gether. Nor is there any discordancy in this evi- dence ; it all tends towards one conclusion, namely, that of the Lower Silurian age of the great mass of schists, limestones, and quartzites of which the north-western Highlands of Ireland are mainly composed. I therefore look forward with confidence 1 Amongst the specimens is one apparently identical with Favo- sites fibrosus, from the ' Bala beds ' (Lower Silurian) of Co. Wexf ord. A specimen of Favistella was kindly sent to the author by Mr. Wal- cott for comparison. Upper Silurian Beds 29 to the general recognition of this conclusion, which seems to determine not only the age of the strata of the north-west of Ireland, but of their representatives in Scotland south of the line of the Caledonian Canal. 1 UPPKR SILURIAN BEDS. These strata are but sparingly represented in Ireland, as there is reason to believe (from evidence which I shall presently adduce) that at the close of the Lower Silurian period the Irish area was generally elevated into dry land and subjected to extensive denudation, while the strata were bent, folded, and sometimes metamorphosed ; and it was only upon the re-sub- mergence of the land, and where deep hollows had been worn out in the older rocks, thus admitting of the deposition of sediment, that strata represent- ing the Upper Silurian period were deposited. In such a hollow were the Upper Silurian beds which lie along the banks of Killary Harbour and the Errif valley in the Galway and West Mayo districts laid down; and their presence there is highly interesting, as throwing light on one of the most important physical problems of British geology, namely, the epoch of the metamorphism of the Lower Silurian rocks of the west and north-west of Ireland 1 On this subject the reader may consult with advantage Dr. A. Geikie's paper on ' Kecent Researches into the Origin and Age of the Highlands of Scotland and the West of Ireland,' p. 11 (1889). 30 Geological Formations of Ireland and of the Highlands of Scotland. 1 For, as we have seen, the rocks of all these districts are repre- sentative of each other, and have been subjected to the same process of transformation or nietamorphism. Now, as regards the Highlands of Scotland, the metamorphic rocks of the Grampians are overlaid directly by the Old Red Sandstone all across from the Clyde to the shores of the North Sea, and the only inference we can draw from the relations of these beds is, that the nietamorphism took place prior to the formation of the Old Eed Sandstone itself. But in the district of West Gal way, we can show that the epoch of nietamorphism must be pushed still further back ; and we are able to assert with precision that it is to be referred to that unrecorded epoch which elapsed between the close of the Lower, and the commencement of the Upper, Silurian period. The basement beds of the Upper Silurian group of this district consist of masses of conglomerate and shingle, with grits and shales, formed from the waste of the metamorphic rocks upon which they rest ('fig. 3) ; these latter having been deposited, trans- formed, crumpled, and ultimately upraised as a coast-line on each side of the deep channel in which 1 This subject has been ably treated by Professor Harkness in his paper on ' The Age of the Rocks of West Galway,' &c. Quart. Journ. Qeol. Soc. vol. xxii. Upper Silurian Beds 31 the shingle now forming the Upper Silurian beds was spread out by the waves and currents of the sea. 1 Above the basement conglomerates there occurs a great series of greenish grits and conglomerates with reddish-purple and greenish shales interstrati- fied, representing probably the Wenlock and Ludlow FIG. 3. Section N.E. of Kylemore Connemara. Mountain Brook A, Metamorphic schists. B, Upper Silurian grits, shales, and conglomerates resting unconformably on and against A. beds of Shropshire. These stretch from the shores of Lough Mask to those of the Atlantic ; and in Muilrea, which stands like a great watchtower guarding the entrance to Killary Harbour, they rise into the highest elevations in the west of Ireland. The formation gives evidence of contemporaneous volcanic action ; for several sheets of felspathic lava, 1 These basement conglomerates are full of fossils of Upper Llandovery types, so that their age is beyond question. 32 Geological Formations of Ireland with accompanying beds of ash, are interposed at intervals amongst the grits. Viewed from the southern shore of Killary Harbour, these beds of lava which maybe seen cropping out in bold relief along the southern flank of the mountain under which they dip, reappear in diminished numbers on the northern flank. 1 It will thus be seen that the Upper Silurian beds have not been subjected to the metamorphic action which has so intensely affected those upon, and amongst, which they have been deposited ; they are fossiliferous, and are in their unaltered condition of shales, sandstones, and conglomerates. Moreover, as the rounded pebbles which are found abundantly in them consist of quartzite, mica-schist, hornblende- schist, &c., derived from the metamorphosed rocks, it is clear that the metamorphic process had been completed before the newer beds were formed. Here we have evidently a great physical ' break or hiatus, 9 during which there were disturbances of the strata, elevation of the beds into land surfaces, and the denudation of the more exposed tracts consequent thereon. Hills and valleys were carved out, and ultimately the whole were depressed beneath the waters of the sea, and the hollows filled with 1 See Map of the Geological Survey, sheets 83 and 84, with Ex- planatory Memoir. Upper Silurian Beds 33 sediment derived from the waste of the older Silurian lands. The only other locality of sufficient importance to be noticed, where Upper Silurian beds occur, is the extremity of the Dingle promontory. Here along the coast a fine section is opened out in a series of shales, grits, limestones, &c., highly fossiliferous, which probably represent the whole of the Upper Silurian series of England and Wales, from the Llandovery beds upwards to the Ludlow inclusive. Amongst these are also to be found sheets of fels- pathic trap and ashbeds of volcanic origin, which were vomited forth from submarine vents during the process of deposition of the fossiliferous strata. 1 These beds again dip beneath a series of grits, shales, and conglomerates, several thousand feet in thickness, called by the late Professor Jukes * the Dingle Beds,' as they occupy the greater part of the promontory of this name. They are apparently comforinable to the recognised Upper Silurian beds below, and are overlaid in a highly discordant manner by the red sandstone and conglomerate of the Upper Old Eed Sandstone. Not having hitherto yielded any fossils, they have been placed by the Government surveyors in a kind of neutral territory, and are provisionally 1 See Explanation to sheets 160 and 170 of the Maps of the Geo- logical Survey. 34 Geological Formations of Ireland unattached to either of the formations with which they are in contact : but it will probably be admitted that their physical relations suggest their associa- tion with the Upper Silurian rather than with the Upper Old Red formation, to which they are strongly unconforinable. 1 The discordance in the stratifica- tion of the Dingle Beds and Upper Old Red Sand- stone is shown in several of the coast sections of the Dingle promontory, and is illustrated in the Explana- tory Memoir of this district from drawings by the late Mr. Du Noyer. DKVONO-SILURIAN (in part Lower Devonian) BEDS. The rocks which rise into the highest elevations in the south-west of Ireland, culminating in the Reeks, and in the Dingle promontory, stretching far out into the Atlantic, were named by the late Professor Jukes ' the Glengariff Grits and Schists' a name adopted by Sir Eichard Griffith, who was inclined to connect them with the Silurian series. 2 Sir Eoderick Murchison referred them to the Lower Devonian ; 3 1 This is the view suggested by Sir E. Griffith in his communica- tion to the British Association in 1857, ' On the Relations of the Rocks below the base of the Carboniferous Series.' Trans, of Sec- tions, p. 67. 2 Mep. Brit. Assoc., 1857. Trans, of Sections, p. 67. 3 Siluria, 4th edit. p. 175 et scq. Murchison very clearly de- scribes how in the cliff section of the Dingle promontory the Ludlow beds with fossils are seen to pass gradually and conformably up- wards into the greenish and purple slaty beds, forming the base of Ihe great series termed ' Glengariff Grits and Schists ' ; hence he very Devono-Sihirian Glengariff Beds 35 and, finally, the author has suggested the name ( Devono-Silurian,' to indicate their relationship to the Devonian system on the one hand, and the Silurian on the other, believing them to be what is known as ( passage beds ' exceptionally developed, and represented in the British area by the Downton Sandstone group of Hereford and the Foreland grits and slates of North Devon. 1 In the Dingle promontory, in the almost con- tinuous coast-section below Mount Eagle, the Upper Silurian beds, with fossiliferous bands representative of the Wenlock and Ludlow stages, may be clearly seen passing upwards into the unfossiliferous series of green and purple slates and grits which go to form the Devono-Silurian group, and which attain a total thickness of at least 7,000 feet. This great group in Mount Brandon, the culminating height of the Dingle promontory, reaches an elevation of 3,126 feet, and gives origin to a grand mountainous tract, dipping beneath the waters of Dingle Bay and reappearing in the still more elevated masses of the Keeks of Kerry. However clear may be the relations of the ( Dingle properly inferred that they represented the basement- beds of the Devonian system. 1 Quart. Journ. Geol. Soc. May 1882, p. 200. See also the author's Physical History of the British Isles, p. 68, &c., with physical maps of the period, plate iv. figs. 1 and 2. D 2 36 Geological Formations of Ireland Beds ' to the Upper Silurian series, not less clear are those which they maintain regarding the Upper Old Eed Sandstone. We have seen that they are clearly conformable to the former series ; but the reverse is the case as regards the latter. Nowhere are there to be found throughout Ireland two groups more decidedly unconformable and transgressive than are the Upper Old Eed Sandstone to the Devono-Silu- rian. This is illustrated by the well-known section on the coast cliffs at Sibyl Head, drawn by the late Mr. Du Noyer, and reproduced in ' Siluria ' ; where the whole of the ' Dingle Beds ' are overlapped and the Upper Old Eed conglomerate rests on the up- raised and truncated edges of the Upper Silurian series. But throughout the whole of the Dingle promontory similar unconformable relations are to be observed; the base of the Upper Old Eed passing transgressively over all the older strata, such as might be expected in the case of formations of different geological periods, but unusual in the case of members of the same system. 1 From the above statements it will be evident that throughout the region north of Dingle Bay the great J)evono-Silurian series, with of course the Upper 1 See ' Explanation ' to accompany sheets 160, 161, 171, &c. of the Geol. Survey Maps, by G. Du Noyer ; and the maps, sheets 161 and 172. Devono- Silurian Beds The Reeks 37 Silurian beds themselves, were subjected to much disturbance and denudation after their deposition, and that upon their resubmergence the conglomerate and sandstones of the Upper Old Red formation were deposited over their truncated and denuded edges. Thus we are brought face to face with the fact that there is a blank (or hiatus) in the succession of the strata between the Lower and Upper Old Red, or Devonian, series in the west of Ireland, due to the absence of the Middle Devonian series of Devonshire, Belgium, and the Rhine. 1 Region of the Reeks and Killarney Mountains. Not less definite is the evidence of an hiatus between the Glengariff Grits and Slates (Devono- Silurian beds) and the Upper Old Red, and Carboniferous groups in the region extending from the extreme western promontories of Kerry and Cork eastwards to the coast of Waterford. In this region, however, owing to the great amount of flexuring and folding, the result of lateral pressure, to which the strata were subjected after the Carboniferous epoch, and also to the absence of such sections as we find in the 1 This subject is more fully developed iu my paper *0n the Geo- logical Age of the Rocks forming the Southern Highlands of Ire- land,' Quart. Journ. Gtol. Soc. vol. xxxv. p. 699 ; and also in another ' On the Relations of the Carboniferous, Devonian,, and Silurian Rocks of the South of Ireland to those of North Devon/ Scien. Tram. Hoy. Dull. Soc. vol. i. (new series), 1880. 38 Geological Formations of Ireland Dingle promontory, the unconformable relations of the Glengariff beds to those which succeed them are not so very clear, and hence they were supposed by Griffith and others to be conformable and continuous. But a detailed re-examination of this region carried on a few years ago by the officers of the Geological Survey has conclusively shown that the hiatus, or break in continuity of stratification, in the region here described is even more remarkable than in that to the north of Dingle Bay. For, not only is the Middle Devonian stage unrepresented, but often (at various points) the Upper Old Eed Sandstone, and even the Lower Carboniferous slates and grits, which sometimes reach a thickness of 2,000 feet, have no representatives ; so that the Carboniferous Limestone is found almost directly reposing upon the Glengariff beds. Such is the case at Rough ty Bridge in the Kenmare valley; and at Macroom in Co. Cork, where the Carboniferous Limestone rests almost immediately on the Glengariff Slates. But, usually in the districts of Glengariff, Kenmare, Sneem, and along the Coomhola Eiver the Lower Carboniferous strata are found resting directly on these beds with- out the intervention of any representative of the Upper Old Eed Sandstone, estimated to attain a thickness of 3,000 feet in the eastern and northern parts of Cork and Waterford. The explanation Relations of Formations in Kerry, &c. 39 which I have offered in order to account for the remarkable relations of the strata in this part of Ireland may here be briefly recapitulated. At the close of the Lower Devonian period, after the de- position, either in the ocean or in a large inland lake, of the Glengariff Grits and Slates, the whole region was subjected to elevation and disturbance ; that tying to the north of Dingle Bay having been highly disturbed, that to the south only slightly so ; having in effect been upraised into the air in the form of a low arch or dome. In this condition the whole region remained throughout the middle Devonian period, 1 but at its close was again sub- merged under the waters of an extensive lake, and as submergence progressed, the conglomerates, or shore-beds of the Upper Old Eed Sandstone (or Upper Devonian), were successively deposited along the borders of the land, gradually giving place as subsidence proceeded to the finer sediments now forming the ' Kiltorcan Beds,' and ultimately upon further depression, when the waters of the ocean invaded the region, being succeeded by the Lower Carboniferous grits and slates, which contain numerous marine forms. By some such hypothesis, 1 During which the Ilfracombe limestone beds with their as- semblage of marine forms were deposited, but which are wholly unrepresented in Ireland. 4O Geological Formations of Ireland which seems quite compatible with what we know of terrestrial movements, we are able to explain the peculiar relations which subsist between the Devono- Silurian beds on the one hand, and the Upper Devonian and Carboniferous on the other, through- out the whole region of the south of Ireland. I have already endeavoured to explain that Devonian rocks, which are the marine representa- tives of the Old .Red Sandstone, do not, as far as we know, occur in Ireland ; but, under its lacustrine or freshwater type of Old Red Sandstone, the for- mation is extensively represented in the south-west, and less so in the centre and north. It consists of a great accumulation of sandstones, conglomerates, shales, and slates, which have been deposited below the waters of a large lake (or inland sea) as shown by Mr. Godwin- Austen ; or, more probably, in two lakes separated from each other by a ridge of Silurian land which occupied the central portions of Ireland. This dividing ridge includes the district of Connemara, and in the opposite direction probably stretched across the Irish Sea to include North Wales and Central England. The existence of such a ridge will account for the absence of beds of this formation between the base of the Carboniferous series and the Lower Silurian rocks in the counties of Wicklow,.Westmeath, Monaghan, and Armagh. To the north of this tract Upper Old Red Sandstone 41 the formation was deposited in a basin which stretched into the centre of Scotland; and to the south, in a basin the western and southern shores of which lie below the Atlantic waters. UPPER OLD RED SANDSTONE, including the c Kil- torcan Beds. 9 This upper division of the Devo- nian series is represented in Devonshire by the ' Pickwell Down Sandstone,' and in Belgium by the ' Psamite du Condroz.' In these districts, however, it rests quite conformably upon the Middle Devonian beds ; but in Ireland, where these latter are unrepre- sented (owing 1 to the hiatus above described), the Upper Old Red rests indifferently upon various divisions of the Palaeozoic series, from the Lower Silurian up to the Lower Devonian inclusive ; and necessarily with greater or less unconformity of stratification. Thus, in the Dingle promontory the Upper Old Red Conglomerate is found crowning in a fine escarpment the summit of Caherconree, at an elevation of 2,713 feet, resting successively upon beds belonging to the 'Bala,' 'Ludlow,' and ' Dingle' series, and, following the northern coast, it dips beneath the Lower Carboniferous strata of Tralee Bay. 1 Throughout the southern parts of Co. Cork, owing to the unconformable overlap already referred to, only the uppermost beds are represented, and 1 See Geol. Survey Maps, sheets 161 and 172. 42 Geological Formations of Ireland these often very thinly; but further north, in the counties of Limerick, Tipperary, and Waterford, the lower beds emerge and form a fringe surrounding the dome-shaped masses of Lower Silurian grits and slates of Slieve Aughty, Bernagh, Mauherslieve, Galtymore, and Slievenaman. In the Knockmealdown and Commeragh range, the Upper Old Red Sand- stone, with a conglomerate base, covers nearly the FIG. 4. Section on the left bank of the Suir above Waterford. c, Dark purple conglomerate of Upper Old Eed Sandstone resting on light-grey and blue banded slate of Lower Silurian age. Beds nearly vertical. whole back of the ridge reaching in Knockanaff rin an elevation of 2,478 feet ; but the Lower Silurian beds emerge from beneath along the eastern side of the mountain, stretching away towards the valley of the Barrow at Waterford. On the banks of this river above the city a very fine cliff section is laid open, in which the Upper Old Eed Conglomerate may be seen resting on the Lower Silurian grits and slates, Kilt or can Beds 43 with, of course, a marked unconformity. It is here made up of well-rounded pebbles of grit, slate, and vein -quartz ; while at Dromore the pebbles are sub- angular, forming a breccia, composed chiefly of trappean rocks. 1 These changes in the composition of the basal conglomerate are due to the changes in the composition of the older rocks in different localities from the waste of which the conglomerate beds have been constructed. Kiltorcan Beds. As we ascend from the basal beds the formation becomes less conglomeritic, and ultimately passes into a series of fine- grained yellow, grey, and light-red sandstones and shales, evenly bedded, sometimes ripple-marked, indicating quiet deposition and gentle currents, while the surfaces of the slabs are often covered over with large worm- tracks. That these beds were formed under the waters of a lake is clear from the occurrence of a large freshwater bivalve discovered in Co. Cork by Professor Jukes, and named after him Anodonta Jukesii (Forbes). 2 In addition to this shell are remains of fishes such as Coccosteus, Pterichthys, Glyptolepis, and Asterolepis ; while magnificent fronds of a primseval fern, the Palceopteris Hibernica, and 1 * Explanation ' to sheets 167, 168, &c. of the Geol. Survey by G. V. Du Noyer. 2 Report on the Fossils from Kiltorcan Hill, by W. H. Baily, Proe. Roy. Irish Acad. 2nd series, vol. ii. 44 Geological Formations of Ireland fragments of Sigillaria and other forms, bearing affinity to the oncoming Carboniferous flora, enable us to infer that the marginal lands of this upper Devonian lake were clothed with a rich sub-tropical vegetation. The thickness of the Upper Old Eed Sandstone, including the Kiltorcan beds, may be taken at 2,000 feet. 1 The northern parts of Ireland were probably in a condition of dry land during the deposition of the Devonian beds over the southern area, except in the case of several detached freshwater basins where strata, referable to the Devonian period, were deposited at successive intervals. These deposits were undoubtedly disconnected from those of the south by a ridge of older rocks, extending from the western highlands of Gal way, Mayo, and Sligo across to the eastern coast. To the Devonian period are referable in the north- ern area (1) the Eed Sandstone and Conglomerate of Curraun Achill, a mountain rising from the shore of Clew Bay to a height of 1,784 feet, which rest unconformably onmetamorphic schists and quartzites, and dip beneath the Carboniferous beds forming the margin of this bay. 2 (2) The Eed Sandstone and 1 The ' Kiltorcan beds ' are the Yellow Sandstone ' of Griffith ; in some publications they have been called the ' Upper Old Red Sandstone,' but in reality are only the upper part of that formation. 2 Geol. Survey May, sheet 74. Old Red Conglomerate 45 Conglomerate, with, contemporaneous felstone and ash, extending from Ballaghadereen in the county of Eoscommon, by Boyle to the banks of the Shannon. (3) The large tract lying between Lough Erne and the Silurian rocks of Pomeroy, overlain unconform- ably by Lower Carboniferous strata along the south, and having a massive conglomerate base resting on metamorphic strata, well laid open near Irvingstown. This conglomerate consists of large rounded pebbles FIG. 5. Section near A, Schists. B, Old Red Conglomerate, c, Purple Sandstone, &o. x , space unseen. D, Basaltic Dykes. of quartzite, felstone-porphyry, and schist which remind us of the conglomerate base of the Old Red Sandstone on the shores of Loch Lomond, Loch Long, and the banks of the Firth of Clyde. These basal beds are succeeded by dark purple or chocolate- coloured shales, flagstones, and pebbly sandstones of great but unknown thickness, and traversed by re- markable basaltic dykes. There can be no doubt, from 46 Geological Formations of Ireland their unconformity to the overlying Carboniferous strata, that these beds are the representative of the Dingle and Glengariff series of Lower Devonian or Devono-Silurian age. (4) A fourth representative of this epoch is the great conglomerate of the east coast of Antrim between Cushendall and Cushendun. This rock is finely laid open to view in the old sea-caves of the headland, resting on metamorphic schist, and pass- ing upwards into less pebbly sandstone. The pebbles consist of vein-quartz, quartzite, mica-schist and felstone-porphyry. On the coast near Cushen- dall the blocks of porphyry, which are rounded or sub-angular, attain to a size of three or four feet in diameter ; ! and the whole mass lias its counterpart in the conglomerate of the coast of the promontory of Kin tyre, not so far distant across the strait. 2 (5) The last representative of this period requir- ing notice is the remarkable isolated tract lying 1 Expl. Mem., sheet H of the Oeol. Survey Maps, by E. G. Symes and A. McHenry, p. 12 (1886). 2 I have endeavoured to represent the physical condition of this (the Devono-Silurian) epoch in plate iv. figs. 1 and 2, of the maps in the Physical History of tlie British Isles, from which it will be seen that the Fermanagh, Tyrone, and Antrim beds were deposited in the old basin, called ' Lake Caledonia,' by Dr. A. Geikie. 'On the Old Red Sandstone of Western Europe,' Trans. Roy. Soc. Edin. vol. xxviii. Knockalla Conglomerate 47 amongst the mountains of North Donegal in the Fanad promontory between Lough Swilly and Mulroy Bay. This tract was discovered during the progress of the Geological Survey in the district, and is of special interest, as the preservation of the beds is altogether due to the fact that they have been vertically lowered (or ' let down ') relatively to the surrounding strata by a large fault which ranges in an E.N.E. direction along the northern base of Knockalla, a quartzite FIG. 6. Section over Knocltalla t Co. Donegal. Q, Quartzite and Schist with band of Diorite D. c, Old Red Sandstone and Conglomerate, resting unconformably on Quartzite. x , Fault with downthrow to the North. mountain rising to an elevation of 1,196 feet direct from the sea. The beds consist of chocolate-coloured sandstones and shales, sometimes pebbly, with a conglomerate base, resting discordantly upon the metamorphic strata. It is probable that this little tract, so far removed by mountains and valleys from its representative in the counties of Fermanagh and Tyrone, was deposited in a lake originally continuous 48 Geological Formations of Ireland with that in which the Old Eed Conglomerate of the Great Glen was deposited, called by Geikie the 6 Lake of Lome. 5 CARBONIFEROUS SERIES. The Carboniferous strata occupy about one-half the entire area of Ireland, but are represented chiefly by their lowest members, namely, by the Lower Carboniferous slate and grit and the Carboniferous Limestone. That formation, which from its physical position in Central England is called ' the Mountain Limestone,' is in Ireland the formation of the plains ; except in the north-western districts of Leitrim, Sligo, and Fermanagh, where it rises into bold escarpments and isolated hills. The upper members of the Car- boniferous series, including the Coal-measures, occur in a few detached outliers in the north and south ; remnants of a once widely extended formation, which probably covered at least three-fourths of the entire surface of the country, but has since been almost entirely swept away by denuding agencies. To this subject we shall return when we come to discuss the origin of the great central plain of Ireland. The basement beds of the Carboniferous group consist in the south of Ireland of cleaved slates and grits ( c Coomhola grits'), which swell out to a great thickness in Cork and Kerry, and are full of marine Lower Carboniferous Beds 49 fossils (Cucullcea, Posidonomya, &c.) ; they are con- formable to the Upper Old Red Sandstone, which, being a freshwater or lacustrine formation, affords a well-recognised upper limit by which the Devonian beds can be separated from the overlying Carbonife- rous, of marine origin. In the centre and north of the country these basement beds consist of yellowish conglomerates, grits, and shales, with earthy lime- stones, representing the * Calciferous Sandstone series ' of Scotland. A fine section in these beds is laid open along the precipitous cliffs which border the Atlantic west of the entrance to Killala Bay. The thickness of the series must here be about 1,500 feet. Eesting on the basement beds above described comes the Carboniferous Limestone, stretching across the central plain from sea to sea, and north- wards to the base of the Donegal Highlands, skirt- ing the mountains of Mayo and Galway, then encircling the isolated domes of Silurian and Old Red Sandstone which rise from the central plain towards the south-west, and finally, after passing beneath the Coal-measures of Kanturk and the upper valley of the Blackwater, emerges around the shores of the Killarney Lakes at the northern base of the mountains of Kerry. Owing to the numerous foldings into which the Carboniferous and Old Eed E 5'0 Geological Formations of Ireland strata have been bent, the limestone is found filling the centres of several narrow troughs in the south of Ireland,, lying along approximately east and west axes, and bounded by anticlinal ridges of Old Red Sandstone. Of these synclinal troughs, the most important are those of the Kenmare River, of Cork and Middleton, of Tallow, of Dungarvan and Lis- more, and of Carrick-on-Suir- The Limestone formation generally consists of three members a lower, middle, and upper; the middle (or 4 Calp beds ') being generally more earthy than the other two,, which consist in the main of pure fossiliferous limestone ; the whole formation attains the thickness of 2,500 to 3,000 feet. In some localities,, as in Co. Limerick, it has been found difficult, or impossible, to distinguish these three divisions from each other ; but Prof. De Koninck has pointed out that the three stages which he has recognised by the occurrence of certain fossil forms in Belgium can also be co-ordinated with those of this country, and it would seem that they correspond very nearly with the threefold division above described. 1 In the lower 'niveau' he places the Limestone of Armagh with Orthis Michelini, Athyris Roysii, Spirifera tornacensis, Conocardium fusiforme,. Phillipsia pustulata ; in the middle, the 1 Ann. de la Soc. geol. de Belg. t. ix. p. 50. Carbonifero2i Limestone 51 Limestone of Bathkeale, A dare, with Amptexus co- ralloides, Spirifera striata, Syringothyris cuspidatum, Conocardium Hibernicum and G. Koninckii; and in the upper, the Limestone of Galway with Productus yiganteus, P. striatus, P< cora, Orthis resupinata, Spirifera glabra, and Cardiomorpha oblonga. On the other hand, in the north-western counties of Sligo and Fermanagh the formation is entitled to its English name of ' Mountain Limestone,' for in no district of the British Isles are there grander escarpments and terraces than in that which lies between Sligo Bay and Lough Erne, overlooking the southern shores of Donegal Bay. Over this tract the Upper Limestone, resting on the softer alp series, rises into a tableland, broken off along walls and bluffs of rock, jutting out in bold headlands with scarped faces, from 1,500 to upwards of 2,000 feet above the level of the adjoining ocean. Close examination of hand specimens, and espe- cially thin transparent sections placed under the microscope, will show that this great calcareous formation consists almost entirely of the shells and skeletons of marine animals, such as corals, cri- noids, foraminifera, and molluscs. Even the denser masses which to the eye exhibit no appearance of organic structure, when examined under the micro- scope in the manner above described, are almost E 2 52 Geological Formations of Ireland certain to show a field full of animal forms, chiefly those of foraminifera and crinoids. The formation, therefore, notwithstanding its great thickness, must be considered as the work of marine animals which lived in the waters of the clear ocean of the Car- boniferous period, generally far removed from land, and uncontaminated by muddy or sandy sediment. Deep sea-soundings in recent times have shown that similar deposits are in course of formation over vast tracts of the ocean bed of the present day. The Upper Limestone is remarkable for contain- ing numerous beds, and great amorphous masses of a siliceous material resembling flint, known as 'chert.' Sometimes this material completely replaces the original limestone ; and we find corals, crinoids, and other forms which were originally constructed of calcareous material frequently preserved in chert. Thin translucent sections which I have examined under the microscope have shown that the original forms of crinoids, foraminifera, &c. can be more or less distinctly recognised ; so that we may infer that the chert is a product of replacement by silica from solution of the original calcareous material of which these structures were composed. 1 Dr. Hinde, how- 1 On this subject see paper ' On the Nature and Origin of Chert- beds in the Carboniferous Limestone of Ireland,' by E. Hull and E. T. Hardman. Abstract, Geol. Mag. Sept. 1877. Shore Beds of the Limestone 53 ever, has shown that the masses of chert are largely composed of spicules of siliceous sponges ; and this is true also of the same formation in England. 1 The general succession of the Carboniferous Limestone series in the N.W. of the country is illustrated by the adjoining section though a portion of Counties Sligo and Mayo. (Fig. 7.) 2 In South Donegal we find the clearest evidence that the mountains of this county originally formed the margin of the great basin in which the formation was deposited. Thus along the southern flanks of Barnesmore, a granitic mass rising about 1,500 feet above the sea, the limestone formation is found to pass into a calcareous conglomerate and breccia the beds of limestone gradually thinning out as we approach the older rocks, and being replaced by f shore beds ' of sand, gravel and boulders. On the other hand, the once wide extension of the Carbon- iferous strata over the older metamorphic rocks of West Donegal may be inferred from the occurrence of two outliers of conglomerate containing impres- sions and casts of stigmaria and annelid tubes on the summit of Slieve League at a level of 1,964 feet above the ocean surface beneath. 3 1 Geol. Mag. October 1887, p. 435. 2 In the collection of the Rev. Dr. Grainger, Rector of Brough- shane, are some remarkably fine fossils from this formation. The collection is especially rich in forms of Cephalopods. 3 Journ. Roy. Geol. Soc. Ireland, Feb. 1886. 54 Geological Formations of Ireland ,3 3 *~ f . v o> a 11 1 I u 2 S -I I S II o a 'Carboniferous Volcanic Rocks 55 The Carboniferous Limestone of the south-west of Ireland, like that of Scotland and Derbyshire, is characterised by the occurrence of volcanic rocks. The rich pastoral plain of Counties Limerick and Tipperary, called the Golden Valley, is diversified by a range of scarped hills composed of volcanic rocks, arranged in a somewhat circular form, outside and beyond which are several isolated bosses rising from out of the limestone plain, both to the north and to the south of the circular range. These latter are composed of felstone-porphyry^ and, judging from their position, are clearly the old necks, now filled with solid rock, through which the ancient lavas and ashes forming the hills were erupted. On examina- tion it is found that there were two successive out- bursts of volcanic materials. During the earlier, felspathic lavas along with quantities of ash and lapilli were extruded ; during the second, which occurred after a considerable interval, and just at the close of the Carboniferous Limestone period, augitic lavas which have consolidated into basalts and melaphyres were poured forth. The following section drawn through Knock Roe, a hill about 672 feet in elevation, will serve to illustrate the succes- sion of volcanic materials of the earlier outburst. (Kg. 8.) In this section we have no fewer than ten different 56 Geological Formations of Ireland Sfo l fa 43 SI'S III a 5j -a^'S/a O-f P J3 O o3 OQ 0~ -2 g . 1^1 2 . ti_. *cc bCTS 1^1 02 O S /-" ** > iO O t- GO Ci '^^ CO O) I Ki- s 1 1^_ Jg BQ JQ l remains belonging to the labyrinthodont amphibians, from the ' Jarrow coal/ of peculiar characters, have mad-e FIG. 9, Section across the Leimter Coal-field.. W. Castlecomer. Carlow. B.. X, 5 StF S L G, Granite. L, Carboniferous Limestone. S and P, Yoredale Shales and Carlow Flags, c, Lower and Middle Coal-measures in centre of Basin. this coal-field somewhat celebrated amongst palaeon- tologists. They have been described by Professor Huxley and Dr. Percival Wright. 2 Coal-seams of the Castlecomer Basin. 1 . Three-foot coal (worked out) . . . .30 2. Four-foot, or Jarrow, coal .36 3. Upper Toiclerton Coal , . 1 ft. 6 in. to 2 4. Lower Torvlerton Coal . . . do. 2 1 The Upper Coal-measures, which in Lancashire attain a thick- ness of 2,000 feet, are not represented in Ireland. If they were ever formed, they have since been completely denuded away. - Trans. Hoy. Irish Acad. vol. xxiv,(18G7). 64 Geological Formations of Ireland Terrestrial Changes at the close of the Carboniferous Period. The events which followed the formation of the Carboniferous rocks over the Irish area may now be touched upon. After the Coal-measures had been deposited the surface must have presented the appearance of a vast lagoon, but little, if anything, elevated above the level of the sea, and (as we may judge from the similarity in the succession of the beds) physically connected with the Carboniferous tracts of England. In the north-west and south- east the older rocks rose above the surface of the lagoon, and formed portions of the un submerged lands from which the sedimentary materials compos- ing the Coal-measures were derived. 1 But at the close of the period terrestrial movements, for a long time in abeyance, arising apparently from contrac- tion of the earth's crust in a direction nearly north and south, set in. The forces thus brought into action influenced the whole of the British Isles and the west of Europe, and to them are due the numerous flexures and lines of geological boundary which may be observed ranging in approximately east and west directions in these countries. As regards the Irish area, they produced their most 1 I have endeavoured to represent the positions of the submerged and emergent tracts of the Upper Carboniferous epoch over the British Isles in the Physical History of the British Isles, plate vii. (London, 1882). Flexures of Strata in Kerry 65 powerful and striking effects over the southern and northern districts, while the centre was only par- tially and locally disturbed and elevated. Over the former districts the beds were forced by lateral pressure to arrange themselves in numerous foldings, very evident, upon an inspection of the geological map, by the narrow bands of Carboniferous beds alternating with those of Old Eed Sandstone. So powerful, indeed, were the tangential forces that along the base of the Killarney Mountains the strata appear to have been thrust back upon themselves ; so that the Carboniferous Limestone of the plain dips towards the Devono-Silurian beds of the moun- tains of Kerry; but to this subject we shall re- turn when we come to treat of the mode of formation of the mountains themselves. 1 Meanwhile, as the anticlinal ridges were forced into the air, and as the whole of the central plain was probably somewhat elevated into a land surface, the agents of denuda- tion commenced to wear down the exposed surfaces. Thus was begun the process of destruction to the coal- formation which was continued through so long a period that (as we have seen) only a few isolated remnants have been left as monuments of the ruin which has overtaken this once widely-spread forma- tion. 1 See page 166 F 66 Geological Formations of Ireland The amount of denudation in the north of Ireland accomplished even before the Permian period properly set in (or at least before any Permian strata were deposited) was very great ; for, as I shall now proceed to show, we find beds of this formation resting directly on the Carboniferous Limestone, the Upper Carboniferous strata having been stripped off previous to the deposition of the Permian strata. Similar events took place in the north of England, which at this early period of the earth's history appears to have been physically connected by con- tinuous land with the north and centre of Ireland. 1 The formations which remain now to be de- scribed are restricted to the north-eastern districts of Ireland, where they are collected within a narrow- compass, and generally represented upon a small scale. It is easy to see that the presence of some, or all, of these beds is due to the solid sheets of basalt which, during the Miocene period, were poured over this district, affording a protection to the strata that were still at the period of eruption, to some extent, preserved. This great sheltering cap has itself been subjected to a large amount of waste. Its original area extended far beyond its present limits; but, seeing how the softer Mesozoic strata are grouped 1 For a restoration of the physical geography of this epoch, see plate viii. in the PJiys. Hist. Brit. Isles. Permian Beds 67 closely around its skirts, we may ask ourselves whether, in its absence, evidence of their former presence could have been left sufficient for us to infer that they ever had been there ? PERMIAN BEDS. The representatives of this for- mation occur only on a very small scale, but they are of much, interest as showing that the physical conditions prevalent in Britain during the close of the Palaeozoic period were extended over the N.E. of Ireland. The formation is represented by beds belonging both to the lower and upper divisions, the * Eotheliegende ' and c Zechstein ' formations of Germany, the 'Lower Permian' and 'Magnesian Limestone' of England; and these we shall now .proceed to describe. Lower Permian Beds. The only representatives of this division that we are at present acquainted with are found underlying the city of Armagh. At the time of my visit in 1872 they had not been re- cognised ; but, being familiar with the various types of Lower Permian strata in England, I had no diffi- culty in recognising the nature of the peculiar formation which is laid open to view in the marble quarries a short distance to the west of the city. The upper portion is a boulder deposit, and the lower a limestone breccia; and as the former is overlaid by the Boulder-clay of the Glacial (or Drift) period, F 2 68 Geological Formations of Ireland both had doubtless been confounded together by previous observers. The breccia had probably been considered as a broken condition of the Carboniferous Limestone. In this quarry, therefore, we have the curious concurrence of two boulder formations, of different and widely separated periods, superimposed one upon the other (see fig. 10). Though somewhat similar in appearance, there is really a difference FIG. 10. Section of Permian beds at Marble Quarry, Armagh. B, Boulder-clay (Drift), d, Permian boulder beds 2 ft. o, Red stratified conglomerate, or breccia resting on & compacted sandy breccia of limestone fragments. L, Carboni- ferous Limestone. between them which the practised eye may easily Jetect; and the divisional line between the two formations may easily be followed along the face of the cliff. 1 The Permian beds are of a deep red colour, rudely stratified, and at the base where they rest on the limestone consist of a consolidated breccia of limestone pebbles in a sandy base like the 1 Prof. Sir A. Ramsay, who subsequently visited the section with me, fully concurred in the identification of these beds. Magnesian Limestone 69 ' Brockram ' of the Cumberland district. The blocks of rock in the boulder bed consist of purple grits and felspathic sandstones, sometimes calcareous, which may have come either from the Silurian or Old Eed Sandstone districts to the north or west. They attain a size of two feet in diameter, and, though not showing any actual glaciated surfaces, have the appearance of ice-transported boulders. 1 The beds, in fact, have a strong resemblance to those of Shropshire and Worcestershire, from an examina- tion of which Sir Andrew Earn say was led to infer the prevalence of glacial conditions in parts of the British Isles during the earlier Permian period. The representatives of the Magnesian Limestone occur in two localities one near Cookstown, Co. Tyrone, and the other at Cultra, on the southern shores of Belfast Lough. They have been described by Dr.Bryce and Professor King, who has established their correlation with the Upper Permian beds of England by the occurrence in them of shells of the genera Schizodus, Bakevellia, &c. It is only at ebb- tide that these beds can be observed at Cultra, where they rest directly on Lower Carboniferous shales and limestones. 2 1 ' On the Permian Beds of Armagh,' Quart. Journ. Geol. Sec. vol. xxix. p. 402. 2 The bands of magnesian limestone were formerly quarried for chemical purposes, so that little is left visible above the sands of the beach. 7Q Geological Formations of Ireland From these instances it will be apparent that what I have stated above is borne out by the evidence before us, namely, that before the Permian period there was denudation on a large scale of the recently formed Carboniferous strata, throughout a period of long duration, during which the rains and rivers, and perhaps the waves and currents of the sea, were undermining and carrying away the materials of which that formation is composed. CHAPTER II MESOZOIC FORMATIONS TEIASSIC BEDS New Red Sandstone and Marl. The beds of this formation occur as a narrow band encircling the basaltic region of Antrim and Derry. Occupying generally low ground and valleys, such as those of the rivers Blackwater and Lagan and the estuary of Belfast Lough, they rest unconformably on all the formations older than themselves, and in the geological classification form the base of the Meso- zoic series. The lower portion, or Bunter Sand- stone, consists of soft bright red or variegated sandstone with marly bands; and the upper (or Keuper Marl) consists of bright red and grey lami- nated marls with irregular bands of gypsum, which are well shown in the railway cuttings between Larne and Carrickfergus. Near this latter place extensive beds of rock salt occur, and are worked by means of vertical shafts. Sections in the Bunte* Sandstone are laid open along the north shore of 72 Geological Formations of Ireland Belfast Lough and south of Dungannon, where the beds have yielded remains of fish. One of the most interesting exhibitions of the New Eed Sandstone strata occurs at the quarries of Scrabo Hill, near Newtown Ards. The rock is quarried for building purposes, and consists of light yellow and reddish freestone, lying probably at the base of the Keuper division. On the surfaces of the strata sun-cracks, rain-pits, and ripple-marks may often be observed, as in the case of the Stourton quarries in Cheshire. They are capped by masses of spheroidal dolerite, and are evidently in proximity to an old volcanic neck (or vent), as the sandstones are penetrated by horizontal sheets and vertical dykes of basalt, intersecting each other in various directions, and seriously injuring the quality of the stone in immediate contact with them. As the quarries are very extensive, and the excavations ex- pose a large section of the hill-side, there are few places where the phenomena of igneous intrusion can be more advantageously studied than at this spot. 1 KH^ETIC AND LIASSIC BEDS. The representatives ' Detailed sections and descriptions of these beds are given in the Explanatory Memoir to accompany sheet 37 of the maps of the Geological Survey. Another small patch of New Eed Marl with gypsum occurs at Carrickmacross, in Co. Louth. This is the most southerly place where the formation is represented. The beds have been deposited in an old depression amongst the Palaeozoic rocks, xpl. Mem. sheets 81 and 82, p. 8. The Lower Lias 73 of these formations occur together in a few narrow bands at intervals around the base of the Chalk escarpment of Antrim ; namely, at Larne, Collin Glen, near Belfast, and at Portrush on the northern coast, where they have been indurated by contact with a sheet of intrusive dolerite. At Larne the beds are well laid open on the coast north of the town. The upper belong to the Lower Lias, and consist of light blue, grey, and dark clays with thin earthy limestones, containing Gryphcea incurva, Ostrea Liasica, Ammonites planorbis, &c. Below these are dark shales with Avicula contorta resting on light greenish calcareous sandstone, somewhat pisolitic, belonging to the Ehsetic series ; below these are the Keuper Marls. The main portion of the Lias, together with the whole of the Jurassic (or Oolitic) series, is unrepre- sented, as the Cretaceous beds immediately overlie those above described. If the Jurassic beds were ever deposited over the N.E. of Ireland, as was pro- bably the case, they were subsequently swept away before the Cretaceous period; there is, iherefore, a great unrepresented hiatus in the geological series between the beds just described and the upper Creta- ceous strata by which they are succeeded at Larne. 1 1 For a further account of the Liassic beds, see papers by Messrs. Tate and Holden in the Quart. Journ. Geol. Soo. Land. vol. xxiii. and Portlock's Geology of Londonderry, p. 55, &c. 74 Geological Formations of Ireland CRETACEOUS BEDS. The Lower Cretaceous beds being absent, the Upper, consisting of the Upper Greensand and Chalk, are the sole representatives of this great group of strata. These beds, which are soft and easily disintegrated, occur as a very narrow fringe cropping out from beneath the dark basaltic sheets, to which they owe their preservation, and with which they form a remarkable contrast; the whitest of all rocks being in immediate contact with the darkest. The Upper Greensand is similar in appearance to that of England, as it consists of a greenish cal- careous sandstone, soft, very friable, and sometimes pebbly. Its colour is due to small grains of silicate of iron, which under the microscope are seen to be round or oval, and are probably of organic origin. 1 Good sections occur in Collin Glen and at Larne, and their thickness when greatest may be placed at thirty feet ; but it is generally less than this. The Chalk formation which succeeds is precisely similar to the upper portion of its representative in the south and east of England. It consists of com- pact white limestone with bands and nodules of flint. Amongst these are some of large size, of the 1 Similar strata were discovered by the officers of H.M.S. ' Chal- lenger ' to be in course of formation over portions of the deep ocean bed ; and this occurrence corroborates the views of the late Professor Ehrenberg, that the grains are the casts of foraminiferal shells. The Chalk Formation 75 shape of sponges, and perforated through the axis, the orifice being filled with chalk. 1 Fossils, such as Echini and molluscs, are locally plentiful, but in reality the whole mass is of organic origin. The Antrim chalk is usually sufficiently hard to allow of its being cut into thin transparent slices, which when placed under the microscope show numerous sections of foraminifera, plates and species of Echinoderms and other forms, imbedded in an impalpable calcareous paste. From specimens ex- amined by Prof. T. Rupert Jones and Mr. W. K. Parker the following genera of Foraminifera were determined : Lituola (?) Valvulina, Dentalina, Buli- mina, Textilaria, Vemeuilina, Globigerina, Planor- buUna, and Pulvinulina.* The sections of flint present somewhat similar forms under the micro- scope, except that they were less definite. 3 The thickness of the Chalk formation varies exceedingly, as its upper surface had undergone extensive erosion previous to the overflow of the basaltic covering. Between the basalt and chalk there is generally an irregular bed of flint-gravel, 1 These large fossil sponges are locally called ' Paramoudras ' a name not to be found in zoological dictionaries. 2 Journ. Roy. Geol. Soc. Ireland, vol. iii. (New Ser.) pp. 88-9 (1872). 3 Zinc has been discovered by Mr. Hardman in the Chalk of Co. Tyrone. Ibid. p. 159. 76 Geological Formations of Ireland with red ochre, filling the hollows in the surface of the rock. These flints have undoubtedly been derived from the beds of chalk, which originally were superimposed on the existing beds, and which have probably been carried away in solution by atmospheric waters, while the insoluble flints have been left behind. As this flint-gravel was formed FIG. 11. Cliff Section of Chalk overlaid ~by Basalt. Co. Antrim. j a, Upper Greensand, cropping up from below the chalk. &, White chalk with bands of flint, c, Flint-gravel resting on an eroded surface of the chalk. B, Basalt with spheroidal structure. chiefly over a land surface of the early Tertiary period, it might have been expected to contain remains of the animals then inhabiting the surface ; none, however, have yet been found. The maximum thickness of the formation is about 350 feet, In the interior of the chalk-flints there often occurs a white powder, which Mr. Joseph Wright of Belfast has subjected to close scrutiny, resulting in The Chalk Formation 77 the discovery of a large number of beautifully formed genera and species of Microzoa an illustration of the way in which the most apparently worthless objects in nature repay investigation and study. 1 The Antrim coast often displays fine sections in. the Chalk capped by dark masses of basalt, or amygdaloidal lava. The beds are generally nearly horizontal, and are often traversed by basaltic dykes, and in some cases by the necks of old volcanoes filled with masses of lava. In other cases the old necks are choked up by bombs which have been blown into the air, and on falling back have filled up the throat. The rock on which stands Dunluce Castle is one of these, and another example may be seen in the cliff by the road-side east of Portrush. 2 Sections are also opened in numerous quarries along the escarpment between Larne and Lurgan, and in the interior of the basaltic area. The Chalk is brought to the surface through the agency of faults at Dunwater quarry and Temple-Patrick. The occurrence of the Chalk formation so far in advance to the north-west of its position in England is one of the most curious facts in the physical geology of the British Isles. Were it not for its 1 Trans. Brit. Assoc. (1874), p. 95 ; also Pep, Belfast Nat. Hist. Field Club (1873-4). 2 This I now consider somewhat doubtful. 78 Geological Formations of Ireland presence in the north-east of Ireland, it would scarcely have been suspected that it had originally extended into that region. Judging from the abruptly truncated form of its margin, except in the direction of Armagh towards the south-west, it must originally have been spread far beyond its actual limits. 1 In truth, its composition gives little evi- dence of proximity to land, for pebbles of older rocks scarcely ever occur, and are only numerous in the Greensand formation, as at Cushendall in County Antrim. 2 Recollecting that it has undergone two denudations one before the basaltic sheets were poured over its surface, the other since that time, in conjunction with the basalt itself we may well sup- pose that its original margin was far in advance of its present limits. The portion now preserved is, in fact, the central part of a wide and shallow basin, into which form the beds were tilted after the Miocene period ; this we infer from the fact that they are found to dip generally from the margin inwards. We may, therefore, venture to assume that the formation originally stretched as far as the mountains of Donegal towards the west, and those of Slieve Gullion, Carlingford, and Mourne towards 1 The relative positions of the submerged and emergent areas are represented in plate xi. in Pliys. Hist. Brit. Isles, fig. 2 ; while the distribution of the cretaceous strata is shown in rig. 1. 2 Expl. Mem. sheet 14, by R. G. Symes and A. McIIenry, p. 15. Origin of the Chalk 79 the south. In a northerly direction the forma- tion must have wrapped around the flanks of the mountains of Argyllshire, Kintyre, and Arran, and towards the east, those of the south of Scotland and the Lake district of England. If the formation was originally connected with its representative beds in England, which is highly probable, it was so across the area of the Irish Sea ; and the depression between the Welsh mountains and those of Cumber- land and North Lancashire was probably the line of country through which the connection was made. How far it stretched northwards into the Atlantic Ocean we have no means of knowing, and it imports little to inquire. 1 From what I have said as regards its com- position, it will be evident that the Chalk forma- tion has been built up layer upon layer over the bed of the ocean by the agency of marine animals, for the most part of minute size and low organisation. It contains but little sedimentary matter such as sand or clay, so that it must have been formed far out of reach of estuaries or the mouths of rivers bringing down sediment into the sea. Even the flints are due to organic agency; the silica having probably been secreted by certain forms of animal life from 1 Siliceous beds the representatives of the Cretaceous period have been observed by Prof. Judd in the island of Mull, in Scotland 8o Geological Formations of Ireland the waters of the ocean, 1 or in some cases replacing the original lirne. The representative of this forma- tion at the present day is the foraminiferal, cal- careous, ooze of the central Atlantic bed, and of other oceanic regions far remote from land. 1 Mr. Wright, of Belfast, as above stated, p. 76, has obtained immense numbers of foraminifera from the interiors of flints, but these were no doubt originally calcareous. The forms of marine animals whose tests or skeletons are chiefly built up of silica are those of sponges and radiolaria, together with those of diatomacea. 8i CHAPTER III TERTIARY, OR CAINOZOIC, FORMATIONS Miocene Beds; a chapter in volcanic history. The representatives of the Miocene period in Ireland are not the various forms of sedimentary strata, such as sandstones, shales and limestones, found in the centre and south of Europe, but great sheets of basalt and amygdaloidal lava, with volcanic ashes and lacus- trine formations of iron-ore and 'bole,' such as are frequently found in similar volanic regions. 1 On a former occasion, I attempted to present a connected sketch of the succession of events in the volcanic his- tory of this period as it is indicated for us in the north of Ireland, 2 and I shall follow the same course on the present occasion, only with somewhat less detail. 1 The description of the volcanic rocks in the N.E. of India, by Mr. V. Ball, of the Geological Survey of India, might apply, muta- tis mutandis, to the Antrim district ; so great is the similarity of the volcanic products in both countries. Mem. Geol. Surv. India, vol. xiii. pt. 2, 1877. 2 'Presidential Address to the Geological Section (C.) of the British Association ' at Belfast, 1874. Trans, of Sections, Report, p. 60. 82 Geological Formations of Ireland 1 Plateau-basalt of Antrim 83 The area occupied by tlie volcanic rocks, or sheets of lava, extends over nearly the whole of the county Antrim and the adjoining parts of Londonderry and Tyrone ; being bounded along the north and east by the sea, and in the other directions by a ridge, or es- carpment, marking the commencement of the basalt. The largest lake in Ireland, Lough Neagh, is almost entirely within the volcanic district, and both are drained by the river Bann, which flows northward from Lough Neagh into the Atlantic. The district is generally an elevated plateau, sloping downwards towards the valley of the Lower Bann, and along the north-western and southern borders, overlooking Lough Foyle and Belfast Lough respectively. It is bounded by noble escarpments with precipitous flanks, rising to elevations of 1,000 to 1,560 feet, as repre- sented in the adjoining section taken from one of Gen. Portlock's illustrations (fig. 12). Along the north coast the scenery is often bold and striking ; sometimes, as in the neighbourhood of the Giant's Causeway, the cliffs rise from the sea in a series of terraces of dark columnar basalt, with vertical walls, and separated from each other by bands of reddish bole, or volcanic ash. These great beds or terraces represent successive lava flows, and they differ from one another not only in thickness but'in the size and arrangement of the columns. At G 2 84 Geological Formations of Ireland other times, as at Fair Head directly opposite the Mull of Kintyre (fig. 13), huge columns of basalt descend from the top of the cliff in one or two sheer vertical falls of several hundred feet, while at the base of the cliff the shore is strewn with broken columns of trap heaped up in wild confusion ; a Titanic breakwater which the waves of the sea have FIG. 13. Basalt Cliffs of Fair Head, Co. Antrim, 630 feet above the sea, seen from the south. Carboniferous beds in the foreground. treared up against their own advance. The slopes of the .escarpment overlooking Lough Foyle are cha- racterised by enormous landslips of trap (see fig. 12), illustrating the progress of atmospheric waste, as jthose .of (the coastline at Fair Head illustrate the destructive effects of wave action. In the former case the great sheets of basalt surmount softer Cretaceous and Triassic strata, and as these are The Basaltic Plateau 85 undermined by the percolation of water from springs, or by rains, the soft foundations give way, and the heavy superstructure breaks off along lines of joint- age, or the faces of the columns ; the whole then slips down the hill-side and lies a shapeless mass, till it has been still further disintegrated by frost, rain, and streamlet, and carried away pebble by pebble into the ocean. The central portions of the basaltic district are often rocky ; and near the eastern coast, where in- dented by valleys running inland from the sea, are diversified by terraced escarpments, but are usually destitute of striking features. 1 In this respect they may be contrasted with the volcanic region of Auvergne in Central France, where the plateau is surmounted by numerous cones and cup-shaped craters, from which the lava streams were originally poured forth, and have spread over the tracts at their base. But in the case of the north-east of Ireland, all the original volcanic cones and craters, though only a little more ancient (geologically speaking) than some of those of Central France, have entirely disap- peared ; and along with them a considerable portion of the sheets of trap on which they were planted. 2 1 In the centre of Co. Antrim the basalt is generally overspread by boulder-clay and gravel. 2 The massive dome of Sleamish, which rises above the plateau to 86 Geological Formations of Ireland The country has been swept by planing and levelling agents from which Central France has been exempt since the volcanic fires became extinct ; hence the contrast in the scenery of the two districts. What these planing agents were we shall enquire further on. I can only here mention their names the waves of the sea, river-streams, and sheets of ice. And now let us pass on to consider the nature of the volcanic products of Antrim, and the order in which they were erupted. As I have shown on the occasion above alluded to, 1 the age of volcanic activity is divisible into three stages, between each of which there was probably a considerable lapse of time, which may be clearly recognised by differences in the volcanic products. The earliest of these is characterised by highly sili- cated felspathic products, such as trachyte porphyry, pearlstone, and pitchstone ; the next, by sheets of amygdaloidal basalt with bands of bole, surmounted by stratified ash-beds with plants, lignite, and piso- litic iron-ore ; and the newest by more solid sheets of columnar basalt and dolerite, in which the preva- lent minerals are augite, Labradorite felspar, olivine, and titano-ferrite (or titaniferous magnetic iron-ore) . a height of 1,475 feet above the sea, is the nearest approach to an old volcanic cone, but it has no crater It is formed of dolerite rich in olivine. 1 See p. 81. Trachyte Porphyry 87 (1) Earliest stage. The lavas of this stage con- sist of trachyte porphyry, originally described by Dr. Berger in his memoir ' On the Geological Features of the North-east of Ireland ' under the name of * porphyry of Sandy-brae,' 1 and are well laid open to view at Templepatriok, Tardree and Brown Dod hills. Near the town of Antrim, and in the centre of the volcanic district, the rock consists of a light- greyish felspathic matrix in which are enclosed numerous grains of smoke-quartz and crystals of sanidine and plagioclase, together with minute cry- stals of biotite, magnetite and apatite. In the little cavities, which are numerous, a very rare form of silica, called ' tridymite,' has been discovered by an eminent German mineralogist, the late Professor A. von Lasaulx, who states that it is so abundant in some places that he was tempted to call the rock a 6 tridymite trachyte.' 2 This mineral consists of little thin hexagonal plates, generally intersecting in groups of three, and of a yellowish colour ; it might easily be overlooked by any but a practised mineral- ogist. The trachyte rises from beneath the basaltic 1 Geol. Trans. Lend. 1st ser. vol. iii. p. 189. - This discovery was kindly communicated to the author by Prof, von Lasaulx, in a letter written shortly after his return to Breslau- in the autumn of ,1876. See ' Petrographische Skizze aus Irland,' by this author, fitmeralogiscJien u. petrographiwhen Mi - theilungen (Wien), 1878, p. 410 88 Geological Formations of Ireland sheets, being the oldest and lowest of the volcanic rocks, and may possibly belong to the latter part of the Eocene period. At any rate, such is its contrast to the overlying sheets of basalt and amygdaloidal trap of Miocene age, that I am constrained to infer a considerable lapse of time between their respective eruptions. A similar rock occurs at Templepatrick and west of Hillsborough. These highly silicated and felspathic lavas are of very limited extent as compared with the augitic masses which succeed them, and they were poured forth over an irregular and much-eroded floor of Chalk, or of the still older rocks where this formation (as in the S.W. parts of the district) has been entirely removed. (2) Middle stage. The beds of basaltic lava re- presenting this stage consist of vesicular or amyg- daloid basalt, generally showing a spheroidal struc- ture. Bands of bole (red clays resulting from the decomposition of the lava) are frequent, and the rock, when amygdaloidal, contains in its cavities various zeolites and carbonates or chloritic grains. These sheets attain in some places a thickness of 600 feet, and are surmounted by bands of bole and volcanic ashes, with bombs of trap, well seen along the Antrim coast near Dunluce Castle, marking the close of the second period of eruption. That they were erupted directly under the air, from vents or fissures situated Miocene Plant-beds 89 at internals over the whole area, appears probable ; and this period of volcanic activity was followed by one of repose, marked by the formation of peculiar beds of iron-ore and lacustrine plant-bearing strata, which have enabled geologists to determine the geo- logical age of the formation. The plant-remains are seen to best advantage in the stratified ash-beds of Ballypalidy, and belong to the genera Sequoia, Cup- ressites, Rhamnus, Quercus, Pinus, &c., as determined by Mr. W. H. Baily, from specimens discovered by the late Mr. Du Noyer. The whole assemblage indicates a Miocene (according to Mr. J. S. Gardner, an Eocene) flora, and is similar to that detected in the basalts of the Island of Mull by the Duke of Argyll in the year 1851. ! The bed of pisolitic iron-ore, which is now being extensively worked at intervals over an area ranging from the northern coast near Pleaskin Head 2 to Ballypalidy on the south, is very peculiar, and often rich. It is composed of small grains of red haematite, of the size of a pea, or a bean, cemented by red 1 The plant-remains have recently been examined and compared with those from other districts by Mr. J. Starkie Gardner, who con- cludes ' that they might belong to any age between the beginning and the end of the warmer Eocene period ; and that they cannot be of earlier, and are unlikely to be of later, date.' Palceont. Soc. Trans. vol. xxxvii. (1883). 2 Where it was originally observed by the Rev. Dr. Hamilton in 1790. 9O Geological Formations of Ireland ochreous paste ; and is sometimes three or four feet in thickness. The beds of bole which lie below it are also rich in iron, and exhibit traces of lamination. On the whole, I am disposed to consider these beds as of lacustrine origin, formed in shallow water, and in a depression of the basaltic area due to the sinking of the surface at the close of the second period of volcanic activity. The streamlets which drained the surrounding uplands appear to have carried down the leaves and branches of the plants which grew on the margin of the lake. Owing to the decomposition of the basalt their waters also carried down the iron dissolved as carbonate, which on entering the lake was precipitated in the form of oxide of iron. Sec- tions of these deposits may be seen at Port Fad Mine on the north coast, at Broughshane, Shanes' Hill, Glengariff, Ballypalidy, and Island Magee. (3) Latest staye. After a period of rest, the volcanic fires again burst forth, giving birth to vast sheets of solid augitic lava, which were poured over the lacustrine deposits just described, and now form successive tiers of columnar basalt, which may be studied to great advantage along the cliffs of the northern coast. At Port Fad, and Shanes' Hill near Larne, these upper basaltic sheets may be observed rising in rows of massive columns from the upper surface of the band of iron-ore, or of a thin seam of Volcanic Necks and Dykes 91 lignite which is sometimes interposed. This upper series attains a thickness of about 400 or 500 feet ; so that the entire thickness of the plateau basaltic lavas of Antrim may be taken at about 1,100 feet ; a thickness which, owing to denudation, is less than the original, but is much exceeded by the masses of contemporaneous trap in the Island of Mull. Volcanic 'Necks' and . Basaltic Dykes. Although the actual craters and cones of eruption have been swept from off the surface of the country by the ruth- less hand of time, yet the old ' necks ' by which the volcanic mouths were connected with the sources of eruption can occasionally be recognised ; they some- times appear as masses of hard trap, columnar or otherwise, projecting in knolls or hills above the upper surface of the sheets through which they pierce, as at Carmoney Hill near Belfast, 1 or that remark- ably abrupt mass of trap called Sleamish which dominates the surrounding country near Ballymena. Tn other cases, the neck consists of a great pipe choked up by bombs and blocks of trap, more or less consolidated, bombs which have been shot into the air, and have fallen back again. To one of these 1 The basalt of Carmoney Hill is remarkably rich in olivine in its unaltered condition, and it contains a black -looking mineral of secondary formation, considered by Mr. Hardman, who has analysed it, to be 'new, which he has called 'Hullite.' Pruc. Roy. Irish Academy, 2nd ser. vol. iii. p. 161. 92 Geological Formations of Ireland near Portrush on the northern coast I have already referred, but another not far from this is not so generally known as such, namely, the rock in which stands the ruined castle of Dunluce, the ancient fortress of the MacDonnells, its base washed by the ocean. This rock is formed of bombs of all sizes up to 6 feet in diameter, of various kinds of basalt, dolerite, FIG. 14. Diagrammatic Section through Dunluce Castle, showing the supposed old Volcanic ' Neck.' N B, Amygdaloidal Basalt in sheets, resting on the Chalk formation (c), and penetrated by the old ' Neck ' N, filled with agglomerate. and amygdaloid firmly cemented, and presenting a precipitous face to the sea. The white chalk through which it rises may be seen below the clear green ocean water at low tide, and the coast cliffs, sepa- rated from the castle-rock by a broad chasm, are of the same formation. The site must have been a remarkably strong one in the days when cannon were unknown to the warlike tribes of Ulster. The Basaltic Dykes 93 section above (fig. 14) will probably afford a better idea of the relative positions of the rocks at this place than pages of description. 1 Basalt Dykes. The whole area of the north-east of Ireland is traversed by basaltic dykes, piercing the different formations in every direction, but more frequently converging towards the region of the basaltic plateau. Some of these are of more ancient date than the Miocene. In the district of Carling- ford, where there appears to have been a focus of eruption, most of the dykes are probably of Carbon- iferous, or Permian, age. To the same periods are referable the large number of trap dykes, which traverse the Silurian rocks along the coast S. of Newcastle at the base of the granite mountain Slieve Donard, but which terminate at the margin of the granite itself, being of earlier date. 2 Similar in- stances are of frequent occurrence around the gran- 1 Since the above was written I have had an opportunity of re- visiting the coast of Antrim in company with Professor Ramsay and Mr. W. A. Traill ; and a further examination of the Rock of Dunluce Castle and of the cliffs adjoining leads us to suspect that we have here, instead of old volcanic necks, simply pipes, formed by filtration out of the chalk, into which the basaltic masses have fallen or slipped down, thus giving rise to their fragmental appear- ance. Sept. 1877. 2 These are marked in Griffith's Geological Map, and in more detail on the Geological Survey Map, sheet 61. But so numerous and varied are they that it is only on the 6-inch maps that they can be properly shown. 94 Geological Formations of Irelani itic district of the Mouriie Mountains. But, as Mr. Trciill has shown, along with the basaltic dykes of greater antiquity than the granite there are some others which traverse the granite itself and are therefore of more recent origin, and we shall pro- bably not be wrong in referring these latter to the period of the Miocene eruptions of augitic lava. It is, however, on approaching the base of the basaltic escarpment that dykes of undoubtedly Ter- tiary age become most abundant. These are found penetrating not only the Triassic and Cretaceous beds, but even the sheets of basalt and dolerite them- selves. It is only occasionally, however, that they are found traversing the uppermost beds of lava except on the northern coast. 1 It is very instructive to walk along the northern shore of Belfast Lough during ebb tide, and trace the course of the dykes as they protrude in little rocky ridges, running out from the shore into deep water, and cutting across the beds of New Bed Sandstone and Marl, which they sometimes convert into a sort of Lydian stone. Some are of large size ; such as that on which the solid walls of Carrick- 1 See account of the Basaltic sheets and dykes of Scrabo Hill, near Newtownards, p. 72. Dr. Geikie has forcibly dwelt on the phenomena connected with the wide distribution of the Tertiary basaltic dykes, and considers that much of the lava forming the sheets has been extruded through them. Loc. cit. p. 70 et seq. Sir A. Geikie s Views 95 fergus Castle have been reared, vjing in solidity with the basaltic foundation itself. Similar remark- able dykes, together with intrusive sheets, penetrate the Carboniferous rocks at Ballycastle Bay, where their relations to the strata can be advantageously studied ; l and if we cross the narrow channel which here separates our island from Scotland, we shall find their representatives along the coast of Kintyre, the islands of Arran and Bute, or the district of the Glasgow coal-field. The views here expressed regarding the relative ages of the trachyte of Antrim and the basaltic sheets have been taken exception to by Sir Archibald Geikie in his elaborate treatise on the Tertiary Volcanic rocks of the British Isles. 2 Sir A. Geikie, arguing from a supposed analogy between the relations of these rocks to the granophyres of Mull, which he shows are newer than the plateau basalts of the Inner Hebrides, to the trachytes and basaltic sheets of Antrim, arrives at the conclusion that the trachytes and pitchstones of Antrim are far younger than the surrounding basaltic sheets. 3 He 1 Several narrow dykes traverse the upper as well as the lower sheets of lava east of the Giant's Causeway, leading to the inference that they represent the latest efforts of volcanic activity in the N.B. of Ireland. 2 * History of Volcanic Action during the Tertiary Period in the British Isles,' Trans. Boy. Soc. -Edinburgh, vol. xxxv. p. 170 (1888). 3 Jbid. p. 171. g6 Geological Formations of Ireland also dwells upon the fact as confirmatory of this view, that trachytic fragments have not hitherto been found in the successive sheets of basalt ' which are supposed to have surrounded and buried it.' and which are actually in contact with it, and infers that if the trachyte had been in existence as a dome- shaped mass while the basaltic lava was flowing and accumulating around its flanks, such fragments could not fail to have been found in the lava. I do not admit this conclusion ; because it is quite con- ceivable how flows of viscous lava extended from - surrounding sources, and slowly gathering around the trachytic mass, might be destitute of any such fragments. But whether this be the case or not, Sir A. Geikie's conclusion regarding at least the upper basaltic sheets has been found untenable by a dis- covery made by Mr. McHenry towards the close of 1888, a very short time after the publication of his treatise. I refer to the occurrence of numerous pebbles of Tardree trachyte in the stratified ashbeds with plants which overlie the lower basaltic sheets both at Glenarui and at Ballypalidy. These pebbles are of various sizes, from that of a walnut to a grain of fine sand, and the bands of fragmental matter show distinct lines of bedding, indicating deposition under water. This discovery, clearly showing that the trachyte of Tardree existed as a consolidated Relative Ages of the Trachyte and Basalt 97 rock during the period of outflow of the upper basaltic sheets and of the formation of the stratified ashbeds by which they are separated from the basalts of the earlier stage, makes it almost certain that it was also a consolidated mass before the eruption of the whole basaltic masses. The only alternative view that can be advanced is that the trachyte is of an age intermediate between the upper and lower basaltic sheets. This view seems to me quite in- admissible ; nor is ib supported by any clear obser- vations. The upper and lower basalts, differing essentially in composition from the quartz-trachytes all belong (as it seems to me) to one great, although interrupted, epoch of volcanic eruption ; and it is probable that a wide interval of time intervened between the extrusion of the acid lavas and of the succeeding basic flows. As regards the analogy between the granophyres of Mull and the quartz-trachyte of Antrim, it appears to me misleading. They are very dissimilar rocks, except for the fact that they both belong to the highly silicated group of volcanic rocks. Dr. Hatch, the petrologist to the Geological Survey, states that the Antrim trachyte is quite different from the grano- phires of Mull, and more resembles the trachyte of the Drachenfels and the pebbles in the leaf -beds of Mull. That there were in existence trachytic masses H 98 Geological Formations of Ireland in the region of the Inner Hebrides at the time of the outpouring of the plateau-basalts of that region, is admitted by Sir A. Geikie himself; * if this be so, then the trachytic rocks of Antrim may be of a similar age. This is a view which would seem to satisfy the requirements of the case. As evidence in favour of the views here main- tained, it may be stated that in no single instance has a dyke or protrusion of the trachytic rock into basaltic sheets been observed over the area of its occurrence, while, on the other hand, the trachyte is traversed by numerous basaltic dykes. These facts may be used as a set-off against the argument derived from the non-occurrence of fragments of trachyte in the basaltic sheets near the junction, which may be explained from the fact that the actual junction is never seen around Tardree Hill, and the visible approximate sections are few. Several of the basaltic knowls which surmount the trachyte of that district are clearly old volcanic necks filled with consolidated lava, which have burst through the older trachytic mass. The remarkable section of these rocks shown in the quarry at Templepatrick, of which a representation is given 1 Geikie, loc. cit. pp. 92, 144. These pebbles occur in the gravel beds of Ardtun, and were noticed by Mr. G. Cole. Quart. Journ. Geol. Soc. 1887, p. 277. The Temp lepat rick Section 99 in fig. 15, appears to rne to confirm the view I have taken. On a full reconsideration of the evidence, and after a fresh examination of all the sections, in- cluding that of Templepatrick, made in company with Mr. McHenry in the summer of 1888, I see no FIG. 15. Section in Quarry at Temj>lej)atrick, Co. Antrim. i ^J^-"! - "*** ~X"* ~ :i^ -V - T j,. \ C, Chalk with bands of flint. G, Bed of flint-gravel. T, Quartz-trachyte with columnar structure. B, Massive solid basalt intruding itself into the flint-gravel, and forcing the trachyte into a vertical position. reason to recede from the conclusion I had come to as far back as 1874, that the quartz -trachyte is the oldest of the Tertiary Volcanic rocks of Co. Antrim.' 1 The section at Templepatrick which shows in one place the prismatic trachyte resting on the flint gravel overlying the chalk, and, in another, a mass of basalt invading the flint-gravel and apparently forcing into a vertical position the base of the trachyte appears to me to support my view, though not conclusively ii taken by itself. H 2 ioo Geological Formations of Ireland Pliocene Clays of Louyh Neagh. Lough Neagh is not only the largest fresh-water lake in the British Islands, but it is the oldest still surviving. Many of the existing lakes owe their origin to glacial agencies, or to solution of the strata by water, and have been formed during, or since, the Glacial (or Post- Plio- cene) epoch. But Lough Neagh is older than the Glacial epoch ; and still survives as a lake, though in diminished size, notwithstanding the physical changes to which its basin has since been subjected. The evidence of this I shall presently endeavour to give; meanwhile let us examine the proofs of the former more extended range of its waters. All along the southern portion of the lake, includ- ing the eastern and western shores as far as Sandy Bay and Arboe Point, there occurs a tract of country not much elevated above the surface of the lake (perhaps 80 or 90 feet at the most), formed of grey, purple, and blue stiff clay, with thin laminated sand- stones and bands of lignite. In the neighbourhood of Dungannon the clays are used in the manufacture of pots and pans (or f crocks ' as they are called in that country). The clays have been pierced in several places to considerable depths ; * and the late 1 In one of these, to a depth of 294 feet, including 30 feet of Drift, at the Townland of Annaghmore. Griffith, ' Second Report to the Kailway Commissioners,' p. 22. Pliocene Clays of- L. 'Ntagk '101 Mr. E. T. Hardman, who refers them very properly to the Pliocene stage, estimates their maximum thick- ness at not less than 500 feet. 1 These Pliocene clays have a slight dip towards the lake, and they rest on an eroded surface of the Miocene basalt, their junction with which was very clearly observed some time since by Mr. Hardman and myself along the banks of the river Crumlin, a short distance above its entrance into Sandy Bay. The lowest beds were seen to consist of a conglomerate formed of pebbles of the basalt, evidently an old shore gravel beach ; and then over this were formed the bluish clays, stiff and plastic, in which my companion detected bivalve shells resembling a species of Unio, or Mytilus, 2 the first that had been discovered in these beds. While the clays rest upon an eroded surface of the basalt, as has been ascertained both by observa- tion and boring experiments, thus proving their age to be more recent than the Miocene stage, they are themselves often covered over by lower Boulder Clay and deposits referable to the Post-Pliocene or Glacial epoch. This is illustrated by the accompanying 1 ' On the Age and Mode of Formation of Lough Neagh,' Journ. Roy. Geol. Soc. Ireland, vol. iv. pp. 174-5 ; Expl. Mem. sheet 35 of the Geol. Survey Maps, p. 77. 2 So considered by Mr. H. Woodward, F.R.S., but Mr. Baily thinks they are Uniones ; if the former be correct, it would show that the sea had access at an early stage of the clay-formation. 1O2 Geological Formations of Ireland section (fig. 16), drawn at the eastern shore of Sandy Bay, where the relations of the different formations, namely, the Boulder Clay, the Lough Neagh Clays, and the basalt were very clearly determined by Mr. Hardman and myself on the occasion of our visit in 1876. 1 The clays are, therefore, clearly of an age intermediate between the Miocene of the basaltic sheets and the Boulder Clay of the Post-Pliocene ; FIG. 16. Section across the Eastern Shore of L. NeagJi cct Sandy Say. 1. Miocene Basalt-sheets. 2. Pliocene Clays resting against their flanks. 3. Boulder Clay oyerlapping both. in other words, they are probably of Pliocene age. They appear to be of an age intermediate between one of intense igneous action, and one of intense glacial action; but in their composition they indicate no immediate connection with either. On the other hand, their laminated, silty character, the frequent bauds of lignite, and the impressions and bark of 1 See Expl. Mem. to sheet 35 of the Geol. Survey Maps by E. T. Hardman, p. 89. Old Lough Neagh 103 plants (Sequoia, Alnus, Quercus, Fagus, and Salix), indicate deposition under tranquil waters, in a warm climate, and with occasional alternations of swampy lagoons choked with vegetation. That they have been deposited under the waters of what we may call ' Old Lough Neagh,' to distin- guish it from the existing lake, and probably where an ancient river entered this lake, there can be no doubt ; and it is also clear from the wider extension of these beds beyond the margin of the existing lake especially in the direction of Dungannon and Armagh that the waters of Old Lough Neagh oc- cupied a considerably wider area towards the south than at the present day. The original banks, or margin, of the lake south of the river Crumlin can be clearly traced, by the abrupt and steep ascent which the basalt produces ; and against the base of which the beds of clay have been deposited. Towards the south, however, this original margin is not so clearly definable, it having been formed of the softer strata of the New Red Sandstone, which (along with the clays themselves) are generally thickly covered by a deposit of Boulder Clay. Here, then, we have a formation of an age but sparingly represented in the British Islands, and in which a warm and equable climate prevailed, as indicated by the plant-remains. Ere it set in, the volcanic fires had smouldered away. IO4 Geological Formations of Ireland It was an age of calm repose, separating the period of volcanic activity on the one side from that of frost and ice on the other; and during its continuance the ordinary agents of denudation rain, rivers, and sea waves carried on their operations without unusual interruption, but with marked effect in modifying the physical features of the north and adjoining dis- tricts of Ireland. I reserve to a future page the discussion of the question how Old Lough Neagh was formed, and now proceed to describe the characters of the de- posits which represent the period succeeding that of the Lough Neagh clays, and known amongst geologists as the 'Post- Pliocene, 5 'the Glacial,' or ' the Drift,' and which are widely represented in this country. 1 1 Flint implements have been found in great numbers along the shores of Lough Neagh, and particularly at Toome Bridge, on the north shore, which was probably a productive fishing-station in pre- historic, as in recent, times. See Dr. J. Evans' Ancient Stone In^le- ments, p. 258 ; also Wilde's Catalogue* Mils. Roy. Irish Academy, p. 10. CHAPTER IV POST-PLIOCENE, OR DRIFT, DEPOSITS Post-Pliocene, or Drift, Deposits. These are the newest deposits requiring our attention to any great extent, and are the most widely distributed of any existing in Ireland. It is probably not too much to say, that, in one form or another, they cover three- fourths of the entire surface of the country, resting indiscriminately on all the older and more solid formations, and rising from the plains up the flanks of the mountains to elevations of between 2,000 and 3,000 feet. They are the representatives of a period generally characterised by extreme cold, ' The Great Ice Age,' in which the rains and rivers of the present day were represented by snow, sheets of ice, and glaciers ; while the relations of land and sea were subjected to considerable modification as compared with those now prevailing. The deposits of this period may be arranged under three divisions corresponding with those of the north of England, if not of a much larger area; io6 Geological Formations of Ireland namely, (1) The Lower Boulder Clay, or Till; (2) the Middle Sands and Gravels ; and (3) the Upper Boulder Clay, this being the newest member of the series. These divisions have been identified over a large district extending from Kilkenny on the south to the borders of Tyrone and Londonderry on the north, and may be considered to represent three stages of the Glacial period or phases of climatic conditions ; the lower and upper being essentially arctic in character, the middle being temperate. I shall now endeavour to give a short account of the deposits representing each of these divisions in as- cending order. (1) The Lower Boulder Clay. This consists of a very stiff solid clay, of a dark blue, or reddish, colour according to locality, and containing blocks, pebbles, and fragments of various rocks imbedded therein, and in every possible position. These blocks are of all sizes, either angular or rounded, and often have their sides planed down and covered by scars or groovings, evidently due to their having been forcibly rubbed over hard substances, such as pointed rocks or sharp stones. These scars and scorings are generally well preserved on blocks of limestone which have been freshly disengaged from their beds. The clay itself is very seldom laminated, like those of aqueous formation ; on the contrary, it is generally Lower Boulder Clay 107 entirely structureless, and tlie stones and boulders may be observed standing in vertical, or inclined, positions; and not, as in the case of those which have been strewn under water, lying on their flattest surfaces. These peculiarities serve to prove that this remarkable deposit must have been formed in a manner differing from that in which ordinary beds FIG. 17. Coast Section near Ardylass, Co. Down, showing the Relations of the Drift-beds and liaised Beach. Silurian Rocks. 1. Lower Boulder Clay, resting on a glaciated surface of the Silurian rocks. 2. Middle Sand and Gravel, resting on the Lower Boulder Clay. 3. Raised Beach of gravel 15-20 feet above sea. of gravel or conglomerate have been accumulated ; that some agent other than water has been engaged in its construction. Now, the only agent we are acquainted with capable of producing such a deposit is ice, either in the form of a glacier, or of a sheet spread over the country and moving in certain directions. io8 Geological Formations of Ireland This view gains additional confirmation upon observing the surface of the solid rocks from which the Boulder Clay has recently been stripped off. It is frequently found that these surfaces (especially when the rock is hard and compact) are smoothed down into flat, or mammillated forms, and are remarkably polished, or covered by parallel scorings and flutings. In such cases the surface of the rock is said to be ' glaciated.' Instances of this are of such frequent occurrence that it is scarcely necessary to refer to examples ; but in the neighbourhood of Dublin they may be observed on a surface of lime- stone along both shores of the narrow neck of land which unites Howth Hill with the mainland, and again to the south of Dublin at Killiney and Bally- brack Hills. They may also be observed along the eastern coast of Co. Down north of Dundrum Bay. In such cases the direction of the grooves and scorings indicates the direction in which the ice sheet has moved. 1 The tendency of the Boulder Clay to arrange itself in parallel ridges has been noticed by many observers in nearly all parts of the country. 2 These 1 It is not my purpose to enter into the proofs that these phenomena are due to the action of ice : the subject will be found sufficiently discussed in various works dealing with physical geology. 2 A list of these observations has been furnished by the Kev. Maxwell Close. Journ. Roy. Geol. Soc. vol. i. p. 209. Lower Boidder Clay 109 ridges are very well shown on the hill-shaded Ordnance Survey maps of the neighbourhood of Dublin, and the Eev. Maxwell Close has clearly indi- cated their connection with the directions of the ice- scorings of the rocks in their immediate vicinity. This connection will be recognised on referring to the 'Map of the General Glaciation of Ireland,' which accompanies his well-known paper on this subject. 1 As I shall have to return to the discussion of this matter in a future page, I will not further dwell on it at present, except to observe, that the phenomenon here alluded to affords additional evidence that the Boulder Clay is the result of glacial action. These ridges must not be confounded with those of the Esker Gravels, which are due to entirely different causes, and are composed of different materials, namely, water-worn and stratified masses of gravel and shingle, which are of much more recent origin than any of the true glacial de- posits, and have been constructed out of these very deposits themselves. The Lower Boulder Clay is the most extensively distributed of all the Post-Pliocene deposits in Ireland. Being the oldest, it has suffered less from denudation than the more recent Gravels and the Upper Boulder Clay, while it frequently underlies 1 Kev. M. Close, Journ. Roy. Geol. Soc. plate, p. 224. 1 10 Geological Formations of Ireland these newer strata, forming the immediate covering of the solid rocks. It may be observed in this position along the shores and islands of Cork Harbour on the south, as well as along the coast of Antrim and Donegal on the north ; and along the borders of the Irish Sea, skirting the coast of Wexford and Wicklow on the east, as along the shores of Galway Bay on the west. Its extension into these districts shows the prevalence of glacial conditions over the whole country, as also indicated by the scorings on the rocks ; and thus we have side by side, or in actual contact, two witnesses to the wide extension of glacial conditions at a former epoch in the physical history of Ireland. 1 As might be inferred from what I have said of its mode of formation, the Boulder Clay occurs in greatest mass in the lower grounds and deeper valleys of the country. The plains and valleys having been in the main formed and hollowed out before the Glacial period, the Boulder Clay natur- ally filled up the hollows and protected portions of the surface, often choking up the deep gorges, and entirely obscuring the underlying rocks. Many of these valleys have been re-excavated ; and the exist- 1 A map, showing: the general glaciation of the British Isles, will be found in pla j e xiii. in my Physical History of the British Isles, London (1882). Middle Sand and Gravel 1 1 1 ing streams have worn down their channels to the original solid floors. From the plains and valleys, the Boulder Clay rises on the flanks of the mountains and hills to elevations of upwards of 1,500 feet in many cases, when it either becomes thin and spar- ingly distributed, or entirely disappears. It is curious also to observe, that on some comparatively low grounds, such as that at Inniskeen in Co. Louth, the old rocks are entirely free from any covering of Glacial drift over a considerable area, while it occurs in great thickness a few miles further east, in the neighbourhood of Castleblaney. Such instances show either the great irregularity in the original extension of the ice-sheet, or the extent to which the Boulder Clay has been locally denuded since its formation. In mountainous districts, such as those of Galway, Mayo, and Mourne, the Boulder Clay assumes the appearance of local moraine matter, made up of angular blocks mainly of local rocks confusedly heaped together with earth or shingle ; and it is often difficult, except where fragments of foreign rocks are enclosed, to distinguish such masses from those originating in local glaciers. (2) The Middle Sand and Gravel. Inter-glacial Beds. This division is very largely distributed over the central plain of Ireland, constituting what is generally known as ' the limestone gravel,' because 112 Geological Formations of Ireland largely made up of pebbles of Carboniferous Lime- stone. It also rises high upon the mountain slopes of Wicklow and Dublin, and is distributed along the eastern coast of Dublin, Wicklow, and Wexford, where it contains numerous species of marine shells. 1 This formation is very different in character, con- sequent on differences in the mode of accumulation, from the Boulder Clay which it succeeds, and upon which it frequently rests. As its name imports, it consists of stratified sand or gravel, of water- worn pebbles, sometimes of large size ; and, as it contains marine shells in various places, may be regarded as a formation of marine origin, which has been strewn over the bed of a comparatively shallow sea. In these waters local currents appear to have been prevalent, as they have left their traces in the numerous instances we find of * current-bedding,' or 6 oblique lamination.' The fine sections in these deposits along the coast at Bally brack, south of Dublin, show remarkable examples of such oblique bedding as I have described. 2 Again at Howth, 1 The shells from the Wexford gravels were named by the late Prof. Edward Forbes, and considered by him and Sir C. Lyell to be of Pliocene age, owing to the occurrence of Fusus contrarius, &ic. But this evidence is inconclusive, and there can be little doubt the beds are the representatives of those here described. See Rep. Brit. Assoc. 1887-8 on the Wexford gravels, by Mr. Alfred Bell. 2 This section, accompanied by a woodcut, is described in the Gi'ol. Magazine, vol. viii. II ere the Upper Boulder Clay, the Middle Shells from Interglacial Beds 113 these beds, containing numerous marine shells which have been named by Dr. Scouler, may be observed overlying the Lower Boulder Clay, and are ultimately surmounted by traces of the Upper Boulder Clay on the flanks of Howth Hill. The Middle Gravels rise to considerable elevations on the flanks of the Dublin and Wicklow Mountains, where they also contain shells, most of which live in the adjoining seas : they have been ably described in such positions by the late Mr. John Kelly, 1 and in greater detail by the Rev. Maxwell Close. 2 By these observers they have been found at elevations of 1,300 feet (Caldbeck Castle), and others not quite so high up, opposite Ballyedmonduff, on the road from Stepaside to Glencullen ; and in the Killakee valley. The species from Ballyedmonduff have been determined by Mr. W. H. Baily, and are as follows: Trophon muricatus, Fusus (part of colu- niella), Turritella communis, Ostrea edulits, Pecten (two species), Cardium edule, C. echinatum, Astarte com- pressa, A. elliptica, A. sulcata, Cyprina Islandica, Artemis lincta, Venus striatula, V. casina, Lutraria elliptica, Mactra stultorum, Tellina ? My a truncata, Sand and Gravel, and the Lower Boulder Clay may all be observed between the base of Killiney Hill and the Martello Tower of Bally- brack. 1 Journ, Gcol. Soc. Dub. vol. vi. p. 133. 2 Journ. Roy. Gcol. Soc. Ireland, vol. iv. p. 36. I i T 4 Geological Formations of Ireland Pholas crispata, Balanus balanoides, Annelid perfora- tions. Mr. Close 1 observes that though all the above individual species live in the neighbouring seas, yet as a group they present a rather more boreal fades than those of the present coasts, and than those of the low gravels described by Professor T. Oldham ; but, on the other hand, a decidedly less northern fades than those from the Drift beds of Moel Tryfaen, in North Wales, which occur at 1,360 feet above the sea level. 2 Mr. Close supposes the gravels at Ballyedmonduff to have been carried thither by floating ice, and not to have been deposited where they occur, as they contain pebbles of limestone and other travelled stones. At the same time, re- collecting the elevation of those on Moel Tryfaen, which are probably representative and have all the appearance of having- been formed in situ, and the prevalence of gravel beds at elevations but slightly below that just referred to, it seems probable that these beds have not been transported from any great distance. 3 Still, without some such agent as that 1 ' On the more recent Geological Deposits in Ireland,' Joiirn. Ceol. Soc. Dub. vol. iii. p. 61. 2 First discovered by the late Mr. Joshua Trimmer, F.G.S. * Mr. Close admits that these shell-bearing gravels belong to tlie division of the Drift series (middle sands and gravel) here de- scribed. They contain limestone pebbles, which must have come from lower ground. Interglacial Beds 1 1 5 suggested by Mr. Close, it is difficult to account for the presence of limestone pebbles. In the north-east of Ireland, as pointed out by- Professor Harkness, beds of stratified gravel and sand occur, with sea-shells belonging for the most part to existing species, but indicating somewhat colder conditions than those which obtain at present* 1 In Co. Antrim, near Ballycastle, these beds form conspicuous terraces rising on the flanks of the hills to an elevation of 600 feet. They rest on Lower Boulder Clay, and appear to be overlaid by an upper similar deposit. 2 A similar succession of beds occurs near Glenarm, as I am informed by Mr. Traill. Besides the localities already mentioned, shells have been found in this gravel in several localities inland, and at Muff on the hillside opposite Lough Foyle, by General Portlock ; 3 so that there can be no doubt the gravels were formed over the bed of the sea which was co-extensive with the plains, and rose to very considerable elevations on the flanks of the mountains. In England they have their representa- 1 Geol. Mag. vol. vi. p. 542. 2 A good section of these beds may be seen in the banks of the river Carey, four miles south of Ballycastle. 3 Gen. Portlock considers thesj shells to be of Pliocene age, but there can be little doubt they are referable to the interglacial period here described. Geol. Hep. Londonderry, &c., p. 165. i 2 1 1 6 Geological Formations of Ireland tives in the shelly sands and gravels between the Upper and Lower Boulder Clays, bearing the same name as originally proposed by the author when describing the Drift Deposits of Lancashire and Cheshire. They have been found at various eleva- tions in these and the adjoining counties west of the Pennine Chain, climbing the flanks of these hills and of the Welsh Highlands. These facts lead us to infer a great depression of the land, extending over the northern portion of the British Islands, and gradually decreasing southwards. The general ab- sence of erratic blocks, except such as have been washed out of the Lower Boulder Clay, indicates the disappearance of glacial conditions, such at least as prevailed during the preceding period ; so that the deposits which were formed may properly be termed ' inter- glacial.' Assuming the greatest depression to have reached 1,500 feet below the existing level, the Irish area must have presented the appearance of an archipelago of islands ; as the higher portions of the mountain groups could alone appear above the general level of the waters. 1 (3) Upper Boulder Clay. This deposit is spar- 1 The southward drift of the waters of the Interglacial sea is proved by the presence of pieces of chalk and chalk-Hints in these beds, both in the south-east of Ireland and in the western parts of England. Mr. D. Mackintosh has found (apparently) Irish chalk fragments at Ellesrnere, and Mr. G. Maw at Strethill in Salop. Drift Section at Kilkenny 1 1 7 ingly distributed as compared with the two prece- ding, as, owing to its position, it has suffered from denudation to a greater extent than they ; neverthe- less, its presence has been determined, particularly by the late Mr. E. T. Hardman, in several localities, both in the north and centre of the country. It is clearly displayed in the coast cliff, south of Killiney Hill, where it may be seen resting on an eroded FIG. 18. Section of .Drift Deposits at the Marble Quarry, Kilkenny. C. L. 1. Lower Boulder Clay. 2. Middle Gravel. 3. Upper Boulder Clay. Length of Section about 50 yards. C. L. Carboniferous Limestone. surface of the c Middle Sand and Gravel,' and sloping downwards towards the plain which opens out on Killiney Bay. Its presence is, however, much more extensive than is generally supposed, as it may easily be mistaken for the Lower Boulder Clay. This will be evident from the adjoining section at the marble quarries of Kilkenny, where it may be seen resting on the Carboniferous Limestone, while the two lower members of the Post-Pliocene series 1 1 8 Geological Formations of Ireland are found cropping out on the banks of the river Nore. 1 (Fig. 18.) The formation consists of reddish stiff clay with boulders, and bands of gravel and silt. It is some- times a little sandy ; in consequence of which, when the underlying gravels have some intermixture of clay, the distinction has been often overlooked. To the practised eye, however, the distinction is suffi- ciently apparent, especially after the observer has studied the succession of the Drift deposits in the north of England. The great thickness to which this formation is capable of attaining is shown by the following section taken while sinking the shaft of the Modubeagh Colliery, near Carlow : Section of Drift Deposits near Carlow. Upper Boulder Clay Stiff red clay with stones . 84 feet Laminated clay ( ' Book clay ') . 5 Middle Sands, &c. Sand, sometimes clayey, with pebbles of limestone, &c 25 Lower Boulder Clay Strong clay with stones . 8 Coal Measures (Shales and fire-clay) . , . 122 feet Mr. Hardman has shown that the Upper Boulder Clay is well represented in the County Tyrone, cap- ping the summits of the Drift hills where it has escaped denudation. He also shows that it occurs 1 This section has been given by Dr. James Geikie in the 2nd edit, of his excellent work, The Great Ice Age^ but is too important to be omitted here. Erratic Blocks 1 1 9 in numerous places in the Carlo w district, resting on the Middle Gravel, and containing rounded and sub- angular blocks of rock, with polished and striated faces. 1 It is exceedingly probable that in some parts of the country, especially in the west of Ireland, the Upper Boulder Clay has never been deposited ; but though itself absent it is sometimes re- placed, or represented, by large erratic blocks strewn over the upper surface of the interglacial gravels. Such blocks, consisting of large slabs of Carboni- ferous grit, are strewn over the surface near the vil- lage of Kilkelly, south of Swinford in Co. Mayo. So numerous are these in some places that they might be mistaken for the outcroppings of the solid rock, were it not for sections in pits and banks of streams which reveal thick beds of gravel lying under- neath. At other times the clay of the formation has been washed away ; but the large boulders which it once contained are left strewn on the surface of the subordinate beds of gravel. Thus in the valley of the Yellow River, which flows through the range of the Ox Mountains in Sligo, the stratified sands and 1 Journ Roy. Geol. Soc. vol. iv. p. 73 ; also Expl. Mem. sheet 35 of the Geol. Survey Maps, p. 78. 1 20 Geological Formations of Ireland gravels form terraces and ridges along the sides of the valley, and are generally strewn with large boul- ders of local, or foreign, rocks, such as Carboniferous grit from the south. In other places, however, these blocks are seen to be imbedded in reddish clay rest- ing on very coarse gravel, which I consider to be the representative of the Upper Boulder Clay near its westerly limit. 1 It will be evident from the above description that the Upper Boulder Clay is due to the recurrence of glacial conditions in some sort, but probably not to anything like the extent, or exactly of the same kind, as existed during the period of the Lower Boulder Clay, or Till. We have seen that the phenomena connected with this latter deposit can only be fully accounted for by the supposition of an ice-sheet spreading itself over the land. But in the case of the newer deposit, there is great difficulty in ad- mitting this view, as a second ice-sheet would prob- ably have ploughed out all the older deposits of the Post-Pliocene age, and left them confusedly piled against the flanks of the mountains, or even carried out to sea. But the Upper Boulder Clay generally rests on the interglacial beds in such a manner as 1 I am not aware of any instances of this deposit west of the Ox Mountains. Formation of Upper Boulder Clay 121 not to show evidence of forcible attrition. The sur- face of the gravels is often eroded as by water action, but seldom displaced as by a solid mass of ice. 1 In the north of England the Upper Boulder Clay is some- times stratified. In this country, as noticed by Mr. Hardman, it also sometimes shows traces of stratifi- cation. It is, in fact, just such a deposit as might be supposed to have been formed under the sea, the waters of which were laden with ice-rafts and bergs bearing stones and boulders which would constantly fall to the bottom as the ice melted away, while these waters were themselves rendered turgid by streams from glaciers entering from various directions. It is in this manner, as I conceive, the Upper Boulder Clay was really formed. A similar deposit is prob- ably now in course of formation over the floor of the Greenland Sea, as far south as the banks of New- foundland, due to conditions such as those above described. If this be so, it is clear we must assume a considerable elevation of the land (corresponding to a shallowing of the sea- bed) at the close of the period during which the interglacial gravels were formed. Such an elevation would be necessary in 1 In some cases, however, the interglacial gravels show evidence of displacement, as in the neighbourhood of Balbriggan ; but it is probable (as suggested by the late Prof. Jukes) this is due to the stranding of floating icebergs. 122 Geological Formations of Ireland order to allow of mountain groups of sufficient extent and elevation for the formation of glaciers in their valleys, and in order to become centres of dispersion, from which bergs and rafts of ice could float off and melt in the waters of the surrounding sea. 1 It is not, perhaps, difficult to restore the physical features of our country under such conditions. I am unable to state the actual, or original, limits of the Upper Boulder Clay above the sea, but if we assume from 800 to 1,000 feet we shall probably not be far from the truth. If such be the case, all the hills and mountains above the level of 1,000 feet must have formed groups of islands from which icebergs and rafts were dispersed over the surrounding waters ; 2 the Donegal and Derry Mountains on the north, those of Mayo and Gal way on the west, those of Kerry, Cork, Tipper ary, and Waterford on the south-west, those of Wicklow and Dublin on the south-east, and those of Down and Armagh on the north-east appear to have formed an archipelago of 1 Mr. Joseph Wright, F.G.S., states that Foraminifera occur in the Boulder Clay of the north of Ireland, and that at Woodburn, near Belfast, forty-eight species were found. Proo. Belfast Nat. Field Club, March 1 880. I am uncertain whether this is Upper or Lower Boulder Clay probably the former. 2 A map representing the position of land and sea of the Irish area during the period of greatest depression will be found in Ly ell's Antiquity of Man, 4th edit. pp. 325-8 (fig. 42) ; also in The PJiys. Hint. Brit. Isles, plate xiv. figs. 1 and 2. Later Glacial Conditions 123 snow-clad isles, represented in the British area by similar groups. 1 1 If the depression along the flanks of the Wicklow Mountains during the stage of the Middle Gravels amounted to 1,500 feet (as measured by the existing sea margin), and that during the stage of the Upper Boulder Clay to 1,000 feet, there would have been a rise of 500 feet in the sea-bed between these stages, 124 Geological Formations of Ireland CHAPTER Y POST-GLACIAL DEPOSITS Mountain Terraces, ~Eskers, and Local Moraines. With the close of the formation of the Upper Boulder Clay the history of the Glacial Epoch properly ends. At its close there was a gradual elevation of the land from beneath the waters, accompanied probably by pauses ; the climate became milder, and the glaciers retreated, little by little, up the mountain glens, until they disappeared altogether under the genial influences of a warmer sun and less arctic tempe- rature. Into the causes of this change it is not necessary for me to enter here, the subject having been sufficiently handled by others better qualified than myself to deal with it, and to their writings I must refer the reader. 1 The physical evidence of a colder climate preceding the existing temperate one in these latitudes obliges us to infer a transitional 1 Lyeirs Principles of Geology, Croll's Climate and Time, James Geikie's Great Ice Age, will probably be found to contain sufficient information to satisfy the inquiring reader. Mountain Terraces 125 period, during which the change from the one to the other, and from the former conditions of sea and land to the present, must have been gradually pro- gressing. On the whole, the land was gaining on the sea; and as lower slopes amongst the mountains successively rose into the air and were exposed to wave action, terraces composed of the recently strati- fied materials would naturally be formed whenever a pause took place in the upward movement. Mountain Terraces. Such terraces are not un- common amongst the Irish mountain groups. One of them may be very clearly observed, when looking up the valley of the Kilkeel Eiver from the south, skirting the base of Slieve Lough Shanagh in the Mourne Mountains at an elevation of about 1,000 feet above the sea. Amongst the mountains of the west of Ireland similar terraces are of frequent occurrence, and can often be better seen at some little distance than when standing immediately on their surfaces. Thus two distinct terraces may be observed on the flanks of the mountains running up from Killary Harbour to Delphi when they are looked at from the south bank of the harbour, but are not so conspicuous when the hill-side itself is examined; the observer not being then in a favour- able position. Not far, however, from this spot, and facing the head of the harbour above Leenane, 126 Geological Formations of Ireland there are two very well-formed terraces of gravel, one at an elevation in the upper surface of about 60 i'eet, and the other at an elevation of about 200 feet ; this latter is very extensive, and is traversed by the road to Cong. Similar terraces, consisting of slightly sloping upper surfaces and abrupt margins, and formed for the most part of Drift or Moraine materials, may be observed at the base of the quartzite mountains near Recess in Connemara, and along the Lough Inagh valley. Mr. Kinahan records similar ( well-marked ter- races which appear to be ancient sea^margins,' on the flanks of Slieve Aughty in the neighbourhood of Lough Graney, the highest being at about 1,200 feet, and the lowest a little above 300 feet. 1 In the Barren country, Co. Clare, there are remarkable terraces formed amongst the nearly level beds of Carboniferous Limestone ; but it is doubtful to what extent they can be considered ancient sea-margins. It is very probable, however, that some of the inci- pient contours may have been determined during the rising of the land. EsJcers. While the land was still being elevated, and fresh tracts were emerging into day, or were being brought within the reach of surface waters, 1 ' Notes on the Drift of Ireland,' Journ. Roy. Geol. Soc. vol. i. p. 198. Eskers 127 it may easily be imagined that the tidal and other currents, being forced to oscillate within narrow channels bounded by the ridges of the un submerged land, would act with considerable effect on the soft materials of the Drift, both in sweeping them away and in piling them up along tortuous lines in the form of embankments. And such may the Eskers be regarded. They consist of long mounds or banks of gravel, formed for the most part out of the inter- glacial gravels, often running for miles, and assuming directions depending on those of the adjoining hills. They are confined to the plains, and their upper surfaces are frequently strewn with large erratic blocks, probably derived from the Upper Boulder Clay, which had itself been washed away during the emergence of the land. These Eskers are found at intervals over the great central plain, and as far north as the valley of the Lagan on the borders of Down and Antrim. 1 Mr. Kinahan observes that the Eskers in the strip of country lying between Dublin and Galway form a compound bar, consisting of well-defined ridges or ' bar-eskers,' and in other places of shoal- eskers. The bar-esJcers from Galway to Tullamore, 1 Some of the Eskers have been represented on the maps of the Geological Survey, and described in the Explanatory Memoirs, One of the most remarkable is that which passes in a northerly direction through Maryborough to the neighbourhood of Mountmellick, 128 Geological Formations of Ireland or thereabouts, are usually on ground under the 250-feet contour line, and from Tullamore to Dublin on ground under the 300-feet contour; while the shoal- eskers towards the west are on ground between the 250-feet and the 300-feet contour, and towards the east between the 300-feet and 400-feet contours. From these facts Mr. Kinahan infers that the land was then between 300 and 400 feet lower than at present, having since risen more to the east than it did to the west. 1 He also considers that icebergs floated about in the Esker-sea, carrying blocks of rock, and strewing them over the sea bed and on the Esker-banks. 2 In this way he accounts for the remarkable stream of erratic blocks of porphyritic granite, with large pink-coloured crystals of felspar, which he and Mr. O'Kelly 3 have traced from the neighbourhood of Oughterard, eastward from Galway Bay by Loughrea and Ballinasloe to the flank of Slieve Bloom. But, as I have already shown, the sea was laden with bergs arid rafts of ice carrying blocks of rock during the period of the Upper Boulder Clay ; and it seems to me much more probable that such trains of erratics as those described by Mr. Kinahan, together with similar ones in other districts, are the 1 On the Drift of Ireland.' LOG. cit. p. 200. 2 Ibid. p. 202. 3 Explanation, sheet 127 of the maps of the Geol. Survey, p. 26. Local Moraines 129 relics and monuments of this epoch ; somewhat as the large slabs of grit which are strewn over the Chalk Downs of Berks and Wilts are the relics and monu- ments of the Lower Tertiary beds with which they were once associated, these beds themselves having disappeared. The Rev. M. Close, in his admirable paper on * The General Glaciation of Ireland,' gives a summary of the observations of such erratic blocks derived from various sources, which shows how extensive has been the dispersion of such blocks by the agency of floating ice ; and to this paper I must refer my reader for fuller details than can be given here. 1 Local Moraines. When describing the conditions under which the Upper Boulder Clay was formed, I showed that we had reason for believing that the mountains of Ireland rose out of the surrounding sea in the form of groups of snow-clad islands, down the valleys of which glaciers descended into the sea, giving birth to icebergs. During the progress of elevation of the land, and the replacement of the glacial by more temperate conditions of climate, we may suppose that these snowfields gradually melted away, and that the glaciers which were fed by them withdrew step by step up the valleys. This retreat of the glaciers was doubtless a slow and lengthened 1 Journ. Hoy. Geol. Soc. Ireland, vol. i. p. 228. K 1 30 Geological Formations of Ireland process marked by pauses, during which moraines would be formed along the sides and at the lower extremities of the valleys. Such moraines are strikingly exhibited amongst the mountains of North Wales and of Cumberland and Westmoreland, and are also to be noticed occasionally, though less con- spicuously, amongst some of the mountain districts of Ireland. I was unable to identify more than one or two amongst the mountains of Mourne, as it appeared to me, when examining this group of gran- itic elevations, that the masses of moraine-like matter to be found on the slopes and amongst the valleys resolved themselves into sheets, or beds, descending into, and connected with, the Lower Boulder Clay. On the other hand, local moraines accompanied by local striations may be observed amongst the moun- tains of Wicklow, Kerry, Waterford, Galway, and Donegal. 1 The cross- striations on the glaciated sur- faces of the rocks in the vicinity of these mountains are referable to' two or more systems of ice movement one of these to the earlier and more general glaci- ation of the Lower Boulder Clay stage, and the others to the more recent action of local glaciers originating in the mountains themselves. They have been ob- 1 Other examples are to be found at the entrance to the Gap of Dunlow, near Killarney, and in the promontory of Dingle, where the valleys open out near the sea-coast. Terminal Moraines 131 served by Mr. Kinahan in Mayo, Gal way, and Clare ; by Mr. Foot in South Leitrim; by Mr. Symes 011 the shores of Killala Bay ; by Mr. Close in Co. Wicklow ; by Messrs. Jukes and Du Noyer amongst the Com- meragh Mountains in Waterford, and the mountain- ous promontories of Kerry; and by the officers of the Geological Survey in Donegal. These local striations require to be carefully distinguished from those of a more general range and earlier date ; and to be fully described would require a separate treatise, together with a glacial map, for each indi- vidual mountain group. The local moraines generally consist of mounds, hummocks, and banks of sandy clay, containing stones and subangular blocks of local rocks confusedly heaped together ; the blocks are sometimes glaciated on their surfaces, and in various directions. These local moraines, when thrown across a valley, as ' terminal moraines/ help to form lakes, by damming up the waters of the streams which enter it above. At other times, how- ever, the river has cut a channel for itself, generally near the end of the moraine where it originally came in contact with the side of the bounding ridge, and thus, the waters having been drawn off, the lake has disappeared. When the moraines are lateral they are piled up along the flanks of the valleys, and are less conspicuous than those above described. K 2 132 Geological Formations of Ireland It would be impossible to do more than give a few out of tlie numerous examples of local moraines to be found amongst most of the mountains of Ireland ; but the following may be mentioned : The remarkably straight and picturesque valley of Glenmalure, which lies along the line of a large fault and is drained by the Avonbeg River, furnishes at least two examples of terminal moraines. On ascending the valley from 'the Yale of Ovoca' through Ballinacor Park we are struck by the large number of huge boulders of granite which have been brought down from the interior of the mountains. One of these near the road measures 12x7x4J feet, and from this a large fragment had apparently been broken off and lies alongside. At the upper end of the park near Strand Bridge, and where a lateral valley enters from the south, immense piles of moraine matter laden with granite boulders lie across the valley, extending for some distance later- ally along the northern side, and cut through by the Avonbeg near its centre. Here we have clearly an old terminal moraine of the glacier which formerly extended down this noble glen, and drained the snow-fields of Lugnaquilla and the neighbouring heights. Above the moraine, the flanks of the valley may be observed to be glaciated to a height of about 500 feet above the bed of the river; above Moraine of Lough Bray 133 which traces of glaciation become indistinct, or entirely disappear. A second and smaller moraine occurs about two miles higher up Glenmalure, near the hotel, in the form of an irregular embankment which has evi- dently extended originally across the valley from side to side. The terraced surface of the valley above the moraine may once have been the bed of a lake which has since been drained, the river having cut a deep channel for itself through the moraine. This is probably one of the latest examples of local moraines amongst the Wicklow Mountains ; formed during a pause in the retreat of the gla- cier towards the head of the valley, while the snows of the surrounding heights were melting away. Lough Bray. The moraine which forms the eastern margins of both the upper and lower lakes is exceedingly striking and well defined. 1 The surface of the lower lake has an elevation of about 1,225 feet above the sea ; that of the upper is about 100 feet higher. Both lakes lie at the base of semi- circular Coombs, or cirques formed of granite which boldly project in naked cliffs in the interval between the loughs. These cliffs rise to an elevation of 1 Described by the Kev. Maxwell Close at a meeting of the Koy. Geol. Soc. session 1877-8. 134 Geological Formations of Ireland nearly 2,000 feet, forming the eastern shoulder of Kippure Mountain. From the extremities of the ranges of cliffs forming the north and south limbs of the cirques stretches a vast moraine of about a mile in length, rising high above the waters of the lakes, the surface being in some places strewn with large blocks of granite. The border of the moraine which bounds the eastern shore of the upper lake is remarkably straight, but along the corresponding shore of the lower lake it is curved into an arc of a circle which stretches considerably beyond the waters of the lake itself, giving space for the pretty cottage and grounds of Miss Crampton. The eastern face of the moraine slopes away downwards into the Glencree River. The bank which is thrown across the lower end of Lough Bray is considered by Mr. Close to be a terminal moraine, and to this we may add the embankment, levelled and fashioned by art, on which are planted the Churches and Round Tower of Glendalough in Co. Wicklow. Leaving the Wicklow Mountains and coming to those of West Galway, there occurs, amongst others, an example of a terminal moraine at the entrance to Glen Inagh where it opens out on the wide Glenda- lough Valley, a favourable position for such accumu- lations. The chain of loughs stretching along the southern slopes of the Connemara Mountains affords Glaciated Rocks 135 numerous instances of rock-basins and moraines, while the bosses of schist and crystalline limestone which rise out of their waters are strikingly ice- worn. The local glaciation has here obliterated the earlier general glaciation; while east of the meridian of Lough Oorid the ice has moved eastward, and to the west of the same line, in an opposite direction. The great tract of undulating moorland which stretches southwards to the shores of the Atlantic is dotted over with loughs which are either rock- basins scooped out by the glacial ice, or pent-up reservoirs where moraine matter has been left by the retreating ice. Thus the picturesque little lough of Glendalough at Recess has been pent up at its Avestern end by a great mound of moraine matter which has been thrown across the original valley, and by which it is separated from Lough Nacoogarrow. Immense masses of moraine matter are strewn along the shores of Lough Inagh and the slopes of Derryclare, probably left behind as the glacier dried up and disappeared. Along the southern shores of Killary Harbour, that remarkable fiord which penetrates the Western Highlands for a distance of about twelve miles from the Atlantic, the glacial phenomena are very striking. The rocks are intensely glaciated, and scored with groovings pointing down the valley, while masses of 136 Geological Formations of Ireland moraine matter with huge boulders are strewn along the shore. The mountains of Donegal present us with several conspicuous examples of local moraines, of which those in Glenfin, Loughs Nacung and Altan in the western part of the county are noteworthy. But perhaps the most marked as well as accessible are those of Barnesmore Gap, consisting of numer- ous mounds of shingle with boulders, lying on both sides of the valley above and below Barnes Bridge. A glacier seems to have descended along the valley from Croaghconnellan and neighbouring heights, and, turning the shoulder into the main valley, now forming the Gap, deposited its moraine matter at several successive intervals as the ice and snow melted away. Through this moraine matter which once extended across the Gap the river Lowerymore has cut a channel for itself. 1 1 These moraines are described and represented in plan in the Expl. Memoir to sheet 24 of the Geol. Survey, p. 39 (1888). 137 CHAPTER VI RECENT DEPOSITS Raised Beaches and River Terraces. Though our coast affords occasional evidence of local depressions, 1 the features which most strike us as bearing on the question of oscillations of level are the raised beaches, occurring in the form of terraces, rising above the reach of the highest tides, and often bounded inland by old coast cliffs. Traces of such terraces may be observed at the head of Killary Harbour, and along the base of the quartzite hills of Cormemara ; along the shore of Kenmare and Glengariff Bays; at Ardmore Point on the coast of Wicklow, and other localities. But the most striking and continuous of all the raised beaches I am acquainted with is one which is traceable at intervals along the northern and eastern coasts of Ireland, and which is un- doubtedly the representative of the c twenty-five feet 1 Such as the submerged peat-bogs in Ballyness Bay, Co. Donegal, and Tramore Bay, Co. Waterford. See Mr. Harte on the Physical Features of Donegal, loo. cit. p. 26. 138 Geological Formations of Ireland terrace ' of the western coast of Scotland, with the peculiarities of which geologists have become familiar by the writings of the late Mr. Smith of Jordan Hill, and, more recently, those of Sir A. Geikie. This raised beach forms a fringe of nearly level gravelly soil skirting the coast of Kin tyre, Arran, Rothsay, and the sinuous shore of the Clyde ; while inland it is generally bounded by a cliff or steep slope sometimes perforated by caves, the bottoms of which are strewn with rounded boulders, just as they were left at the recession of the tide thousands of years ago. Crossing to the Antrim coast, we soon recognise similar peculiar features in the protected nooks and bays of this bold sea-coast. The terrace may not be quite so elevated above the high-water line as on the opposite shore, the average elevation being about fifteen feet, and the sea waves have in some places made great inroads upon its margin, but it is not the less determinate in certain localities. Commencing at the north, we shall follow the course of the beach at intervals along the shore. The coast of Inishowen, in Co. Donegal, some- times has a trace of this beach in the form of a terrace in the less-exposed situations. I have noticed it at Culmore and Culdaff, rising about fifteen feet above the highest tides. On the southern shore of Lough Foyle around Bellarena Station, the Raised Beaches 139 raised beach, extends over a wide area, the ancient sea-margin being indicated by an abrupt bank of drift, or harder rock, lining the terrace inland. 1 Near Castlerock, the upper surface of the terrace is covered by huge sand-dunes, piled up by the westerly winds; and to the west of the terrace con- siderable tracts of rich alluvial land have been re- claimed from the sea by the aid of embankments, resembling the coasts of Holland or of the Lincoln- shire Fens, and bear evidence of the energy and enterprise of the farmers of this part of the country. At Portrush it occurs as a shelly gravel resting on the dolerite and the indurated Liassic beds of that district. But it is along the eastern coast of Co. Antrim that its features are most marked. That the coast has here been raised is evinced, not only by the presence of the narrow marginal terrace, but by the old coast cliffs perforated by caves now well out of reach of the waves. Sea-stacks of rock, standing erect as they were left when placed out of reach of the breakers by which they had been dissevered from the interior solid masses, are conspicuous monuments. One of these stands above the harbour of Ballycastle, and another is represented in fig. 19, 1 Mr. Joseph Wright, of Belfast, has discovered Foraminifera in great abundance in this deposit, including species of Lagena, Casi- dulina, Lingulina, Polymorpliina^ and Discorbina. Proo. Bel/. Nat. Field Club, March 1880. 140 Geological Formations of Ireland from the east coast of Island Magee. The caves may be seen penetrating New Bed Sandstone at Red Bay and Glenarm, and basalt along the coast north of Larne. Three of them at Port Ballintoy were explored some years ago by Mr. James Bryce 1 (the late Dr. Bryce of Glasgow), in company with Dr. McDonnell, and yielded bones of horse, ox, deer, sheep, goat (?), badger, otter, water-rat, and several FIG. 19. Old sea-stack of basalt. Island Magee. birds. Another cave on the west side of Carricka- reede Island yielded similar remains. The best section of the raised beach, perhaps, to be obtained on the Antrim coast is that which is laid open in some gravel-pits near Larne Harbour. The old sea-bed is here elevated into a terrace about fifteen to twenty feet above high-water line, and is seen to be composed of stratified and water-worn gravel, 1 Trans. Brit. Assoc. 1834, p. 658, Raised Beach of Kilroot 141 with numerous blanched marine shells, and (what is still more remarkable) with flint-flakes of human workmanship. The shells belong to species inhabiting the neighbouring sea, and the worked flints are of that rude form and finish known as ' Palaeolithic.' Another excellent section may be observed at Kilroot on the north shore of Belfast Lough, where the shells and worked flints are exceedingly abundant. It is here that these works of ancient art were first dis- covered by members of the Belfast Naturalists' Club ; they were afterwards described by the late Mr. G. Y. Du Noyer, who, judging by the great number of the chips of flint accompanying the arrowheads or spear- heads, came to the conclusion that the shore of Kilroot had been an ancient Palaeolithic workshop where weapons of war or of the chase were made from the chalk-flints of the adjoining hills. 1 The following species of shells have been identified by the Rev. Dr. Grainger from the raised beach at Larne Kurran : Anomia ephippium, Ostrea edulis, Pecten varius, Cardium edule, Kellia sub orbicular is, Lucina borealis, Tapes pallustra, Tellina tennis, Corbula gibba, Saxicava rugosa, Patella vulgaris, Trochus magus, Turritella terebra, Littorina obtusa, Pur pur a lapillus, &c* 1 Evans' Ancient Stone Implements of Great Britain, p. 258. 2 Trans. Brit. Assoc. (Belfast meeting, 1874), p. 73. 1 4 2 Geological Formations of Ireland Extending our observations further south, we occasionally find traces of the beach in the form of a narrow terrace from twelve to fifteen feet above high- water mark along the Downshire coast, as at Killough Bay near Ardglass (see fig. 17), and in Dundrum Bay. They may also be observed at Green ore, forming a terrace of shelly gravel about ten feet above the sea at the entrance to Carlingford Bay. Under the town and neighbourhood of Dundalk the terrace is rather widely spread, formed of shelly gravel. It may also be noticed at intervals along the coast southwards ; as, for example, at Lowther Lodge near Balbriggan, 1 and towards Dublin Bay, where it almost descends into the sea, the elevation being only five or six feet at Clontarf, and along the narrow neck which joins the Hill of Howth to the mainland. In Dublin Bay it merges into the old estuary of the river LifFey, forming the level terrace on which the Custom House, the Bank of Ireland, .the University buildings, and Sackville Street are erected. South of Dublin Bay, it is represented by the terrace at Bray, and the remarkable shingle beach, 1 Here the following shells occur : Litorina litoriiis, Pecten maccimus, Purpura liyrillus, Cardium ecliinatum, C.edule,Rostellaria pes-pelicani, Mya arenaria, Patella vulgaris, Troclius, Dentalium Turritella communis, Turbo cinerew. River Terraces 143 partly piled up by the waves and currents, which stretches for several miles along the coast north of the town of Wicklow, and is again represented in Arklow Bay. The elevation here is, however, only slightly above that of the high- water line ; and, comparing this with the levels along the coast of Antrim, it is clear that the surface of the terrace has a general slope downwards from the north to the centre of Ireland. The 1 5-feet beach of the north-eastern coast may be considered as the most recent representative we can point to of coast and land elevation. From the occurrence of buried canoes in this terrace in the Clyde valley, and the presence of the worked flints, associated with the shells in the stratified gravels at Larne and Kilroot, it is evident that the coast has been raised since the occupation of the British Islands by prehistoric tribes. This brings the history of physical changes as far down towards our own times as it is necessary for us to follow it. It is at this point that the archseologist and historian takes up the pen which the geologist lays aside. River Terraces. In connection with the subject of the elevation of the coast and raised beaches, we are naturally led on to refer to the formation of river terraces. These may be observed along the sides of the principal rivers of our island, where they approach 144 Geological Formations of Ireland the coasts and enter upon the last stage of their course, which is generally smooth and tranquil. The terraces may be observed forming almost level sur- faces bounded by abrupt descents which lead down to the alluvial flat still liable to floods. Where sections are exposed, it will be found that, like the alluvial flats themselves, the terraces are composed of river gravel, generally overlaid by a thin stratum of silt ; and we therefore infer that they are themselves old alluvial flats which were once occupied by the waters of the river, but which are now forsaken, the stream having deepened its channel. Sometimes two or more of these terraces may be observed, indicating successive deepenings. Now, such changes in the river bed must evi- dently follow the rising of the coast ; for where the coast rises the rivers are converted into rapids, or waterfalls, which commence to cut back their chan- nels inland, until a balance between the eroding power and the inclination of the river bed is esta- blished. Meanwhile, the channel being lowered, the former alluvial flats are laid dry, and new ones in a lower position are constructed. The old terraces which are so conspicuous along the valley of the river Boyne, in the neighbourhood of Navan and Slane, may therefore be traced back to the period when the seacoast itself was elevated, as shown by Old River Terraces 145 its raised beach ; and on the opposite coast of Ireland, the remarkable terraces of the Errif Valley, above Killary Harbour, may have some connection with the old sea terraces or beaches which have already been described as occurring in that neighbourhood. I have already referred to the old terrace along the banks of the Liffey at Dublin, which merges into the slightly raised beach of this part of the Irish coast. 1 Amongst the river valleys of Wicklow, numerous instances may be observed of old river terraces ; as, for example, at the lower end of the vale of Glen- malure above the junction of the rivers Avonbeg and Avon more. But of these the most remarkable and interesting, from its historical and architectural asso- ciations, is the terrace at the lower end of Glenda- lough, at its confluence with the vale of Glendasan, upon which stands the Eound Tower and the Church of St. Kevin. This terrace is composed of stratified gravel of rounded pebbles and sand banked up against an old moraine which had been thrown across the valley during the Glacial epoch. The upper surface is level, and it rises about twenty feet above the bed of the Glenealo River. As the site for the interesting ecclesiastical buildings by which it is surmounted, it was admirably chosen. After 1 See Gcol. Surrey Map, sheet 112. T, 146 Geological Formations of Ireland the ruins of lona, which Dr. Johnson has immortal- ised, perhaps there are none in the British Islands which impress the visitor with more interest than those of 6 the Seven Churches/ rising in chaste sim- plicity amidst the solitudes of these deep valleys, monuments of the faith and piet} 1 " of a past age. 1 1 A very interesting little handbook and guide to the ruins of Glendalough, with a historical sketch, has been written by Mr. Joseph Nolan, M.B.I.A. of the Geological Survey of Ireland. PAET II PHYSICAL GEOGRAPHY OF IRELAND CHAPTER I MOUNTAINS WHEN a traveller explores a new countoy, the objects which are calculated to arrest his attention are, first, the mountains, then the plains and great valleys, and ultimately the rivers and lakes ; and in the following attempt to elucidate the origin of the physical features of Ireland I shall adopt the order here indicated, as it seems to correspond, not only with the usual process of observation, but with the order in which the physical features themselves were developed. The origin and geological ages of the respective mountain chains, or groups, first demand our consideration. But it is desirable that I should state at the outset what I mean by ' the geological age ' of a group of mountains ; for, as we shall see, some of the Irish mountains have been either sub- L 2 148 Physical Geography of Ireland merged beneath the ocean, or partially buried be- neath newer strata after they had been in existence many ages before. Thus it happens that the High- lands of Scotland have at one time been deeply buried beneath strata of Devonian and Carboniferous ages. The mountains of North Wales have been, in part at least, entombed in Carboniferous, Triassic possibly Jurassic and Cretaceous beds ; even the central core of the Alps has more than once almost disappeared beneath enormous masses of Secondary and Tertiary formations. But after all these vicissitudes the mountains have emerged again into day ; changed, indeed, in form and feature, but ever grand and venerable ; the noblest objects on which man can gaze, and made use of by him as emblems of the majesty of the Great Creator Him- self. After various vicissitudes and periods of ob- scurity, they now stand before us in all the beauty and loftiness of maturity, and in all their wonderful variety of form, shadow, and colouring ; and as we gaze upon them our thoughts find expression only in such words as those of England's great epic poet : * These are Thy glorious works, Parent of Good ! Thyself how glorious then ! 1 Still grander is the language of the Psalmist of Israel when he points to the mountains as emblematical of the eternity of Jehovah : ' Before the mountains were brought forth, or ever the earth was made, from everlasting to everlasting Thou art God.' Ps. xc. 2. The 'Birthday' of Mountains 149 When we speak, therefore, of the age of any of the Irish mountains, it must be understood that we mean their actual birthday (so to speak) and not any subsequent renaissance, or redevelopment after a period of obscurity. Now, there is only one way in which such an epoch can be determined in the case of any particular group of mountains, namely, by observing the relations of the strata of which the group is composed to the newer formations with which it comes in contact, and determining the geo- logical age of each. Thus, if we find a mountain group composed of strata belonging to the Lower Silurian system (as indicated by their fossils or other- wise) to be overlaid along its base unconformably l by strata of the age of the Old Red Sandstone, we have here evidence that the beds forming the moun- tain group have been forced out of their originally nearly horizontal position, and subjected to denuding action before the newer beds were deposited over them. Now, as denudation can only take place at the surface o'f the ocean, or under the atmosphere itself, we are led to conclude that the denuded beds had, in such an instance, been converted into a land- surface be fore the epoch of the Old Red Sandstone; and this was, in geological language, the date of their birth. 1 That is, when the strata of the newer formation rest on the disturbed and denuded edges of those of the older. 150 Physical Geography of Ireland Upon the principles above laid down, we shall now proceed to determine (as far as the evidence admits, for it is not always conclusive) what may be the respective ages of the different mountain groups of Ireland, and the special features connected with their development. The description already given of the formations themselves, and of their mutual relations, will have prepared the reader for what is to follow. Before, however, entering upon an account of the geological age of the mountains of Ireland, it will be desirable for me to endeavour to sketch out the leading physical features of the country as repre- sented by its mountains and valleys. Ireland may be described as an island consisting of a great central plain, bounded in various direc- tions near the coast, but not entirely surrounded, by groups of mountains. Along the west, south, and north the coast is deeply indented, and the central plain is diversified by numerous lakes and sluggish rivers which find their way out to sea at the head of the bays and estuaries. A line drawn across the centre of the country from Dublin or Dundalk Bay on the east, to Galway Bay on the west, will meet with no higher elevation than that of about 250 feet above the sea ; but a section in every other direction will be found to cross a mountainous ridge bounding Mountain Groups 151 at each, extremity the central undulating plain. This plain is formed of Carboniferous limestone, which occurs in beds not much removed from the horizontal position, except in the neighbourhood of local disturbances, or of several eminences which rise above the general level, towards the southern portion of the country, and which are composed of older formations. For the purposes of classification, the mountain groups may be divided into the following: (1) The North- Western Highlands of Donegal and Derry ; (2) the Western Highlands of Mayo and Gal way including Connemara; (3) the South- Western Highlands of Kerry and Cork, with the outlying elevations of Maugherslieve, the Devil's Bit, Slieve- naman, Knockmealdown and Galtymore, which all physically belong to this group ; (4) the South- Eastern Highlands of Wicklow and Dublin ; and (5) the North-Eastern Highlands of Mourne, Carlingford, and Slieve Gullion. Besides these there are, of course, many minor hills, variously composed, and of different ages, which are not of sufficient magnitude to entitle them to rank as mountains ; though it must be confessed that it is often difficult to say where the ' hill ' merges into the ' mountain,' and, conversely, where the mountain merges into the hill. I have already given some account of the geo- 1 5 2 Physical Geography of Ireland logical structure of these mountain groups when describing the formations of which they are com- posed, and our principal object is now to endeavour to ascertain their actual and relative geological ages, bearing in mind those principles which I have laid down for our guidance in this inquiry. 153 CHAPTER II NORTH-WESTERN AND WESTERN HIGHLANDS IN searching amongst the groups I have named above for the most ancient, we naturally refer to the form- ation of greatest antiquity, and we find ourselves amongst the north-western, the western, and the south-eastern Highlands, formed, as I have already shown (p. 12), of Lower Silurian beds generally con- verted into crystalline schists, quartzites, and gneiss by that deep-seated hydrothermal and dynamic pro- cess which we call fc metamorphism.' I have already shown that the metamorphic rocks of Donegal, Mayo, and Gal way belong to one great geological system continuous with that of the Central Highlands of Scotland, and that the epoch at which this trans- formation of the original strata took place is ac- curately determined for us by the position of the Upper Silurian rocks on both sides of Killary Har- bour, and in the direction of Lough Mask. 1 Here it 1 Some account of the physical features of these districts has already been given at p. 12, and need not, therefore, be repeated. i 54 Physical Geography of Ireland is that the Upper Llandovery beds, consisting of red and grey shales, grits, and conglomerates, are found resting discordantly on the eroded edges of the meta- morphic rocks ; the former being well stored with fossils by which their age has been determined, and often containing rounded pebbles and blocks of quartzite, schist, and gneiss derived from the meta- morphosed Lower Silurian beds. These newer beds rise in a series of ledges and terraces into the summit of Muilrea, 2,688 feet, and form a range of hills ex- tending to the shores of Lough Mask, including the remarkable tableland of Slieve Partry, or ' Joyce's Country.' From the position of these beds with reference to those upon which they rest, it is evident that they have not shared in the metamorphism to which the Lower Silurian rocks have been subjected. The metamorphism is, therefore, of older date than the Upper Silurian, and belongs to a long, unrepresented, period intervening between the Lower Silurian on the one hand and the Upper Silurian on the other. I have dwelt somewhat at length upon this point because of its importance. The period of meta- morphism was accompanied by great disturbances of the Lower Silurian strata ; they were bent into folds and flexures, subjected to lateral thrusting, sheared and faulted, elevated out of the waters of the sea in North- Western and Western Highlands 155 which they had been deposited, and subjected to the operation of denuding agents the sea waves, the rains, and rivers of the period which swept away large masses of the more exposed portions, hollowing out valleys, and thus in reality creating the moun- tains. One of these hollows lay along the line of the Upper Silurian hills in West Mayo already de- scribed. On the resubmergence of the land the sides of this valley were subjected to wave action, and shingle beaches, now occurring in the form of con- glomerate beds, were strewn over the floor of the sea. Here, then, we have the first indications of mountains that is, of old land surfaces, raised, we know not how high, into the air, and bounded by valleys which ultimately became arms of the sea, and were filled up as the land subsided by beds of shingle, sand, and clay derived from the waste of the neighbouring lands. It has also been shown that the rock masses of the Donegal Highlands are of similar age and composition to those of the West Gal way and Mayo Highlands, and that they have been subjected to similar changes and at the same period of geological time; so that from all these considerations we arrive at the conclusion that the epoch of the formation of the North- West and Western Highlands is c Pre-Llandovery,' to use a technical phrase ; in other words, an epoch immediately pre- 156 Physical Geography of Ireland ceding the formation of the Upper Silurian series, the base of which is formed of Upper Llandovery beds. Such seems to be the age of the oldest mountains of Ireland, unless we suppose the existence of a still earlier range formed of the Archaean rocks of South Donegal and Tyrone. 1 1 The relations of the Lower Old Eed conglomerate to the meta- morphic strata in Tyrone and North Donegal (Fanad) show that the Donegal rocks were upraised and metamorphosed before the epoch of the Lower Devonian. 157 CHAPTER III SOUTH-EASTERN HIGHLANDS THE range of mountains extending from the shores of Dublin Bay through Wicklow into the valley of the river Barrow, in Carlow, is formed of granite, which penetrates the Lower Silurian beds in the manner already described (p. 15) ; these latter being considerably altered, or metamorphosed, in proximity to the granite. 1 It is remarkable that although the Cambrian rocks occupy a large tract of country along the seacoast, the granite never penetrates these older rocks, but is limited to the Silurian area. Along the central line of the granite the moun- tains attain their highest elevations, rising by long sweeping moorlands, often, however, bounded by pre- cipitous or steep cliffs and escarpments, into the culminating points of Kippure (2,473 feet), Duff Hill (2,364 fret), Douce Hill (2,384 feet), and Lugnaquilla (3,039 feet). These mountains give 1 This is indicated by the development of mica and other minerals along planes of foliation. 158 Physical Geography of Ireland birth to numerous fine streams, such as the Liffey, the Vartry, the Avonmore, and Avonbeg, which, uniting at the well-known ( Meeting of the Waters,' go to form the river Ovoca, and, this stream being still further augmented by the confluence with the river Aughrim, enters the sea at Arklow. The southern portion of the range is drained by the river Slaney. These rivers traverse tracts of Silurian and Cam- brian rocks, along valleys remarkable for their depth ; while to the naturally varied character and ruggedness of the landscape there has been imparted an aspect of richness and softness owing to the extensive woods with which the banks of the valleys are clothed. Some of these river valleys are very ancient, dating probably from the Carboniferous period, or even earlier r 1 to these I shall have occasion to refer again, and now pass on to consider the matter more imme- diately in hand, namely, the date at which the moun- tains of this region themselves were f brought forth.' In order to do this we must assume that the birth of the mountains corresponds with the intrusion of the granite. This rock is of intrusive origin ; that is, it was thrust in amongst the Lower Silurian rocks from below with great force ; and its formation was (we 1 See Mr. Kinahan's paper ' On the Estuary of the Kiver Slaney,' Journ. Roy. Geol. Soc. vol. iv. p. 60. Age of the Leinster Granite 159 may suppose) accompanied by a considerable eleva- tion of the Silurian rocks, forming a ridge which maintains its pre-eminence, though subjected to ex- tensive denudation, down to the present day. Now, we have no very precise information re- garding the age of the Leinster granite further than this, that its formation was antecedent to the period of the Upper Old Red Sandstone. This is clear from the position of this latter formation on both sides of the river Slaney above New Ross, where the Upper Old Red Sandstone, and then the Carboniferous Limestone, gradually overlap the Lower Silurian beds, and finally rest on the eroded surface of the granite itself near Bagnalstown. 1 The granite, and, as we may infer, the mountains which are formed of it, is therefore older than the epoch of the Upper Old Red Sandstone ; but, having reached this point in our inquiry, our evidence ends as regards direct super- position of strata ; and perhaps it would be wise not to venture on indirect evidence in order still more closely to determine the date of irruption. But on looking at a geological map we cannot close our eyes to the fact that the granite is pro- truded along a certain definite direction. This gene- 1 That the granite was consolidated before the Carboniferous period we have abundant evidence in the neighbourhood of Dublin, where pebbles of granite have been found imbedded in the lime- stone. 1 60 Physical Geography of Ireland ral direction is E.N.E. and W.S.W. ; and it will be observed that it corresponds nearly with the outbursts of granite and trap in Donegal, the date of which we have ascertained to be immediately preceding the formation of the Upper Silurian beds. I cannot help attaching importance to this parallelism of direction, because it is intimately connected with parallelism in the action of terrestrial forces, which, from the days of Elie de Beaumont, is admitted by physicists to be some evidence of contemporaneity. 1 I do not insist very strongly upon the application of this principle in the present instance, and will only go so far as to say that the granite mountains of Wicklow are certainly older than the period of the Upper Old Eed Sandstone (or Upper Devonian), and probably older than that of the Upper Silurian. 2 We now pass on to the consideration of the age and origin of the remaining groups, amongst which we shall be brought into contact with natural opera- tions of a different character from those we have been hitherto considering. 1 M. de Beaumont may have carried his theory too far, but it certainly contains a germ of truth. 2 And therefore synchronous with those of Donegal, Mayo, and Gal way. CHAPTEE IV SOUTH-WESTERN HIGHLANDS Mountains of Kerry, Cork, and Waterford. The origination of the ranges of hills and mountains we have hitherto considered is, as we have seen, con- nected intimately with hydrothermal action result- ing in metamorphism of the strata ; but in the case of the ranges now to be considered no such action is observable. Here we are brought face to face with the results of mechanical forces arising from the contraction of the earth's crust, and resulting in the production of flexures, foldings, slaty-cleavage, and even inversions of the strata, along lines ranging in approximately parallel and definite directions. The study of a geological map of this part of the country will probably do more to bring the structure of the strata vividly before the reader's mind than any verbal description. It will be seen that the rocks of this part of Ireland are disposed in long parallel, or sharply wedging bands, ranging nearly east and M 1 62 Physical Geography of Ireland west, 1 and coloured differently according to the for- mation. The narrower bands are formed of Carbon- iferous rocks ; the broader, of Old Red Sandstone. And while the former are compressed into narrow syn- clinal, or trough-like folds, forming the valleys and arms of the sea, the latter rise into mountainous tracts, and project as headlands far into the Atlantic, while the strata are bent into great arches ; more or less modified by plications somewhat after the fashion of a Moorish arch. A moment's consideration will suffice to show that the whole of this mountainous tract must once have been covered by Carboniferous strata, and that where the Upper Old Eed Sandstone forms the sur- face of the ground, it is only because the newer beds have been removed by denudation. And when we further consider that these strata were once horizon- tally spread in parallel layers over the whole of the south of Ireland, 2 but are now bent into a series of nearly parallel folds and flexures, such as I have described, it will be seen that they occupy much less horizontal space than they did originally ; that they have, in fact, been crumpled and bent into these foldings by tangential forces acting in a direction perpendicular to the axes of the flexures ; that is 1 More correctly, a little south of west, and north of east. 2 That is, at the close of the Carboniferous period. Flexured Strata of the Reeks 1 63 to say, from north to south, or from south to north. It is impossible to conceive, and much less to calculate, the amount of force required to bring about such results. It far exceeds that of gravitation, and can only be accounted for on the theory of the secular cooling of the earth's crust. Such a process causes the crust to contract upon itself; and where lines of weakness previously existed, forces the strata into a less horizontal space, and thus to range themselves in parallel folds. These folds can be frequently seen amongst the bare, or slightly clothed, mountains of Kerry ; as for instance, in the rugged tract south of Kenmare Bay, and wherever the passes cross the ridges transversely. The massive grits and bands of slate may, in such positions, sometimes be observed to be bent completely over in grand arches, or folded in synclinals ; or, lastly, to rise at high angles into the air. The district now under consideration includes the loftiest elevations in Ireland. The broken and somewhat serried ridge of Macgillicuddy's Eeeks reaches at Carntual * a height of 3,414 feet above the sea, and there are several elevations in the same district not much below this. The range stretches 1 Sometimes spelt ' Carrantuohill,' which Dr. Joyce states to mean ' the inverted reaping-hook.' 1 64 Physical Geography of Ireland westward to the ocean between Dingle Bay on the north, and the long narrow channel of the Kenmare River on the south, and rises abruptly from the borders of the beautiful Lakes of Killarney, where we have a combination of natural features rarely to be met with, and rendering this (as all who have visited it in early summer must acknowledge) the most delightful spot in the British Isles. 1 The rugged heights of the Reeks, as they slope down- wards towards the water of the lakes, are clothed with forests of timber and underwood, chiefly natural, amongst which the Arbutus is conspicuous ; and at the base stretch away the placid lakes, studded with islets, and their banks clothed with verdure. Those islets and rocks which rise out of the lakes are often intensely glaciated, and appear above the waters near Muckross Abbey, in long smooth backs of limestone polished and striated. 2 The lakes are situated in the Carboniferous Limestone, but send a long arm southwards into the heart of the mountains of Lower Old Red Sandstone, which terminates in the Black Valley, a gloomy and savage cul-de-sac, bounded by steep cliffs stretching along the eastern shoulder of the Reeks. 1 On the high authority of Lord Macaulay. See History of England, vol. ii. p. 307 (edit. 1872). 2 Amongst which the Osmunda regalis grows in fronds of extra- ordinary size. ^ I 1 I 9S - S " 1 IMl to O g- 3 I g 8 1 66 Physical Geography of Ireland By the Gap of Dunloe, a narrow gorge which strikes across the ridge into the higher part of the Black Valley, a fine section of the rock forming the northern flanks of the mountains is obtained. It is here, indeed, that the wonder of the geological observer is excited ; for he finds the great beds of green and purple grit of Lower Devonian age dipping southwards into the heart of the ridge, and away from the Carboniferous Limestone of the plain; and, when he conies to examine the sections of the limestone in the islands and shores of the lake, he finds that this formation dips in a similar direction that is to say, the newer formation of the plain appears to dip below the older formation of the mountains. (See fig. 20.) Here, evidently, is a problem requiring explana- tion, and one which has occupied the attention of several geologists of eminence, without a perfectly satisfactory solution having been arrived at. The actual junction of the two formations is, I believe, not visible, owing to the presence of a thick deposit of Glacial Drift, and we are obliged to have re- course to theory to account for these phenomena. Two explanations are possible : either the boun- dary between the Carboniferous Limestone and the Lower Devonian is a fault (or fracture of the strata) along which the former formation has been Inverted Strata of the Reeks 167 brought down (or displaced relatively to the latter), or else the strata have been folded back upon them- selves. This latter is the view adopted with some hesitation by the late Professor Jukes, 1 and is doubtless that which is supported by the greatest amount of evidence. It is probable, however, that at some points along the boundary, which runs in a slightly sinuous line from the southern shore of Dingle Bay eastward, the reversed fold is accompanied by a fracture, or fault. We infer, therefore, that along such a line of fold or fracture, or both, the beds were thrust up in the form of a grand Moorish arch, the apex of which lies between Killarney and Keninare Bay, where it bends down again to form a narrow trough. Again the strata rise into another crenellated arch between Kenmare Bay and Bantry Bay. Between this bay and the southern coast the strata are forced into the arches forming the headlands of Muntervary, Mizen Head, and Cape Clear, while the intermediate synclinal flexures occupy the valleys which termi- nate in Dunmanus Bay, and that of Crookhaven or Eoaring- water. The long promontories which jut out into the Atlantic along the southern coast are the western 1 See Explanation to sheets 163 and 174 of the Geol. Survey, p. 10 ; also, Journ. Roy. Geol. Sac. Ireland, vol. ii. (new ser.) p. 90. 1 68 Physical Geography of Ireland extremities of mountain ridges of Lower Old Red Sandstone, which stretch far inland and eastward into the County Cork. Amongst these, the serried ridges of Glengariff washed by the waters of Bantry Bay are perhaps the most striking ; and when seen from the opposite shore against the glowing back- ground of an evening sunset afford studies of shade and colour for the painter, not often surpassed in depth and richness of tone amongst the British Isles. The outlying mountains of Cork and Waterford, which rise above the limestone plain, often in the form of a dome, or of an oval-shaped ridge, belong to the same system of flexures. They include Slieve Bernagh and the Silver Mine Mountains, elevations which were originally united, but are now intersected by the deep gorge of the Shannon above Limerick. The Maugherslieve, Devil's Bit, and Slieve Bloom Mountains which rise from the central plain, and the grand mass of Graltymore, which is the highest of the group, and reaches an elevation of 3,015 feet, have all a very similar structure. A central core of Silurian rocks throws off all around beds of Old Eed Sandstone and Conglomerate, which in their turn plunge below the Carboniferous Limestone of the plain. In the case of the Galtees, although the general structure is dome-shaped, the form of the Geological Age of the Killarney Mountains 169 mountain is elongated in an east and west direction corresponding to that of the system of flexures we have been considering. Geological Age. And now it is time to consider the question of the Geological Epoch at which the powerful system of terrestrial forces came into play, owing to which the great east and west flexures and foldings of the strata were originated. As the strata which have been influenced by them belong to the Carboniferous, as well as the Devonian, group, it is evident, in the first place, that the epoch referred to was more recent than the Carboniferous. But if we were to go further and seek to determine the actual time by reference to the south of Ireland alone, we should be left completely in the dark, because there are no formations in that part of the island with which to compare their relations newer than the Carboniferous, until we come down to those of the Post-Pliocene period. On examining, however, the geological structure of the adjoining districts of South Wales and Eng- land, and extending our investigations still further into the Continental areas across the English Channel, it becomes evident that the terrestrial disturbances so strikingly represented in the south of Ireland belong to a system which has influenced the Palaeozoic strata all over the south of England and 1 70 Physical Geography of Ireland Wales, and the borders of France and Belgium to, and even beyond, the valley of the Ehine a line of about 850 miles in length. It will be found, also, to correspond in direction with several of the moun- tain ranges of the Continent, such as those of the Alps, Pyrenees, and Carpathians ; for although these mountain ranges have been affected by disturbances of much more recent date, their early movements probably corresponded with those we are now con- sidering. As far, however, as the British Isles are con- cerned, we have very precise data for determining the epoch of these movements. In the south-west of England, the New Eed Sandstone or Trias is found resting upon the upturned and eroded edges of these older Carboniferous and Devonian rocks. This is the case along the borders of the South Wales and Somersetshire coal-fields, and the shores of the Bristol Channel. The New Eed Sandstone formation is, therefore, evidently more recent than the epoch of the disturbances and erosion of the Carboniferous beds, and, consequently, we must refer the epoch of such disturbances to one preceding the Triassic. But we may even go a step further ; for in the north of England and Ireland we find the Carboni- ferous beds arranged in folds and flexures parallel Geological Age of the Killarney Mountains 1 7 1 to those of the southern districts. We have, there- fore, the evidence of parallelism of direction in support of the view that the flexures are contemporaneous. But, as the relations of the Carboniferous strata to those of the Magnesian Limestone, of Permian age, in the north of England clearly show that the E. and W. flexures in the former were produced during an epoch anterior to the deposition of the latter formation, we arrive at this general conclusion with regard to these great east and west lines of flexure in the Carboniferous rocks over the British Islands, that they belong to the period, unrepresented by strata, between the Carboniferous on the one hand and Permian on the other. This was an age of general disturbance and of great denudation of the previously formed Carbonifer- ous beds, of which thousands of feet in depth were swept off the surface of the land before the deposition of the Permian sandstones and limestones. It was an epoch, therefore, of long duration, but entirely unrepresented by existing records. It is a blank in the stratigraphical series ; a leaf in the history of creation torn out. 1 1 In the north of England the Permian beds are found resting directly on the Carboniferous Limestone and Yoredale Series,' in which cases all the coal-measures and millstone grit series, amount- ing from 5,000 to 6,000 feet of strata, had been denuded away. In the north of Ireland at Armagh this formation (the Permian) rests on the Carboniferous Limestone (see p. 67). 172 Physical Geography of Ireland CHAPTER V NORTH-EASTERN HIGHLANDS Mountains of Mourne and Carlingford. This group of mountains is full of interest as illustrating plutonic and volcanic phenomena of past geologic times, and is remarkable for the number and variety of rocks and minerals which it produces. The Carlingford range is separated from that of Mourne by Carling- ford Lough, a deep and picturesque arm of the sea. There is a third line of elevations lying to the north of the Mourne Mountains from which it is separated by a valley of Silurian grits and slates, extending from the Newry Canal towards the north-east, called the Slieve Croob range, attaining to an elevation of 1,755 feet, which is composed of granite apparently much more ancient than that of Mourne, 1 but our at- tention must here be specially directed to the latter. The mountains of Mourne are composed of a peculiar granite, of which Dr. Haughton has deter- 1 The granite of Slieve Croob is probably of the same age as that of Donegal or Wicklow, and consists of quartz, orthoclase, and black mica. The Mourne Mountains 173 mined the chemical and mineral composition. It consists of orthoclase, albite, quartz and mica, and is full of little cavities which often contain beautifully formed crystals of smoke-quartz, orthoclase, topaz, and rarely of emerald. It is of intrusive origin, sending dykes and veins into the Lower Silurian rocks with which it comes in contact, and is itself traversed by dykes of basalt, felstone, mica- trap, and porphyry. There are other dykes, however, of basalt and diabase which are of older date than the granite, and are abruptly terminated at its margin. Thus its age is intermediate between that of the one set of dykes and that of the other. These pheno- mena may be well observed on the hills between Carn Mountain and Slieve Muck, where the contorted Silurian rocks are found capping the granite along the crest of the ridge in a very peculiar manner. The Mourne Mountains consist generally of a series of conical or dome- shaped elevations, of which Slieve Donard, itself forming the culminating summit, is a fine example, rising from the margin of the sea to an elevation of 2,796 feet. This is generally the form assumed by granite in mountainous districts. Still, serried ridges and peaks are not absent from this range, examples of which we find in Slieve Bingian (2,449 feet) and Slieve Bearnagh (2,394 feet). The ridge of the Carlingford Mountains presents a 174 Physical Geography of Ireland peculiarly rocky and serried aspect when seen from the opposite shore of the Lough. It runs parallel to the southern shore of the Lough, and reaches in Slieve Foye an elevation of nearly 2,000 feet. This ridge is formed of a remarkable olivine gabbro, largely crystalline granular, composed of anorthite, diallage, olivine, and magnetic iron-ore. 1 The rugged ridge FIG. 21, Diagrammatic section to shorn horv the Syenite brealts through the Pyroxenic Rock. Barnaveve. 1,022 feet. D s, Syenite intruded amongst masses of Olivine Gabbro, D. D (on left) Dolerite. of Barnaveve is capped by a similar rock supported and penetrated by syenite of more recent age, com- posed of flesh-coloured crystalline felspar with crystals of hornblende and a little quartz. At the western base of this ridge there is a mass of largely crystalline dolerite. 1 Rev. Dr. Haughton has determined the species of the felspar to be anorthite ; Prof. A. von Lasaulx refers the pyroxenic mineral to be diallage rather than hypersthene, as had previously been supposed. The olivine I had observed in thin sections under the microscope. The rock may, therefore, be called ' olivine gabbro.' The Carlingford Mountains 175 The whole of this group of hills is formed of felspathic and pyroxenic rocks of several forms and varieties graduating into one another, and which may be regarded as the representatives in time of those of the Mourne Mountains on the opposite side of Carlingford Lough. Thus we may classify the two sets as under : Pyroxenic group (the more ancient), consisting of micaceous dolerite of Slieve Gullion, diorite of Trumpeb Hill, dolerite and olivine gabbro of Barna- veve and Slieve Foye. Felspathic group (more recent), consisting of varieties of felstone-porphyry, and syenite of the Carlingford Mountains, and representing the granite of Mourne. These rocks are penetrated by innumerable intru- sive sheets and dykes of trap, generally basaltic ; some of which, as in the case of the Mourne Mountains, are of Tertiary age, others much older, probably of Upper Carboniferous age. These intrusive sheets, jutting out along the flanks of Carlingford Mountain (Slieve Foye) and cutting the Silurian beds trans- versely, impart to it that rocky, terraced aspect which may be noticed even from the northern side of the Lough. The Carboniferous Limestone laps round the southern base of Slieve Foye, near Greenore, and at 176 Physical Geography of Ireland the quarries here the basalt is seen to traverse the limestone, both as intrusive sheets and dykes. It is possible that the sheets may be of Upper Carbonifer- ous, and the dykes of Miocene age belonging to the epoch of volcanic activity in the north of Ireland. 1 Geological age of the Mourne and Carlingford Mountains. These mountains are so isolated from strata newer than the Carboniferous that much FIG. 22. Carlingford Quarry. Horizontal and inclined dykes of basalt traversing Carboniferous Limestone. uncertainty exists regarding the date of eruption of their rocks. They may undoubtedly be considered as the roots of volcanic mountains, the trunk and branches of which have been removed by denuding agents ; just as if a mountain like -ZEtna were to be cut down into a group of hills rising 2,000 or 3,000 feet above the level of the Mediterranean. The intru- 1 The great numbers of dykes in this district has led Dr. Haugh- ton to conclude it was a focus of volcanic action. Age of the Mourne and Carlingford Rocks 177 sive character of the rocks ; the association of fels- pathic and pyroxenic varieties, as in the case of more recent volcanic mountains ; the innumerable dykes of trap which radiate from, or traverse, the whole dis- trict ; all point to this region as having been the seat of great volcanic activity. Nor ought we to omit reference to the remarkable mass of agglomerate, made up (as on the southern flanks of Slieve Gullion) of bombs of granite, which have been torn up from the granitic basis of the hill below, and blown through the throat of an old crater, as conclusive evidence that these rocks in some places were erupted at the surface of the land of the period. What then, let us inquire, may have been the epoch of all these volcanic operations? Now, as we find on both shores of Carlingford Bay the Carboniferous Limestone traversed, as already de- scribed, by basaltic dykes which seem to be connected with the larger masses of Slieve Foye : and as on Slieve Foye itself, Dr. Haughton, and more recently the officers of the Geological Survey, found the limestone converted into crystalline marble at its contact with the olivine gabbro, the more ancient of the volcanic products, it is clear that these rocks are themselves of more recent age than the Lower Carboniferous. But beyond this, we have no direct evidence as to age ; as the Mesozoic strata nowhere K 178 Physical Geography of Ireland come in contact with the Mourne or Carlingford rocks. It should, however, be stated that Sir A. Geikie has assigned a much more recent age to the Mourne Granite than that above suggested. 1 He recognises in this remarkable rock a resemblance to the granites of Arran Island and of the Western Isles of Scot- land, particularly in the occurrence throughout all these masses of a tendency to the development of minute irregular cavities, giving here and there a ' carious aspect ' to the rock ; and in which well-formed crystals of quartz and felspar have been developed. This is certainly a peculiar characteristic of the Mourne Granite as distinguished from other granitic masses in Ireland, and is doubtless due to the presence of large quantities of gas and aqueous vapour at a high temperature in the viscous mass during consolidation. To all the above rocks Dr. Geikie assigns a middle Tertiary age, and this opinion, however startling, is entitled to great weight. It is improbable that the actual age can ever be directly determined; but whatever it may really be, it cannot be doubted that the granite of the Mourne Mountains and its representative felspathic masses of Slieve Eoye are amongst the most recent igneous rocks of Ireland. 1 * History of Volcanic Rocks, &c.' Trans. Roy. Soc. Edin. vol. xxxv. pp. 148-50. Mountain Groups of Ireland 179 We have now passed in review the different mountain ranges, or groups, of our island, and deter- mined, as far as the evidence admits, the composition of their rock masses, their mode of formation, and date of elevation in the sense already explained. It may be useful to place the conclusions in a tabulated form before we close this chapter, and the following table has been drawn up with this object : Mountain Groups of Ireland. NAME. 1. North-eastern High- lands of Mourne, Carling- ford, and Slieve Gullion. (Most recent.) 2. South-western High- lands of Kerry, Cork, and Waterford. 3. Western Highlands of Connemara and West Mayo. 4. North-western High- lands of Donegal and Derry. 5. South-eastern High-) lands of Wicklow, &c. ORIGIN AND BISECTION. Plutonic or Old Volcanic. Mechanical. E. and W. Metamorphic. E. W. Metamorphic. NE. S.W. Metamorphic and intrusive. NE. S.W. GEOLOGICAL AGE. Later than Carboni- ferous ; close of Mesozoic, or pos- sibly Middle Ter- tiary. j Between Carbonife- | rous and Permian. J Between Lower and | Upper Silurian. (Between Lower and | Upper Silurian. [ Probably between j Lower and Upper I Silurian. x 2 180 Physical Geography of Ireland CHAPTEE YT ORIGIN OF THE CENTRAL PLAIN NEXT to its mountains, the Great Central Plain is the most striking physical feature in Ireland. As already stated, it stretches right across the island from the coast between Dublin and Dundalk Bays on the east, to Galway Bay on the west, between which it seldom exceeds 300 feet in elevation, the average being less. Towards the north, bounded by ranges of hills generally of no great elevation, it stretches from the foot of the Ox Mountains in Mayo, along the southern slopes of the table-land of the Sligo and Fermanagh Hills, eastwards to the coast at Dundalk; the margin nearly coinciding with the uprising of the Lower Silurian rocks in the Counties of Monaghan and Cavan. Along the west, the boundary is very determinate, as it follows the western shores of Lough Mask and Lough Corrib to Galway Bay, beyond which it assumes the form of a terraced range of low hills in the Burren district of Clare, and thence sends an arm southwards to the Formation of the Central Plain 181 estuary of the Shannon at Foynes Harbour. To- wards the south, it skirts the group of isolated hills of Old Red Sandstone and Silurian rocks, which, as already described, are the advanced outworks of the mountains of Kerry, Cork and Waterford, and in- clude Slieve Boughta, Slieve Bernagh, Keeper Mountain, Slieve Bloom; and further south, Galty- more and Slievenaman. Along the east, the boun- dary skirts the granitic range of Wexford, Wicklow, and Dublin, reaching the coast at Dublin Bay along the base of the Three Rock Mountain and Killiney Hill. 1 Throughout the area included in this wide circuit, several isolated hills rise above the general surface of the plain, such as the table-land of the Castlecomer and Killenaule Coal-fields, the Chair of Kildare formed of Upper Silurian beds, and a few small detached hills towards the north-west. The Castlecomer table-land remains as a monument of the former elevation of the plain. 2 The latter is in some degree due to direct elevation. Geological Formation of Central Plain. Through- out the greater portion of its area, the Central Plain is underlaid by the Carboniferous Limestone. 1 The Hill of Howth (Ben Edair), Lambay Island, and the granitic bosses of Eockabill would (if the bed of the sea were laid dry) form a continuation of the eastern margin of the Central Plain. 2 See fig. 9, p. 63. 1 82 Physical Geography of Ireland Indeed, to such an extent is this the case, that the limits of the Plain, except in the north- western districts, might be taken to coincide with the boun- dary of the limestone itself, as laid down upon the Geological Maps. As already stated, the strata are nearly horizontal, except near the margins, and in proximity to local disturbances. To the north of Dublin, near Malahide, the beds of the ' Calp series ' are greatly disturbed; often violently contorted. Such contortions in limestone are sometimes due to the formation of subterranean caverns and the fall- ing in of the superincumbent beds, and are uncon- nected with ordinary changes in the earth's crust. The limestone of the Plain, however, is itself only occasionally visible, as the greater portion of the surface is overspread by beds of gravel, or boulder clay, belonging to the Post-Pliocene formation already described, 1 or by shallow lakes and sluggish streams ; and the extensive peat-mosses, which ex- tend over large areas, are a still more recent cover- ing, and generally occupy the positions of former shallow lakes or swampy forests. That the Coal-measures (forming the upper members of the Carboniferous group) once over- spread all the area now occupied by Carboniferous limestone, is a proposition which a geologist is pre- 1 Seep. IWetseq. Formation of the Central Plain 183 pared to accept as soon as stated, but to the ordinary reader it will not, perhaps, be so clearly manifest. I may perhaps be allowed to use an illustration drawn from works of human art, in order to make the subject more palpable. If a traveller, visiting the regions of early civilisation in Egypt, Syria, or Babylonia, observes the basement walls of palaces or temples, and the foundations of arches or piers, while numerous blocks of hewn stones are strewn around, it requires no history to convince him that he may be standing on the ruins of a Thebes, a Palmyra, or a Babylon. He knows that where there were the foundations, there also must have been the superstructures. Now the limestone is the basement of the Carboniferous superstructure ; and the unvary- ing sequence of beds, at least within the limits of the British Islands as proved by observation, leads us to this conclusion, that representatives of the upper members of the Carboniferous group were always originally present where the basal beds had been laid down ; and that when the former are absent it is only in consequence of their removal by denudation. In addition to the evidence derived from analogy with other districts, such as those of England and Wales, that fairly deducible from direct observation in Ireland itself will prove satisfactory to one who is 184 Physical Geography of Ireland at all familiar with the process of geological induc- tion. Thus, in several places either on the margin of, or rising from, the Central Plain we find rem- nants of the Upper Carboniferous strata (or Coal- measures), which owing to special circumstances (chiefly that they occupy stratigraphical basins or synclinal troughs) have escaped destruction, and like solitary columns amidst the ruins of ancient temples, are monuments of the decay and waste which has reigned around. In this way the little coal-fields of Castlecomer and Killenaule in the south, and those of Arigna, Slieve-an-Ierin, and Tyrone in the north, are interesting as showing what kinds of strata originally overlay the Carboniferous Limestone between their widely separated positions. Now, if we compare the succession of the beds in these places we shall find that they are strictly re- presentative ; and the manner in which they are broken off and abruptly truncated along their out- crop, proves that they originally extended to unde- fined distances beyond their present limits. Let us then compare the succession of the beds from the Carboniferous Limestone upwards in the three dis- tricts above specified, and we shall see how strictly analogous it is, in each case, though slightly varying in details : Comparative Carboniferous Sections 185 COMPARATIVE CARBONIFEROUS SECTIONS. 1 UPPER CARBONIFEROUS SERIES. Leinster Coal-field. Middle Coal-mea- sures ; with beds of coal and freshwater shells (Anthracosia). Arigna (N. W.) (Absent through de- nudation.) Tyrone (N.E.) Middle Coal-mea- sures of Coal Island, with seams of coal and bivalve shells (Anthraeosia). 2 MIDDLE CARBONIFEROUS SERIES. Lower, or ' Gannister beds,' with thin coal seams and marine shells ( Goniatites, Aviculo-pecten, &c.). Flagstones and shales. Carlow Flags.' 'Yoredale beds,' or Upper shale series, with marine shells. 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