——_—_-—_--—---- Leen i SINR LN NAD | SRE eee 1) Sean nL Oe Olly Re ZOE noe are eS Tee Roe nie Seer 2 Pe pes y ; ap. ie’ cx é cues TN axe \ ‘ ‘ a - } , } a Bs os Jf y F : t DELVERSITY AF ILLINOIS OBJECTS OF GEOLOGICAL INTEREST IN THE el Myewonanengiat | | | VICINITY OF DUBLIN. DRAWN UP AT THE DESIRE OF THE ROYAL DUBLIN SOCIETY, PREPARA- TORY TO THE MEETING OF THE BRITISH ASSOCIATION IN AUGUST, 1835. | | | Bd Sa | DUBLIN: R. GRAISBERRY, PRINTER TO THE ROYAL DUBLIN SOCIETY. 1835. eee en eo ; ‘ ‘ a —=SS4 5 ~ i! ( leuamal ~S Williamstown = = : z = kein ae Merrion — E 4 = : y ; 44i 14 Salioraann fF /)) Te 4 Garrucastle Ballybrack y ~S Mountams SS Putter Mowitan Douce 5 Mountaty OG f A Sandymount = : Ss. Pooterstown Roek eee Taght House Seaport 3 ‘i ) Kingstown, Sas Lin base store, 2 Sieeletunansy bay Patteru SSN eld ~ marines RNG QS SSAA S alle ek Harkour OF THE VICINITY OF Dublin ENGRAVED BY J. KIRKWOOD DUBLIN. Irish Miles Limestone GEOLOSAL Map | MEMORANDUM OF OBJECTS OF GEOLOGICAL INTEREST IN THE VICINITY OF DUBLIN. DRAWN UP AT THE DESIRE OF THE ROYAL DUBLIN SOCIETY, PREPARA- TORY TO THE MEETING OF THE BRITISH ASSOCIATION IN AUGUST, 1835. DUBLIN: R. GRAISBERRY, PRINTER TO THE ROYAL DUBLIN SOCIETY. VIMTTHE TADIIO. ’ % ; ar Pe Weck Yate 40° Saae % INTRODUCTION. Tue vicinity of Dublin offers a great variety of interesting matter for the study of the geologist. Within a very limited distance from the capital, we are presented with an important series both of primary and secondary rocks. To the south of the bay of Dublin, Primary Rocks alone occur; which are re- markable not only from their variety, but from the indications of violence exhibited in the contortions of the strata, the intrusion of granitic veins into the micaceous schist, and the chemical changes which the schists have suffered when in contact with the granite. The primary rocks of the vicinity of Dublin consist. of a central ridge of granite, on each side of which the micaceous and argillaceous schists, the quartz rock, and mountain limestone are arranged. This granite chain extends from Kingstown on the North into the county of Waterford on the South, a distance of nearly sixty miles. Inthe vicinity of Dublin the course of the granite chain is well ascertained : it A2 4 extends from Dalkey Island to Black-Rock, and from thence passes southward to Dundrum and Rathfarnham; it then crosses the Military Road behind Montpellier hill, and running across the northern extremity of Glenismaule, forms the basis of Seechon, and consequently supports the schist which constitutes the greater portion of that hull. On the east, that is, next the sea, the boundary of the granite is very apparent; from Dalkey it runs along the shore to Killiney, from thence it runs inland to Rochestown hill, extending in nearly a right line to the Scalp, passing on to Glencree and Lough Dan, holding a southerly course. This central granite ridge includes some of the loftiest hills in the vicinity, they are, however, rivalled by the adjoming quartz mountain, called the greater Sugar Loaf,* and the schistose mountains of Seechon and of Djouce. This granite ridge is destitute of the sharp and spiry outlines which so often charac- terize mountains composed of this rock; a circum- stance apparently dependant on the inconsiderable elevation of the hills, and also on the very decom- * See Appendix for table of the heights of these mountains. 5 posible nature of some of the kinds of this rock, which disentegrate rapidly with exposure to the weather. The mineral nature of the granite in general ex- hibits, nevertheless, but little variety, and is almost completely free from hornblende or other ingredients, not essential to its character. The felspar is for the most part of a pearly whiteness, and forms a striking contrast with the black mica. The stone is much employed for architectural purposes in Dublin and the vicinity, and considerable quantities of it are ex- ported to Liverpool, and there employed for paving the streets. Near Killimey, at the junction of the granite with the schist, the quality of the former is rather different from that obtained in the quarries near Kingstown. It is harder; and the mica, in- stead of occurring in plates, has assumed the form of plumose mica. At Gleneullen, Glenismaule, &c., the granite is more coarse-grained and the mica is of light colour, forming large hexagonal plates, sometimes half an inch in breadth. This variety is less compact than the granite of Killiney, and contains more felspar and mica; hence, perhaps, its more decomposible nature. In the vicinity of Glenismaule the granite is often completely disinte- grated for a depth of four feet or more; and the de- 6 cay of the rock would proceed with great rapidity, if the covering of peat did not afford a protection against the destructive effects of the weather. This decomposed granite sand is brought to Dublin under the name of freestone, and is employed for scouring and other domestic purposes. The mass of granite, whose limits have been de- fined, is almost every where in contact with the mi- caceous schist, both on its western and eastern flanks, and the junction of the rocks may be studied at Kil- liney, the Scalp, and Rathfarnham. In the first of these situations, the schist is seen resting on its up- turned edges, on a basis of granite, and traversed by numerous veins of that substance. As the granite veins run in two directions they often intersect, and one set runs parallel to the lamination of the schist, while a second set cuts across the strata. “Many of these veins contain fragments of the schistose rock. Along the line of junction of the two rocks, the schist is much curved, and contains abundance of crystals of chiastolite arranged in stelliform groups. The schist 1s not the only rock which is in contact with the granite; for, from Black-Rock to Dundrum, the limestone succeeds the granite, and consequently the whole series of primary strata are absent. The ac- 7 tual contact of the two rocks has not been observed; but at Black-Rock they are within a few yards of each other; and the limestone is extremely compact, consisting of angular fragments, as if it had been shivered into small pieces and subsequently reunited. The quartz rock of Shankhill, if not in actual con- tact with the granite, is only separated from it by the intervention of a thin film of. micaceous schist ; and at Ballinascorney, the argillaceous schist is not far re- moved from the granite; but, as the two schists gra- duate into each other, it is not easy to characterize them, in every instance, by precise mineralogical dis- tinctions. The Micaceous Scuist occurs both on the eastern and western flanks of the granite; on the east it commences at Killiney, occupies the eastern side of Rochestown hill, and extends from thence to the Scalp, where it is seen reposing on the granite, much con- torted, and containing crystals of Andalusite. rom the Scalp, it passes to the west of Enniskerry, and constitutes the rocks of Powerscourt waterfall; and still continuing its southerly direction, it passes by the head of Glencree, constitutes Djouce mountain, and may be seen in contact with the granite at the upper extremity of Loch Dan. 8 On the western side, the micaceous schist com- mences at. Rathfarnham, and. the junction of the two rocks may be seen, on the road side, near the com- mencement of the Military Road ; it then runs across Glenismaule and forms the mountain of Seechon. The micaceous schist exhibits the usual mineral characters of that rock, and consists of a mixture of quartz and mica, in variable proportions. Sometimes alternating lamine of the two ingredients are so fine that the mica appears to preponderate, and the quartz is not so apparent: on the other hand, the quartz sometimes attams the thickness of an inch, and almost excludes the mica. Not unfrequently the quartz is replaced by argillaceous lamine, and thus the rock passes into an argillaceous schist; which, when in contact with the granite, is sometimes changed into hornblende schist. At Kallmey the schist exhibits a peculiar mode of decomposition, which it is diffi- cult to expla. At first little circular depressions may be observed in the schist, and as these enlarge, little cavities are formed, often the size of an orange, and giving the rock a remarkably corroded appearance, as if it had been an amygdaloid which had lost. its mineral nodules. This, however, is not the case in the present instance, for the ‘cavities are not caused 9: by the fallmg out of nodules or portions of conglome- rate, but appear to depend on some ill understood concretionory structure. The mica schist is followed by AreGrLLaceous Scuist and Quartz Rock; the former occurring on both sides of the granite chain, whilst the latter is only found on its eastern side: quartz rock also appears on the north side of the bay, constituting the peninsula of Howth. The schist occurs in continuous strata, which may be traced over a wide extent of country, but the quartz rock is found only im detached portions. On the eastern side of the granite ridge, the argil- laceous schist, bemg the outermost of the rocks on that side, is bounded by the sea. The other margin of the clay strata is bounded by the micaceous schist, and may be defined by a line drawn from Shankhill and passing to Enniskerry, and to the west of the great Sugar Loaf, and continuing in the same direc- tion beyond Loch Dan. It includes the country around Bray, the Dargle, and Glen of the Downs; and also includes several extensive masses of quartz rock, such as Shankhill, the two Sugar Loafs, Bray Head, the Glen of the Downs, &c. 10 On the western side of the granite ridge, the commencement of the argillaceous schist may be seen, beyond Rathfarnham, where. it is bounded by the river Dodder, which separates it from the micaceous schist ; it then passes to the west of ‘Seechon till it reaches the sources of the Liffey. ‘There is often considerable difficulty in tracing the junction of the two schistose rocks, as they pass into each other by insensible gradations, and have both been greatly disturbed and contorted. The lower parts of the argillaceous schists often pass into greywacke schist, viz., into schist containing fragments of schistose rocks, which are fine in some cases, as near Bray, while they are coarse conglomerates near the Tallaght Hills. Near the granite, these rocks undergo a very remarkable change; and as we trace them they gradually lose the stratified appearance, and even their schistose structure; they have become, in short, hard and compact, passing into a very close-graimed green stone, consisting of hornblende and. felspar, and where the crystals of felspar attain a larger size, a green stone porphyry is the result. In the ravines, portions of schorl in acicular crystals are very common, but they have not been traced to their source. 11 Lambay Island, to the north of Dublin, may be included under the head of argillaceous schist. The island consists of strata of schist and beds of green stone and porphyry. The schistose strata are much indurated and are contorted in a most intricate manner, and these contortions occur both on the minute and the great scale. These strata often lose their stratified appearance and pass into green stone and porphyry. The porphyry is sometimes amygdaloidal, containing nodules of calcareous spar. The crystals of felspar often exhibit a very peculiar laminar structure. The Quartz Rock exists in two states, either al- ternating with schist, and in that case decidedly stratified, or destitute of all foreign intermixture, and in these examples the stratification is very indistinct. The hills composed of quartz rock are easily recog- nized by their conical outline, a circumstance which has served to give names to some of them. The chief masses of quartz are Bray Head and Howth, in which it alternates with schistose strata; Shank- hill; and the greater and lesser Sugar Loaf, in which no schistose strata occur. The quartz of the peninsula of Howth exhibits 12 the phenomenon of contorted strata in a very beau- tiful manner. The stratification is very obvious, and the schistose beds exhibit a great diversity of hues from purple to red, thus rendering the contortions more apparent. . Some of the strata rest on thei edges, others are undulated, and sometimes curved upon themselves, so as to resemble the concentric crusts of some spheroidal concretion. The same phe- nomenon is observable at- Bray. . The only secondary rock that occurs m the vi- cinity of Dublin is the Mountain Limestone, which constitutes all the country beyond the primary strata ; occupying the counties of Meath and Kildare, and greater part of the county of Dublin. No lime- stone is found in the county of Wicklow, and the farmers of that county, on the eastern or sea side, obtain their supplies from Howth or from the beds of stratified calcareous alluvium, the only condition under which limestene occurs in that county. On the opposite side of the county the supplies of lime for building and agricultural purposes. are chiefly drawn from the county of Carlow. The limestone exists in two very distinct states m the vicinity of Dublin; in the one it has the charac- 13 ter of the ordinary carboniferous limestone, con- taming the usual organic remains; but near the pri- mary strata it is very impure, has a schistose struc- ture, contains but few organic remains, and is the Catp of Kirwan. The calp is distinctly stratified, the strata seldom exceeding two feet in thickness and being separated by thin beds of slate clay. This limestone, which is much used for architectural purposes, occurs in many localities around Dublin, and every where exhibits marks of contortion and violence, which may be observed in almost every quarry around Dublin. At Lucan there is a beau- tiful example of contorted limestone strata; and equally interesting instances may be seen at Portrane, where the sea coast has exposed numerous sections, in which the nature of the calp is fully displayed. Besides the calp, magnesian limestone occurs in a few localities, as at Howth, near the junction of the primary and secondary strata, and on the Dodder between Milltown and Classon Bridge. This lime- stone contains no organic remains, but occasionally, as at Howth, we find it contains imbedded fragments of the mountain limestone. ‘ bas & iis Ai tsdt owt Yalhodexs in ataiP asotstaddinn! ait in 651 tila tid sities i regio Faden Sire oan ete baat Sairgntyoti 43) ‘ae of ar’ ‘Bas attiniiet ‘oii to Woe: bal Matis @ Yoni a glad of T° stew aT “ERIS ide Yo aback aide” ed" Prsitoobidgre a) ‘Deer dosine a inh Bano esitilesok vitens Hi 8 a hiss Bolte adr09) Lo’ pabtcce aig oeaels via. ry dedi nae ho raed! ad’ eau lg “Head & et ated “netted BAP wildlo Bas fait ‘anotamtl Hohottes" to ¢ efile Srl is Oo u592 ad ‘Ula eosin | TH autieioaa and miarie frowns gail \ <: aly ib vila 3 al re ied ibs ahs ac SE RR a 2 Oe WEEE bok Wein: MEMORANDUM, &e. &e. The following is a List of those Localities most deserving the Attention of the Geologist. Norru oF DvuBLIn. Porrranu.—This peninsula consists chiefly of lime- stone, which is remarkably contorted, the strata resting’ on their edges, and curved in a very com- plicated manner. Lampay Istanp.—This island consists of conglome- rate rocks of different kinds, chiefly of argilla- ceous schist, including fragments of other rocks. There is also a stratum of sandstone conglome- rate at the northern extremity of the island. In some places the schist is greatly contorted. Green- stone and porphyry are extremely abundant, al- ternating with and passing into grey-wacke. Maranipe.—The country in this vicinity consists entirely of mountain limestone. The quarries of Malahide, Feltram, and St. Doolagh, afford numerous organic remains. 16 The following is a list of organic remains collected from different situations : Articulata.—Calymene _ sp. | 'Terebratula lineata. common everywhere. {—— resupinata. Mollusca. ao acuminata. Bellerophon hiulcus. —— reniformis. costatus. Spirifer cuspidatus. Ellipsolites ovatus. =< attenuatus. Nautilus carinferus. ——— trigonalis. biangulatus. glaber. Ammonites spheericus. —__—— obtusus. Orthocera striata. striatus. fusiformis. Cardium hibernicum. Amplexus coralloides. _ | Sanguinolaria gibbosa. Cirrus acutus. Zoophytes. Euomphalus pentangularis. | Turbinolia Fungites. Turitella sp. Caryophyllea affinis. Buccinum acutum. Lithotrotion floriforme. Tsocardia oblonga. striatum. Productus comoides. , Tubipora catenata. — sulcatus. KiLLesTER, near Clontarf.—Impressions of organ- ized bodies occur in the upper beds of limestone ; these impressions are however so obscure, that nothing more than their vegetable nature can be inferred. CiLontarr.—A vein of lead occurs in the limestone, which was worked a few years ago, but is now. abandoned. ot Howtu.—The peninsula of Howth and the adjacent island of Ireland’s Eye, consist chiefly of quartz 17 rock. The phenomena of contortions are here exhibited in great variety and distinctness. The quartz is mterstratified with schistose rocks of a great variety of colours, rendering by their con- trast the curvatures of the beds very apparent. The following minerals occur in this locality : Tron pyrites. Oxide of manganese. Copper pyrites. Earthy black cobalt ore. Galena. Beds of Macnesian Limestone occur at the north eastern extremity of Howth, at Sutton. These beds occur near the junction of the blue limestone with the primary strata; are interstratified with it, and include loose angular fragments thereof. Near the town of Howth there is an extensive mass of stratified alluvium, attaining the thickness of about 100 feet, and containing fragments of ma- rine shells of the same species as those which still exist in the adjoining sea. IMMEDIATE VICINITY OF DUBLIN. Lucan.—The contortions of the limestone (calp) are displayed with great distinctness at the quar- ries near Lucan. The strata are perpendicular, some of them. broken across, others folded and convoluted in a very complicated manner. In the vicinity of Lucan there is a mineral. spring, characterized by sulphuretted hydrogen. Donnysroox.—In the quarries at Donnybrook there are numerous strata of calp, passmg imto the B 18 ordinary limestone, and containing organic re- mains. Mitittown.—Between Milltown bridge and Classon “bridge there is a portion of magnesian limestone, which is included in the ordinary limestone. Granite is visible at a short distance from this place, but its contact with the stratified rocks has not been detected. Ratuaar, Crumuin, and Rounptown.—The ope- rations of quarrying have disclosed an extensive series of strata of calp limestone. The calp al- ternates with strata, and numerous such alterna- tions may be counted. In all these quarries the limestone is highly inclined, and exhibits other indications of disturbance. -Souty oF DvuBLIN. Briacx-Rocx.—On the coast near the railway there are a series of rocks which are best observed at low water. ‘The granite may be seen within a few feet of the limestone, but the actual contact of the two rocks cannot be observed. ‘The lime- stone is hard and crystallme, and appears as if it had been shivered into angular fragments, which have been subsequently united. Kinestown.—The country around Kingstown and the Island of Dalkey consists entirely of granite. Almost every block of granite is traversed by concretionary veins of the same substance, dif- fermg from the general mass in the texture, 19 colour, and relative proportions of the usual in- gredients. The following mimerals have been found in the gra- nite : Spodumene. | Garnet. Kallinite. Tourmaline. Beryl. Apatite. Fluor. Rutile. € These extremely Copper pyrites. Sphene. rare. Tron pyrites. Orthite. Galena. KILLINEY.—On the sea coast, immediately below the obelisk, the junction of the granite with the mica schist occurs. The edges of the schistose strata repose on a basis of granite. The schist is much contorted, and sometimes so convoluted as to form concentric crusts. At the line of junction the schist abounds in crystals of Andalusite grouped in a stelliform manner. Numerous veins issue from the granite, and intersect the micaceous schist ; some of the veins run pa- rallel to the lamination of the schist, others run parallel to the direction of its stratification, and consequently one set of veins intersects the other. ; In one instance a heave has taken place, and the two portions of the granite vein are displaced. These veins frequently contain fragments of mica- ceous schist. RocuEestown Hitut.—The line of junction of the 20 schist. with the granite may be traced for a quarter of a mile, and is remarkable for its clearness and precision. ) In this locality the spheroidal structure of the granite may be observed. Scatp.—A deep ravine has cut across the granite and schist, so that their contact may be easily ob- served. The schistose rocks recline against the granite, and are much contorted. Crystals of Staurotide occur near the Scalp. Batiycorus.—At a very short distance from the Scalp a vein of lead ore occurs near the janotion’ of the schist and granite.* Galena, sulphate of barytes and carbonate of. lead are found here. SHANKHILL.—This hill is composed of quartz rock, and its stratification is not very apparent. On the west side of the hill, where it approaches the granite, the quartz is changed into hornstone. Bray Heap consists of quartz rock and supers schist m frequent alternation. The strata, in many places rest on their edges, and are turned and contorted in every direction, exhibiting phe- nomena analagous to those observed at Howth. *This vein has been worked by different mining companies, and is at present in the possession of the Mining Company of Ireland, who have works for smelting, and for rolling and drawing pipes. Shot is also manufactured here. 21 To the south of Brav there is an extensive mass of alluvium containing broken shells. GREATER and Lesser Sucar Loar.—These hills consist of quartz rock without any argillaceous schist, and their stratification 1s obscure. RATHFARNHAM.—The junction of the granite and micaceous schist may be observed near the com- mencement of the Military Road. GLENISMAULE.—The junction of the granite and schist may be traced to a considerable distance on the southern side of the valley. In this situation there are extensive beds of alluvium, consisting chiefly of rolled fragments of limestone, and covering the base of the hills to a height of 100 feet. BaLLinascorny.—Strata of green-stone and green- stone porphyry may be seen in the deep natural sections with which this part of the country abounds. ‘These green-stones graduate into the schistose rocks. Tattacut Hixzs consist of clay slate with grey- wacke slate, with occasional beds of green-stone. e : sehty ah nits on ci cube ski ee 4 tt Rs, oa aldets shina is Sai! Ab tata m9 ols ¢ eat rs | gh APPENDIX, TABLE OF HEIGHTS OF MOUNTAINS ABOVE LOW WATER MARK IN DUBLIN BAY. Names. Authorities. LUGNAQUILLA, . . «. ~ Mr. Richard Griffith, Hrs Vattotien so dene te gl Heap oF Kippurge, .«. . Messrs. Griffith and Weaver, conjointly, . DaGUCBie oc She. eee Ditto, . SEECHON, . . . - « Mr. Hugh Hamill, GreaT Sucar Loar, . . Dr. Taylor and Mr. Weaver, * TuHrEE Rock Mountain, Dr. Fitton and Mr. Weaver, . LessER Sucar Loar, . Messrs. Griffith and Weaver, . Beat OA ss (bs fog ee Hitt or Howtn, . . . Dr. Fitton, Feet. 3070 3045 2527 2392 2150 2004 1585 1183 807 978 NotE.—It was hoped, that the measurements of some of these heights, by the officers engaged in the Trigonometrical Survey, would have been obtained in time for this publication, but it was of necessity sent to press without them. * Later observations have placed the height of the greater Sugar Loaf below this. ALPHABETICAL NOTICE Of Places mentioned in the preceding pages, and of some others in the Vicinity of Dublin ; with their Bearings and Distance from the Castle of Dublin, in Irish Miles. N. B.—The proportion of Irish to English miles is as 11 to 14. BALINASCORNEY. A mountain village, six miles, S. W. Buacx-Rockx.