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DRESSER, M.A. OTTAWA PRINTED BY S. E. DAWSON, PRINTER TO THE KING'S MOST EXCELLENT MAJESTY 1902 1»-L No77« 1 To Robert BELt, M.D., LL.D.. 8c. D., F.R.8., Acting Director of the '•■ological Survey of Canada. Sir, — I '^eg to submit a report on the Geology and Petrography of Shefford Mountain, Quebec, the field-work for which has been done chiefly under the auspices of the Geological Survey at intervals during the aeaions of 1897-8-9. The accompanying map is based on the topo- graphic survey of the late N. J. Giroux for the Montreal sheet of the Eastern Townships maps, published in Volume VII of the Annual Report of the Geological Survey of Canada, 1894, in connection with the report of Dr. R. W. Ells on the areal geology of the district. My warmest thanks are due to Dr. F. D. Adams, Logan Professor of Geology at MoGill U niversity, for valuable aid and advice in many parts of the work ; also to Mr. M. F. Connor, B.A. Sc, Radnor Forges, Quebec for three rock analyses. I have the honour to be. Sir, Your obedient servant, St. Francis College, HicHMOMD, Que., May 1, 1901. JOHN A. DRESSER. |1 13_L— IJ REPORT ON THE GEOLOGY AND PEIROGIIAPIIY OF SHBFFORD MOUNTAIN, yOEBEC IIV JOHN A. UKESWER, MA. The bro«d valley ot the St. Lawrence river, which crosvea the southern part of the Province vt Quebec in a north easterly direction, Hoparates the T^urentian highlands on the north {nm that part . the Appalachian system to the south, known as the On en mountains in Vcruiont, nnl as the Notre Darae, orShiclcshock, rangti in the Province of Quebec. The valley is a nearly level plain, and in the western part T(.i««rapliy. of the province is about eighty miles in width, It is underlnin by strata, which are often nearly horizontal in position and are of Prtlit'ozoic age, ringing from Cambrian to Devonian. In its topo- graphy, this regio.i issharpiy distinct from the undulating Laurentian which emerges from underneath it at the northwest, but is less defi- nitely separable from the >iiliy country on the southeast, into which it gradually passes. Certiun strnta along the main a«'.'s of the Notre Dame mountains n' now clasaod . pr "■-Cambrian, but the Palwotoic M members become brought into vi< .« ighly con' ted n jftny places before the.se are Across this valley a single line of hi course at about the latitude of .M<>iitt - va's of ten to twenty miles, often ri or more, above the surrounding ^^liiii that part of the valley iu which the\ intrusive character have been long ku by Logan in the " Geology of Canadu, extends in a general easterly These hills Jippear at inter- 'ii'ie;li; il' a thousand feet conspicuous features of I lieir igneous origin and MoiitcifKiiui fhey were thus de«<;ril)ed > {p. 655) : HIIU •Annual Reix)rt Geol. .Sur. Cnn.. Vol. Ill N > tin a recent cim.munication upon the subject ' Can. 19<)2), Dr. ¥. V. A first *. ' A few miles to the south of Belcpil is Mount Johnson o' Monnoir, another intrusive mass ; which, although out of t»"» rango of those just mentioned, apparently belongs to the same series. The mineral com- position of these rocks varies greatly, not only for the different hills, but Bn.mT' ""'' '*"■ '^'^•'^'" P*""^ o' t*»« ""o one- Thus, Shefford and Brome moun- Uins conaist of granitoid trachyte ; while the succeeding one of Yamaska, and Rigaud at the other extremity of the line, are partly of trachyte and partly of diorite. Monnoir and Belceil are made up of diorite ; while Rougemont, Boucherville.f and Mount Royal consist in great part of dolerite ; presenting, however, many varieties in onipo- sition and sometimes passing into pyroxenite. The dolerite". of Kouk^ ■ montand Mount Royal are cut by dykes of trachyte; simila.- iiykes also traverse the dolerite of Yamaska, and may pe-'-^os be cor^. -. fd with the trachytic portion of the mountain. It i. n> ->hable, jnd^ing from some specimens from Rougemont, that the dolerita is there inter- sected by veins of diorite ; some of which resemble that of Beloeil and others that of Mount Johnson. Dykes both of trachyte and dolerite are also fojnd traversing the sedimentary strata in many localities in the vicinity of these great eiuptive masses.' •The recent inventigstionn of Mr. LeRoy, (Bull. (;e<>l. .Soc. .\m., 1900), show that Kigaud may belong to the !*• irentiana, which would thu» shorten the line of intru- Bives, Mount Koyal, the next undoubted n.en.ber of the series being al>out fifty miles from Shefford.— .J. A. p. t or Montarville. mountains. ClKOLOf'V AMP PBTitOORAPIIY Tha roouiiUin* of Itromoftmi Hh-^ftord, with whi^!^ lii« >i*ri«- apj-wrx to end towardi theeMt, •'-« Ncpar«t«d frtiiii encli other by Hiinif four ntilen of Mtrktirted rock, -tnd V«m«HkA, the next inount*in of the seri««, i* eleven niilei diita. /rtm S' "ord. Concerning the reUtivf p**! I'»ii>"ii tionn of th*»e mountain)! Dr. •' . i»\i: ' Itrome and Shefford wcur 'irKiu. '^ •long the line of contact iMtwnen th Cnnihro Hiiurian «nd Cimhrian rooks, while Yamanka mountain ia lituatei] on the line of fault between the Sillery divinion of the Cambrian And the Ijower Trenton forma- tion. It is probable that the 8h(>fford and Iironi<' fxtrunion is also alnnn a fault line, the presence of which is not to cli>arly indicatfd an t' 'n which Yamatka mountain lies, although the amount of dioritic u\' > is much greater at Uromc' iiip extreme length of Shefford, which is the smallest of thesn thrt« Ent. mountain*, in three and a half miles, it.-> greatest breadth two and a half, and its ar«>a somewhat lesi than nine square miles. The mean altitude of the surrounding plain is approximxtelj tive Altitude. hundred feet above sea level,** but the mountain rises in two ridges from one thousand to twelve huiidrp] feut higher. The intervening ' notch ' between thene ridges is, on the south and west sides, about seven hundred feet above tin bas« of the mountain, or nearly twelve hundred feet above sea-level. The xedimrntary strata which surround the igneoui part of the Surroimclin? mountain, consist of quartsites, conglomerates and light end dark " ' gray slates. The last mentioned rocks are descrilied by Dr. Ells as belonging to the Lower Trenton formation, while the others are referred by him to the Cambrian system. All these rocks strike in a N. N.EL direction and dip at varjring angles ; always, however, above 45° towards the W.N.W. They have a nearly vertical cleavage, which agrees in the direction of its strike with the stratification. Near the contact with the igneous portiou of the mountain, the sedimentary rocks "pit«h " or dip longitudinally away from it at high angles. They wrap around the base of the mountain, mantling it with a hardened contact zone, to a height of three hundred to a thousand feet aljove the plain, *loc. cit., varying with the amount of glaciation. The mountain thus attests the enonnous amount of erosion which Rt-lief due to the whole region has suffered, its present elevation above the surround- •''™"'"- * Annual Keiiort Geol. Surv., Can. Vol. vii, (N'.S ), p. 73 J. ** Went Shefford xtstion .1 the Canadian Pacific Railway, ii 440 iVet above mean n.'a-level. Can. Pac. R7 irofile, P. Alex. Pet«Tson Chief Kngineor. The other elevations given are ther inn of several aneroid measurements from this station. I 8 1 QUEBEC A laccolite. ing plain being evidently due to the greater resistance to denuding agencies that is offered by the igneous than by the little altered sedi- mentary rocks. For, in addition to the facts just cited, there is a large mass of black slate, similar to that which was said to be of Trenton ago, on the highest ridge of the mountain above Knotts' corner It is scarcely more altered than that near the contact at the base, and is penetrated by dykes from the underlying igneous rocks, which, in the area beneath it, are of two different ages of intrusion. This slate covers an area of not less than a quarter of alquare mile and Its thickness near the head of Plamondon's wood slide was esti- mated at one hundred feet. From these facts, together with the entire absence of any tuff-like material, and the holocrystalline character of the Igneous rocks, it is inferred that Shefford mountain is an uncovere.l laccolite rather than the denuded 'neck' of a once active volcano This view 18 also corroborated by the occurrence of smaller patches of sedimentary rock in several other places on the igneous portion of the mountain. THE CONT.\CT. Kktomorphic contact phenumena. An approach to the contact of the sedimentary with~the igneous rocks is generally well indicated in the former by the development of a rusty-brown colour. In the quartzite the alteration is comparatively inconspicuous, the zone of discoloration being never more than a few yards in width, while in the black slates the rusty colour and other evidences of local metamorphism are quite distinct at a distance of one hundred yards from the contact. Where they are especially altered the slates often resemble fine trap rocks so closely that their actual character cannot bo ascertained without seeing a freshly fractured surface. i^r M Along the actual contact they are commonly i educed to a ' gossan- like ' mass resulting from the oxidation of the large amount of iron sulphides which have been .leveloped in the slates bv igneous meta- Pyrrhotitc. morphisra. These are usually in the form of pyrrhotite and are extremely oxidized. These sulphides, where unaltered, are sometimes evenly disseminated throughout the rock, but at others are segregated in small masses, which on alteration produce rusty or brown spots on the weathered surface four or five inches in diameter, (plate iii.). 'J THE CONTACT 9 L A apetiraen of tho black slate was taken about two hundred yiiids from the contact with igneous rocks near Dounan's quarry. It is a dark steel-gray rock with a good slaty cleavage and shows a rusty colour along the joint planes. The specimen when taken was thought to be entirely unaflFected by the intrusion of tho mountain mHSS. On the sawn surface fine lixht gray lines are quite apparent, which Altcml were not previously noticed, and under the microscope these are found to be crystalline feldspar. The greater part of the dark-coloured lay- ers consists of ma^'netite, with possibly a little graphite, and feldspar grains. Shreds of mica, generally colourless, are also present. Besides the light coloured, feldspathic band.«, the rock contains numerous rounded spots also of a lighter colour. These are so small as not to be noticeable to the naked eye. They differ from the rest of the rock in showing no banded structure, the dark minerals which are in smaller grains being evenly distributed in them. The spots are there- fore regarded as an incipient form of alteration which has evidently taken place since the foliation of the rock was well advanced, if not quite completed, and hence as probable contact phenomena. The light-coloured bands appear to be only miniature veins of felds- par, such lis might occur in any part of the regionally altered rocks, and are not thought to lie due to contact metainorphism. Four specimens from the slate cap on the top of the mountain, which was referred to amongst the evidences of laccolitic structure, show that rock to consist; of feldspar and brown mica with smaller amounts of magnetite and pyrite. In thin sections from specimens near the contact, the feldspar and mica are recrystallized and form a mosaic-like structure, the individual grains being bounded by poly- gonal outlines. The oxidation of the pyrites in this mass, as in the contact pre- viously described, gives much of the rock a reddish-brown colour. A specimen from the light-coloured mica-schist near the base of the Micaschist. mountain, was taken about twenty yards from the contact liear lieau- regard's corner, is similarly discoloured in streaks by the alteration of pyrites which here occurs in cubical grains about ,',y inch in diameter. The feldspar is in part little altered from its original clastic character, while in other parts it is recrystallized, presenting much the same appearance as in the previous specimen. The mica here is in shreds. 10 L QUEBEC Quartz ite. Conglomerate Few dykes Ll^n^^.^T"""' "***''■ '•"*' " P'"«°' " '« "t'ing-Hke forms following the cleavage of the rock. Quartzite comes in contact with the mountain for a short distance on the east side and also on the west. It is best seen on th nonh s.de of the road leading f,^m McCatcheon's corner to the mountdn whereacontams a considerable amount of feldspar, and hrratLe; the composU.on of quartzose sandstone. It is but s ightly altered a a distance of fifteen yards from the contact. The principal sedimentary rock, besides those already mentioned is a conglomerate, which may be seen at West Shefford, on the Sm™ Vermont Railway. It consists of feldspar, biotite, quartz au« e hornblende and magnetite in order of imporUnce. The i2L7Zs are comparatively few, and ai. either feldspar, which may'^fZ plagioclase or orthoclase. or more commonly, quart. No composi e grams of larger size were distinguished. Much quartz apperrTal ^zzir^Trrr ""' ^^^'"^'^'"-^ «"«- But e'e'^ these t: often broken and faulted, sometimes showing very distinct strain shadows, the result, of pressure subsequent to their deposition. The dark minerals, of which biotite is the chief, in places make ud nearly half the rock. The structure is highly schislse, hTlager nodules giving the appearance of a rather fine augen gneis^. The sedimentary rocks are invaded by dykes from the main mass of the mountain in several instances, but on the whole somewhat"; ly while dykes, more recent than the mountain, cut both it and the S' mentary strata much more frequently. Fragments of the surro^^d. .ng sediments are occasionally included in the margin of the ignZs mass and the stratified rocks are much contorted £ many p f^eTa he con act. yet on the whole the intrusion of the body of the m ulrn ph°:::clr "'^^^'^^"^^^"-''-^^ 'y-y very^ioWtcaLr ^■' Kinds of j'neous rock. IGNEOUS ROCKS. fielf'l'hTfirst'*"" ""'^'T.^' ''"''^ '^'■^ ^-""^ distinguished in the field. The first is a rock of dioritic aspect, which weathers to a dark brown and shows a predominance of dark minerals. Detailed exam' uon shows It to belong to the £sse.ite group. The second is ZZ wholly composed of rather coarsely crystalline feldspar. This s classed as Jfordmarkite. ^ ] IGNEOUS ROCKS 11 L The third resembles it, but generally shows a somewhat porphyritic structure, and near the contact with the other rocks becomes still finer in texture and ol t>ii assumes a greenish shade. It is Puhukite. Their structural relations are clearly defined, each being the product of Structural a separate irruption. The first is penetrated in many places by dykes of each of the other rocks, and the second by a large number of dykes of the third. The contacts of the different masses with one another can be seen and in all cases corroborate the evidence of the dykes. The second mass has generally been intruded along the former line of contact between the earlier igneous and the sedimentary rocks, although it also divides the former into two parts, while the third has been injected between the other two igneous rocks. These relations can be most easily seen by a reference to the accompanying map. EssEXiTE. — This is a rather coarsely crystalline rock of granitic Essoxite. texture, dark-gray in colour and weathering to a dull brown. On a fresh fracture, feldspar is seen to be the most abundant of any one class of constituents, and by the aid of a pocket lens part of it can be seen to be striated by polysynthetic twinning and hence is triclinic. The most conspicuous of the dark minerals present is hornblende. Hornblende. which is of a black or dark-brown colour and varies considerably in amount. In some of the contact phases it makes up fully half of the rock, but in general it is quite subordinate in amount to the feldspar. In typical parts of the essexite it is also exceeded in amount by a light coloured variety of augite which it is difficult to discern in the rock by the naked eye. Brown mica is often closely associated with the hornblende, probably by intergrowth due to contemporaneous crystal- lization. A mechanical separation of a specimen of this rock (No. 179) plate Feld8|)ar. iv, was made by Mr. O. E. LeRoy at the petrographical laboratory of McQill University by means of Thoulet's solution, and the follow- ing specific gravity determinations of feldspars were obtained. When the specific gravity of the liquid was reduced from 2 689 to 2-6.51, much feldspar fell; between 2-651 and 2-62, much feldspar both in clear and also in turbid grains ; between 2'583 and 2-524, a smaller amount of feldspar, all turbid. There were no lighter constituents. Specimens of the powder taken at 2-524, 2-62, and 2 651 were mounted in Canada balsam and gro'md for microscopic examination. 12 L Ortlioclane. anciesine. Labradorite. Microscopic featurps. QUEBEC The first 8how8 no striation ani in orfinary light is quite turbid It » presun>.bl3r all orthocla«e. Of that which fell at 2 62 the clea E *th 1' '''■"''' '' Polyynthetic twinning, to the ;>: o andesme The turbid grams of this weight are mostly composite bexng made up of orthoclase and a heavier constituent, but'Te^rre found to show a bite twinning and an extinction angle of at le^rso" These are doubtless altered labi^dorite. The grains having a specific gravuy between 2 651 and 2 689 showed the same difftences t d.aphane.ty as those taken at 2-62. but the clear grai^ are he e labradorue since the extinction angle on the twinning lamellae riLI whIreThfr T'^"!, """' *'"' ^ ^'«»-«-«hed in the thin section, Tytf mall ^^ ''""""' ^'™'''"" ""'' '^'"^ '*''-'^°-*« by Its small angle of extinction, finer twinning lamellae and more anotnomorphous outline. The labradorite constitutes the largest and best formed of the feldspar crystals. ^ T -J" i*!'7"i'?f?°'^'"°"*^P'*'^^P*''*«°^*'>«™<=k (specimens 179 177 Essential constituents ; blende. Plagioclase, orthoclase, augite, biotite, horn- Augrite. Acce=sory constituents: Magnetite, sphene, apatite, quartz (rare^ • with leucoxene as a secondary constituent. ^ ^ ' ' The structure is hypidiomorphic, and the order of crystallization the normal one for plutonic rocks, that of decreasing \a«idtyTh crro.magnes.an constituents enclose the usual basic aLssorie-s.^apatite i>phene and magnetite, and are themselves generallv of .n crystallization tha„ the feldspars. Of the .atteTthe le ba c pCo-^ place when the plane of either nicol ^^^Zt^^^^^^^ .. nearly a right angle, in sections approximately parallel to O J, wht DKIUIIb j lONEOUS ROCKS 13 L the angle of extinction on the vertical axis rises to 45' as the zone of the clinopinacoid is approached. Hornblende is triohroic, ranginjr in colour from deep chestnut to yellowish brown in ordirary light, its scheme of absorption being ra. The greatest 8 iigle of extinction, c ^ r, that was observed was 27% Biotile frequently encloses augite and seems to have generally crystal- lized later than that mineral. In some specimens (No. 147) an excel- lent micropoikilitic structure is thus produced, a number of augite individuals being set promiscuously in the larger crystals of biotite*. -Vccci-Mi irv Sphene is an abundant accessory and occurs in characteristic wedge- conatitiunts. 8hape ^ 14 QDIBBC The analysis has been made by Mr Radnor Forges, Quebec. M. P. Connor, B.A.Sc., of 53 15 1 62 17 tM Tio" ... Ai,(i, ■.;.::.:: ,^ Feb"' 310 Mn O. . . CaO.... B«0 ... ngo...::: K,o Na,. O pA ;::.:.::.••■ C Oj so, CI..'... H, O..... ■'■•.■ 4 y peculi. mineral intergrowths produced by the nearly contemporaneous crystallization of minerals which usually form in succession, as quirtz and feldspar granitic pegmatite, which produce the structure known as graphic granite, or in microstructure, as granophyre, or microgranite. In such cases the relation of quartz to feldspar in respect to their changed order of crystallization is quite analogous to that between feldspar and hornblende in. the quartz-free basic rock of Shefford. Basic or Ampiiidolitic Sroreoationh. IrreguK.r patches of fine dark rock material occur frequently in the essexite. Hornblende can be distinguished in them with the unaided eye, and under the microscope is found to be the only constituent of importance besides plagioclase feldspar. There is a smaller amount of magnetite also present. The hornblende is generally green, and the feldspar always plagioclase. They have thus the composition of Ar-pliiMitc amphibolite, but have not the schistose structure which is a common though not an invariable characteristic of that rock. These masses appear in irregular string-like forms, ranging in size from a few inches in either dimension, to bodies commonly fifteen or twenty feet in length and perhaps a foot wide. One such mass was found which was exposed for over one hundred yards in length and about fifteen in breadth. It is intersected in many places by offshoots from the enclosing essexite in different directions giving it the appearance of an *\V.C. Briigger.— ' Die Mineralien der Syenitptgmatitgange dcr Hiidiiorwegisohiii Augit-und Nephelinsyenit,'' I Theil, pp. 216'^, trar-'latwl by X. N. EvanH, Cana- dian Record of Science, Vol. VI, Xo». 1 & 2, p. 33-4B ft 61-71. A. Harker.— 'Petrol'jgy for Students,' Cambridge, 1«»7, p. 25. G. H. Williams.— "On the Origin of the Maryland Pegniatiten," XV. Kept. U. .S. Geol. Survey, pp. 675-694. A. E. Barlow.— Annual Report (ieol. Surv. Can.. Vol. X, (N.S.) i>art I, pp. 61 <"■'. W. O. Crosby ard it L. Fuller. -Techno'.ogy Quarterly, Vol. IX, Dec. 1896, ], pluto ii. The minf-ral cc.inp.>Hition of thete uiiwws, however, m-ftm to be lo widely diffonnt from that of any of th.. «edim.-riUry rockn of the locality ai< to nrnko it a more pro- Jtabli. theory of their origin that th.-y are due to primory Hegregation. Yet it in one that rei|uir««it Htill further evidence to be fully veriflfd. Di«intf({r»- tioii. I Kffwt (111 vpgi'ttttiiin. S|>lier(>i(l'»l weattierinif. Nordiiiarkitc descritied liy Logan. WKATIIItRI!irr>. Thn e8«'xit.' !-. generally frcnh in ■pecimen', obtained at any i„n- Mderable depths, as in op^n quarries, yet the disintegration is •trongly marked at a depth of eight feet in the excavotioim for the Granln- waterworks, near Couplunds lake, at a jK.int where gla.ia- tion appears to have been .juite m heavy as usual. Wherever deconi- p..dtiou is seen, calcite seems to be an import-mt resultant pr.Kluct This, together with the release of the potassium attendant on the breaking down of the orthoclase molecule (for orthoclase l)eside8 being the last to cr „allij!o is ihe Hrst to yield to disintegrating agencie..)givas a considerable degree of fertility to the rock waste. Little of the virgin forest (c( nifers) remains, but a thrifty second growth (deciduous) is borne by a soil of angular grains of feldspar all but destitute of the j'i,»?htest vestige of leaf mould. Roots can be seen penetrating the joint cracks of the rooks, and by their growth in size forcing the divisions of the rock apart. This healthy vegetation IS, however, attributable in no small degree to the regular rainfall of the district, which amounts to about forty inches per annum. Pomo- logical experts claim that the apples which are raised in great abundance on these eruptive hills are distinguishable by their flavour from those on the surrounding plain of stratified rock. The tendency of the essexite to weather into spherical forms by casting of concentric shells is well shown in the mountain road, near Lavignrs brook. Here rectangular blocks have been llr-ely reduced to rounded forms, leaving less surface in proportion to their volume. 2. NoRDMAHKiT«._The second variety of intrusive rocks in order of age is described by Logan (Geology of Canada, 1863, p. 656), as ' being made up in great part of a crystalline feldspar, with small portions of brownish black mica, or of black hornblende, which ore sometimes associated. The proportion of these two minerals is never above a few hundredths, and is often less than one hundredth. The other mineral species are small brilliant crystals of yellowish sphene, and J WKATIIk.MNO 19 I. othen of niagn>>tic iron, Amounting togetlifr |irol)«bly to on«> thou- Mkndth of tlie innitit.' It varii'n from a iij{ht-Kmy, uliiioat wliit«, to • fawn colour, anil in Home parts showi very noticwubie amounts of the darki-r minorals, Ijeing c»ftBn atainml by oxith) of iron t^' a liKht bull colour. Though not (liiicerriibln by tue lakid eye, then' is a lu-arly lolourU'sa augito jiri'iM-nt, which commonly enuais the hornblenilo in iimount. anil ii« a ratlier more jit^raiiitent c-nstituent, In thin sections of sjHJcimeiiH that are coniiidenil typical, Iho e»«pi,- tiiil constituents are feldspar and auKitf, the latter snarcely riHiiii: .ilKjve accessory proportions (plate V, fig. 4). Th« fdiispar in mu.1i - i.s wholly in the form of the intereHtinn orthoclasi' albite n microperthito. The following ana'.yHJs shows its compositio parison -nalyses are added of the original perthite froi Ontar. yptoperthite from Laurvik, Norway, and an from the koratophyre of Marblehead, Mansachusetts. N< and V arc taken fiom the GeolDgy of Canada, 1HG3, >n 657 ; III is cited in 'IgneouM Rocks of ArkunsaH,' Vol. JI. Hurvey, Arkansas, 1890, p. 00, from " 8yenite-pegraatit;.i> Broggor, p. 524 : IV is given in Hullctiu of the Museum t tive Zoolo;'y, Cambridge, .Mass., Vol. XVI, p. 170, 'k from Marblehead Neck,' •!. J I. 8<'ars. The microphotograph numl)ered IV is from a speciiiH^ laksn p* bably from the same exposures as that analysed uniler N >. wliil* i the locality assigned to Nos. II and Ila the only form A Miinjav ' 'mt could be found is al-o microperthite. All the analyses are of feldspar only. In colum 1 the - opt. mate theoretical composition is given of a soda-pot feldsj* up of albite and orthoclose ro' lecules in the proportion of 3:2. r-u-ns M icro|ifnisitf lase ila .f.y liiiKiiii. II. Fildsiiar— Bi-oiiif. " Iln. Ffl'l»i>ar— Bnmif. III. Krvi'loiierthite-Iittiirvik. " (iiii.'liii " Kn«KiT. IV. Anorth.w!a«" -Marblfht-ad N.-ck. Analyse.1 l.y Chatani. Df^ribeil hy Seam. V. J'erthitp ISiirgf^-.. " l'"'"- " f*"'"' VI. Avproxiniatethi-orBticalconipo^itiimof f.-l.Upar having the furiimla Ab,, Or iiiitai^ jy«*. i3_L— 2^ 90 L Au|{ite. Hornblende. Biotite. Quartz. HI O,.. . AI.A, F«;().. MbO.. c»o . MfO. . K,0 . N», O . H, O... sons 7a II AS 70 »ao nvaato lb man W 70 84 IM « iW ««7 00 III 6 M 11 11 la 90 BU JOO 70 1 WW IV ft*> Ml ■io im Trm*. IS «7 •IM 6 BH 6 StI :i 21* ( IW) ftl m 44 IH 3b 1 Ml •17 «:t7 ft M 40 Vi 07 OB 10 tn A 98 700 !« »« l« tW • Incunipli-te. t At I ltion is accordingly f<||< a, and the maximum extinction angle that was obxerved, c A t -■ 26 , practically the same as in the brown hornblende It is sometimes more abund- ant than augite, and at others nearly or altogether wanting. The same may be said of the occurrence of biotite. This mineral is of a deep brown colour in ordinary light and polarizes in brilliant tints, probably indicative of a larger proportion of iron that, jsual in its composition. Sphere fi-equently appears, and occasionally a few grains of quartz are seen, (Noa. 166, 118, 188). One of these shows an uniaxial cross and positive sign, thus clearly identifying it. In structure the rock is coarsely granitic, but the abwnce of quartz in sufficient amount to form a cementing material for the other con- • Quoted by Prof. Ronenbmch ir ' Elemente de Geatein»lehr».' TMK COMTACT PAClia 31 •tituanta U>n<\» to nnA«r th« rook frUble. On exp«»u.-* to th« •tmo«- plim it U wwily 22 li SB 506 03 ■0(1 73 IS 30 li B" 50 100 IM •M 100 14 on 90 I. Nordiimrkite-Sheffor(l-(N"- »•'"•!)• Analysis by Connor. ,_.,„, II .. (gray) Tonsena*, near Chriatiania. Cited by Ry Hillf- brand. U.S.G.S. Bull. 148. V. Feldi»i«r—Sheffottl— Already quoted. Contact Facies. The contact zone of the nordmarkite is commonly distinguished by an increase in the dark minerals. Hornblende and biotite rise to IT fcas^SSSSSS^^ 29 QUEBEC 3- r Oomian'H quarry. the importance of essential constituents, while the microscope shows hndmnorplnc larger and more numerous sphenes. Nepheline is also occasionally present around the periphery of the mass, though only in very sub- ordinate amount. The feldspar becomes more Knely laminated until the perthitic structure is all but lost, becoming discernible only in parts and under high power. This gives th« mineral, which has otherwise the general aspect or orthoclase, a peculiar mottled appear- ance, answering apparently to the characters of kryptoperthite. A few grains of finely twinned pliigioclase also appear. In a mechanical separation made by Mr. LeRoy of a specimen (No. 145) from Dounan's quarry, about thirty yards from the contact, the feldspathic constituents fell at the densities 2-62, 2-583, and 2-56t!. The specific gravity of the specimen selected for analysis I of the feldspars, cited from the Geology of Canada, 1863, (p. 657) was 2-561. The darker colour which these rocks acquire in proximity to the contact often gives them a marked value for decorative purposes. It is also noticeable that while the changes in the mineral consti tuents which characterize the endomorphic contact zone are such as to denote a more basic composition, yet certain dykes (No. 188, 135) radiating from the mass appear to be more acid than the normal rock. In them there is no increase of ferromagnesian constituents but rather an increase of quartz, a variation which has not been found in any other part approaching the contact. Basic Segregations. The nordmarkite also contains dark masses in which the iron- magnesia minerals are prominent. The feldspar in these be- comes finely granular and sometimes shows perthitic structure under high magnifying powers. The leading dark constituent is biotite, deep brown in colour with high double refraction. It is rather dis- a«icma.«.H. tinctly idiomorpl in form. Dark green hornblende with th(> usual pleochroism is abundant in these masses, but usually in small crystals. No augite appears. The dark minerals together occupy rather less than half the rock, so that it is only by comparison with the light gray, or fawn-coloured nordmarkite that these patches appear as dark-coloured rock. In mode of occurrence they are similar to those found in the essexite already described, and are probably analogous to them in origin. •J HASIC SBOKEOATIONS 23 L 3. PuLASKiTE.— The third class of these eruptive rocks differs from I'ul'^'kite. nordmarkite principally in having hornblende as the characteristic bisilicate instead of augite, and in its structure, wliich tliough holo- crystalline, is of a porphyritic trachy tic character. It is rather variable in appearance, but is usually of a gray colour and tine texture, being too fine to admit of mechanical separation of the mineral constituents. It is often coated with a dark, almost black, o.xide of iron which tends to obscure these features. Small crystals of black hornblende can he occasionally detected in the finer feldspathic groundmass, and coarse nodular masses, from six inches in diameter downwards, are commonly seen. Some of these are darker than the enclosing rock, while others [^'.''y"^';;'^;,^';^;,. are considerably lighter. In the marginal portions of this rock and in the numerous dykes which it sends off into the adjacent roiks it is more distinctly porphyritic. The groundmass in such cases assumes a dull greenish shade, due evidently to the increased proportions of the ferro-magnesian minerals, and small feldspar phenocrysts are plamly seen in it. A few specimens nearer the central portions of the mass show fine greenish specks which under the microscope are found to 1« a-gerine- augite, while in others an occasional blue spot of sodalite is apparent to the unaided eye. By the aid of the microscope, this rock is found to consist of felds- ^/^^-J^'^J^.^^^ par, hornblende, augite, biotite, magnetite, sphene, sodalite and apatite. The first four, only, are in essential amounts, and the feldspar is by far the most important of all except in the endomorphic contjict zone. The structure varies from coarsely trachy tic in the central portion of the mass to porphyritic along the margin. The phenocrysts include both orthoclase and plagioclase as well as hornblende and occasionally augite. Of the feldspar phenocrysts, orthoclase is far the most abundant in the interior, while plagioclase largely predominates in the peripheral portions of the area and in the dykes. The increase of plagioclase «ia| at the expense of orthoclase in the phenocrysts seems to be analogous to that described by Cross* in the Game Ridge trachyte, the last mem- ber in the sequence of important eruptive rocks in the Rosita Hills. The feldspathic portion of the groundmass consists of short rather stout prisms packed together, often in parallel arrangement (fig. 5) Kidge trmliyte. "t;ii)l.iKy 'if Silver Clitf and K.pHita Hills, Cclonid.i Anmial Kei«>rt U. S. (i. S., 1805 C, p. :MHi. Wliitnian C'nwH. 24 L QUBBEC Ortboclaite. with a little all otriomorphicfeldspar. A few striated grains appear, which, as they extinguish parallel to or at very low angles with the tvvinning lines, are probably oligoclase, but fully ninety -five per cent show no striation, have a generally parallel extinction and are doubt- less orthoclase. Crow partinK. A cross parting is often noticed in the smaller phenocrysts which at first was thought to be a fracture due to pressure exerted upon the rock subsequently to its crystallization. But, although other evidences of dynamic metamorphism are seen, no displacement of the parts of these crystals could be discerned. For instance, in figure 6, the five parts into which the largest orthoclase crystal appears to be divided, extinguish simultaneously. The hornblende is chiefly green in colour, though a few of the largei individuals are brown, resembling the hornblende of the essexiie, while the green is like tl At of the nordniarkite. Both are trichroic having the same scheme of absorption, viz : f>ib>a, and extinction angles, , as high as 26" — 27° have been observed in each. Augite, u uen present is colourless, and in one instance was seen to Bisilicatea. have a fibrous rim of hornblende. Both these bisilicates are almost wholly replaced in one part of the rock by aegerine-augite (No. 1 87, fig. 6, plate vi), and in the same portion both a colourless and a blue sodalite mineral are quite prominent accessories. The former is distinguished by its rounded or polygonal outlines, its isotropic character, and frequent dust-like inclusions. The latter occurs in strings and small interstitial patches, and is bright blue in ordinary and wholly dark in polarized light. Biotite occurs in comparatively few, but large, well formed individuals. Both basal and prismatic sections are seen, but present no features which calFfor especial notice. Sphene is a rather abundant accessory mineral in some portions. Needles of apatite are also frequently found. A little undetermined matter occurring interstitially amongst the feldspar was thought to be altered nepheline, but may be kaolinized orthoclase. The structure and mineral composition of the rock ally it with the Pulaskite type of hornblende syenite, the original occurrence of which was described by the late Dr. J. F. Williams, from Fourche Mountain, Arkansas. Its chemical relation to that rock is well shown in the following analyses, as well its resemblance to the allied mica-free sub-class, umptekite. Accessory minerals Fourche Mountain. NODULES 25 L II III SiOj I WMM} TiOj ' ■«■' Al,6, ! ll»lli Fe'Oj 1 1 H". Feb.! ' i-7:i Mn O , ■»'•» CaO 2-24 BaO ' 12 MgO ■fi-'i Kro ; 491 Na,() ! CIW V/O. H CO., iioiif. so, 08 CI 14 H,0 11" 20 7<> 4 (II trace. 80 5 4H 5 9fi 07 ■5!t ilO.itl 101 07 12 it'J!»8 IV 5'J 01 58 70 81 traw. 18 18 : 19 2f! 1 «:i 3 37 3 (» 1 ■.■rf< 03 1 10 2 40 1-41 •7(i 4-53 10 '_> ti4 lot) 00 I. I'ulaskite. Shefford. AiialyHis by M. F. Connor. II. " Fourche Mt., Arkansas. Anal, by Brackett and Smith. III. Umptekite, Red Hill, MoultenliorouKh, N. H. Cited by Rownbuiicli, loc. cit. IV. Tinguaite var. Solvaberffite, Crany Mountains, Montana. Described by Wolff and Tarr. Bull. Mus. Comp. Zoiilogy, 1893, under the name "acmite tra- chyte " and later renamed by Dr. Wolff as above in accordance with BruK^er'a classification. In the aegerine-augite-bearing portion of this mass, the texture appears rather finer and the structure is that characteristic of trachyte (fig 6, plate vi). It then closely approaches the Solvsberg type in appear anc?, and, as is shown by analysis IV, does not dififer radically from it in chemical composition. Mineralozically, however, it differs from Crazy , , , o ,,..,. . ... . mountain. the Crazy mountain type in the character of the bisihcate constituents, which are chiefly aegerineaugite at Shefibrd, instead of acmite and aagite intergrown with aegerine. ,*■■ KODULES. i The lighter coloured of the two classes of nodules that havp been mentioned consist almost entirely of orthoclase feldspar, or possibly kryptoperthite, and are somewhat similar to parts of the nordmarkite. The other is composed es-sentially of brown hornblende with a small amount of feldspar. The hornblende is occasionally intergrown with biotite to a small extent. Nodules of this class decompose more Complement- . , « • 11 ... -i . ary nodules. readily than the inclosing rock, thus forming small cavities or pits in the surface, sometimes two inches in depth. No order could be dis- 26 L QUEBEC i Primary origin. ^ cerned in the distribution of eiiher class. Thoy are frequently, but not always, rounded or ellipsoidal in form, and in the latter case have the longer axes parallel to the plane of the foliation of the rock. They show no evidences of radial or concentric structure as in nodules of concretionary or spherulitic origin. Tl.eir inineralogical composition shows them to be, generally speaking, complementary parts of the pulaskite magma. In one class of nodules the only essential mineral is feldspar similar in character to the phenocrysts of the main rock, while in the other the feldspathio constituents are of minor importance to hornblende and biotite, the former of which preponderates. They appear to be best accounted for by the segregation of their component minerals during the cooling of the general magma, perhaps analogous on u small scale to the common differentiation of an alkaline magma when it produces both bostonite and comptonite dykes from the same mass. Comparison of Igneous Rocks. Igneous rocks comiAred. In order to give a comparative view of these rocks, their mineral con- stituents and chemical composition are repeated in tabular form : — MlNERALOGICAL COMPOSniON. — Ensexite. Nonlmarkite. Pulaskite. Essential constitu- PlaKioclose. Orthoelase. Micriiperthi'e. ( )rtliiiclase. ents. Pliigioclase. Hornblende (brown). Augite. Hornblende (gn in Hor. 'lende (green). and brf)wn). Augite. Aug ite (sometimes legerint-augite). Biotite (variable). Biotite. Biotite (variable). Acceisory constitu- .\|)atite. Magnetite. Magnetite. ents. Magnetite. Sphene. Plagicxlase. Sphene. Sphene. -Vpatite. Leucoxene. Apatite. Sok was intruded first, and that highest in silica flu wical second, while the third in order of age is intermediate in composition. All are comparatively rich in alkalies, and the greatest variation in the proportion of the bases is in lime and magnesia. The extreme range of silica is 1228 %, alumina, 216 %, lime 430 %, magnesia 2-72 %, potash 2-26 %, and soda 1-98 %. The mean between the composition of essexite and nordmarkite, in equal proportions, approximates quite closely to the composition of pulaskite, thus : I Mean of Kssexite I and Xordniarkite. Si O3 . . . Ti O, . . . Ai;<5,.. Fe, O, I Fe ) MnO... Ca O. . . . MgO... K,0... Na, O . . 59 29 ■84 17 30 .-) 41 43 3 .Jl 1 .-.K 4 23 5 47 Pulaskite. r)9 9<> •ot; 19 12 3^r.H 49 2 24 O.'i 4 91 98 28 L qVBBBC Distribution. The areal distribution of these rocks, which is most conveniently shown by a reference to the map, cannot, however, be taken as a basis for any quantitative calculation. Nordmarkite occupies about as large an area as both the others, but it appears in places to overlie the essexite, and thus the origined by Prof Kemp*. But as the term was employed by Prof. Kemp to emphasize their occurrence remote from any known volcanic centre, it has been thought better to use the term trachyte as the generic one in this case where dykes c. r at the seat of two intrusions of a syenitic magma. The term bostonite is accordingly restricted to those specimens in which the ferromagnesian silicates are present in less Trachytes. than essentia] amounts. In such cases too, the trachytic structure appears to be less marked. Two classes. Lamprophy- res. i '•The Trap Dykes of Lake Champlain,' .T. K. Kemp and V. F. Marten, U. S. Geological Survey, Bulletin No. 107, pp. 18 and 22. .jmmjaiiJ Iateb nVKES 29 L Lamprojifiyres, The number of tine-grained dark dykes is large and specimens have l^J|irk ;[y!'''^| been taken from a comparatively small numlier of those whose macros- copic appearance is quite uniform. Plagioclase feldspar appears in all with one or more of the ferro- magnesian minerals. Hornblende usually preb>a. The maximum value observed for c A C was 13'. The Nepheline has crystallized rather later than the feldspar and is Xiphelinc thus the last mineral constituent to form ; consequently its out- lines are almost wholly allotriomorphic. Much of it is decomposed, the alteration product of which Dr. Barlow has determined to be 'an aggregate consisting of a radiating zeolite which possesses the optical properties "-f natrolitc, in association with which there is also a con- 30 I. gUKIIKC •iderablo quantity of lolourlusK brilliantly polarizing niutcovito. The nepheline also woathera to a dull colourless mineral dimly polarizing substance which is proltttbly kaolin.' Ill 'mount nephclinc is nearly eriual to feldspar. The other minerals ,)ie.sent iio fi-aturi's worthy of note. The structure of the rock U hypidiomorphic with a noticeable tendency towards ideomor|>hi8m on the part of tho lar^or and probably morn bisic feldspars. Culllpiollltt). Iilioiiii>r|iliiu homl>Irii(l<'. Fc'ld»piir. Undi»tnrlwcl occurrencf. t'nmi>lonile. This rock is characterized in the thin section by an abundance of hornblende, always in distinct idiomorphic crystals varying in the length of prismatic .sections from 2. mm. to .2 nmi. In fresh crys s the lolour is deep brown, with the usua'. pleochroism, but the greater part of the hornblende is somewhat altered, and grayish brown in Colour, showing little if any pleochroism. It is .studded with minute rounded or irregular grains of magnetite. Feldspar, generally of a rather indistinct chariicter, Ijcing »ome- wliat turbid, is present in amount about e<|UHl to the hornblende. Lath-shaped individuals are, however, of quite frecjuent occurrence, and have an extinction angle of as much as 20° with the prin- cipal axis. Polysynthetic twinning is distinctly seen in a few CHses. A little biotite and apatite are the remaining primary constituents. Secondary ca'.cite occurs intcrstitally in considerable amount. An isolated occurrence of this rock was found, the position of which is indicated on the [accompanying map as ' 151.' It is there exposed in the bed of a small stream for about one hundred feet or more, at a distance of some five hundred yards from the is;neous part of the mountain. It is inclosed by mica schist, and seems likely to be a sheet rather than a dyke. It appears to diflFer from the other rooks of the mjuntain in showing no deformation, but the area exposed i.s too small to furnish very reliable evidence of the difference in age which its unaltered character seems to suggest. THACHYTEa coloiircil dykef*. The dyke rocks that have the general characters of trachyte are uniformly fine in texture and of a light-gray, or bufiF, colour. When of the latter shade they pass into the bostonite type. TKACIIYTKS ii I T i I t i 111 ihin set-lion, feldnpar ii. iiIwbvh largely in oxcomi of tlio oilier conslitURiiiH, iiiul in soiiui ini«U»nefis coiiHlilult's iiiiiPly lo iiiiiciy-five per cent, of the eniiie took (113, \AK, 20'>). TIipmc are thu l;o«toniU'B l...-t..iiit". Ill an aviMugi; specimen (No. 103) feldspar occupies alioul thrce-fourlliH of the field and oi'cura in siimll coliiiiiniir cry-stals of iiiiiforni siw and parallel arrangenionl, and u few larger individuulM which are nuarcely rUntinct enou{;h to >»■ called plienocrysts. They conform lo the parallel arranneinent of the liithshapod niicrolileM, which given to the rock a deeided Mow stiucturo. Slender )>rismalic Hections of hornblende and n little biotitc can be recognized in ordinary light, a» well as a few grains of magnelitp, and a coii'scriiied by can be ascertained, conformable to these in strike 1 dip. In a sec- tion it is seen to be composed of a microcrystalline groundmass. S2 L (JURBBO Dykes cooled t.lowly. holding numerouH large crytUli of feidiipAr MAtlernd through it. These feldapar oryatAlit have, under the mioroecope, m Neing present in the groundroass, lo that the quartz may be a secondary product. Disseminated through the groundmsss, and in smaller amount in the imbedded crystals, there are numerous opaque black grains generally irregular in shape, but sometimes occurring in little cubes. These are probably an iron ore. Associated with these grains, at a few places in the groundmass, there is a strongly pleochroic mineral, the colours changing from light yellowish-brown to a dark-brown, and with the greatest absorption parallel to a very good cleavage. Betwee crossed nicola, extinction takes place when the plane of polarization of either prism coincides with this cleavage, so that the mineral is probably a magnesia mica. In a section, the groundmass appears of a light-brownish tint, the colour being due to a yellowish-brown mineral which is finely dis- seminated tlirouf;h it and which also occurs, though in much smaller quantity in the imbedded crystals, either in little patches or runnin),; with their cleavage lines." Condition of Coolino of Dykes. The prevailingly coarse texture of the dykes of Sheflford mountain and the absence of glassy material in them point to their having coolefl slowly, presumably due to their solidification at greater depth or to a heated condition of the side walls at the time of the injection * ComiKire Rcnenbiisch, Mikroskopisch Physiographie der Mawigen Gcsteine, p. 129, ed. 1877. MIKI*- nVMAMIO METANOIfFIII'M r.\ L ot il....l>k.. maleriil. In the cm of «'vi.ri»l dykf^ of i.onlt.mrku.. (i:Jl, l.'«'> Pi «l.), whicli tut e»>i«xite, narrow off *ho..t». «-urci-ly « 'Hi»r tcr of H.i inch in wi.lth. xtriko off for a .liii.uic of t«.-nty to tw.-i.ty ft%e f«l, l)Ut urn -canvly \v»» fo«r»e in -truclur.- tlmii th» .lyke, themiclv.w whii'li firefruiii thrw lo fixe fwt wi.if. In neither th«. ilyke^ nor thnr off»h.H.tH in tli.-i-.- imv api.romli t.. a porj.hy.itio ■truoiurf. The »»nie charata«r of . r_\>.taliziitic.ii iip|«ttr< in thf lat.T tlyk."« a» (V rule, thi- .hie! exception \mug in the .une of .lyke. i.rr.lml.ly In- longing to the pula-.kil« nines. These, likr the contiiot facie, of that rock, have either a pori.hyritio or. more ...nnnonly, » jx.ri.hyritic trachviii' ntruo le. hVNAMlC MbTAMOHPIII'M. All the ig-»ou» ro kM ,on.po«in« the n.a,s of SheHoni nioutUain. "^i;*",',;;;;; with one possible exception, tlinpUy more or le*« iliHtinct foli.ition in a direction parallel to the folding of the (.edimenlary rocks of ihe.li*- trict. Foliation is frecU there is a .linht ' rift ■ or tendency to cleave, always in the same direction, In thin section from the least altered parts of the rock f.-ldspiir crystals are occasionuliy found which are diHtorted imd whiih show distinct strain ahadows, thus attesing to the subjection of the rock to metamorphic a«encie8. at least in the final stages of the App.du.hiah foldin<{. The ilykes also ahow evidence of subjection to pressure giiico tlieir golidificatiou. A slaty cleavage is more or less perfectly develoi>ed in Cl.nvu^.-. both classes of dykes. That this is due to pressure is shown l>y the microscopic sections in which distorted crystals and grains hiiving a wavv extinction ticcasionally appear. The cleavage is l«st shown in dykes which run obliquely to the axis of foliation. In such cases an apparent differential movement of the side walls has produce.l a peculiar twisted frncidre crossing the dyke obliquely that is easily noi i.ed. plate II. Faulting too is well seen in numerous narrow vein-like dykes which Ka„Hin({. appear in "the essexite in Lavignes brook along the mountain road. 13— !.— 3 34 L QVEltir III ininrral cumpoaitum nmi mirroitrurture they kk**** C(ini|»otoly with the boaUinitd dykPN that hnve {wn cieicribeil. Tlie faultioKof mir, nlxKit mi inpri IX i mutely to )ii.-ulf, th«> wiijth of thf v<>in Iwing muni'whitt oxii;;- KcrntKl. Intriiiiion«. Owing to the intrusion of the various large iiias8C>< at dift'eroiiL periods each ro-.-k is liable to have suffered luor^ or leas deformation from the forces which caused or nccoinpaniwl cacii xubsequent in- tru ion. But since the latest of thoie possesses a cleavage scarcely less distinct than the earliest, it too must have Ijecti in its present pocition before the causes of the regional foliation ceased. Tiiert- is thus ample proof that the rocks of Slietlbrd mountain have shared in the foliation of the entire region, viz., that of the Notre Dame, or Green Mountain, range of the Appalachian system. The only possible exception is the camptoiiite already mentioned. AoE OP Intrusive Rocks. Important data is thus nTorued for determining the age of intrusion of these rocks. AOR ur IXTRl'MVK H(M HM 35 I. The UUmt ntxiinirnU niui>n){it wliirli thry liKve bntn intruiiinl i« tltHt Kniup u( th« lower Trantiin furuiatiun known m the Fiinilmni Itlack KlntaM (I) 3u, map tii itcouropnny PnrtJ, Aniiu«l Ur|H)rt, (iro lut{icnl Survry ttt CMiiiciii, I8'J4), whilf lln< ♦•luliftt iiifiiibur* >'f ihe Palit^zoic NyNtpiii in nnsturn North A'nei ioit that hnvn nut licfn ilin- tui'lM*ii hy thf ApiMilnchinn iiphft uie the IVinio-mrhimift-roUN of l'rin<'« hilwmil IxlunW and the Hiljoininx niainUnt). AfCortl that th« linal foUliii); thri>iit{hoiit itll parts of Ihf northern Appalii>-hi>in'4 look pliice ^iinultJinwiusly, thf intrunion of the Whefforfi inaM< would t'ceniMirily have occurred liftween «"arly Trenton aiul later C'krlx>nif«rou> time, liut the HiiiiultHneoy* t'olding of ho xreal i belt a.s thi' Appalachian ^yRteni here > ompriwM cannot b« safely aMHunied without a better correlation of its complex structural details than ix at present poiwible, and in cVKb: SHA. Vol.. XIII., I'ART li, PlATB III. 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