umn-AXCs: Class j^c. .v5:cr^ Cost Qate.lr.ZP.^Sl.e. »^ » »■» » » » » » SXI01S 'sauBjqn auinioA sup aipueq » » »»»»^«»-»-» Digitized by the Internet Archive in 2009 with funding from Boston Library Consortium IVIember Libraries http://www.archive.org/details/geologyofgreenmoOOpump LIBRARY CATALOfUTK SLIPS. United States. Deparlimnt of the hilcriur. ( r. ,S. yeoloi/U'dl surveii.) Department of the interior | — | Monograi)lis | of tlie | Unit(^(l States geological survey | Volume XXIII | [ Seal of tlie depart- meut] I WasUiugtoii | government printing ofiHce | 1S91 Second tUle: United States geologieal survey | .1. W. Powell director | — | Geol gy | of the | Green monntain.s | in | Massa- chusetts I by Raphael Pumpelly, J. E. Woltf, and T. Nelson Dale | [Vignette] j Washington | government printing office | 1894 4°. XIV, 206 pp. 2;! pi. Pumpelly (Raphael) and others. United States geoh)gical survey | .1. \V. Powell director | — | s Geology | of the | Green mountains | in | Massachusetts | liy Z Raphael Pumpelly, J. E. Wolff, and T. Nelson Dale | [Vignette] | a Washington | governnu'ut printing office | 1894 5 4°. XIV, 2U6 pp. -23 pi. [United States. Deiiart ineni of the intcrinr. (P. !i. rjeoloijical .iiiroey.) Monograph SXllI.j United States geological survey | .1. W. Powell director | — | Geology I of the | Groen mountains | in | Massachusetts | by Raphael Pumpelly, J. E. Wolff, and T. Nelson Dale | [Vignette] | Washington | government printing office | 1893 4°. XIV, 20li pp. 23 pi. [UxiTEi) States. Drpartinent u/ the interior. {U. *. ijeoloijical aiuoey.) 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'sJ^ WASHINGTON GOVERNMENT PRINTING OPFICB! 1894 CONTENTS, Page- Letter of transmittal xi Preface xili Part I. — Geology of the Green mountains in Massachusetts, by Raphael Pumpelly. General description 5 Age and structure 7 Correlation 9 Part II. — The geology of Hoosac mountain and adjacent territory, by J. E. Wolff. Introduction 41 Topographic work 41 Topography 41 Description of rocks of Hoosac mountain 44 The Stamford gneiss 45 The Vermont formation 48 The Hoosac schist 59 The Stockbridge limestone 64 Amphibolites 65 Geology 69 The Hoosac tunnel 69 The region embracing the central part of Hoosic mountain 72 The northern and eastern schist area 86 The region south of Cheshire and of the Hoosic valley 88 Hoosic valley schist 97 The region around Clarksburg mountain and Stamford, Vermont 98 General conclusions , 102 Part III.— Mount Greylock: its areal and structural geology, by T. Nelson Dale. Outline of this paper 125 Historic 131 Physiographic 133 Structural 136 Types of structure 138 Correlation of cleavage and stratification 155 Pitch 157 Structural principles 157 Structural transverse sections 158 Lougitutional sections 175 Resumd, structural 177 Lithologio stratigraphy ,,..,, 179 V VI CONTENTS. Page. Pctrograpliy 181 Areal and structural 191 Relation of geology to topography 192 Appendix A: Stone hill near Williamstown 197 Appendix B: New Ashford 202 ILLUSTRATIONS. Page. Pl-ATK I. Map of Greylock and Hoosao mountains Frontispiece. II. General map showing the relation of the Greylock series to the Hoosac mountain rooks 10 III. Structural relations of the Hoosac series 14 IV. Detailed map of western crest and slope, Hoosac mountain 40 V. Geologic profiles, Hoosac mountain 70 VI. Geologic profiles, generalized, Hoosac mountain 80 VII. Thin sections, white gneiss 110 VIII. Thin sections, white gneiss and albite schist 112 IX. Thin sections, diorite and amphibolite 114 X. Thin sections, quartzite conglomerate and crumpled metamorphic conglomerate 116 a . View north over crest of Hoosac mountain 118 ■ b \ Profile of Hoosac mountain from Spruce hill south, looking west . . ^ 118 XII. Mount Greylock, eastern side 130 XIII. Mount Greylock, western side 132 XIV. Southern summit of Mount Greylock 134 XV. Southern side of Mount Greylock 136 XVI. Southern end of Ragged mountain 160 XVII. The north-south part of Hopper 192 XVIII. Greylock sections, A, B, C, D XIX. Greylock sections, E, F XX. Greylock sections, G, H, I XXI. Greylock sections, J, K, L, M XXII. Greylock sections, IJ, O XXIII. Greylock longitudinal sections, P, Q, R Fig. 1. The Stamford dike, showing the Cambrian conglomerate deposited in dike fissure 11 2. The Stamford dike, plan 11 3. Correlated columns of the Hoosac and Greylock rocks 13 4. Anticlinal arch across Hoosic river between North Adams and Briggsville 15 5. Ideal section east of Cheshire, showing lateral transition of limestone to schist 17 6. Diagram of structure, summit of the Buttress 22 7. Crumpled structure in albite-schist 23 8. Map showing the varying character of Cambrian rocks around the Hoosac core 31 9. View from Hoosac mountain 42 10. Profile of Hoosao mountain ( western crest) 43 11. Profile of Hoosac mountain ( western slope) 44 12. Granitoid gneiss 45 VII a a, o VIII ILLUSTRATIONS. Page. Fig. 13. Metamorphic conglomerate showing crushing 48 14. Metamorphic conglomerate, showing shape of pebbles 49 15. Metamorphic conglomerate; flattened pebbles 50 16. Metamorphic conglomerate ; round and flat pebbles 51 17. Metamorphic conglomerate, banded variety 53 18. Metamorphic conglomerate, typical 55 19. Metamorphic conglomerate, showing large pebbles 57 20. Conglomerate ; cliff 58 21. Albite-schist, Hoosac schist 59 22. Albite-schist, Hoosac schist 61 23. Albite-schist, Hoosac schist '. 62 24. Mount Holly amphiliolite _ g5 25. Mount Holly amphibolite .-...- _ 66 26. Mount Holly crumpled amphibolite 67 27. Contact of grauif old gneiss and metamorphic couglomerate 73 28. Contact of grauitoid gneiss and quartzite, Stamford dike, looking north 100 29. Contact of grauitoid gneiss and quartzite, Stamford dike, looking east 101 30. Northwestern side, Mount Greylock 136 31. Albitic sericite-schist in contact with limestone 138 32. Sericite schist with two foliations, in contact with limestone I39 33. Sericite schist ; specimen with two foliations 139 34. Thin section illustrating origin of cleavage I.j0 35. Sketch of ledge south of Sugarloaf, showing cleavage in both limestone and schist 140 36. Limestone block with cleavage ; Sugarloaf 141 37. Limestone ledge with cleavage ; east of Sugarloaf 141 38. Weathered limestone from East mountain 142 39. Polished surface of limestone shown in Fig. 38 142 40. Weathered limestone with mica in cleavage planes 143 41. Specimen of sericite-schist showing stratitication and cleavage. Bald mountain 144 42. Specimen of sericite-schist showing only cleavage, Symonds peak 144 43. Section of specimen shown in Fig. 42 I45 44. Section of specimen of sericite-schist, top of Mount Greylock I45 45. Microscopic drawing of sericite-schist, top of East mountain 146 46. Specimen of sericite-schist, one-fourth mile south of Mount Greylock 147 47. Diagrams showiug relation of quartz lamiuiv to cleavage 148 48. Ledge of sericite-schi.st, junction of Gulf and Ashford brooks 148 49. Part of ledge shown in Fig. 48 I49 50. Section of sericite-schist with quartz lamin;e ; from Goodell hollow 150 51. Ledge of mica-schist in Readsboro, Vermont, svith ([uartz in both foliations 151 52. Sericite-schist with two cleavages, Goodell hollow 1,52 53. Section of sericite-schist, one- fourth mile south of Greylock summit 153 54. Sericite-schist, one- fourth mile southwest of Greylock summit 154 55. Diagram showing fault between schist and limestone t 154 56. Section of sericite-schist. Bald mountain spur 155 57. Diagram showing relation of cleavage to stratification 156 58. Diagram showing relation of cleavage to stratification 156 59. Quartz laminae in schist, west side of Deer hill 157 . ILLUSTRATIONS. IX Page. Fig. 60. Minor pitching limestone folds 157 61. Cross-section G 160 62. Section of syucline at soutli end of Ragged mountain 161 63. Cross-section H 166 64. Cross-section I 166 65. Cross-sections A, B 169 66. Cross-section F 171 67. Cross-sections J, K, L 172 68. Structure in schist, south side of Saddle Ball 173 69. Cross-sections M, N, O 173 70. Structure in schist, west of Cheshire reservoir 174 71. Longitudinal sections P, Q, R 175 72. Continuity of the folds ou the Grey lock sections 178 73. Albitic sericite-schist : typical Greylock schist 188 74. Outline sketch of Round rocks 194 75. Sketch of Greylock mass from southwest 195 76. Cross-sections S, T, U. Stone hill 198 77. Sketch of protruding limestone anticline. New Ashford 202 78. Diagram map of Quarry hill. New Ashford 202 79. Cross-section of Quarry hill. New Ashford 203 LETTER OF TRANSMITTAL. Department of the Interior, U. S. Geological Survey, Archkan Division, Newport, E. L, January 18, 1892. Sir : I have the honor to transmit herewith a memoir on the Greology of the Green mountains in Massachusetts. Your obedient servant, Raphael Pumpelly, Geologist in charge. Hon. J. W. Powell, Director U. S. Geological Survey. XI PREFACE. The following memoir is the result of the fieldwork of the Archean Division of the U. S. Geological Survey in northwestern Massachusetts, during the years 1885, 1886, and 1887. The conclusions put forth were all arrived at before 1888, but the publication of them was delayed until they should be either confirmed or corrected by the results of further study in southwestern Massachusetts and in central Vermont. The progress of our survey of western New England has fully con- firmed our interpretation of the facts observed in the Hoosac mountain and Grreylock area. It has been our intention to keep wholly clear of the Taconic controversy, and to confine our efforts to accurate study and inter- pretation of structure. In the first part I have given a statement of the sequence and bearing of the results and have advanced some theoretical views in explanation of the sudden disappearance of the Lower Silurian limestone against the western base of the Grreen mountain anticline. I have also advanced a hypothesis, supported by observation in the northern and southern Appalachians, to explain (through the presence of a previously deeply disintegrated land surface) the apparent conformable transition between Archean or pre-Gambrian gneisses and Cambrian quartzite. This almost insuperable difficulty is met with in many of the great crystalline areas of the world, in passing from Archean or eruptive masses to the clastic crystalline schists. The second part treats of Hoosac mountain — the central or crystalline range of the Grreen mountains. The field work was performed by Dr. J. E. Wolff, Mr. B. T. Putnam, and myself. The analysis of the results, the petrographic study, and the presentation are by Dr. Wolff. Mr. Putnam XIV PREFACE. had contributed largely to the sum of the work. His early death in 1886 deprived the Survey of one of its most accurate and thouglitful geologists. The third part deals with the Grreylock synclinorium — made up of the Cambrian-Silurian quartzite, limestones, and schists, which are the offshore time equivalents of the white gneisses and schists of Hoosac xnountain. The held work was done by Mr. T. Nelson Dale, assisted in part of the area by Mr. William H. Hobbs. The analysis of the results and the pre- sentation are by Mr. Dale. As during the first two years we had not yet the benefit of the new topographic map of Massachusetts, our work was delayed by the necessity of making our own maps. This was done in part by Messrs. Putnam and Wolff, assisted by Mr. Yocum. Later, Mr. Josiah Pierce made a detailed topographic survey of the western flank of Hoosac mountain which forms the geographic basis of PI. iv. Mr. C. L. Whittle was also connected with the work under Dr. Wolff during the season of 1887. Mr. William H. Hobbs acted as assistant to Mr. Dale during one season and a part of another in the work on Greylock and was engaged inde- pendently during the rest of the second season on the coloring of the northwestern part of the Greylock sheet. I have mentioned in its proper place the fact that we owe to Mr. C. D. Walcott the determination of the age of our basal quartzite. R. P. PA^RT I GENEIUL STRUCTURE AND CORRELATION. By RAPHAEL PTJMPELLY. MON XXIII 1 CONTENTS. Page. General description : 5 Age ami stnieture - 7 Correlation 9 ILLUSTRATIONS, Page Pl. I. Map of Greylock and Hoosac Mountains Frontispiece. 11. General map showing relation of the Greylock series to the Hoosac. mountain rocks 10 III. Structural relations of the Hoosac series - 14 Fig. 1. The Stamford dike, showing Caoibrian conglomerate deposited in dike fissure 11 2. The Stajnford dike, plan 11 S. Correlated columns of the Hoosac and Greylock rocks , . 13 4. Anticlinal arch across Hoosic river between North Adams and Briggsville 15 5. Ideal sectiou east of Cheshire, showing lateral transition of limestone to schist 17 6. Diagram of structure, summit of the buttress 22 7. Crumpled structure in albite-schist 23 8. Map showing the varying character of Cambrian rocks around the Hoosac core 31 3 GEOLOGY OF THE GREEN MOUNTAINS IN MASSACHUSETTS. GENERAL STRUCTURE AND CORRELATION. By Raphael Pumpelly GENERAL DESCRIPTION. The Green mountains, nearly coinciding with the prolongation of the axis of the Archean core of the Appalachians through western New Eng- land, stand between the less disturbed fossiliferous Paleozoic strata of New York and the highly crystalline rocks of New England. They consist of three principal structural elements : The Green mountains (Hoosac moun- tain) ; the Taconic range, lying several miles to the west ; and, between these, the great valley. But the whole region between the Hudson and the Connecticut has very properly been placed by Dana in one mountain sys- tem. I shall therefore follow Dana and distinguish between a central or axial ridge, flanked by an eastern belt extending to the Connecticut, and a western belt extending to the Hudson, though what I shall have to say refers mainly to the central belt and the neighboring portion of the western belt. The Green mountain range is composed of crystalline schists, which our results show to be of Cambrian and Lower Silurian age, resting on pre- Cambrian rocks, and it was long ago shown by Edward Hitchcock to have an anticlinal structure. The Avestern edge of this axial range is, for long stretches, marked by a loft}^ brow of quartzite, and for this reason the mountains present a very steep flank on the west. At the base of this western flank lies what is known as the valley of Vermont or, in Massachu- 6 (iREKN MOUNTAINS IN MASSACHITSETTS. setts, the Berkshire valley. This valley has a floor of crystalline limestone, often a saccharoidal marble of Cambrian and Lower Silurian age, on which stand long island-like ridges of schist, of Lower Sihman age, and it extends with a breadth of several miles from northern Vermont to Alabama. The schist is everywhere underlain by the limestone, which is marked by the fertility of its soil ; and, along its whole length, its wealth of limonite ores has for more than a century formed the basis of important iron indus- tries. Li the folded strata of this valley belt in Vermont and Massachusetts, subsequent erosion has left island-like mountains, sometimes of anticlinal, but generally of svncliual structure, with more or less pitch in their axes. Instances of the latter are Eolus (Dorset), Anthony, Greylock, Everett, etc., rising to 1,500 or 3,000 feet above the valley, and surrounded to a greater or less height above the base by the limestone, and hea^^ly capped with the weather-resisting schist. Instances of anticlinal structure are the less elevated pine hill near Rutland and the ridge which connects it with Danby hill in Vermont. On the west, this limestone valley has for a wall the Taconic moun- tains, with peaks rising 1,500 to 2,500 feet above the valley. This is a synclinal range of the same Lower Silurian schist, but, having its trough at a lower level, the limestone foundation appears only at the base. Turning now to the region east of the axis of the Green mountain anticline, we find no great and continuous depression comparable to that of the valley of Vermont until we reach the Connecticut valley ; and this is occupied by much later strata — Triassic resting on Devonian. This eastern region is a very roughl}' mountainous mass of schist, and, though of plateau origin, is crossed by deeply cut transverse valleys, which receive longi- tudinal tributaries, whose courses are determined in the main by the geologic structure of the territorj^. All along the eastern edge of the axial belt of the mountains there occur such narrow, longitudinal valleys, and as they contain, more or less continuously, beds of limestone of either Cambrian or Lower Silurian age, they define the eastern limit of the Green mountain range proper, with less topographic but with equal geologic sharpness. GENERAL STliUCTURE AND (JORKBLATION. ^ AGE AND STRUCTURE. In Ijeginniug work on the geology of New England, two facts were apparent — that from the Grreen mountains eastward the rocks were all highly metamorphosed and crystalline; and that onlj" in two or three localities had fossils been found, and in these places the rocks were so much disturbed that it seemed hopeless to use them as starting points for the general work. I became convinced that our hopes of determining the age of the New England rocks lay in using the Green mountains as a bridge. In following this plan we were immediately met by the fact that on the main ridge — our proposed bridge — the rocks are not only highly metamorphosed and their structure the reverse of simple, but that the western edge of the ridge marks an abrupt lithologic change between the character of the rocks of the mountain and those of the valley, with the exception of the younger schists, which in places cap both the axial range and the valley hills. On the west the great limestone and an underlying great quartzite come eastward to the base of the mountain, while a careful reconnaissance showed no trace of these rocks as such upon the mountain, nor of such a combination on the eastern side. This difficulty, which met the earlier surveys, had led to various hypotheses in which faults and overturns played an important part. And while* the rocks of this main ridge were assigned by different eminent geologists to ages ranging from the Sillery^ to Huronian and Laurentian,^ the residuum of opinion has been of late in favor of Archean or at least pre-Cambrian age. The problem was undoubtedly too difficult to be solved without more ample means than were at the disposal of our pre- decessors. It was evident that our first and hardest work would be to find the key to the structure of the range. For this purpose I sought a region where the western edge should present, instead of a straight line, as many bay-like curves as possible, and where the structure of the ridge itself should show folds with pitching axes. I hoped in such a region to eliminate the difficul- ' Logan colors them as Sillery on the Geological map of Canada, 1866. " C. H. Hitchcock : geological sections across New Hampshire and Vermont. Bull. Am. Mus. Nat. Hist., vol. I, New York, 1884 8 GREE:sr MOUNTAINS IN MASSACHUSETTS. ties introduced by possible faults, as well as the temptation to infer their existence; and also in case of pitching folds to get, through radiating cross sections, a knowledge of the true order of bedding. These conditions were found well presented in the northwestern corner of Massachusetts. Here the western edg-e of the main ridg-e coraino- down from Vermcfnt makes a sharp turn eastward around Clarksburg mountain; then after resuming for several miles a straight southerlj^ course it curves back westward to bend around the Dalton hills. Opposite this bay stands Greylock mountain, which Emmons and Dana had shown to be a great synclinal mass. The greater and higher part of this Greylock mass of Lower Silurian rocks rises to the east of the chord of the arc that is formed by this bay-like curve. Again, Hoosac mountain, east of this bay, exhibits a variety of distinct rocks in folds, the axes of which show a persistent northerly pitch. And in addition to this I hoped for much aid from the great tunnel, which, in 1865, I had examined for the state of Massachu- setts. With a length of nearly 5 miles, it pierces the mountain through its whole breadth at a depth of over 1,000 feet, and the fact that the tunnel was driven from both ends and from two intermediate shafts gave assurance that the dumps would supply unaltered material for the petrographic study of the various rocks in all their variation of habit. As there Avas then no topographic map of the i-egion we were obliged to locate all of our work by transit survey. During the first two seasons, in company with my assistants, Mr. B..T. Putnam and Mr. J. E. Wolff, I made thorough recon- naissances of the area in question, and, to obtain as much light as possible, these excursions were extended southward to the Highlaiids east of the Hudson and northward to centi-al Vermont. We had found that the mass of Hoosac mountain consists of a core of coarsely crystalline granitoid gneiss, overlain in some places by a conglom- erate, in others by line grained, white gneisses. Above the conglomerate and white gneisses Ave had found a great thickness of biotitic and sericitic schists, containing either macroscopic or microscopic albite, in both un- twinned and simple twinned crj^stals. At all the contacts of this whole series there appeared distinct structural conformability. On Clarksburg mountain Ave had found the same coarse granitoid gneiss, GENERAL STEUCTURE AND CORRELATION. 9 covered, apparently conformably, by a true quartzite. At the base of the Dalton hills the quartzite Avas found to conformably underlie the great Cambro-Silurian limestone, wliich in its turn forms the base of Greylock, and this. limestone was found to Be conformably overlain on Greylock by a great thickness of scliists, identical in character with those overlying* on Hoosac — here the conglomerate and there the white gneiss — with no inter- vening limestone or quartzite. Again, we had found that these white gneisses contained apparently iuterstratified beds of these same schists. CORRELATION. Having made it a rule that all correlation of strata and interpretation of structure should be decided solely upon observed structural relations, there was nothing to be done but patiently to work out the structiu'e, step by step, using lithologic similarities as clews only. The reconnaissances showed that the Green mountains are wholly made up of crystalline schists, and that one or more of the horizons of these must vary in the most jjrotean manner in the external habit of its rocks, while on either side of the range' the rocks retain their respective characteristics with relatively little change. One of the earlier observa- tions on the western brow of Hoosac mountain had been the superposition of the coarse granitoid gneiss over the white g-neiss at a well-marked con- tact and with structural conformity of lamination. On the other hand, in the tunnel, this same granitoid gneiss appeared as a central core, fiirtiier east than the geologic meridian of the surface outcrop. This core Avas found in the tunneP to be flanked on each side by the conglomerate over- lain by the albitic schist. If the structure were as simple as the tunnel section seemed to indicate it would point to two horizons of the granitoid gneiss, and connect this rock and the white gneiss in age. An important ' Diuui pointed out iu 1872 tlie abrupt lateral transitions between the quartzite and schists of Berkshire county. (Ami. Jour. .Sci., 1872, p. 368.) 2 This tunnel is lined with masoury at irregular intervals to such an extent that a large part of the rock, especially of the more interesting western half, is hidden. The walls are covered to a depth of an inch with soot. In addition to this, geologic work was made extremely dangerous by the fact that the smoke was so dense that even our thirteen torches were invisible across the tunnel, and the noise of trains running 30 miles an hour was not audible until the engine was within a few yards from us. Notwithstanding these difficulties we managed to find the important contacts, except at the ■western end, where they were bricked over. 10 GREEN MOUNTAINS IN MASSAOHUSETTS. point was therefore gained when the hypothesis advanced to us by Mr. Putnam that the surface exposure of granitoid gneiss was a flat, overturned anticlinal fold was corroborated by Mr. Wolff. Mr. Wolff also discovered that the schist beds in the white g^neiss on the western flank belong to the series above the wliite gneiss, and are simply remnants left in compressed troughs overturned to the west under the overtmnied anticline just men- tioned. The next step was made by Mr. Wolff in the determination that the white g-neisses are clastic rocks, while the coarse granitoid gneiss shows no trace of clastic origin. This pointed to a closer relation between the white gneiss and the conglomerate, from the fact that one or the other was found to overlie the granitoid gneiss. This question also was settled by Mr. Wolff by tracing out the lateral transition from the conglomerate into the white gneiss. Finallv the upward transition from the white gneiss and from the con- glomerate into the schist was observed. Messrs. Putnam and Wolff had observed, and I had traced later at several points on Clarksburg mountain, a strict conformability between the lamina- tion of the granitoid gneiss and that of the overlying conglomerate and quartzite, the continuation of the great quartzite belt of Vermont ; and later Mr. Walcott had found, near the same contact, numerous casts of OlcneUus, showing the lower part of the quartzite to be of Low^er Cambrian age. Later, Mr. Wolff, in tracing this quartzite northward along the eastern flank of the granitoid gneiss of Clarksliurg mountain, found it to pass by lateral transi- tion along the strike into well-defined white gneisses like those of Hoosac mountain. Later still a similar transition was observed between the true quartzite and the Hoosac white gneiss on the northern side of the Dalton hills. There still remained to be explained the nature of the relation between the granitoid gneiss and the overlying clastic rocks, and the conformability that exists between the stnicture of the granitoid and that of the overljang rocks. Prof. Emerson, working on the map in Hinsdale, found an area of granitoid gneiss overlain by the conglomerate, and concluded, from the re- lation of the two rocks over broad areas, that they are there structurally U.S.GEOLOGrCAL SURVEY: MONOGRAPH XXII PL II- SHOWING THE RELATION OF THE GRFiXOCK SERIES TO THE HOOSAC MOUNTMN ROCKS. BY RAPHAEL PUMPELLY. Scale, 125000 =iMiles 1891. GENERAL STRUCTURE AND CORRELATION. 11 unconformable. At about the same time Mr. Wolff had found the two dikes of eruptive basic rock in Stamford in the g-ranitoid gneiss and at its contact with the quartzite. (Figs. 1 and 2.) J? 'i N A Fio. 1. — The Stamford dike, showing the Cambrian conglomerate deposited in dike fissure; (7, conj;l"iiifriite; c. l(^^ve^ layers of conglomerate rendered schistose by admixture of material from the altrre COREELATIOK 13 been excavated as the well-kuown Berkshire sand. About 100 feet east of this we find an outcrop of vitreous qiuirtzite. The next outcrops — okler and 300 or 400 feet eastward and dipping 50° easterly — show a schistose quartzite overlain by an older and more slaty bed of the same; and this by a very coarsely feldspathic quartzite followed by another bed of schistose quartzite, and this by a feldspathic biotite-schist. Representing these in their normal succession we have — Stockbridge liiuestoue. Flaggy quartz-schists. Schistose calcareous quartzite. , Saudy (juartzite (friable). Vitreous quartzite. Covered (300 or 400 feet). Schistose quartzite. Schistose quartzite, more slaty. Very feldspathic quartzite. Schistose quartzite. Feldspathic biotite-schist. HoosacMt. GrvjlockMt. Mclloyvspipc 2jtm£S6oTW , Sei^k.?}Ltre Sch LsL f^rtrr-/;,-!--: Howe 'ScfUst. Ifoosfxc Schist, Quiu-tzUeConfflomerate., Sfajri/oT'cL Gneuf^. Fig. 3.— Correlated columns of the Hooa 'C and Greylock rocks. We now had both the Hoosac and Greylock columns complete, and both springing from the same conformably underlying Cambrian quartzite (see Fig. 3). A glance shows one point of difference — the entire absence of limestone in the Hoosac column. But on the other hand, we have the observed con- 14 GREEN MOUNTAINS IN MASSACHUSETTS. tiuuity of deposition from the quartzite upwai'd in each column, and we have also petrographic identity in the schists of the two columns. Prof Emmons attempted to explain the similarity of the Greylock schists to those of Hoosac mountain by deriving the supposedly younger Greylock beds from the destruction of the supposedly older Hoosac rocks, but Mr. Wolff finds, under the microscope, not only no evidence to sup- port the idea of such derivation for the Greylock schists, but that the principal constituent minerals of these schists were in each column all crystallized in place. Early in the course of the work it was proved that the limestone was not present as such in the Hoosac column. But at two points near Cheshire harbor, and east of North Adams, we found schist outliers. extending out from the Hoosac column, and at the extreme western ends conformably related to the great limestone; in one case occupying a ' synclinal trough in it, and in the other either capping it or interbedded in it. Almost at the beginning of the survey, altliough we had as yet none of the proofs above given as to the equivalence of the valley quartzite with the Hoosac conglomerate and white gneiss, the strong possibility that at least a part of the Greylock column was contemporaneous with a part of the Hoosac column had presented itself to me. This possibility was strength- ened when we had correlated the quartzite with the white gneiss and con- glomerate beds as equivalents. The truth of this hypothesis could be tested only by finding beds showing lateral transition to bridge the narrow belt between the Stockbridge limestone and the Hoosac schist. In the progress of om- survey we found, at various points between the valley and the mountain, and always east of the limestone, outcrops of a peculiar rotten schist — quartz and mica with some feldspar, with the mica arranged in long narr(jw flakes and with sufficient calcite to show the cause of the decomposition. The occurrence of this peculiar calcareous rock along the boundary between limestone and quartzite, as on Tophet creek and below the albitic schist in the western end of the tunnel, shows that it belongs in the horizon of the vertical transition between the quartzite and the limestone, and it seems to represent also the lateral transition zone in this horizon Ijetween the Hoosac and Greylock columns. East of North Adams, on the road to Briggsville, the river cuts longitu- in JZ to > 1*3 ■OC a to ca ►J «> (1 ■t) y g 5 M'l'. ' ^ 1 ^'VV' J3 SYiiliii'iIi^l i J ^§ Wi'"",' * li/'"i,'i 1-^ .\^ ^-\ H' nil J-ijA GENERAL STEUGTURE AND CORRELATION. 15 diually through an anticline ; a few huu tired feet west of the river there is a massive anticline of marble exposed in large quarries ; the eastern end dips toward the river, but a sharp anticlinal fold, slightly overturned to the west, brings the strata, up near the west bank of the stream, in interstrati- lied beds of limestone and schist. The arcli springs over the river, and its easterly dipping limlj forms a high cliff on the eastern bank. In this eastern limb the limestone is represented by calcareous siliceous-micaceous schists and very impure limestones. The whole arch is exposed near by, in a cliff in the bend of the river (see Fig. 4). This is the most eastern exposure of limestone, and there can be no doubt that we are here in the zone of lateral transition between the condi- tions that produced in the same horizon the Stockbridge limestone and part of the Hoosac schist. Again, along the north base of the Dalton hills, in Fig. 4 — Amiclinal arch across Hoosic river between North Adams and Briggsville, ill the zoiie of lateral transition hetween Stockbridge limestone and Uoosac scliist: a, limestone more or less micaceous and siliceous; 6, calcareous and siliceous sithist witll thin layers of limestone; an, interstratified siliceous and micaceous Jimestonc, calcareous quartzite and mica-schist; 66. less calcareous garnetiferous schist. Cheshire, Mr. Wolff found a schist consisting of calcite, mica, quartz and simple twinned albite, which, from its position and nature, vmdoubtedly represents this zone of lateral transition from limestone to schist. If the reader will turn to Plate ii he will see that the Stockbridge limestone sends a broad rectangular bay southeast in Cheshire to conform to the embayed topography of the Dalton- Windsor hills. In the middle of this embayment he will observe a detached area of Berkshire schist of an irregular sha[)e, suggesting a long-eared rabbit. There is no question as to the continuity of the schist over the area as represented. The long rabbit- ear-like area lies upon the limestone in a synclinal trough. The structure of this area is not simple ; it is that of a small synclinorium, the axes of the north-south running folds pitching toward the center, and the folds at the northern end being more or less overturned to the west in conformity with 16 GREEN MOUNTAINS IN MASSACHUSETTS. the general overfolding of Hoosac mountaiu and the Daltou-Wiudsor hills. Tlie limestone proper borders the whole western side of the area and ex- tends well into the bay east of Cheshire. On the east side it also extends visibly down from the north for some distance, but it then disapjjears under a heavily drift-covered area. Going south from the limestone on this east side, the first exposures we find belong to a continuous belt of the schists connecting the Cheshire schist area with the tongue of schist infolded in the Cambrian white gneiss farther east at the base of Hoosac mountain. There is neither any trace of the limestone nor any room for it. On the south of the Cheshire schist area the Cambrian quartzite covers the Dalton- Windsor hills, the topography of which is formed by the undu- lations and intervening sharp folds of this hard mantle. The dip of the undulating quartzite beds and the pitch of their sharp folds are both toward the center of the Cheshire schist synclinorium. The Cheshire schist hills are separated from the higher Dalton- Windsor quartzite hills by a narrow valley, which curves around the southern end of the former with few exposures. But at one point quartzite and schist are very near together, and it is evident that there is no room: for the limestone as such. In this valley there are large numbers of gr.eat angular blocks and at least one ledge belonging to a transitional scldst formation. I repeat here L)r. Wolff's description of this important rock: It resembles a micaceous white limestoue fiUed witli little dark grains or imper- fect crystals of feldspar. Under the miscroscope, in thin section, it is composed of a mass of calcite grains, with here and there single grains of quartz, or an aggregate of several grains, plates of muscovite and often of chlorite and biotite, and large por- phyritic feldspar grains in single crystals or simple twins, very rarely showing poly- synthetic twinning. These feldspars contain inclusions of mica, quartz, iron ore, rutile, and calcite. and are in every way identical with the albites of the albitic schists, although the exact species of plagioclase has not been determined. The calcite seems to play the part which the quartz does in the schists: it sends tongues into the feldspars or cuts them in two, and gives one the impression by its inclusions in the feldspar, and its occurrence with the quartz and mica, that it is of contempora- neous origin with the feldspar, mica, and quartz. This schist represents the landward transition of the Stockbridge lime- stone into the Hoosac albitic schist. Thus the Cheshire schist area is at its GENERAL STRUCTURE AND CORRELATION. 17 Fig. 5. — Ideal section east of Cheshire, showing lateral transition of limestone to Hoosac schist; S, Berkshire schist; i, Stockbridge limestone: ^, Lower Cambrian qnartzite of Dalton- Windsor liills; Ci/, cal- careous quartzite: transition quartzite to limestone; OS, calcareous feldspathic scliist in lateral transition from Stockbridge limestone to Hoosac schist. northern end simply the Berkshire schist resting upon the Stockbridge limestone, while as we go southward we find it representing not <^nly the Berkshire schist, but also the whole thickness of the limestone itself, and as we go eastward we find through continuous exposures its connection and identity with the tongues of schist infolded in the Cambrian quartzite gneiss of Hoosac mountain. In Fig. 5 I have attempted to represent, in a somewhat ideal section, the transition from limestone to schist at the south end of the Cheshire hill. The transition is clearly quite abrupt, and might easily occur within the space represented by the eroded folded arch between the limestone and the infolded schist along the west base of Hoosac mountain. See c, PI. iii. The western end of the Hoosac tun- nel lies in the belt of this lateral transi- tion of the Stockbridge limestone into the Hoosac schists; but it is now completely hidden by the brick arching ren- dered necessary by the decomposed condition of the material. Indeed, it acted for several hundred yards from the portal as a quicksand, and the tunneling work had to be preceded by small tunnels incased in closely matched planks, so fluid was the decomposed water- saturated rock. I have attempted to represent the structural facts at this point on the west flank of Hoosac mountain in D, PI. iii. At the time of my examination of the tunnel, in 186.5, the limestone was exposed in open cuts and tunnels — nearly parallel to the present open cut — ^for nearly 700 feet east and west. The exposure showed in this dis- tance two rather flat anticlines. The eastern limb of the easternmost anti- cline dipped east and was for a short distance concealed by masonry. East of this was an open cut, for nearly 400 feet, in the decomposed rotten schist, which seemed to show faintly preserved indications of an easterly dip. Just east of the middle of the cut a less altered bed showed a well-defined syncline with an anticline on the east and having the eastern limb of the latter exposed in the heading with easterly dipping structure. MON XXIIT- 18 GREEN MOUNTAINS IN MASSACHUSETTS. From the above description it will be seen that the actual nature of the relation of the limestone to the rotten schist was hidden. But just west of where the contact should be I found the limestone conformably overlain by a few feet of Hoosac schist. Farther east is a small shaft, from which was hoisted some of the rock excavated between the western headings of the "west" shaft and the open cut; this rock is a more or less rotten calca- reous feldspathic mica-schist, having the same elongated structure parallel to the axes of the folds as in the rotten transition schists of this zone, and marked by the same similarly an-anged long, narrow flakes of mica. It recalls in structure, also, at once, the calcareous gneiss associated with the limestone on its eastern border near South Adams, and also the noncalca- reous and rather less feldspathic mica-schist of the "Buttress" core. I think that, taken in coimection with the facts observed south and east of Cheshire hill, we have in this rock the upward transition from the quartzite to the limestone brought to the tunnel line in an anticlinal arch, and that we have, in the wholly decomposed material of the former open cut, the lateral transition from the rest of the limestone into the Hoosac schist. A few hundred feet, from east to west, would span the whole lateral passage from limestone to Hoosac schist. This transitional calcareous schist decom- poses much more easily than the limestone and is therefore more rarely seen. Nevertheless, as stated above, it is found exactly where it should occur as such a transitional form, not only in the western end of the great tunnel, but at several points along the western base of Hoosac mountain above the quartzite and west of the infolded schists. While the rocks of the zone of lateral transition, in the horizon of ver- tical transition from quartzite to limestone, were tolerably hard, they suc- cumbed to disintegrating agents much quicker than the quartzite proper. But the rocks of the zone of lateral transition between the limestone and Hoosac schist, being calcareous schists, were adapted to the most rapid destruction, and we therefore find them only where the conditions for their preservation have been exceptional. From Cheshire hill northward this zone covered anticlinal folds turned over to the west, which have been to a great extent eroded down to the harder beds towards the true quartzite. It does not seem improbable that GENERAL STEUCTUEE AND COEEELATION. 19 the zone of lateral transition of limestone to Hoosac schist was a zone of weakness which had ranch to do with the overfolding along the west base of Hoosac mountain. These anticlinal axes are inclined gently to the north. About a mile south of the tunnel, at the "Buttress," the core of one is visible as a hard, white gneiss, but at the tunnel line it has sunken to where the erosion surface cuts the beds representing the lateral transition of limestone to schist, where they mantle around the pitching anticline, and before they disappear under the younger schist beds which stretch out from the mountain. While the equivalence of the Greylock column with a large part of the Hoosac column can be thus asserted, I am not yet in a position to make a correlation reaching into details. It is not possible with our present data to subdivide the Hoosac column into equivalents of the two schists and two limestone horizons of Greylock. There is, indeed, in the eastei'u half of the Hoosac mountains a rather sharply defined plane of division, separating the feldspathic schist on the west from the practicall)^ nonfeldspathic schists on the east, and these latter are distinguished further by the fact that their quartz is distributed in thin, even layers, instead of occupying lenses, as in the rocks to the west. This plane is used by Prof Emerson as the base of his lower hydromica-schist, and forms an impoi-tant horizon of reference in his work east of the mountain. The thickness of the albitic schists between this plane and the conglomerate has not yet been determined, as the structure is masked by the cleavage. It is certainly not more than 5,600 feet, and probably not less than 2,500 feet. If there are no faults or foldings, it is probably about 4,000 feet. We are equally ignorant of the real thickness of the Greylock beds, after allowing for the effect of lateral pi'essure and increasing local thickness. But it is quite possible, if not probable, that these nonfeldspathic schists belong wholly above the Greylock rocks. In the study of Greylock mountain Mr. Dale, by patient search for the traces of the original stratification, which have here and there escaped the general obliteration caused by cleavage, has been able to work out the details of surface structure quite closely, and to obtain a general idea at least as to the maximum thickness of the two limestones and two schists. But the compressed foldings have so altered the thickness of the 20 GREEN MOUNTAINS IN MASSACHUSETTS. strata that it is impossible to give the real vertical dimensions. His estimate is: Greylock schist 1, 500-2, 000 Bellowspipe Umestoue 600- 700 Berkshire schist 1, 000-2, 000 Stockbridge limestone 1, 200-1, 400 These are, however, based on measurements of beds that have been subjected to strong lateral compression, and, as Mr. Dale observes, although the aggregate maximum of the thickness given above is below that assigned to the Lower Silurian in the Appalachian region, it is probably far in excess of the real thickness, which maybe considerably below the maximum above given. ^ The sediments which in vast thickness form the substance of the Grreen mO'Untain system have been subjected to intense lateral thrust, which has produced numerous folds. These, as a rule, are more or less compressed and overturned to the west, in places indeed forced over until the axial plane lies almost horizontally, or compensations have taken place through overfaulting. The sections and map of the Hoosac-Grreylock ret,ion illus- trate the structure in its generality. From these it will be seen that on Hoosac mountain the granitoid gneiss and the overlying conglomerate gneiss-quartzite and albitic schists have been folded into a low anticlinal arch, the western side of which has been forced over to form an overfold to the west. An examination of the longitudinal sections on Plate vi accompanying Part 11 (Mr. Wolff's report) shows that the southern end of this arch is over- folded in the same manner, but to the south. We have thus the remarkable occurrence of an overturned anticline abruptly turning a right angle. A glance at the map (Plate ii) will show that this is repeated by the next over- folded anticline to the west, which bends equally abruptly around to run eastward, and that the inverted trough between these anticlines is still marked by the infolded band of schist. Groing from this southward, Ave come immediately upon another east and west trough of schist, also over- turned to the south. Still further southwest, we find along the northern part of the Dalton- Windsor hills the quartzite gneiss beds thrown into GENERAL STEUOrUEE AND COREELATION. 21 overfolds, but witli the axes striking northwest to southeast ; while still farther westward they are overfolded to the west, but with the axes in the normal position of the Grreen mountain folds — nearly north and south. Looking at the map and sections of Grreylock, Pis. i, xviii, xxiii, we find a great basin-bottomed mass, thrown into numerous more or less overturned folds, with axes in the normal Green mountain position, and inclined from each end toward the middle. Again, if we look at the eastern border of the map, we find in the observed strikes and dips of the conglomerate gneiss and schist east of the granitoid, no trace of a departure from the general Green mountain direction. This local modification in the structure of Hoosac mountain must be due to some local cause, which I think must be sought in the pre-Cambrian topography. The Greylock basin of sediment was guarded on the north by the large mass of granitoid gneiss of Clarksburg mountain, and on the south by the great body of pre-Cambrian rocks which are now masked by the Dalton and Windsor quartzite. I imagine that the lateral thrust to which the foldings are due 'met with greater resistance opposite these mtire rigid granitic masses than in the interval, and that the abnormal overfoldings to the south, described above, are the result of compensatory movement. The Hoosac mountain cross sections show a much more marked overturn than is observed to either the east or west of it. The axial plane of the principal overturned fold on the west side of the mountain lies very flat. We may suppose the greater rigidity of the granitoid gneiss ' to have caused it to yield as a unit to the contracting force. Only its relatively nari'ow top participated in the actual folding and was carried over to form, with the leeward, protected beds, a flat-lying, compressed syncline. A similar overturn, though not so flat, was observed by us on Sumner mountain, in Pownal, on the west of the Clarksburg mass of granitoid gneiss. Section a on Plate iii was made by Mr. B. T. Putnam. - I have added my interjjretation in dotted lines. This outlier is separated from Clarksburg mountain by Broad brook, this interval being occupied by the quartzite. The large Clarksburg mass of granitoid gneiss remained a dome mantled by the Cambrian quartzite, and showing the effect of the folding force only in the induced lamination common to itself and the 22 • GREEN MOUNTAINS IN MASSACHUSETTS. quartzite, while in the smaller mountain to the west, which has a grani- toid gneiss core, this core is pushed up in the form of an overturned anti- cline upon which the quartzite lies, in normal position on the east, while on the west the granitoid is underlain, in inverted order, by the quartzite and the limestone. A careful study of the western flank of Hoosac mountain shows that its structure is not that of a simple, great, overtm-ned fold. It consists of a series of parallel, crumpled folds, one or more of which have a greater depth than the others. All of them are overfolded, with their axial planes dip- ping eastward and with their axes pitching about 10° northward. The average chord-plane of these folds dips westward 15° to 20°, forming thus, as a whole, a comparatively flat, though much crumpled, western limb of Fig. 6.— Diagram of structiu'e, summit of the Buttress, ou west flank of Hoosac mountain, about one mile south of Hoosac tunnel, a, Buttress rock, upper part .» of Cambriau white gneiss ; b, Hoosac schist. The exposure at the east end is part of the long trough infolded along the whole front of Hoosac mountain. the main Green mountain anticlinal arch. This structure is shown in nu- merous preserved fold-cores, and is illustrated in the section through the "Buttress" (Plate in, c) and in the annexed diagram of the summit of the same hill (Fig. 6). The " Buttress"— a high hill on the flank of the mountain about one mile south of the tunnel— is the southerly extension of one of the larger of these crumples, where the axis in rising to the south brings up the harder core of Cambrian white gneiss. The structure is marked both by the preserved fold-core at a, just west of the summit (Fig. 6), and by the small infolded troughs of younger schist at b on the summit and h on the western flank. Further north, as at the tunnel line, where nearly the whole flank of the mountain is covered by the schist, the crumpling is much greater, as one would expect in this material, and is marked by the crumpled layers of quartz (Fig. 7). " Toward the south end of the mountain, near where the GENERAL STEUCTUEE AND COEEELATION. 23 great schist trough is seen on the map to turn sharply to the east, the evidence of this same structure is preserved in several minor iufoldings of schist. In the tunnel the rotten rock of the old open cut, and that which I have described as the Buttress-core rock and as forming below it the upward transition from quartzite horizon to limestone horizon, are con- cealed by masonry. But from a point several hundred feet west of the "west" shaft we find the Hoosac albitic schist, which extends some 1,400 or 1,500 feet fui-ther east till we reach its contact with the underlying con- glomerate-white-gneiss (See PI. Ill, d). This last-mentioned rock extends some 2,000 feet farther east to its contact with the pre-Cambrian coarse crystalline gneiss of the Hoosac core. On both its eastern and western sides w Fig. 7 Crumpled structure in the Hoosac schist above the " west shaft" on Hoosac mountain, a, cleavage foliation ; b, stratification lines marked by crumpled quartz layers. the contact planes show that the Cambrian white gneiss is overturned in a flat-lying anticline. Leaving, now, the tunnel and climbing to the opening of the "west" shaft on the flank of the mountain we find that the upper part of the shaft is in the Buttress-core rock — quartzite-limestoue transi- tion rock — and that the same formation crops out upon the mountain until we reach the Hoosac schists, several hundred feet higher up. Climbing above this point we find the Hoosac schists, with evidence that they occupy an inverted syncline. Fig. 7 shows the structure at this point on a small scale. Above this the dips observed on both sides of the summit show that the crest is a simple open syncline. The presence of the Buttress-core rock at the top of the "west" shaft and its projection so far westward Qver the Hoosac schist of the tunnel 24 GREEN MOUNTAINS IN MASSACHUSETTS. below can be explained only by introducing an overthi'ust fault or by sup- posing that the inverted anticline was pushed out thus far without rupture. The former explanation seems the more likely one and accords better with the thickness of the Cambrian gneiss and the dips in the schist observed in the tunnel. The bed of white gneiss— 600 to 800 feet thick— when ex- posed to the great thrust which overfolded the Hoosac rocks, would, it seems, be less able to adapt itself by minor foldings than the more readily yielding schist, and would be more likely to find its compensation in a rupture and an overthrust fault. At the tunnel line the axes of the folds are still pitching to the north. Immediately north of the limestone is a mass of folded Hoosac schist, under which the limestone is earned by the pitch of its folds and which is seen at several points to be younger than the limestone. The zone of lateral tran- sition is also Qan-ied under this hill, and this fact explains the peculiar areal geology of this part of the map (PI. i) on which the color for the Stock- bridge limestone extends along the west side of the schist hill, that for the Vermont formation along the east side. The obscurity disappears on PI. ii, where I have separated these transitional rocks from the quartzite and given to them and to the lower part of the limestone a separate representation as Cambrian. It is not easy to determine the extent to which overthrust faulting has entered into the building of the Grreen mountain range in northwestern Mas- sachusetts. Along the eastern side of the pre-Cambrian core of Hoosac mountain the movement flattened the coarse pebbles of the conglomerate and granulated their quartz and large feldspars to the point of obliteration. But the great Cambrian conglomerate-gneiss bed as it curves around the core shows no break due to faulting. It is not until we come to the west side of the pre-Cambrian gneiss-core that we find evidence of a rupture in the flat fold, where the hard Cambrian white gneiss has been pushed along an overthrust fault ont(i the younger schists as far as the west shaft. Here it seems, probable that the rupture was favored by the fact that the troughs of the folds, both above and below the middle limb, were on the lee side of the less yielding pre-Cambrian core, as will be seen from the section (PI III, u). Now this is the same fold that incloses the trough of schist all GENERAL STEUCTUKE AND COEEELATION. 25 along the west side and south end of the mountain, and it is not impossible that it may be accompanied there, as here, by the same ruptvire. If this is so, then the position of this overthrust plane would lie above and to the east of the schist trough shown on the Buttress (PI. in, c, d). Having sketched thus briefly the general relation of the crystalline schists of the main ridge of the Green mountains to the fossiliferous rocks lying to the west, let us now return to the main ridge. We have seen that the Cambrian white gneiss rests with a time break on the coarse granitoid gneiss In places on Clarksburg mountain we find the micaceous quartzite more or less conglomeratic at the base, resting on the gi'anitoid gneiss, the two rocks sharply distinct. In others, as on Hoosac mountain, a conglomerate rests on the granitoid gneiss with sharp definition. But this simplicity is not always present, especially at the meeting of the white and granitoid gneisses. In general there intervenes between the well-defined coarse gneiss and the well-marked white gneiss a zone of beds of luore or less coarse gneiss, often alternating with finer grained biotite schists. It is not easy in such places to draw the line between the Cam- brian and pre-Cambrian formations, though, as I will show further on, in some instances there is good reason to draw the line at the base of the transitional beds where these show alternating strata of varying character. One thing appears certain : the dynamic action which has folded these rocks has impressed upon them not only their cleavage and plication, but also the I'emarkable simulation of conformity in bedding and of vertical transition. The pre-Cambriau core of the Grreen mountains reappears at frequent points along the range. In places it forms almost island-like masses of old, hard gneisses surrounded by the Cambrian quartzites and allied rocks, as in the northwestern corner of Connecticut. In others, as on Hoosac and Clarksburg mountains, it appears as limited, oval, dome-like areas of granitoid gneiss. Again, as in Chittenden, Vermont, it consists of a long, narrow line of coarse gneiss, at eroded points in the backbone of the range. Finally, as between Clarendon and Ludlow, in Vermont, where the height of the range has been cut down by the removal of the younger rocks, the core of the folded range shows itself in a variety of old granitic and gneissoid rocks, cut by intrusives and with extremely irregular structure 26 GEEEN MOUNTAINS IN MASSACHUSETTS. We have done but little work towards the study of this old core. A valuable clew was found by Prof. Emerson in what he considers to be a threefold division of the pre-Cambrian iu southern Berkshire, where, according to his observations, chondroditic limestone separates a coarse, blue quartz gneiss — possibly the Stamford granitoid — from a still older gneiss. The massive granitoid gneiss which forms the core on Hoosac and Clarksburg mountains is in places separated from the overlying Cambrian quartzite gneiss series by beds of coarse, light-colored gneisses, which have interbedded layers of finer grain and darker color from the greater propor- tion of biotite. These "transitional coarse gneisses" between the granitoid and white gneisses are probably, to a great extent at least, Cambrian. They are detrital, containing pebbles in places, as in the tunnel. Their coarse feldspar is identical with that of the granitoid gneiss, except that in this transitional zone it is white, while in the granitoid it is reddish. While the granitoid gneiss is preeminently a massive rock, this "transitional" zone is bedded and contains micaceous layers. On the east side of the granitoid area on the surface of Hoosac mountain it occupies the place of the quartz- ite-white-gneiss-conglomerate and is overlaid conformably (as seen at the contact) by the albite-schists. The granitoid gneiss was probably much disintegrated at the time of the Cambrian transgression, and in the differ- ent conditions of character of disintegrated material and of breaching and sedimentation lies, perhaps, to a considerable extent, the explanation of the fact that this horizon is here quartzite and there gneiss, and presents itself under a great variety of aspects, due to alternating layers with vary- ing proportions of quartz, feldspar, and mica. But in the field it is often very difficult to distinguish, in the absence of true pebbles and of alter- nating sediments, between the redeposited detritus of disintegration, which has been subjected to the action of chemical and dynamic metamorphism, on the one hand, and beds which, simulating these, have been produced by the action of these same metamorphic agencies directly upon the older gneisses, granites, or basic eruptives. I imagine that the Cambrian transgression found an Archean elevation forming the western border of an Archean dry region. To the west of this GENERAL STKUCTURE AND CORllELATION. 27 lay the great Paleozoic ocean of America. I imagine, also, that the rocks of this dry area had become disintegrated to a greater or less depth and that the products of this action varied from kaolin and quartz at the surface to semikaolinized material with fresh cores at depths. The depth of this action would vary according to varying lithologic and topographic condi- tions, as I have shown elsewhere.' While the abrasion of the deeply disintegrated rock was progressing along the advancing beach line the detritus of sand and pebbles arising from this disintegrated material was deposited with varying proportions of its constituents in a continuous sheet in progressive "transgression" over the previously -dry land;^ for I think the evidence offered by the erosion of the Stamford dike is sufficient to show that the region owed its absence of older sediments to its having been an area of dry land instead of an "abyssal" area. During the progress of this removal and deposition of ready-prepared material there would be places where the underlying unaltered rock would be washed clean and re-covered with sand and gravel. There would be others where the material removed from the disintegrated mass would be derived from the zone of semikaolinized fragmentary disintegration, and places where this material would be deposited without having been much rolled and in beds alternating with finer material. And again there would be places where the disintegration was deeper — in basins as it were — and where this material escaped removal and was covered by the sedimentary beds. The recognition of these premises would, it seems to me, aid in the explanation of many of the difficult points observed in the field. Take, for instance, the schistose lamination of the Stamford gneiss on Clarksburg mountain, where this structure is most highly marked near the contact with the overlying quartzite. The lamination is parallel in both rocks. The quartzite here bends around the mountain and is highly crinkled, this structure being defined by the micaceous constituent, and for some distance ' Secular rock disintegration, etc. Am. Jour. Sci., vol. 17, 1879, pp. 133-144. Also the applica- tion and extension of the ideas advanced in that paper. F. von Richthofen: China, vol. 2, p. 758. '^ F. von Richthofen has called attention to the fact that toe little importance has been attached by geologists as a rule to the breaching and abrading action of the ocean when the beach line is advancing landward. China, vol. 2, p. 768. 28 GEEEN MOUNTAINS IN MASSACHUSETTS. inward the same structm-e is similarly defined in the granitoid gneiss and is perfectly conformable in the two rocks, although we have here, in the conglomeratic character of the base of the quartzite and in the pre-Cambrian erosion of the Stamford dike, evidence of a time-break. If we imagine the granitoid gneiss to have been deeply disintegrated and to have been abraded only to the semidisintegrated zone, or even to the lower zone in which only the integrity of the micaceous element had been attacked, then the material of this zone would have presented itself to the force that produced the crinkling and lamination in much the same physical condition as the sand and pebbles of the quai-tzite. Again, take the coarse gneisses with blue quartz which occur at many points along the core. Mr. Wolff finds them to contain the same feldspar with the same inclusions as that of the granitoid gneiss, except that they are light colored, while those of the granitoid are reddisli, and thev have fre- quently the same blue quartz. But they are bedded and have alternating layers of finer schists, and a2)2jear as transitions conformable to the under- lying granitoid and overlying white gneiss or other equivalents of the Cam- brian quartzite. The granitoid gneiss consists of large crystals of feldspar — perhaps averaging one by three-quarters by one-third inch in size — and flattened lenses of blue quartz and thin, irregular layers of mica. I imagine that these materials, taken from the zone of semidisintegration and quickly deposited, would, in their new arrangement, produce our "transitional coarse gneisses," while the material of the upper zone of complete decay would furnish the sand and clay for the quartzite and finer sediments. If this reasoning be correct, we should in many instances include in the Cambrian quartzite series the coarse, more thinly bedded gneisses, with then- iuterbedded, finer grained schists. But in the present state of our knowledge of the Grreen mountains the granitoid gneiss appears to be only one of the constituents of the old core, and perhaps a subordinate one. From our recent work in Vermont it seems that the pre-Cambrian area will be found to contain a variety of granites, gneisses, and schists, as well as basic rocks, which will need to be studied in connection with the rocks of both the New York highlands and the Adirondacks. It therefore remains to be discovered whether the old core contains any rocks of the periods be- GENERAL STRUCTURE AND CORRELATION. 29 tween the Archean (Laurentiau) and Cambrian. Thus fai* only some ob- servations that will serve as clews have been made in this direction/ One apparently negative piece of evidence may be seen at the place where the Archean rocks of the New York highlands suddenly end near Poughquag, Dutchess county, New York. Here the highlands end in a promontory of nearly vertical beds of old gneisses, against which the Cambrian qtiartz- ite lies with a very flat dip. Toward the correlation of the Green mountain rocks with the fossilif- erous strata of New York, the paleontologists have given us some facts. Mr., Walcott's discovery of Olenellus casts in the quartzite of Clarksburg mountain, about 100 feet above its base, caused him to assign that rock to the Lower Cambrian. The many findings of Lower Silurian fossils in the limestone of Vermont have shown that limestone to include Calciferous, Chazy, and Trenton horizons, and it is inferred that, since the limestone is Trenton and is capped by schists, the latter are of the age of the Utica and Hudson River slates. I have shown above that the white gneisses and conglomerates of Hoosac mountain are the equivalents of the Cambrian quai'tzite and that the albitic schists of Hoosac mountain represent in time both the limestone and schists of the valley, and therefore range from the Cambrian into or through the Hudson River. It seems probable that the limestone must reach down well into the Cambrian and that all of the Cambrian that is not represented by the quartzite must, in the valley, be included in the lower part of the limestone and its downward transition beds;^ while on the mountain it must be in- cluded in the lower beds of the albite schists. We have yet to discover whether the nonfeldspathic schist of the eastern portal of the tunnel (Rowe schist) represents Hudson River, or, Ijerhaps, Medina time. Geologically above the nonfeldspathic schists of the eastern portal, and coming in successively to the east to build up the old plateau region that forms properly the eastern belt of the Green moun- ' Since this was written we have fonnd Algonkiau schists at several points along tlie Green moun- tains. - This has been confirmed by recent discoveries of Cambrian fossils in the lower part of tlie lime- stone near Rutland and Clarendon, Vermont, by Messrs. Foerste, Wolft', and Dale. 30 GREEN MOUNTAINS IN MASSACHUSETTS. tain system as far as the Connecticut valley, there is a series of schists having a great aggregate thickness. Prof Emerson, to whose report the reader is referred for the descriptions and for the views of our predecessors, has been able to divide these schists into several distinct formations with persistently defined characters and boundaries. Above the nonfeldspathic Rowe schists comes a horizon of hornblende schist (Chester amphibolite) often with serpentine, varying from a feather edge to 3,000 feet in thick- ness, overlain by over 9,000 feet of "upper hydromica-schist" (Plainfield schist). This in turn is overlain by the "Calciferous mica-schist" of the Vermont survey (Conway schist), which obtained its former name from the presence of occasional large lenses of more or less biotitic limestone, which latter has beds of hornblende-feldspar-schist in places along its bottom and top. Above this again is the heavy bed of Leyden argillite, with inter- calated quartz-schist. Next above and unconformably supeq^osed are the representatives of the Devonian. The age of these different fonnations still remains uncertain, though at least the Leyden argillite and the Conway schist ("Calciferous mica-schist") are supposed by Prof Emerson to belong to the Upper Silurian. While the Green mountain system includes the whole region between the Connecticut and the Hudson, its characteristic features consist, as we have seen, of the central anticlinal ridge of the Green mountains proper on the east, the synclinal range of the Taconic mountains on the west, and a succession of high, synclinal, island-like masses rising from the intermedi- ate valley. The results of the survey in northwestern Massachusetts lead to the supposition that the central or main ridge was in pre-Cambrian time outlined as a mountain range of highly crystalline rocks on the western border of an area of dry land. During long exposure to the action of atmospheric agencies and of the products of vegetable decay, the rocks of this region had become decomposed at the surface and disintegrated at depths. The breaching action along the advancing shore line of the Cambrian sea found ready prepared the materials which the water assorted and distributed to form the great sheet of Cambrian rocks. While these deposits of detritus were accumulating over the shallow areas, the mateiials for the future lime- stone were gathering offshore to the west. As the positive movement GENERAL STRUOTUEE AND CORRELATION. 3] deepened the water shoreward, the calcareous materials accumulated above the earlier detrital beds, so that we may imagine that, while the later beds of the Cambrian were being made of sand and gravel in shallow water, the lower beds of the great limestone formation were being deposited offshore. Later, with a change of some kind in the conditions, there came the deposit of finer material over the previously shallow region, while the accumulation of limestone, with Lower Silurian organisms, still continued offshore. Still later, by another change in the conditions, the deposit of finer detrital material extended far to seaward, covering everywhere the limestone accumulations. u Stanibrrl TransvUonat Gneiss Gn^ss FiQ. 8.— Map showing the varying character of the Cambrian rocks in con- tact with the pre-Cambrian granitoid gneiss mass on Hoosac mountain. As we are not yet able to say to what depth into the Cambrian the lime- stone may extend in the Hoosac valley, so, also, we are unable to say to what extent the lower beds of schists on Hoosac mountain may represent Cambrian time. Mr. Wolff has shown that the Cambrian quartzite horizon, which is a true conglomerate on the top of the arch at the north end of the granitoid gneiss area, consists on the eastern and easterly dipping limb of coarse gneisses, showing only occasional pebbles, as in the tunnel, while on the western and crumpled limb it is represented by finer-grained white gneiss. These relations are shown in Fig. 8. We may suppose an island of coarse granitoid gneiss with a disintegrated mantle, and imagine this latter to have been abraded down to its less disintegrated zone, and the resulting 32 GREEN MOUNTAINS IN MASSACHUSETTS. coarser material to have beeu laid down, during the positive movement, over the gneiss area. In the subsequent folding I imagine that the rigidity of the unaltered granitoid mass offered far greater resistance to the folding than any of the superposed material, and that, as a result of this resisting, inverted wedge, the material of the eastern limb was subjected to the slip- ping or shearing movement producing the coarse laminated structure of these gneissoid rocks, while the similar material on the west limb, having a more rigid base which yielded less readilv to overfolding, was forced into minor overfolded crumples and crushed into a finer grain. Beneath the gneisses remade out of the conglomerate by dynamic action during the folding, there would be formed more or less similar transitional rocks through the action of the same dynamic processes upon the semidisinte- grated surface of the older rock. This is what is found at many points along this contact in Hoosac mountain. From what has just been said it is evident that the high degree of metamorphism of the Paleozoic rocks is intimately connected with the folding. It is also a salient fact that, while the schists and limestone are wholly recrystallized throughout the whole folded area beginning west of the Taconic range, the change of the underlying Cambrian quartzite to a crystalline rock — a white gneiss — does not begin until, in going east, Ave reach the central, main range. In this sense the metamorphism of the schists is regional ; that of the quartzite has the apjDearance of being local. Both the quartzite and the ovei'lying schists contain tourmaline, crys- tallized in situ, and frequent lenses or faulted veins of quartz, feldspar, and tourmaline. The schists contain in places needles of rutile. As we follow the quartzite in its transition to white gneiss we find here and there peg- matite veins, more often near its contact with the core of older gneiss. If we could go back to the original character of the sediments we would find west of the western flank of Hoosac mountain a column of fine sedi- ments, probably argillaceous, with, in places, calcareous bands, resting on a thousand feet or more of limestone, and this on six or eight hundred feet of Lower Cambrian grit — here a quartz sandstone. On the eastern side of the western flank of Hoosac mountain we would find many thousand feet of the same fine sediments resting on, and passing downward into the Cambrian grit — here a coarse conglomerate abounding- in detrital feldspar GENERAL STRUCTURE AND CORRELATION. 33 in its cement. We would find the limestone of the western column repre- sented only by more or less calcareous matej'ial in the fine sediments of the corresponding part of the eastern column, and by a rather abrupt lateral transition through flaggy limestones and marls, containing more quartz sand at the bottom and more clay at the top. Above this horizon we would find the fine sediments alike common to both columns and extending far both to the east and the west. Analyzing the different horizons we find along the west side of Hoosac mountain different conditions of sedimentation aff'ecting the horizons of both the grit and the limestone. To the east the grit becomes a conglomerate abounding in granitic pebbles and in detrital feldspar. To the east also the limestone passes into shoreward argillaceous sediments. Higher up we find in the uniformly widespread fine sediments the evidence of changed condi- tions, which through a long period excluded to a great extent the formation of limestones over the whole region. Such in a general way was the differentiated character of the rocks upon which the processes of metamorphism acted. These processes resulted in changing the quartz sandstone of the Cambrian grit into a quartzite, and the shoreward feldspathic sandstone into a highly crystalline gneissi. The Cam- bro-Silurian limestone, the limestone proper, was changed to crystalline limestone; its shoreward transitions into more or less calcareous gneiss and its more eastward calcareous shales into a garnetiferous variet}^ of the albitic schist, into which the whole column of Cambro-Silurian fine sediments above the lower Cambrian grit has been changed. In the finer sediments, the uniform character above the horizon of the limestone resulted in a uniform change into a mica-schist characterized by the general presence of albite in macroscopic or microscopic crystals. We do not yet know to what depth these rocks were buried. They ha.ve in themselves an aggregate thickness of 5,000 feet or more. Certainly if they were covered by the great thickness of material represented in the schists between Hoosac mountain and the Connecticut river, they were buried to a point of load and temperature sufficient to satisfy these condi- tions of metamorphism. Throughout the whole region all the rocks above the pi-e-Cambrian MON XXIII 3 34 GREEN MOUNTAINS IN MASSACHUSETTS. have been subjected to the action of great lateral pressure, throwing them into folds and along certain lines into compressed and ruptured overfolds, subjecting the constituent particles to crushing or shearing and to move- ments which are now marked by the crinkling of the original stratification lamination, and by the predominant cleavage resulting from movement. There were therefore present the three factors of load, temperature, and attrition of particle on particle produced during the folding movement. These factors were essential in the process of metamorphism, but they could not change ordinary clay sediments into schists consisting largely of mag- nesia and potash micas and abounding in soda-feldspars, nor could they change a grit of quartz and microcline detritus into a gneiss consisting largely of soda-feldspar. Either the original sediments must have contained all of the elements required to form by recrystallization the present constituent minerals, or a part must have been contributed from elsewhere. The extreme rarity of observed eruptive dikes and of pegmatite veins outside of limited areas makes it hard to explain the difference between the chemical constitution of the schists in their great breadth and thickness and that of ordmary argillaceous sediments by ascension from below. It would there- fore appear more likely that the original sediments were of an exceptional character. They may have been deposited under conditions favorable to the preservation of magnesium and alkaline salts — conditions which we know have at ^various times existed over large areas. In the case of the Lower Cambrian grit the action of mineralizing processes originating below is more pi-obable. Where the rocks have been subjected to the different forms of readjustment of particles during the great folding of the strata, a change occurs from a gi'it containing much detrital microcline to a highly crystalline gneiss with a predominant soda feldspar, Avhich bears evidence of being crystallized in situ. Along these zones we find veins and "flames" of pegmatite, and in the crushed quartzite proper perfect little crystals of tourmaline often appear in great abundance. The very feldspathic veins along these zones of extreme folding in the grit may stand related causally to the lenses of qiiartz and tourmaline, with and without feldspar, which occur rather frequently in the higher schists along the west flank of Hoosac mountain; also along the zone of extreme folding. \ F^RT II. THE GEOLOGY OF HOOSAC MOUNTAIN ADJACENT TERRITORY. J. E. M^OLFF. 35 CONTENTS Page. Introduction 41 Topograpliic! work 41 Toijography 41 Description of rocks of Hoosac mountain 44 The Stamford gneiss 45 The Vermont formation : 48 The Hoosac schist 59 > The Stockbridge limestone 64 Amphibolites 65 Geology 69 The Hoosac tunnel 69 The region embracing the central part of Hoosac mountain 72 The northern and eastern schist area 86 The region south of Cheshire and of tlie Hoosic valley 88 Hoosic valley schist 97 The region around Clarksburg mountain and Stamford, Vermont 98 General conclusions 102 Descriptions of plates 109 37 ILLUSTRATIONS XI. ^ Page. Plate IV. Detailed map of western crest and slope, Hoosac mouutain 40 V. Geologic proflle.s, Hoosac mountain 70 VI. Geologic profiles, generalized, Hoosac mountain 80 VII. Thin sections, white gneiss 110 VIII. Thin sections, white gneiss and albite schist 112 IX. Thin sections, diorite and amphibolite 114 X. Thin sections, quartzite conglomerate and crumpled metaraorphic conglomerate 116 ( View north over crest of Hoosac mountain 118 B \ Profile of Hoosac mountain from Spruce hill south, looking west 118 Fig. 9. View from Hoosac mountain _. 43 10. Profile of Hoosac mountain (western crest) 43 11. Profile of Hoosac mountain (western slope) 44 12. Granitoid gneiss 45 13. Metamorphic conglomerate, showing crushing 48 14. Metamorphic conglomerate, showing shape of pebbles 49 15. Metamorphic conglomerate, flattened pebbles 50 16. Metamorphic conglomerate, round and flat pebbles 51 17. Metamorphic conglomerate, banded variety 53 18. Metamorphic conglomerate, typical 55 19. Metamorphic conglomerate, showing large pebbles 57 20. Conglomerate, clitf 58 21. Albite-schist, Hoosac schist 59 22. All)ite-schist, Hoosac schist... 61 23. Albite-sohist, Hoosac schist 62 24. Mount Holly amphibolite 65 25. Mount Holly amphibolite 66 26. Mount Holly crumpled amphibolite '. 67 27. Contact of granitoid gneiss and metamorphic conglomerate 73 28. Contact of granitoid gneiss and quartzite, Stamford dike, looking north 100 29. Contact of granitoid gneiss and quartzite, Stamford dike, looking east 101 39 3 GEOLOGICAL SURVEY MONOGRAPH XXin PL T/ THE GEOLOGY OF HOOSAC MOUNTAIN AND ADJACENT TERRITORY. By J. E. Wolff. INTRODUCTION. The territory embraced in this report extends from the Hoosic valley in the west to the meridian of 73° on the east, and from the state line on the north to the valley in the south which runs east from Pittsfield through Dalton. It covers the easterly half of the " Greylock sheet" of the new map of Massachusetts. It is an area about 18 miles in length, varying from 10 to 4 miles in width, and covering about 120 square miles. TOPOGRAPHIC WORK. As the extreme complication of the field required great accuracy in the location of ovitcrops, at an early stage in the work a base-line 7,000 feet long was meastired on the Boston and Albany railroad in Hoosic valley and a sufficient number of points were established by triangulation to allow the accurate vertical and horizontal topographic determination of important out- crops, which were then plotted on a large field map on a scale of 1,000 feet to the inch. Subsequently the plane-table sheets of the state map (scale 2 inches to the mile) were utilized, and a special topographic map of that part of Hoosac mountain near the tunnel (on a scale of 1,000 feet to the inch) was prepared. At many places accurate section lines were run by the stadia and the geological points incorporated in the general map. TOPOGRAPHY. Hoosac mountain is the name applied to a part of the Green mountains situated in the northwest corner of the state of Massachusetts, near the Ver- mont boundary. This region forms the watershed between the Hoosic and 41 42 HOOSAC MOUNTAIISr. Deei-field rivers, branches of the Hudson and Connecticut, respectively. At its southern end it is di-ained by branches of the Hoosatonic and by the Westfield river, a branch of the Connecticut. The entire mountain mass is cut through at its central part from east to west by the Hoosac tunnel, nearly 5 miles long, the tunnel passing almost directly under the highest point of this part of the mountain, a knob one- half mile north of Spruce hill, which is 2,600 feet above the sea. At the extreme north of the field, half a mile south of the Vermont line, the highest point is found to be 2,800 feet. Where the tunnel crosses the central part of the mountain the outline is that of a double crest with a central basin or Fig. 9.— View looking west from alope of Hoosac mountain, east of North Adams. This gives a general idea of the topography of the valley. depression (see Profile iii, PI. v), the two sides joining at the north end to form the high north point and to terminate the basin. In its southern-central portion the mountain loses the north to south ridges and drainage. It is tliere characterized by flat, rounded summits and gentle depressions, and a frequent east to west trend of the valleys. A glance at the strike and distribution of the formations will show that the frequent east to west strike and extreme crumpling of the white gneisses which occupy this region cause this diff"erence in the topography. In the southern part of the field a north to south strike of considerable regu- GREEN MOUNTAmS IN MASSACHUSETTS. 43 larity again comes in, causing a more ridge-like topography, until the deep east to west valley of Dalton, a mile or so south of the map (PI. i), bounds the region on the south. On the east the mountain joins the hilly country extending to the Connecticut river; on the west the bi'oad Hoosic valley, running noilh and south, bounds Hoosic mountain and separates it from the mass of |Grreylock mountain, the highest in the state. (See Fig. 9). ( The relations of topography to geological stnicture are ofbfen notice- able. The whole eastern border of the area shown on the map is covered Fig. 10 — Profile of part of west crest of Hoosac mountain, looking cast from Hoosic valley opposite Adams. This shows the continneil northerly pitch of the axissome miles sonth of point sho\Yii in PI. XI, B. The snmmit in the right center is of white gneiss (Vermont formation) with a little indistinct minor ridge of the Hoosac schist trough, both slanting to the left (north). by the schists, characterized by a uniform north to south strike and steep easterly dip of their structural planes; and the ridge topography, with deep cross-goi'ges for the streams, is evidently due to that structure. The long crest of Hoosac mountain, forming the main watershed, coincides in direction and position with the axis of the northerly pitching fold which forms the principal feature of the mountain, and with the axis of the central core of granitoid gneiss. The presence of the limestone in 44 GEEEN MOUNTAINS IN MASSACHUSETTS. the Hoosic and Dalton valleys determined these depressions, as is always the case with that rock. The profile of Hoosac mountain shows plainly the northerly " pitch" ^ of tlie formations by the gentle slopes to thQ north and the bluffs facing south. (See Fig. 10 and PI. xi, b.) The western slopes of Hoosac mountain running down to the Hoosic valley are steep, but have a marked series of buttresses or benches. (See Fig. 11.) The ckift-covered Hoosic valley is Fig. 11. — Profile of west slope of Hoosac mountaiD, from Hoosic valley opposite Adams, looking north. This figure shows the buttressed character of the west slope of the mountaiQ at the left ceuter. These IfuttreBSes are of crumpled white gneiss (Vermont formation), with a gentle easterly dip. comparatively flat, sending branches into the mountain, Avhich are locally called "coves." At Cheshire the valley makes a sharp turn to the west. DESCRIPTION OF THE ROCKS OF HOOSAC MOUNTAIN. The rocks of this region are thoroughly crystalline, but little trace remaining in general of their original elements, whether of detrital or erup- tive origin, but the bedding corresponding to the original planes of deposit is well marked, and, under the proper conditions, we can therefore deter- mine the order of succession. ' Meaning that the axes of the folds are inclined or " plunge " in that direction. HOOSAC MOUNTAIN. 45 THE STAMFORD GNEISS. The basement rock is a coarse granitoid gneiss, which forms the core of Hoosac mountain proper, occupying the surface of the mountain for several miles, .then disappearing below the overlying rock, but cut in Hoosac tunnel for nearly 5,000 feet; hence this rock figures prominently on the dumps of the tunnel shafts. Another area of the same rock under- lies the fossiliferous Cambrian quartzite of Clarksburg mountain, north of Williamstown, continuing some miles northward into Vermont — the "Stam- ford granite" of the Vermont geological report. Fig. 12. — Granitoid gnei8.s (Stamford gneiss), from dump Central shaft. Natural size. Tliis is the variety with a well-marlied gneissoid structure. The dark streaks are composed of the micas inclosing irregularly lenticular areas of feldspar and quartz. In its most typical form the rock is a coarse banded gneiss (see Fig. 12), composed of long lenticular crystals of pinkish feldspar, flattened lenses of blue quartz, and thin, irregular, greenish layers of a micaceous element (biotite or muscovite, or both) mixed with small epidote crystals, which cause in part the greenisli color. We notice at once that tlie broad cleav- ages of the feldspar often do not reflect as one surface, but as a num- ber of little disconnected areas, which are often curved — a well-known 46 GREEN MOUNTAINS IN MASSACHUSETTS. effect of great pressure in crystalline rocks. The feldspars contain little dull grains of quartz, black specks of mica, and crystals of magnetite, and are often crossed by little branches from the layers of mica outside. At the edges the feldspars often pass very irregularly into the quartz, which then forms the narrow parts of the lens of which the feldspai* forms the center (" Augen" structure). The quartz is characteristically blue, but when crushed by pressure in the rock is often white or sugary in appearance, the blue cores then rep- resenting the uncrushed material. In other varieties of this rock it has almost the structure of a coarse granite. The quartz is deep blue, the feldspar colorless and in Carlsbad twins, and the mica layers black. The gneissic structure is almost wanting. Certain other variations occur in the structure of the gneiss. In the bed of Roaring brook, Stamford, Vermont, the gneiss on the weathered surface has numerous rounded elliptical masses which by the absence of quartz and scarcity of mica stand out by contrast with the rock as a whole, and look like pebbles. They are composed of feldspar aggregates and flakes and patches of biotite. The microscope shows that these feldspars are mi- crocline with some plagioclase and perhaps orthoclase; they have the gen- eral structure of the gneiss itself, without the quartz, and are probably of contemporaneous origin. West of Stamford village the rock contains Carls- bad twins of microcline an inch or two across, which weather out from the rock, become rounded by decay, and look like pebbles. The microscopic characters of this rock are quite uniform; the large feldspars are generallv microcline,^ with whatever crystalline boundary they may have once possessed obliterated by the great mechanical changes they have undergone. The crystals are often faulted and the edges crushed; little veins of secondary quartz, mixed with little grains or crystals of an un striated feldspar (albite?) traverse them along the fault lines. (See PI. VII, B.) With a low power the feldspar substance appears cloudy, owing to fluid cavities ajid little prisms of epidote in great numbers. These epidote grains are sometimes arranged parallel to the twinning planes of the feld- ' In the " Geology of Vermont," vol. 2, p. 561, there is an analysis of the feldspar from the Stam- ford granite, according to which it contains from 64 to 66* per cent silica, 10 to 11 per cent potash, and 2 to 3i per cent soda. HOOSAC MOUNTAIN. 47 spar, sometimes not. In some localities the feldspar contains little round red garnets. Flakes of biotite and muscovite and octaliedra of magnetite are common inclusions. The quartz masses show cataclastic changes in the same way; the original cores of the blue quartz, themselves somewhat strained (seen by using polarized light), are surrounded by masses of broken quartz, the derivation of which from the parent mass can easily be traced. The finer grained portions of the rock are composed of little fragments of microcline broken off from the larger pieces, and small simple crystals, often simple twins, of a feldspar which shows but rarely the multiple twinning of pla- gioclase, and which resembles the albite of the schists. The layers of mica are composed of muscovite, often with a greenish color like talc, but easily identified by the large axial angle, flakes of dark brown biotite, rarely altered to chlorite, crystals of magnetite, and the omnipresent epidote in prisms or small grains mixed with the micas or inclosed in them. There are occasional imperfect crystals of apatite and prisms of zircon. Some of the magnetite grains are titaniferous, as can be seen by the yellow border of titanite derived from them. In many slides there are quite large crystals of feldspar which have no multiple twinning, extinguish parallel to the cleav- age, and are perhaps orthoclase. Slides of the large porphyritic Carlsbad twins show that they are micro- cline, filled with irregular bands of a feldspar which extinguishes parallel to QoP do, and is filled with epidote crystals. Aggregates of biotite plates associated with hornblende crystals are common. There are also masses of ilmenite altered in part to titanite. Sometimes circles of hornblende crys- tals and biotite plates, which inclose a core of aggregate quartz, by their shape and occurrence suggest a possible replacement of garnets. Grains of quartz and crystals of zircon are common, so that nearly all the constitu- ents of the rock occur within these cr)rstals. What may have been the origin of this rock it is impossible to say with certainty; it is evident that crushing and the development of mica, quartz, and feldspar, parallel to planes of break and sliding has had a great deal to do With the development of the parallel structure. Viewed from this standpoint it could perfectly well have been an eruptive granite modified 48 GEEEN MOUNTAmS IN MASSACHUSETTS. by metamorphism. On the other hand, its field relations show its close asso- ciation with and frequent transition into coarse gneisses which seem to form part of a detrital series. THE VERMONT FOEMATION. A somewhat varied series of rock overlies this coarse basement gneiss. At one place where there is no possibility of folding (namely, along the pitching axis of Hoosac moimtain (see PL v, Profiles ix, x). The thickness of this series has been measured between a conformable contact with the granitoid gneiss below and one with the albite-schist above; it is between 600 and 700 feet. Fig. 13. — Metamorphic conglomerate (Vermont formation). Dump, Central shaft. One-fiftli natural size. This represents two faces of one block at right angles to each other, the line showing the corner. The pehhles are of granulite and blue quartz, some of them 1^ inches in diameter. The ditferent shape of tlie cross-sections in the two planes is noticeable. By looking closely it will he seen that many pebbles are cut in two hy dark lines (biotite), showing that their present shape is due partly to crushing and the formation of new minerals. This is seen on the right side, but not on the left. This formation contains an infinite series of gradations between coarse gneisses similar to the basement gneisses, finer grained banded gneisses, gneisses composed of quartz and feldspar with but a small amount of the micaceous element, metamorphic gneiss-conglomerates, ordinary quartzite- conglomerates, and quartzites. This series of rocks (represented by gneisses and metamorphic conglomerate) occupies a position in the tunnel section on either side of the central core of granitoid (Stamford) gneiss; while a flOOSAC MOUNTAIN. 49 second nairow belt occurs near the West Portal, adjoining the Hoosic valley (Stockbridge) limestone. On the surface it occupies a large area, especially in the southern part of the field. The quartzites occur generally in or near the Hoosic valley adjacent to the limestone, associated with conglomerates and passing along the strike into the granulitic and gneissic rocks by increase in the amount of feldspar and mica. . Fig. 14.— Metamorphic conglomerate (Vermont formation). Dump, Central shaft. Two-ninths natural size. I'lie larger pebbles are mostly granulile, with some of blue quartz and feldspar. Tliis shows very plainly the shape of the pebbles, which are but little elongated in the plane normal to the picture. Beginning with the simplest rocks, the quartzites, there are vitreous varieties and crumbly feldspathic varieties passing into gneiss. The vitreous variety occurs in large masses with very obscure stratification, and roughly jointed. It varies in color from snow white to yellow, contains often layers of mica and cubes of pyrite. The microscope shows an even MON XXIII 4: 50 GEEEN MOUNTAINS IN MASSACHUSETTS. grained, closely interlocking aggregate of little rounded or irregular quartz grains mixed with considerable feldspar in similar in-egular grains. Broken or rounded crystals of apatite and zircon and perfect crystals of tourmaline and rutile are common. The feldspar grains are in part microcline, plagioclase, and an untwinned feldspar (orthoclase ?). Unless it be the apatite and zircon, no unmodified original clastic elements can be recognized in this rock. According to the usual view of the origin of quartzite, the quartz grains have been enlarged by the growth of new silica, so that the original form is wanting, and the feldspar, judging from its similarity to that of other i-ocks in which it is undoubtedly metamorphic, has probably a similar origin. In many localities the quartzites have a crumbly character, so that Fig. 15.— Metamorphic conglomerate (Vermont formation), near contact with granitoid gneiss. Top of Hoosac moun- tain. Fallen block. One-twentieth natural sixe. This also pbows the production of flattened " pebbles " by crushing and the development of biotite, etc., along crushing and slipping planes. In the right hand of the picture this is especially clear. The pebbles here are granulite, passing into a line grained granite. they can be picked or shoveled out, and are extensively quarried for glass sand. Prof J. D. Dana has called attention to this^ and suggested weather- ing as a cause, and connected it with the alteration and leaching out of the feldspar. In some of the quarries the percolating water cames down filie kaolin, and forms beds of pipe clay in the bottom of the quarry. But some ' On the decay of quartzite, and the formation of sand, kaolin, and crystallized quartz. Am. Jour. Sci., 3d ser., vol. 28, 1884, p. 448. HOOSAC MOUNTAIN. 51 of these crumbly qiiartzites show but little feldspar and that quite fresh, while the quartz grains show in the slide abundant signs of great pressure, or even crushing. Some of these quarries are located at sharp folds of the quartzite, so that the crumbly nature of the quartzite may be in part due '' to a mechanical loosening of the cohesion. The pure conglomerate-quartzites occur often in the quartzite; for instance, the quartzite resting on the granitoid gneiss (Stamford gneiss) of Clarksburg mountain, in which Mr. Walcott has found fragments of trilo- bites.^ contains pebbles of blue quartz, which are often only distinguishable Fig. 16. — Metamorphic conglomerate (Vermont formation). Dump, Central shaft. One-sixth natural aizo. The pehbles are mostly granulitic, but there are some of blue, and some of white quartz. In this type we have round and fiat pebbles ocourring together, the round ones differing but little in shape in the normal plane. This represents the typical variety of conglomerate. by their color from the surrounding quartzite cement. The microscope shows that many of these pebbles are composed of an aggregate of little quartz grains derived from a homogeneous mass by crushing, and hence they easily blend with the quartz cement of the rock. They occur often in flattened elongated forms which it is difficult to distinguish from secre- tions. (See PI. X, B.) Some of the quartzites contain abundant calcite grains arranged in stringers, and scattering flakes of muscovite. Those quartzites in which feldspar becomes more prominent preserve ' Am. Jour. Sci., 3d ser., vol. 35, 1888, p. 236. 52 GEEEN MOUNTAINS IN MASSACHUSETTS. still the appearance of the purely quartzose varieties. The feldspar occurs in irregular grains fitting in between the quartz. It is partly not twinned (orthoclase?), part plagioclase, generally microline. Little crystals of rutile, prisms of zircon and apatite, flakes of biotite and muscovite, masses of iron hydrate, pyrite, etc., occur in nearly all the specimens. The quartz and feldspars often show evidence of mechanical crushing, and part of the quartz has been thus derived from larger grains. The constituents have a nearly even grain. Although garnet is very rare, it is convenient to call this rock the granulitic type of the quartzite. From this rock tlie transition is easy to the white gneiss proper. Sev- eral varieties of this may be recognized; a banded one is common, the color varies from gray, yellow to white; sometimes the banding is very fine or the rock is speckled with biotite or muscovite, or both ; sometimes the feld- spar forms layers separated by layers of mica, or occurs in rounded or irreg- ular masses. The proportions of the elements vary in every conceivable way. A characteristic feature in the slide is seen in the round grains of feld- spar of someAvhat larger size than the average of the rock, inclosed in a groundmass composed of little grains of quartz and feldspar in a most intimate admixture, while plates of mica give the rock its banded struc- ture. The larger sized feldspars are typically of a peculiar rounded shape, occurring either in single crystals or in broad simple twins. They are com- monly entirely without the polysynthetic twinning of plagioclase in polar- ized light and might be taken for orthoclase, but their isolation from the powdered rock by the Thoulet solution shows by the specific gravity that they must be generally a soda-lime feldspar near the albite end of the series, although in some rocks they must cbntain considerable lime, judging by theii* specific gravity. These feldspars are commonly filled with inclusions of minerals found in the rock outside them — little prisms of epidote, flakes of biotite and muscovite, and little rounded grains of quartz, sometimes arranged like a necklace. These inclusions often lie in planes parallel to the arrangement of the minerals outside the feldspar, and entirely inde- pendent of crystallographie directions in the feldspar. (See PI. vii, a, and PI. VIII, A.) It is very rare to find any sign of mechanical deformation in these feldspars. HOOSAC MOUNTAIN. 53 Quartz occurs sometimes in large rounded masses, greatly strained and shattered, and surrounded by a mosaic of small quartz grains. Large pieces of microcline occur, faulted and broken, the cracks filled with an aggregate of little quartz grains and feldspars in simple twins. (See PI. vii, b.). The groundmass of the rock is a closely interlocking aggregate of quartz grains, k \ ^ Fig. 17.— Metamorphic conglomerate (Vermont formation). Dump, Central shaft. One-fifth natural size. The tiy,nre represents the banded variety of the rock, in which vre find it difficult to draw the line hetween true pebbles and forms produced by crushing. A glance at the figure, especially at the right side, shows that the extremely pointed ends of some of the apparent pebbles must he produced by the encroachment of the mica layers. Yet these white masses have a lithological character different from that of the "cement," forming, for instance, the broad baud near the rightside. The former are a fine grained granite or granulite, sometimes blue quartz ; the latter a coarser grained mixture of quartz, mica, and some feldspar. little feldspar gi^ains simply twinned (if at all) and often little grains of microcline of the same form and size. Epidote is often present in large quantities, foiTning microscopic yellow bands in the rock, and inclosed in the feldspars and micas in little prisms and grains, but not in the quartz. 54 GREEN MOUNTAINS IN MASSACHUSETTS. Titanite, rutile, aud tourmaline occur sparsely, as well as little broken prisms of apatite and zircon prisms. The micas occm* in homogeneous plates ; the interwoven sericitic structure is not common. Magnetite occiu's occasionally. Another variety of these gneisses is distinguished by the evenness of its character and its occurrence along the base of Hoosac mountain as the most western band of the gneisses, in close connection with the quartzites and limestones. The rocks thrown out from the "well" shaft, a few hun- dred feet west of the west shaft of the tunnel, are typical of this variety. In the hand specimen the rock is a fine grained, evenly banded gray gneiss; the minerals are arranged in layers and the rock is filled with little squarish feldspars. In the slide these feldspars are seen in gently rounded, eqiiidimensional crystals, in simple twins, according to the albite law. The groundmass is composed of little round or ellipsoidal quartz grains and more angular pieces of feldspar (which are in part in simple grains, in part doubly twinned microcline) mixed with threads of muscovite and biotite, the whole so arranged as to produce a schistose structure in the rock. (See PI. VII, A, and PI. VIII, a.) Sometimes a band of mica and quartz cuts across a feldspar, the two halves polarizing together and being therefore part of one crystal. The bands of the groundmass bend around the porphyritic feldspars in gentle curves. These feldspars are honeycombed with little drops of quartz and flakes of biotite aud muscovite which are often airanged parallel to the structure outside. Octahedra of magnetite are visible in the rock; microscopic crystals of apatite, rutile, and zircon are abundant. In some cases little grains of calcite occur abundantly, even included in the feld- spars, and in some localities we find a calciferous gneiss with this same structure, in which the groundmass contains a large amount of calcite in little grains apparently homologous with quartz and feldspar. This variety occurs at several places in the Hoosic valley near the junction between the limestone and quartzite, and represents the Hoosac mountain gneisses nearest to the limestone. At the base of the white gneiss series the rock in many places passes so gradually into the underlying granitoid gneiss that it is impossible to draw a line between the two. These varieties of the white gneiss are very coarse and feldspathic, but the feldspars are white instead of red as in HOOSAC MOUNTAIN. 55 the granitoid gneiss, and the mica is black. In the shdes the structure is essentially the same as that of the granitoid gneiss: the large crystals are microcline, broken, faulted, filled with fluid inclusions, epidote grains, quartz, mica, etc.; while the groundmass is composed of the usual simply twinned feldspars and quartz, mixed with epidote, muscovite, biotite, and other minerals. At the upper contact of the white gneiss series there are frequent tran- sitions into the overlying albite-schists (Hoosac schist); the transition is caused by the appearance of bands of mica in the white gneiss alternating with bands of feldspar. The latter are often lenticular and composed of the simply twinned feldspars which in the schist are proved to be albite. Fig. 18.— Metamorphic coDglomorate (Vermont formation). From dump of Central shaft. About one.fourth natural size. This is also tlie typical conglomerate. The pebhles are mostly of the fine grained granuUte type. The tine grained layers, of which a good example is seen near the top, are composed of quartz grains, hiotite, and some feldspar. They represent, of course, sand layers in the original sediment which have undergone considerable metamorphism. The last and perhaps the most important member of the white gneiss series is the metamorphic conglomerate. This rock occupies a large area in the tunnel, occurring on both sides of the central core of granitoid gneiss. Nearly all the varieties of the rock are well shown by the dumps of the central and west shafts of the tunnel. On the surface it is found on the crest of the mountain in the line of the axis of the fold, where the rocks have a gentle northerly dip, and measured between conformable contacts with granitoid gneiss below and schist above, it has a thickness of about 650 feet. 56 GEEEN MOUNTAINS IN MASSACHUSETTS. The rock contains pebbles of two varieties : one kind composed of bright blue opalescent quartz, the other resembling a fine grained granite, composed of quartz and feldspar in small grains, speckled with biotite. These pebbles on tlie average are as large as a walnut, though some are much larger, and they diminish in size until undistinguishable from the elements of the groundmass. The shape is sometimes round, sometimes ellipsoidal, angular, or flattened. In Fig. 13, which gives two sides of a large block, the different cross-section of the pebbles in two planes is shown. The groundmass or cement outside is composed of smaller grains of blue quartz, small feldspars resembling the albite of the schist, and biotite and muscovite in large amount. The effects of crushing in the rock are evident; the pebbles are often trav- ersed by parallel breaks or oblique cracks by which bands of biotite pene- trate them, isolating parts of the pebble. Sometimes a pebble is cut in two across its axis by such a band of mica. Thus pebbles, in appearance sepa- rate, may have been parts of one individual originally. This crushing action, combined with the formation of the biotite bands, gives many of these origi- nal pebbles flattened shapes, so that they appear as layers of granitic mate- rial cut off by the biotite bands in planes oblique to their trend. Some varieties of this conglomerate gneiss have a banded structure, due in large part to this crushing actioii carried to an extreme. (See Fig. 17.) In some cases the pebbles are single crystals of feldspar, and this is occasionally microcline. Figs. 13-20 show the character of this rock. Some of the pebbles consist of fine grained granite containing small grains of blue quartz. Fine grained gneissoid layers corresponding to the cement often alternate with pebbly layers (see Fig. 18). In some varieties these granite pebbles lie in a very micaceous matrix, composed of small feldspars resembling those of the schist; in others the pebbles become so small that we get an even banded gnejss containing larger grains of blue quartz, the whole forming the ordi- nary Avhite gneiss previously described. It is then very difficult or impossible to separate the old quartz and feldspar from that formed in situ. The opal- escent blue quartz pebbles always retain their round form and are rarely entered by the biotite outside. This shows perhaps a connection between the formation of the biotite and the feldspar substance. The previous description is based on the conglomerate of the tunnel dumps. HOOSAO MOUNTAIN. 57 On the surface here and there conglomerates are found, often associ- ated with quartzites; in the latter case the pebbles are all quartz and the cement is composed of biotite, muscovite, small feldspar, and magnetite crystals. On the crest o( Hoosac mountain, in Profile ix, PI. v, the conglomerate is represented principally by the finer grained varieties, but toward the base the pebbles are much larger and are in part not pebbles, but fragments of layers broken up by crushing (see Figs. 15 and 27), giving angular forms. When we pass westward from the crest of Hoosac mountain, where the conglomerate lies in its normal position, we trace the rock into the white gneiss series on the slopes of the mountain. The pebbles have lost Fio. 19.— Metamorphic conglomerate (Yermont formation) . Dump, Central shaft. About one-seventh natural size. In this variety the pebbles are of much larger size (over 5 inches long), they have the most perfect beach-pebble shape, and are composed of a very fine grained granite, which contrasts sharply with tlie much coarser gneissoid cement composed of quartz and feldspar grains and mica fl.il£es. The long, white, irregular masses in the center are secondary vein quartz their distinctness, and without the favorable exposui-es on the summit and from the tunnel we would not suspect their nature; they appear as white, flat, lenticular masses of quartz and feldspar, which only in rare places sug- gest a conglomerate (see PI. x, b), but when one has traced this rock foot by foot into the conglomerate he recognizes the pebbly look at once. It is apparent that this change is connected with stretching of the rock, for the conglomerate is folded over and then turned under on the west flank of the mountain. The microscope shows that the quartz pebbles are homogeneous masses 58 GEEEN MOUNTAINS IN MASSACHITSETTS. of quai"tz, wliicli by optical investig-ation are seen to lia^-e been greatly strained; they have a border of l)roken quartz which grades into the ground- mass. (See PI. X, A.) They are identical with the blue quartz pebbles ofthefossiliferous Cambrian conglomerate (Vermont formation) farther west (Clarksburg mountain, Stone hill). The granite pebbles are composed of crystalloids of microcline, plates of biotite, and grains of quartz. The micro- cline and quartz are crushed and faulted. Veins of a later quartz traverse the fissures in the feldspars. Crystals of zircon and apatite and plates of chlo- FiG. 20.— Conglomerate (Vermont formation). Crest of lloosat luuuutiiiu south of Sprace hill. This sbows a large clifl' of the conglomerate as it occurs in place. The pebbles here are largely blue and white quartz and the cement gneissoid. This is in the upper half of the conglomerate horizon. rite occur in the feldspar. There are skeleton crystals of magnetite asso- ciated with the apatite. The cement is quite similar to that of the white gneiss. Without here going into the much disputed question of metamorphic conglomerates in general, which are found in so many terranes of stratified crystalline rocks, ^ it may be said that the reasons for considering this par- ticular rock a true conglomerate and not a gneiss containing peculiar con- ' Cf. A. Winchell, Am. Geologist, vol. 3, pp. 143 and 256. Also C. H. Hitchcock, Am. Geologist, vol. 3, p. 253. HOOSAO MOUNTAIN. 59 cretionaiy forms, are, first, the shape and distribution of these forms (well shown in the fignres) and the alternations parallel to the stratification (deter- mined by contact with other rocks) of bands of coarse and fine material; second, the diverse nature of the pebbles in the same rock (blue quartz, white quartz, granulitic rock, granite, etc.) ; and, third, the frequent transi- tions in the field into quartzite and quartzite-conglomerate. The production of at least part of the mica, feldspar, and quartz of the cement in situ has been indicated, and also the efi'ects produced by crushing. THE HOOSAC SCHIST. The next member of the series is the albite-schist (see Figs. 21, 22, 23, and PI. VIII, b), Avhich confoiTnably overlies the conglomerate on top of Fig. 21.— Albite schist (Hoosac schist). Dump, Central shaft. Ouetwclfth natural size. This is the type with thin Hat quartz layers (the white streaks) and gentle crumpling. Hoosac mountain, extending northward for miles into Vermont. On the east it extends southward along the east side of the conglomerate and on the west in a narrow band along the west slopes of the mountain, curving around so as to almost join that on the east. In Hoosic valley masses of these schists occur adjoining the Stockbridge limestone and then lying between it and the Hoosac gneisses of the Vermont formation. In the tunnel a band occurs several thousand feet wide (see PI. v, Profile in) between the west band 60 GEEEJSr MOUNTAINS IN MASSACHUSETTS. of white gneisses and those of the eastern core, and again at about tne cen- ter of the tunnel, under the central shaft, they come in east of the con- glomerate and fill the eastern half of the tunnel to about 6,000 feet from the east portal, where they are succeeded by the silvery -green schists (Rowe schists) to the east portal. Among the perfectly fresh material found at the tunnel dumps a shiny black glistening rock is typical, containing parallel layers of white quartz which thin out and disappear in the rock. These flat lenses are sometimes very irregular and crumpled by large folds or small puckerings. It is found that they correspond to the plane of stratification of the rock wherever the schist is seen in contact with other rocks. The black, shiny part of the rock is filled with sparkling glassy crystals of feldspar, either in imperfect rounded crystals or in simple twins, which contain inclusions of mica, gar- net, etc. The basal cleavage planes are sometimes bounded by the brachy- pinacoid (M), the prisms T and 1, and the macrodome, etc., but the crystals are in general rounded or even angular. The feldspar twins are according to the albite law, and the crystal is di^'ided into two symmetrical halves, or else the composition-plane is irreg- ular, one half taking up most of the crystal, leaving a small strip to the other. The rock was powdered and the feldspar, separated by the Thoulet solution, analyzed by Mr. R. B. Riggs in the laboratory of the U. S. Greolog- ical Survey at Washington with the following result: SiOi 69-69 AljO-j 18-60 CaO trace MgO 0-20 NaiO 1028 K.O : 0-40 Ignition 0-42 99-59 COj (Combustion), 0-77^0-44C. Basal cleavage pieces with the simple twin give an extinction 4° oblique to the twinning-plane and second cleavage (M). Twins measured in the goniometer give angles of 172° 46' to 172° 50' between the basal cleavages of the two twins. The chemical and physical properties are therefore those of albite. These albite cr3"stals vary from large to small; HOOSAC MOCTNTAIN. 61 they He in planes roughly jjarallel to the schistosity of the rock, but their crystallographie directions have no such relation. Some varieties of the rock at the shaft are filled with red garnets iu dodecahedral crystals. The surface rock has the same characters, but with certain variations due in part to weathering. The shiny black variety is found here and there, but the rock is commonly greenish, indicating a certain amount of chlorite; it varies from light to dark green. Garnets are sometimes contained in the rock, especially at the base, where a gametiferous horizon occurs. Feldspar Fia. 22. — Albite schist (Hoosac srhisti. Dump, Central shaft. One-sixth natural size. Here the quartz lenses are more ii-regular and thicker; tlio little white specks dotting the rock are the crystals of albite. is often present with the garnet. These schists are identical in every detail with the schists of Mount Greyiock. The por])hyritic albites are prominent in the slide. Simple twins are common, but polysynthetic twins rare. Single crystals are common. They have a rounded lenticular or flat shape. The groundmass outside the feld- spar is composed of rauscovite and biotite, or muscovite alone, chlorite, grains and aggregate lenses of quartz, magnetite in octahedra or grains, apatite, tourmaline, and rutile. Ottrelite is found in some localities The 62 GREEN MOUNTAINS IN MASSACHUSETTS. micas of the groundmass bend around the albites in gentle curves (see PI. viii, b), and often a band of mica cuts across a feldspar. The albite contains inclusions of muscovite, biotite, chlorite, quartz, magnetite, rutile, etc., according to their presence or absence in the I'ock. It is common to see them in curving bands parallel to the banding of the same minerals outside the feldspar. These feldspars evidently crystallized contemporaneously with the other minerals in the rock. Fig. 23. — Alliite-srbist (Hoosac schi.sl). Dump, Central shaft. About one-oighth natural size. Here the quartz lenses are agaiu prominent. It i.s found that they are always parallel to the stratitication. The quartz occurs in little grains often arranged in stringers. The mus- covite is either in stout plates or is a mass of interlacing fibers or plates — the structure characteristic of sericite.,, Biotite and chlorite occur in plates or irregular scales; the two minerals occur sometimes side by side in the same piece without any sharp boundary between the two, so that the HOOSAO MOUNTAIN. 63 chlorite has the appearance of an alteration product of the biotite.^ When the chlorite occurs inde})eudently in stout plates it has a marked pleoclu-oism varying fronl green to yellow green, an extinction several degrees oblique to the cleavage and twinning with OP as composition-plane. Tourmaline and apatite occur in imperfect prisms, magnetite in octahedi'a, and rutile in small crystals, often with the heart-shaped twins. In several specimens a little ottrelite has been noticed, and at one local- ity this mineral occurs in such amount that tlie rock must be called an ottrelite-schist. This is interesting in that it still further proves the litho- logical identity of the Hoosac, Greylock, and Berkshire schists, since this mineral is found in all three of these formations. The hand specimen is a shiny, gi-eeuish schist containing crystals of garnet and dotted with little black ottrelite 'crystals. In the slide the ottrelite occurs in comparatively large crystals with the characteristic indigo-blue, vellow, tdive-green ple- ochroism. The extinction is several degrees oblique to the cleavage; it is twinned }iarallel to the base, and basal sections give a faint bisectrix. It occurs associated with irregular masses of black ore; a number of small prisms of ottrelite surround a plate of the ore (ilmenite?). Plates of mus- covite and a few grains of quartz compose the rest of the rock. The ottrelite is filled with little prisms of rutile with the "knee "-twin. Basal sec- tions show the blue color, with vibrations parallel to h (at right angles to the axial plane), and the yeUow green parallel to a; hence it has the ^jle- ochroism of most ottrelites.^ In this schist we recognize no clastic element with certainty and the feldspar, quartz, micas, etc., apjiear to have formed contemporaneously, for the feldspars c(jiitaiu inclusions of the other elements and in turn are some- times crossed bv tongues of mica and qixartz. While the term "schist" is applied to this rock owing to its frequent coarsely crystalline character, yet its great similarity should be noted to crystalline rocks described from Germany and elsewhere as albite-jihi/Uites, which contain porphyritic albites with similar inclusions, micas, magnetite, etc. ' This association of biotite and chlorite is common in the hydromica schists of the Green moun- tains and is often suggestive of hydration by weathering. '^ Cf. Rosenbusch: Physiographie, vol. 1, p. 494. 64 GREEN MOUNTAINS IN MASSACHUSETTS. THE STOCKBRIDGB LIMESTONE. The next rock is the hmestone found in Hoosic valley at the base of Hoosac mountain and covering the valley west to the base of tlie Greylock mountain mass. It occurs in contact with the Vermont quartzite and with both the Berkshire and Hoosac schists at several places in the valley. The rock is generally a coarsely crystalline white marble banded with layers of yellow muscovite or dark graphitic substances, and containing layers of bluish quartz. Layers of quartzite are frequent in the limestone and the change fi-om one to the other is gradual. Microscopically the lime- stone consists of grains of calcite, a few of quartz, flakes of mica, etc. It has been mentioned that one variety of the fine grained white gneiss often contains considerable calcite, thus forming in some sense a transition between the Stockbridge limestone and the Vermont gneiss. A much more perfect transition is found between the limestone and Hoosac schist. The best case of this kind is found in the "Cove," in Cheshire, where the ground is filled with large angular blocks of this rock, which occurs in place in one ledge. These rocks resemble a micaceous white limestone filled with little dark grains or imperfect crystals of feldspar. In the slide the rock is com- posed of a mass of calcite grains, with here and there single grains of quartz, or an aggregate of several grains, plates of muscovite and often of chlorite and biotite, and large porphyritic feldspar grains in single crystals or simple twins, very rarely showing polysynthetic twinning. These feld- spars contain inclusions of mica, quartz, iron ore, rutile, and calcite, and are in every way identical with the albites of the albite-schists, although the exact species of plagioclase has not been determined. The calcite seems to play the jiart which the quartz does in the schists: it sends tongues into the feldspars, or cuts them in two, and gives one the impression by its in- clusions in the feldspar and its occurrence with the quartz and mica that it is of contemporaneous origin with the feldspar, mica, and quartz. Rutile needles, and masses of ore (ilmenitef) occur in curved bands in these feld- spars. Small irregular masses of microcline occur sometimes among the quartz grains of the rock. On the Greylock side of the valley about 300 yards west of Maple Grove station there occur outcrops of a similar feldspathic limestone. Part HOOSAC MOUNTAIN 65 of the feldspar is here in broad simple twins, but part is microcline in simi- lar crystals. The feldspar of this rock needs further investigation. The hne-g-rained silvery green or green schists (Rowe schists) which occu})y a strip on the extreme eastern border of the map, overlying the albite- schist (Hoosac), have not been microscopically investigated by the writer. AMPHIBOLITES. Last to be described are heavy dark rocks, generally fine grained, in which the eye recognizes dark crystalloids of hornblende and irregular Fig. 24.— Amiiliiliolitc. Mount Holly, Vermont. A band of aiii[iliib()Iitc li feet wide, interstratitied with j^neiss and crumpled with it iu a laj-ge double fold. The atructure of the aujphibolite coincides in every detail with that of the gueisa. patches of feldspar and cubes of pyrite. In the finer grained varieties the rock has a glistening surface due to plates of biotite in films mixed with the hornblende, and the rock then has a somewhat schistose structure. They rocks have been found in several localities, in all but one case in beds par- allel to the structure of the inclosing gneisses and contorted with them. These rocks occur abundantly in the Green mountains. The most remarkable occurrence is perhaps near Mount Holly and Wallingford, Vt., MON xxiii 5 66 GREEN MOUNTAINS IN MASSACHUSETTS. 70 miles north of Hoosac mountain. Here, too, the Cambro-Sikirian limestone and Cambrian quartzite (Vermont formation) are succeeded by gneissic rocks in the east, which form the central divide of the Grreen moun- tains. In the region east of Rutland and directly south of the high mountain mass of the Killington peaks there is a marked l)reak in the general topog- raphy in an east to west zone, 10 to 15 miles wide from north to south, which is characterized by the flat character of the hills. The north to south ridge character of the Green mountains is interrupted here, and replaced by gently Fig. 25 Amphibolite. Same locality as 24. The amphibolite is Interatratified here with quartzite. rounded elliptical hills forming an open grazing country. The railroad from Bellows Falls to Rutland crosses the axis of the mountains at this place. We notice that the soil is colored a deep red and soon find that this is due to the decay of masses of these amphibolites, which are interbanded with the highly contorted gneisses of the region. Figs. 24, 25, 26 show this very well. These bands of rock are parallel to the strata of the gneiss in most cases, but here and there send out across the strata tongues which have a fine grain at contact and show that these rocks are intrusions. They have in general a perfectly parallel sti-ucture, which curves with that of the inclos HOOSAO MOUNTAIN. 67 iiig gneisses, but also a marked columnar jointing. The form of the hills and the very existence of this topographical belt seem due to the rapid erosion of these rocks. Their field relations show that they are of intrusive origin — dikes, in fact, injected parallel to the strata and then crumpled and metamorphosed — and their microscopical characters agree with those of similar rocks, described by Lossen, Teale, and many others, which have been recognized as altered dikes. They correspond in part to the "metamorphic diorites" of Hawes.^ They are briefly described by President Hitchcock in the Geology of Vermont, Vol. ii, p. 578, where the remark is make that they "may be only huge dikes." Fig. 26. — Cruniplert ampliibolite, Mount Holly, Vurmout. Natural size. The white bands are feldspar, tlie dark bands hornblende principally. The vertical groovings which coincide with the lino of apices of the folds (the specimen standing as in nature) show but faintly in the figure, and are doubtless caused by rain tlowing over the vertical surface and following the depressions between the small folds. In the hand specimen we see a dark heavy rock, with very faint parallel structure in the coarse vai'ieties. Studied in thin section these rocks have very uniform characters; the least altered forms, of coarser grain, are composed of crystalloids of hornblende and rounded grains of plagio- clase feldspar. The hornblende is a massive brownish-green variety in short irregular crystalloids, the central parts of which are filled with a dark opaque substance, which, with high powers, is resolved into a mass of little crystals of rutile; they sometimes inclose crystals of apatite. In some ' Litliology of New Hampshire, p. 225, 68 GREEN MOUNTAINS OF MASSACHUSETTS. parts of the rock these grains of hornblende fit in between rounded grains of a twinned plagioclase. In other places in the rock the hornblende is seen to have a narrow fringe of light green pleochroic hornblende (see PI. IX, b), massive and not fibrous ; in other grains this entirely replaces the brown hornblende, or only little cores of the latter are left. At the same time the feldspars in those parts of the rock are filled with small acicular crystals of the same green hornblende associated with small grains of pla- gioclase, and minute veins composed of these two minerals often cross the original feldspars by narrow fissures (see PI. ix, a). The extreme change consists in the entire replacement in parts of the rock of the feldspar and hornblende by an agg-regate of these small secondary feldspars, with a little quartz and epidote in abundance. It is plain that the original plagio- clase and brown hornblende has changed to a new plagioclase, green horn- blende, some quartz, epidote (taking part of the lime from the feldspar), and a little calcite. In another form the rock is a fine-grained amphibolite composed of crystalloids of bright green or bluish green hornblende, rarely inclosing small cores of original brown hornblende, and plates of biotite; both these minerals lie in planes, causing the schistose structure. The remaining space is filled with little plagioclases which are rarely polysynthetically twinned and are filled with grains and prisms of epidote. Grains of titanite surround small black cores of original titaniferous iron ore and sometimes the titanite grains run out in stringers parallel to the schistosity. These feldspars contain, in addition to e^jidote, titanite grains, needles of hornblende, biotite flakes, and grains of quartz. In some rocks the little prisms have the characters of zoisite instead of epidote. These feldspars may occur in broad simple twins like the albite of the schists, or may be polysynthetically twinned. The feldspar was isolated from several rocks by the Thoulet solution and found to be always plagioclase, generally toward the albite end of the series. The hornblende contains titanite and epidote; the plates of biotite contain rutile needles. A few of these rocks carry irregular masses of red garnet which alter to chlorite; they inclose masses of magnetite and green hornblende with cores of brown hornblende. The garnet seems to be contemporaneous with the feldspar. HOOSAC MOUNTAIN. 69 One vertical dike of this rock at Stamford, Vermont, contains blue quartz grains and broken crystals of microcline, which have been taken from the country rock of the dike, the granitoid gneiss (Stamford granite). GEOLOGY. For convenience of description the region covered by the map (PL i) may be divided as follows: First. The Hoosac tunnel. Second. The region embracing the central part of Hoosac mountain from the tunnel line on the north to the point in Cheshire where the crest of the mountain makes an offset to the west. Third. The area covered by the schists occupying the northern and eartern parts of the map. Fouilih. The region south of Cheshire and of the Hoosic valley. Fifth. Hoosic valley schist. Sixth. The region around Clarksburg mountain and Stamford, Vermont. THE HOOSAC TUNNEL. This great engineering work is 4f miles long, entering the base of Hoosac mountain from the Hoosic valley on the west, and running in a nearly due east direction across the trend of the range. Two shafts have been sunk; the deepest, the central shaft, near the center of the tumiel, is about 1,000 feet deep, descending from the basin-like depression on top of the mountain. (See PL v, Profile iti). The other, the west shaft, is not quite half a mile from the west portal, and is 325 feet deep. About 1,000 feet west of the west shaft, a small shaft called the "well" was sunk, on the dump of which specimens of the rock are found. The tunnel itself is a large double-track opening, which, starting from the Stockbridge limestone at the west portal, passes through all the rocks of the series at least once. But several tilings combine to greatly lessen its value as a geological section of the core of the mountain. A considerable proportion of the tunnel is now bricked over, and only in the manholes, every 250 feet, can the rock be seen; and secondly, the covering of soot and smoke on the rock is very thick, making it necessary to get fresh sur- faces by hammering. The difficulties of working by lamplight in the smoke 70 GREEN MOUFTAIlSrS OF MASSACHUSETTS. of passing trains are also considerable. Moreover, that part ot the tun- nel wliich would have afforded the most important contact for determining the relations of the Stockbridge limestone to the Hoosac mountain rocks is entirely bricked over; it lies in the decomposed rock which caused so much trouble during the building of the tunnel. Therefore, while the general distribution of the rocks is easily found in the tunnel, much less was done in the way of determining relations by contact than would have been possi- ble under more favorable conditions. In the following description the reader is referred to Profile in, PI. v. Starting at the west end of the tunnel we find the Stockbridge lime- stone of Hoosic valle}^ in the long open cut which leads to the tunnel mouth, and passing under the masonry of the portal ; the dip alternates in a series of small folds, sometimes east, sometimes west. From the portal for 2,700 feet the tunnel is bricked, but at several of the manholes we find rock in place. At a little over 1,600 feet we find in a manhole the first 'occurrence of the fine-grained variety of gneiss with small porphyritic feld- spars, and the same rock again at about 1,900 feet in. Near 2,000 feet the albite-schist (Hoosac schist) is found in all of the manholes to about 3,800 feet. Then by transitional rocks this passes into the white gneisses which extend to 6,000 feet, where by gradual transition they pass into the coarse granitoid gneiss; this rock runs as far as 10,500 feet, then after 250 feet of bricking the conglomerate-gneiss is found at 10,770 feet, and this extends to 12,100 feet, where the albite-schist series is found in conformable con- tact with the conglomerate-gneiss. The albite-schist, succeeded by the Rowe schist, is then found through the rest of the tunnel. We find then in the tunnel, going in from the west: first the limestone, which extends into the tunnel proper a short distance, but is now entirely bricked in; then the fine grained, banded, white gneiss (Vermont formation), extending to about 2,000 feet from the portal; then the albite-schist for 1,750 feet; next the white gneiss (conglomerate-gneiss) series (Vermont) for a little over 2,000 feet; then the granitoid gneiss (Stamford gneiss) for a little over 4,000 feet; then white gneiss-conglomerate for 1,500 feet; and the schist formation (Hoosac schist overlaid by Rowe schist) for the rest of the way, or about 12,900 feet, of which the last 6,000 is occupied by the gi-eenish sericitic or chloritic Rowe schist. TT S.G>:ni,OGIGAJ- SUHATTi'. MONOOnATU 5XIIL pl.t: GEOLOGICU^ PROFILES 0¥ HOOSAC MT. EAST- WEST PROFILES. \. Northern Section Hoosac Mi H.Section from Natural Bridge through Schist ndge W •:^LJ'ortat, Wh 6n Schist. iVhife Gn Granitoid Gnoiss Can^hmeratS AlbitB Sc/iist. IS.. Hoosac Tunnel ant^ Hoosac Mi. ffar\-e Schist W. Sect/on running upl^t CreekS of Tunrrel Line to Spruce Hill. " Buttress TJ.^Sect/on through Buttressto crest Hoosac Mt. Hoosac Mt f the U. S. GEOLOGICAL SURVEY h-ONOGRAPH XXIIl PLATE VII THIN SECTIONS, WHITE GNEISS. PLATE VIII 111 PLATE VIII. A. Fiue graiueil white gueiss (Vermont lormation). Hoosac uiountaiD. Microphotograph. Pol- arized light, X 33. Poi-phyritic feldspar twin (a) containing Inclusions of quartz and mica which are arranged parallel to the minerals of the groundmass outside. B. Albite schist (Hoosac schist). Hoosac mountain. Microphotograph. x 33. The large crystals of albite (a) contain inclusions of muscovite, chlorite, magnetite, and quartz. The gentle curving of the mica of the groundmass between these feldspars is well shown. 112 U, B. QEOLOQICAL SUHVEV MONOORAPH XXart< ■.I'." ^ ■ ■ ^ 13. I ^^ PLATE IX. A. "Amphibolite." Diorite dike. Hoosac moiiutain, south of Cheshire. Microphotograph, x 33. Crystalloids or grains of plagioclase feldspar (a) and of brown hurnblende (fc) are seen around the edge of the figure. In the center we have an aggregate of irregular patches of secondary feld- spar, green hornblende, epidote, etc., forming a confused aggregate, little veins of which are seen to penetrate the feldspars or pass between them. B. Amphibolite. Mount Holly, Vermont. Microphotograph; polarized light x 33. The large black areas are a deep greenish-brown hornblende, surrounded by a fringe of light green hornblende. This shows best in the crystal in the center («) with the fringe (b). The portion between the black crystals is an aggregate of epidote prisms, masses of green hornblende, and feldspar. 114 M ■'NO.j'^APH THIM SECTIONS, DIORITE AND AlllWl»BOL(TE. A. "Amphibolite.' I Crystalloids or graiiiM n the eUg«< of the ';.'"■■ <• > spar, green boi ; penetr.'ite <'■ B. A" T' lack pretM iile. This shd botwcoi. Uiu l)lack cryst )'<-l' ;i il Mi'To|ihotograph, X 33. ■ ndc (fc) are secu around .. |i:itch<4 of secondary t'vhl- tle veiiie of >vlii(h are iu^-5H -■:*.«5rwt.-"-'' "•^^i A. VIEW NORTH OVER CREST OF HOOS«C MOUNTAIN. jl^/^- ^'^^Jf ll^^v^fe- B. PROFILE OF HOOSAC MOUNTAIN FBOM SPRUCE HILL SOUTHWARD, LOOKING WEST. P^RT III. MOUNT GREYLOCK: ITS AREAL AND STRUCTURAL GEOLOGY By T. n'ei.so:n d^le. 119 CONTENTS. Page. Outline of tliis paper 125 Historic 131 Physiographic 133 Structural 136 Types of structure 138 Correlation of cleavage aud stratification 155 Pitch 157 Structural priuciples 157 Structural transverse section? 158 Transverse section G 160 Transverse sections H, I 166 Transverse sections A-F, J -0 169 General jjitch of the folds 175 Longitudinal sections 175 Longitudinal section P 175 Longitudinal section Q 176 Longitudinal sections R' . R" 177 RpsumtS, structural 177 Lithologic strat'graphy 179 Vermont formation 179 Stockbridge limestone 179 Berkshire schist 180 Bello wsiiipe limestone 180 Grey lock schist 180 Petrography 181 The Vermont formation 181 The Stockbridge limestone 181 The Berkshire schist 182 The Bellowspipe limestone 184 The Greylock schist 186 Thickness 188 Geologic age 189 R. Chap. vi. Plutonic and metamorphic rooks; cleavage and foliation. Daniel Sharpe: On slaty cleavage. Quarterly .Journal, Geol. Soc. London, vol. 3, 1847, p. 74. Henry Clifton Sorhy : On the origin of slaty cleavage. ildinb. New Philosophical Journal, vol. 53, 1853, p. 137. John Phillips: Report on cleavage and foliation in rocks, and ou the theoretical explanations of these phenomena. Report of British Association for the Advancement of Science, Part 1, 1856, p. .369. Henry Clifton Sorliy : On slaty cleavage as exhibited in the Devonian limestone of Devonshire. Philosophical Magazine, ser. iv, vol. 12, London, 18.56, p. 127. John Tyndall: On the cleavage of slate rocks. Philosophical Magazine, ser. iv, vol. 12, London, 1856, p. 129. Samuel Haughton : On .slaty cleavage and the distortion of fo.ssils. Phil. Mag., ser. iv, vol. 12, London, 1856, )). 409. 'See Appendix H, Kigs. 77, 78. ■'All comjjass readings in this report are cori'ected for variation. MOUNT GEEYLOCK. 139 CA.SE II. About 150 feet northeast of locality G02 there are two very small folds in the limestone, passing into a very low southwesterly dip on the west. (See Figs. 32 and 78.) In the overlying plumbaginous schist there are corresponding undulations, but these ai'e compounded of more minute ones and crossed by cleavage planes. Where the plications dip 50° south- west the cleavage planes dip 40° to 50° east. Where the former dip 15° to 20° southwest Fig. 32.-Diagraramatic sketch of the north ^ side of a lodge at locality 297, on Quarry hill, the latter dip 35° east, and, again, where the now Ashtowi, showing phimbaghious s.hist in ■*- / f CD f conformalile contact with unucrlying crystal- former are more nearly horizontal the latter ""» "'"'^t'""". ''"'i ■'' cleavage foliation cross- ^ ing the stratification ioliation ot the sc^hist at are vertical. P'ig. 33, taken fi-oni the upper 7*™"^ angles. portion of the section (Fig. 32), shows the relations first described. Fig. 34, taken from a slightly enlarged ])hotograph of a large section of a specimen from the same portion of tlie ledge, shows more distinctly what is but slightly apparent in Fig. 33, namely, that the cleavage planes arise in a faulting along the shanks of tlie plications. In many cases tlie faulting is only incipient. In a specimen from the central part of the leda'c where the cleavasfe planes are ^'ertical they are simple joint-like fractures across the stratification folia- tion ftf the schist, but along one of these faulting has occurred, and the stratification foliation is bent about into the direction of the cleavage. We have here, then, a cleavage which is in jiart a microscopic joint- ing, in })art what Heim has called "Ausweichungsclivage"' (slip cleavage), Fig. 33.— Specimen in inverted position, facing south, from the upper part of thi- rock figured iu Fig. 32, locality 297, Quarry hill. New Ashford. PUiiiibagiuous schist with .a stratification foliation dipping southwest about 50°, crossed by a coarse cleavage foliation dipping 400-50° E. From a photograph. > See Heim, op. cit., vol. ii, ji. 5i, Gesetz 1, aurt Atlas, PI. xiv, Figs. 17, 18 ; PI. .xv, Figs. T, 8, 9, 11, 14. 140 GREEN MOUNTAINS IN MASSACHUSETTS. resulting in a coarse foliation crossing the stratification foliation at angles varying from 45° to 90°, and abutting against the limestone which under- lies th.e schist in conformable contact. Fig. 34. — Thin aection of a specimen from tiie upper part of tbe rock tigured in Fig. 33. locality 297, Quarry hill, Now Ashfurd, enlarged almost 2 diameters, showing cleavage planes arising in slight faults along the aides of the plications. The fractures which occurred in the preparation of the slide are mainly in tbe direction of the stratification foliation, which here dominates. CA.SK III. At the south end of Sugarloaf mountain, one of the subordinate folds of the Greylock mass, a small isolated mass of feldspathic schist over- lies the crystalline limestone. (See map, PI. i, locality 324 and Fig. 35.) Here limestone and schist are seen in contact, lioth distinctly plicated, and FlQ. 35.— Diagrammatic sketch of the south side of a ledge at locality 324, south foot of Sugarloaf mountain. New Ash- ford, showing albitio .schist in confonnable contact with underlying cryataUiue limestone, and a coarse and line cleavage foliation crossing the stratification foliation of both rocks. dipping in a general westerly direction, but really forming part of a minor fold. Where the stratification foliation dips jGO° west it is crossed by cleavage planes dipping 35° east, which in places traverse both rocks. The limestone a few feet away from the schist appears in thick beds. Both schist and limestone are traversed here and there by coarse or fine cleavage. MOUNT GREYLOOK. 141 The presence of both cleavage and stratiticatiun in limestone is also seen in a small mass a little north of this locality (Fig-. '66), probably Fig. 36.~Block of limestone hotograph. Fig. 49 represents the southwest side of the same ledge, together with a portion of its southern side, and also shows the relations of the two folia- tions. The behavior of the cleavage and stratification foliations, when in proximity to a thick quartz lamina, is beautifully shown in Fig. 50, which represents a section from a specimen from locality 184, in Goodell hollow. The general parallelism of the coarse quartz lamina to the minute plications in the schist on either side of it and the cleavage planes arrested by the quartz will be observed. The longitudinal cracks in the quartz are pos- sibly due to strain, as are also the transverse cracks in the quartz lamina in Fig. 40. These facts indicate that the dip of the stratification foliation may be 150 GREEN MOUNTAINS IN MASSACHUSETTS. shown by the general dip of the thick quartz laminae when such lannnte can be distinguished from cleavage foliation quartz larainfe. Locality 207 furthermore shows that stratification foliation may be so completely oblit- erated that cleavage foliation alone is determinable. Fig. 50.— Thin section of aericite-chlorite-schist traversed by a coarsely plicated quartz lamina, from locality 184, Goodell hollow, enlarged 2 diameters, .showing the relation of tlie cleavage to the quartz. lu preparing the slide fractures have occurred along cleavage planes. From a photograph. CASE VII. On the southwest side of Bald mountain, locality 242, the schist is traversed by two sets of foliations with different strikes The stratification foliation, distinguished by its plications, and in part by the continuity of the mineral constituents of the lamina?., strikes north 40° to 50° east, and dips 60° southeast. The cleavage foliation strikes north, and dips 35° to 40° east. The correctness of this observation is coiToborated by one at locality 95, on the northern face of Bald mountain, about 4,000 feet nearly in the direction of the stratification strike as thus deter- mined. There the stratification foliation is indicated by great sheets of quartz striking north 45° east, and dipping about 75° southeast, corre- sponding to the minute plications in the sun-ounding schist, which are crossed by a cleavage foliation striking north 3° to 5° east, and dipping 55° east. The probable correctness of both these observations is still further increased by the trend of the central ridge of Greylock, which, southeast of MOUNT GEEYLOCK. 151 those localities, is also northeast. A large ledge of schist at Readsboro, ill Vermont, in the Grreeii mountain range (Fig. 51), shows on a large scale the two sets of foliations and quartz laminse, with diiferent strikes and dips, and will serve to illustrate what is not uncommon on Clreylock in similar rocks. The parallelism between the strike of the cleavage and the strike of the axis of the great folds has long been recognized in geology When, therefore, the axis of the fold lies horizontally the strike of the sides of the fold will conform to the strike of the cleavage ; but when the axis of the fold is in- clined, i. e., when the fold pitches, the strike of the sides of the fold Fig. 51. — Sketch of west side of schist ledge in Readsboro will not conform to that of the linage, Tt.,showiii{; stratiHc-itiou striking N. 20° E, and dlp- l)ing 250 west, crossed liy cleavage striking N. 15° W. and dip- cleavasre. This, Prof. Pumpelly I'™S 55° east, with quartz laminjB in both foliations. As the '' face of ledge is not parallel with the strike of either foliation suggests, is the most probable ex- tlie app-irent angles of dip are not the tme ones. planation of these differences between the strikes of the stratification and cleavage. The conformity which Heim finds in the Alps between the strikes of the two foliations does not hold here.^ case; VIII. In Goodell hollow, locality 175, southwest of Bald mountain, there is a schist with three sets of planes or foliations, set a striking north 5° east, and dipping 35° to 45° east; set h striking north 20° east, and dipping 40° east; set c striking north 80° east, and dipping 70° north. An enlarged thin section (Fig. 52) shows that the minute i^lications follow the direction of set b, while set a is formed by a slip cleavage more or less pronounced, and set c by the infiltration of dark mineral matter in planes, possibly fractures, traversing the other two sets without altering their structure. This interpretation of this locality is also confirmed by the strikes and dips observed in its vicinity. At locality 132, near the west end and on the ' See his law 13, op. cit., vol. 2, p. 68. 152 GREEN MOUNTAINS IN MASSACHUSETTS. south side of the Bald mountain spur, there" is a small ledge in which the stratification foliation dips 35° west, the cleavage foliation 25° to 30° east, and a secondary cleavage horizontally or very low west. Some vertical joints strike north to south through all these planes. Fig. 52— Thin section of sericite-clilorite-schist from locality 175, Gootlell hollow, the larger figure enlarged nearly 2 diameters and the smaller ahout 10 diameters, showing two cleavage foliations crossing the stratification. In preparing the slide a fracture has occurred along Cleavage I. From photographs. Secondary cleavage foliation occurs here and there in the Greylock area.' ca-sp: IX. Fig. 53 represents an enlarged section of plicated schist from locality 550, about 1,500 feet south of the top of Greylock. The area in the larger and upper fragment measures 1 J by J inches. The specimen from which the section was made showed, in its original position in the ledge, a stratifi- cation foliation about horizontal or dipping west at very low angle, crossed by a cleavage foliation dipping 60° east. From the direction taken by the breaks, which occui-red in the preparation of the slide, it seems probable that in some portions of the rock here the cleavage foliation dominates. Fig. 46 represents a hand specimen from the same ledge in its natural posi- ' Two sets of cleavage planes are uotlced in the slate on Welden's island, Lake Champlain. Geol. Report Vermont, vol. 1, p. 314. A. Baltzer, in the Beitriige zur geologisohen Karte der Schweiz (20te Lieferung, Bern, 1880. Atlas, pi. m, fig. 8, and xiii, figs. 14, 16) figures two cleavage foliations traversing the same rock. Archibald Geikie, in his report on the recent work of the Geological Survey in the northwest highlands of Scotland, describes a double foliation in eruptive gneiss. Quart. Jour. Geol. Soc, London, vol. 44, Aug. 1888, p. 398-400. MOUNT GREYLOCK. 153 tion. The same stratification foliation and cleavage foliation dips recur at locality 549, some 2,500 feet south-southeast, and at locality 539 (see Fig. 54) about 1,000 feet west, and again at the top of Greylock, and may thus be said to characterize the entire eastern portion of the summit of the mountain. If, therefore, the larger microscopic specimen in Fig. 53, which only measures 1^ by J inches, be properly oriented it will correctly represent the structure of an area measuring about two-thirds of a square mile, and probably the entire east side of the highest syncline of the Gi-eylock mass. (See Sec- tions G, H, I, PI. XX.) Fig. 53.— Thin section of sericite-iihlorite-schiat from locality 550, about one-quarter mile south of Greylock summit, enlarged 2^ diameters, sliowinj; a coarse slip cleavajie crossing a very minutely plicated stratification. In preparing the slide fractures occurred mainly in the direction of the cleavage, here the direction of least resistance. From a photograph. The microscopic structure thus often epitomizes the general structure on one side of a fold. This fact agrees with the drift of what Mr. Heim implies in regard to the structure of the Toedi-Windgaellen-Gruppe namely, that physical causes have transformed great masses by transforming the minute particles which constitute them.' This generalization must not be ' Op. cit., vol. II, p. 99. 154 GKEEN MOUNTAINS IN MASSACHUSETTS. carried too far, however, for local changes may occur for a brief space in the direction of the plications and of the cleavage foliation, owing to the pres- ence of quartz nodules ; or there may also be minor undulations on the side of a great fold. Fig. 54.— Specimen of schist from locality 539, about one-quarter mile .southwest of Greylock summit, in its natural position, facing south. Stratification nearly horizontal ; cleavage dip 500-50° east. From a photograph. CASE X. Loc.576 Schist The above cases are sufficient to illustrate the structural significance of stratification and cleavage and the distinction between them in the region under investigation. With the aid of these a fault was detected which would otherwise have escaped notice. Near the west end of the Bald mountain spur there is a somewhat lenticular area of limestone trending north and south, and in contact on both sides with schist. On the west side the contact phenomena are as indicated in Fig. 55. The limestone overlying the schist dips from 45°-60° east, the contact plane between both 55° east; the schist cleavage dips 25°- 55° east, but the plications in the schist dip tvest at a somewhat higher angle. The nor- mal position of this limestone is under the schist; here it is above in conse- quence of a fault. At this point 'the stratification foliation in the schist is very much plicated, and the cleavage faulting divides up the rock into lens Cleavage dip. East. Fig. 55. — Diagram showing the relations of the Beikshire schist and Stockbridge limestone at locality 576, on the Bald Mountain spur, look- ing north. The cleavage of the schist conforms to the stratification of the limestone, but the stratifications are unconformable. MOUNT GREYLOGK, 155 or wedge-shaped masses. This is the typical slip cleavage. The minute structure at the contact, as seen in a microscopic section, corresponds to that represented in the diagram. Fig. 55. The inference from such facts is that while conformable contacts are all-important in determining strati- graphic relations in a metamorphic region they may be entirely misleading unless it can be shown that the foliations which conform to the plane of junc- tion between both rocks are indeed stratification foliation.' CORRELATION OF CLEAVAGE AND STRATIFICATION. The facts adduced naturally raise the question as to the general cor- relation of cleavage and stratification. The relations of the strikes of the two foliations have already been explained under Case vii. As to the dip Fig. 56. — Thin Bection of schist from locality 115, on the Bald Mountain spur, enlarged lA diameters, showing the paral- lelism hetween the cleavage planes and the axial planes of the plications. From a photograph. of the two foliations: The range of the difference in angle of dip between cleavage foliation and stratification foliation in sixty-three observations was found to be from 10° to 120°;' the average difference 62°, 30'. The abso- lute dip of cleavage in ninety-six observations, in which the dip of stratifi- cation foliation was also observed, ranged from 10° to 90°, averaging about 45°; leaving out eleven extreme cases the range was from 25° to 75°, and the average 44°.^ The direction of the dip of the primaiy cleavage in one hundred and nineteen localities, in which that of the stratification was also determined, was distributed as follows: ninety-two localities east or north- east, twelve west, four vertical, one south. The southerly dip occurs at the • Compare J. D. Dana, Taconic rocks and stratigraphy. Am. Jour. Sci., Ser. ill, vol. 33, May, 1887, p. 398, in ■which the possibility of such cases as this is overlooked. ■^Wben the difference is over 90° the direction of the two dips is opposite. 'Where cleavage is horizontal and stratification nearly or quite vertical, as is sometimes the case in the Berkshire county schist, there have probably been two uplifts. 156 GREEN MOUNTAINS IN MASSACHUSETTS. Cleavage dip East W.H.H. north foot of Mount Prospect (Saddle mountain) where there is a well marked southerly pitch. The westerly dips occur in one of the subordinate schist masses, which forms a high cliff west of Cheshire reservoir, and again in a low knoll of schist at the extreme sotith limit of the map, near Berkshire village, and in a similar knoll south of Constitution hill and west of Lanesboro. Besides these there is one isolated observation between Lanesboro and New Ashford, and two others on Ragged mountain. So that the observations indicate an almost universal easterly cleavage dip on Greylock. The question may well be asked how this can be, since the cleavage is so largely associated with the faulting of minute plica- tions in strata which sometimes dip east and sometimes west. The observations indicate that where the sides of a fold dip in a direc- tion opposite to that of the cleavage the axial planes of the small plications are generally parallel to the cleavage planes, and in extreme cases the faulted limbs of the plications lie in those planes (see Fig. 56). Fig. 57 represents this structure dia grammatically, as drawn in the field. Where the cleavage foliation and stratification folia- tion both dip in the same direction, but at different angles, the structure described in Figs. 56, 57 does not occur, and the slip cleavage planes are then either parallel with one or with neither of the limbs of the plications as in Fig. 59, or else there is a com- bination of an extreme form of slip cleavage bor- ,, ^„ T^• ^ . , ^^ t: a FiQ. 58.— DL-igram of p,art of north derinff on slaty cleavage and of the coarse structure, "If "^ '"'""' 'r'^"' ''"'^"'^' ^"' ""'* o J o ^ side ot Deer lull, area 7x5 feet, show- described in Case vi, Fig. 48, and seen also in Fig. 58, *"« ™"™'y p""^*"'' i""'^ '^"""* *^ o ' traversing the schist, which has a in both of which the coarsely plicated quartz laminae ^^^^^^s^ bordering on siaty cleavage. are more or less iudepeudent of the cleavage foliation. Or, the cleavage Fig. 57.— Diagrams ahowing the relation of slip cleavage to stratification at locality 55, north side of Mount "Williams, and 862, ridge south of Sugarloaf. Cleavage parallel to axial planes of plications. MOUNT GEEYLOCK. 157 foliation, as such, may disappear altogether, becoming merged in the strati- fication fohation. Tluis, at the south end of Ragged mountain, there is a minor syncHne, on the east side of which the cleavage has a high easterly dip crossed by plications dipping 90°, or west, at high angle, while on the west . -, p .1 • !• .1 . ,-f i- r 1- • ''"' 5!».— Diagrams showing relation of Side 01 this synclme the stratm cation tohation cieavas,. to stratification in scinstwiure both T ^»i-n . r>/\(- ^ 1 T ■• . 1 foliationsdipin samedirectiou; cleavagepar- dips 25 to 30 east and no distinct cleavage allel to one omeither Umb of plication. foliation is visible. (See Fig. 62.) PITCH. Early in the work my attention was directed by Prof. Pumpelly to methods of detecting the pitch of the axes of folds. Observations of pitch were made in fifty-four localities on Greylock, East, and Potter mountains. In a few places minor pitching folds are exposed, as in the limestone at the south base of Sugarloaf mountain (Fig. 60). But pitch was usually determined by ob- serving the pitch of the axes of the plica- tions of any part of a fold. The angle varies from 5° to 45°, but generally is not over 30°. In one or two instances it was over 45°. The correctness of the method seems to be verified by the general parallelism which exists between the minute and general structure of these rock masses, and also by the opposite directi(Mi of the pitch as thus determined, at the extreme ends of the mountain.' STRrC"rURAL PRINCIPLES. From the foregoing data the following structural principles may be laid down as applicable primarily to the study of the .metamorphic rocks of Mount Greylock, and then to a large part of the Taconic region and to similar rocks and regions. ' See, on the siibjeot. of pitch, Geo. H. Cook, Geology of New Jersey, Newark, N. .!., 1868, p. 5,5; on the inclination of the axes of the flexures in the Taconic region, J. D. Dana, Taconic Rocks ami Stratigraphy, Part 2, p. 399, already cited* Fig. 60. — Minor limestone folds with a northerly l)itch, south foot of Sugarluaf, New Ashford. Rock 50X30 feet. 158 GREEN MOUNTAINS IN MASSACHUSETTS. Tli« lamination in «cliist or limestone may be either stratification foliation or cleavage foliation, or possibly a combination of both. False bedding occurs in limestone also. Therefoi'e the conformability of two adjacent rocks is only shawn by the conformability of the stratification foliation of both. Stratification foliation is indicated by : (a) the course of minute plica- tions visible to the naked eye, (b) the course of the microscopic plications, (c) the general covirse of the quartz laminae whenever they can be clearly distinguished from those which lie in the cleavage planes. Cleavage foliation may consist of: (a) planes produced by or coincident with the faulted limbs of the minute plications, (h) i)lanes of fractm-e re- sembling joints on a very minute scale, with or without faulting of the pli- cations, (c;) a cleavage approaching "slatj^ cleavage," in which the axes of all the particles have assumed either the direction of the cleavage or one forming a very acute angle to it, and where stratification foliation is no longer visible. These forms may all occur in close proximity. A secondary cleavage, resembling a minute jointing, occurs in scattered localities, and, although not yet very satisfactorily observed on Greylock, original cleavage foliation may become plicated by secondary pressure. The degree and direction of the pitch of a fold are often indicated by those of the axes of the minor plications on its sides. The strike of the stratification foliation and cleavage foliation often differ in the same rock, and are then regarded as indicating a pitching fold. Such a correspondence exists between the stratification and cleavage foliations of the great folds and those of the minute plications that a very small specimen, properly oriented, gives, in many cases, the key to the structure over a large portion of the side of a fold. STEUCTURAL TKANSVEESE SECTIONS. On these principles twelve complete and three partial transverse sec- tions have been constructed across the Greylock mass; there are also three across Stone hill, to which reference will be made in Appendix A. All of MOUNT GKEYLOCK. 159 these are oil the same vertical aud horizoutal scale.' The first section, A, crosses the north end of the mass at North Adams; the last, 0, toward its south end, between Cheshire and Berkshire villages; and the others at more or less regular intervals between. See map (PI. i) for section lines, and Pis. xviii-xxii, for sections. The sections show that the range consists of a series of more or less open or compressed synclines and anticlines, which, beginning near North Adams, increase southerly in number and altitude with the increasing width and altitude of the schist area, and then, from a point about a mile and a half south of tlie summit, begin to widen out and diminish in number aud height until they finally pass into a few broad and low undulations west of Cheshire.^ Betvv'een that point and the villages of Lanesboro and Berkshire the folds become somewhat sharper and more compressed, and the schist mass rapidly narrows. The most comprehensive and best substantiated of these sections are those two which, beginning near South Adams, cross the central ridge north aud south of the summit and then follow the two great western spurs aud end near South Williamstown. These sections will now be described in detail. It ' Prof. E. Emmons (American Geology, vol. 1, p. 19) gave a section of Greylock running from Cheshire harlior, across the summit, and Mount Prospect, to Sweet's Corners and Stone hill. Prof. .James Halls section, from Petersburg to Adams, made between 18.39 and 1844, but unpub- lished, showed the synclinal structure of Greylock. Prof. E. Hitchcock (Vermont Report, vol. 2, pi. 15, fig. 5) gave a section similar to, Imt less detailed than that of Emmons. Both of these are drawn on a greatly exaggerated vertical scale, and represent the mountain as a simple syncline. Prof. J. D. Dana, in his paper on "Taconic Rocks and Stratigraphy" (p. 405), reproduces Emmons's aud Hitchcock's sections, and adds several fragmentary ones of his own. On the east side, one west of Nortli Adams (Fig. 47), another west of South Adams (Fig. 44); on the west side, one on the west flank of Mouut Prospect and north of the Hopper (Fig. 45). and another on the south side of tlie Hop- per (Fig. 46); all of whicli simply represent the relations of tlie schist to tlie limestone ou either side of the syncline, along the base of the mountain. In his paper on the " Quartzite, Limestone, and Associated Rocks of Great Barriugton," etc. (1873, p. 273); and again in his paper "On the Relation of the Geology of Vermont to that of Berkshire" (1877, p. 263), he conjectures from the north aud south trend of part of the ''llojiper" depression that the Greylock syncline comprises one or more subordinate folds. - The sections have all been carried down to the top of the ijiiartzite which underlies the Stock- bridge limestone. The observed dips have also been indicated on them to enable the reader to dis- tinguisli between matter of actual observation and of ordinary induction. The cleavage dips have beensimilarly indicated, but ou a separate Hue, and the cleavage foliation has also beeu shown on the drawings crossing the stratification wherever both were observed, but it doubtless traverses the greater part of the mass. 160 . GREEN MOUNTAINS IN MASSACHUSETTS. TRANSVERSE SECTION G. From the Huoaic river at Renfrew milU (South Adams) across Ragged monntain, the central ridge, Symonda peak {Mount Prospect), and the north end of Deer hill See PI, xx and Fig. 61. Between the most easterly and the most westerly outcrops in the lime- stone ai'ea along the east foot of Greylock there is a syncline followed westerly by an anticline. This is corroborated by observations about the quarries a quarter of a mile north. The well-knt>wn relation of the lime- stone to the schists farther up the mountain is not shown here, but may be seen on Section B, PI. xviii, about 1 J miles south of North Adams, locality 28, where the limestone, after fomiing a very small anticline, ruptured and partially eroded, dips, a few feet west of it, at an angle of 15° to 30° west, conformably under the schist, both rocks striking north 25° east. Above, the Hoosic valley limestone comes a mass of schist which forms the lower, more precipitous, and wooded slopes, and which, along this Fm. 61. — Section G, J'rotii the Hoosic river across Ragged mountain, the Central ridge, Synionds jieali (Mount Prospect) and Deer hill. section, dips west at an angle of about 30°. Above these schists is a bench of arable land stretching for several miles along the east side of Ragged mountain. This mountain forms the higher portion of the northern end of the range as seen from Hoosac mountain (PI. xii), but is separated from the central crest by the "Notch," the south end of whicli is called the " Bellowspipe," from the prevalence of wind there. (Bee PI. xvi.) This bench on the east of Ragged mountain measures about 600 feet in width and is marked by outcrops of a micaceous limestone which here dips 70° to 75° west. The bench seems to owe its agricultural value in part to the rapid decomposition and soil-forming quality of this rock, and })robably in part also to the fact that this more deeply eroded strip of the mountain flank has formed a receptacle for sand and soil which would have been drained off a steep slope. At several points on tlie west side of the bench the micaceous limestone comes in close proximity to another mass of schist, but the upper contact is covered on this section. At localities 838, 839, SeC' U. S. GEOLOGICAL SURVEY MONOGRAPH XXIII PLATE XVI SOUTHERN END OF RAGGED MOUNTAIN. Seen from locality 190, about one-half mile south, showing the easterly dipping Greylock schist (Sg) in contact with the Bellowspipe llnnestone (Sbp) on the west side of Ragged nnountain, and the saddle 14 birdsl due to the erosion of the limestone anticline (Sbpl The hollow to the left is the Bellowspipe. The pasture land on the right corresponds to another area of Bellowspipe limestone. From a photograph. MOUNT GREYLOCK. 161 w AibiUc chlar-mtca Schist deavoffe tUp 43 °^ StraUfdip Cleayoffe dip ^ Sdnv tion E, PI. XIX, both rocks dip west, and at 669, Section F, both are Hori- zontal, the limestone underlying the schist in all cases. In ascending the east side of Ragged mountain, over this second mass of schist only westerly dips are met, but on Sections E, C, and again about a mile south of Section Gr (localities 204, 126) there are some well-observed eastern dips following westerly ones and indicating a syncliiie, which, proba- bly being less open at this end of Ragged mountain, escapes observation. Near the top is a narrow belt of calcareous schist forming a north to south ravine across the ridge and connecting the limestone area of the Notch with that on the south. Beyond is a small, isolated schist area which forms the south end of the top of the Ragged mountain ridge. The dips continue westerl}'. In de- scending into the Notch the calcareous schist re- curs, dipping 60° east and indicating another syn- cline. The syncliue of this small schist area is best seen about a half a mile south of the section line, and has already been referred to on p. 157. (See Fig. 62.) On the east side of and close to the schist, the calcareous schist (plicated) dips 90° and west at high angle; the schist (feldspathic and chlor- itic) is also plicated iu the same direction, with a high, easterly cleavage. Again, at locality 733, about 500 feet south of the section, the two rocks come in contact with westerly stratification foliation and easterly cleavage. On the west side of this schist area both rocks are in contact in inverse order, dipping east at a low angle. These easterly dipping beds of the west side of Ragged mountain stand out in prominent ledges which can be clearly seen from the top of a knoll (locality 190) about half a mile south of the Bellows- pipe. (See PI. XVI.) The same syncline occurs on Section F and also con- tinues south of Section Gr, on the knoll just mentioned, in the limestone and calcareous schist area. This limestone is very pyritiferous in places; an assay of the pyrite, said to have yielded a small percentage of gold, led re- 2oort. Gz/c.mica-schist ■ Fig 62 — Section of sniall syncline at south enil Ragged mountain, showing relations of tbe fiiUatinns in the east limb. This section crosses lower part of central mass shown in PI. XVI. MON XXIII- -11 162 GREEN MOUNTAINS IN MASSACHUSETTS. ceutly to some tentative raining here. From the occurrence of the small belt of calcareous schist across the top of Ragged mountain and from the presence of a well-marked syucline in the western part of the small schist area, the structure here has been construed as consisting of two minor folds. The section now crosses the " Bellowspipe." Dip observations both north and south of the line (see map, PI. i), indicate an anticline here. The contact on the west side of the Notch is covered, but along Section F (local- ity 709) the micaceous limestone dips west, and the overlying feldspathic schists occur a few rods west of it with a similar dip. Some 800 feet south of this (Section Gr, locality 589), a quartzite, which frequently replaces or is interbedded with these calcareous beds, dips 60° west ; and in ascending the hill the nearest outcrop of schist (locality 591), about 500 feet west,' also dips west. The relations which occur on the bench on the east side of Ragged mountain are thus repeated on the east side of the central ridge. The section now crosses the schists of the central ridge about 1 mile north of Greylock summit and about a half mile south of Mount Fitch. The low westerly dip was observed at several points along the Grey- lock road north and south of this section and also at 831 south of Sec- tion E. The section then descends into the north fork of the Hopper depression. The high westerly dip occurs in the precipitous ravine which, beginning about a quarter of a mile north northwest from the sunmiit, finally opens into the north fork of the Hopper. Along the 2,100 to 2,200-foot contour and extending down to about the 1,900-foot contour, on the west side of the central crest and in this north to south portion of the Hopper, is a belt of calcareous schist similar in character to that on both sides of Ragged mountain, but less calcareous. Farther south, west of Saddle Ball, this rock passes into the micaceous limestone. At several points westerly dips were found in this belt. It does not recur westward in this portion of the Greylock area. From these facts the central crest has been construed as a sjmchne of schist with a steep west side, a gently sloping east side, underlaid by the limestone and calcareous schist of the Notch and the Hopper. Mount Prospect (Symonds peak, see PI. xvii), consists of an anti- cline, with some minor undulations on the east side and a syncline on its west face. This is confirmed by observations on Section E and also MOUNT GREYLOCK. 163 on Bald Mountain, Section I, PI. xx. The presence of the lower limestone on the west face of Mount Prospect and of the calcareous schist belt in the Hopper, east of it, indicates that its schists correspond to those which, on the east side of the range, intervene between the lower limestone (Stockbridge limestone) and the calcareous benches. On the west side of Mount Prospect (locality 1020), near the contact of the schist with the limestone, there are alternations between the two rocks probably due to the erosion of some minor folds.^ The contact here with the limestone occurs along the 1,600-foot contour, while at the east end of this section it occurs between the 1,200 and 1,300-foot lines, a fact already noticed by Prof. Dana. Between the schist boundary on the west side of Mount Prospect and the Hopper brook is an area about a mile wide, in the eastern half of which there are numerous outcrops of limestone, but the western half of which is covered with di'ift. There is however little doubt, judging from the out- crops north and south of the section, that this area is also underlaid by limestone, and, if so, that it forms several minor folds. (Compare Section I.) It is in the limestone at the foot of Mount Prospect and near the mouth of the Hopper that Mr. Walcott observed "several traces of fossils," one of which, he says, "appears to be the inner whorl of a gasteropod related to Euomplmlus or Madurea? Along the Hopper brook, about a quarter of a mile above its junction with the Green river, is a small area of quartzite long ago noticed by Dewey and Emmons and a,lso referred to by Dana.' In Emmons's section, ' Such interbedding or minor folding near the line of contact occurs also west of Pittsfleld on Hancock mountain, in the Lebanon road. ^ Chas. D. Walcott : The Taconic system of Emmons, and the use of the name Taconic in geo- logic nomenclature. Am. Jour. Sci., ser. iii, vol. 3.5, March, 1888, p. 238. ' Dewey : " On the stream which issues from the Hopper is areua'cpous quartz of a slaty structure, which is an excellent stone for sharpening the chisels used by stonecutters." Am. Jour. Sci., ser. I, vol 1, 1819, p. 341. Emmons : " The outcrop of the quartz occurs again two miles south, near a mill at the junction of the Hopper creek and Green river. A small part only of the mass is exposed, dipping southeast and towards the high range of mountains known as Saddle mountains and Greylock." Am. Geology, vol. 1, part 2, pp. 12-13. Dana: "The quartzite of Stone hill and the quartzitic mica schist of Deer hill in Williams- town may be either of the upper or lower quartzite formation, if judged only by the facts the hill presents. But the position of these areas, in the Williamstown valley, between high ridges of hy- dromica schist, suggests rather that it is the underlying Cambrian." Am. Jour. Sci., ser. Ill, vol. 33, May, 1887, p. 410. . 164 GREEN MOUNTAINS IN MASSACHUSETTS. ali'eady referred to, this quartzite, interbedded with mica schist, is repre- sented as dipping conformably under the Umestone of the west side of Mount Prospect and as separated from the limestone area of the Williams- town valley west of it by a fault.^ This he also represents in another sec- tion (Greology Second District, p. 145, Fig. 46). The outcrop in the river dips about 30° eastwardly, but a few rods southwest up the bank (locality 11) the quartzite has vertical plications traversed by joints dipping south or southwest. Mr. J. E. Wolff finds considerable detrital feldspar in this rock, which distinguishes it from the feldspathic schists of Greylock that overlie the limestone and ally it to the Stone hill quartzites. Mr. Wolff's report on this rock reads as follows : "Si)ecinien l()92fl-. Slide: a fine-grained aggregate of quartz and feldspar. Stringers of muscovite give to the rock a schistose structure. The feldspars occur in irregular, angular giains, part uiistriated, part striated, part microcline. The mica and quartz often so surround and cut across these grains as to suggest secondary origin of the former. Some of the feldspars contain cores of twinned plagioclase feldspar, surrounded by a rim of uutwinned feldspar, or else a core surrounded by a rim of feldspar in a different orieutatiou, suggesting perhaps secondary enlargement. It seems probable that the feldspar in this and similar rocks is clastic (angular shape, different varieties in same rock, etc.) It is noticeable that they do not contain quartz and mica belonging to the groundmass, as the pori)hyritic feldspars of the feldspathic schists of Greylock often do, suggesting a difl'erence in origin. Tourmaline needles occur." ^ When in addition to this we take into consideration the fact that 2 miles south of this l(>cality, on Section 1, there is evidence of faulting, little doubt remains that these quartzites correspond to those of Stone and Oak hills, and are not to be considered as quartzose beds of the Deer hill schists, which are evidently continuous with those on the south side of the Hopper and overlie the limestones. At the Sweet's Corner dam, about a third of a mile north of Section G, the foliation (stratification or cleavage) of the schist strikes north 7° to 12° ' Emmons: " Along the b se of this mountain [Proapect] is a fracture whose direction is nearly north and south, and tlie limestone forming the valley was severed from that of the mountain side by an uplifting force." Report on Agriculture, p. 80. See also Geology Second District N. Y., p. 157, and E. Hitchcock, Report Oeol. of Vermont, vol. 2, p. 598. '^Compare Appendix A, p. 200. MOUNT GREYLOCK. 165 east, and dips SO'^ to 35° east. Immediately east of the Ijridge the land rises 40 to 50 feet, forming what is called Sawmill hill. In the schist along the foot of this hillock the cleavage strikes north 7° to 10° east, and dips 50° to 60° east, but plications are visible here and there, striking east or noi-theast, and dipping south or southeast. The same is time of the out- crops farther up the hill. These observations are confirmed by those at locality 1098, at the north end of Deer hill, along the Green river, where the schist plications dip 45° southeast and are crossed by cleavage planes dipping 40° east in one place and in another 70° east. On the top of the hillock the most northerly outcrop is limestone with somewhat curved strata, striking north 5° east, and dipping 35° to 40° east, underlaid 30 feet west by schist, with a foliation (cleavage) having a like dip. About 100 and 140 feet south of this limestone outcrop there are two small masses of the same rock with coarse, steep westerly or vertical plications. These may be ledges. From all this it has been inferred that the schists of Sawmill hill, instead of underlying the limestone as represented in Emmons's section, are continuous with those of Deer hill, and overlie the limestone; that the super- position of the limestone is the result of an overturn and a fault which have caused the schist to dip southeast and the really underlying limestone to overlie it with an eastern dip ; and that this fault reappears southward, on the east side of Deer hill, where it has brought up the Oak and Stone hill quartzites, which underlie the limestone, to the level of the schists which overlie it, causing a displacement of about 1,400 feet. The section now traverses Deer hill. On the northwest side of the hill, at the Grreen river, layers of calcai-eous schist with blue quartz alternate with a calcareous, ferruginous quartzite, all dipping 40° east. Their exact stratigraphic position is not determinable, but as they are separated from the Stone hill quartzites by a considerable area of limestone, as there is no evidence of a fault there, and as the schists of Deer hill clearly overlie the limestone at localities 7, 8, and 630 on the west, these particular layers have been regarded as representing simply a transition from the lower limestone to the lower schist. The portion of Deer hill traversed by Section G has for these reasons been construed as a syncline, with a fault on its eastern side. 166 GREEN MOUNTAINS IN MASSACHUSETTS. TRANSVERSE SECTIONS H, I. From the Hoosic river above MapU drove station {Soulli Adams) across the central ridge, Bald mountam, a7id the south end of Deer hill, to the Green river. Also Section H, across the summit (see PI. XX and Figs. 63, 64). The observations east of the central ridge along this section are few and unimportant. The lower schist belt measures about half a mile in width, and the area of the overlying limestone and calcareous schist about a mile in width. The latter is not overlain here by a subordinate mass of schist corresponding to Ragged mountain, but ex- tends uninterruptedly, and probably in a series of very gentle undulations, up to the base of the cliffs which form the east face of Greylock proper. The contact between the two rocks, wanting on Section I, can be seen in Peck's brook on Section J, the Fig. 63.-croaa-a6ction H. calcarcous schist Underlying the feldspathic, non- calcareous, micaceous, and chloritic schist, both with a westerly dip. On the face of the cliff, locality 549, the cleavage foliation dips 65° east, and the stratification foliation 15° to 25° west, and low west or horizontal dips prevail to the summit. (See Section H, and Figs. 44, 46, 53, 54.) At the top of the ridge which forms the seat of the saddle between Greylock and Saddle Ball and so also just west of the Greylock summit the dips are high east. The structure of the top of the central ridge here has thus been construed as a minor syncline with a steep east slope on the west side and a gentle west or horizontal one on the east side. The section line now descends a little north of Shattuck flats to the I Fig. 64.— Croaa-aection I. south fork of the Hopper brook. The observations above the flats are not conclusive, but in the most southerly ravine, tributary to the south fork of the Hopper, westerly dips occur, as they do also in the ravine running north MOUNT GREYLOCK. 167 northwest truiii the summit, which cuts deeply into the central crest, and which has already been referred to under Section G. This west dip is also shown on Subsection H. The calcareous schist belt is crossed again and recurs south in one of the forks of Goodell brook, in both cases witli a westerly dip. All this leads to the same interpretation as in Section G, excepting that a small anticline seems to intervene here between the sum- mit and the calcareous belt, the compressed syncline of the central crest having in it a minor fold which does not appear on Section G. The section then crosses Bald mountain. Here a great surface of the lower schists is exposed. A high northeasterly dip is well determined at locality 95 (see Fig. 41), and corroborated at locality 242 on the southwest side of the mountain, both with a strike of north 40° east (see Case vii, p. 150); and an easterly dip recurs high up on the east side of Mount Prospect. East of this locality the dip is east in places, but there are probably minor folds and much thickening. On Section J, PL xxi, which passes along the south side of Bald mountain about 500 feet below its summit, horizontal or low west dips occm*, striking with the much steeper dips of the top, and prob- ably representing the lower and broader part of the Bald mountain syncline. East of this, in the Goodell hollow ravines, there are high westerly dips. These facts, and the situation of the calcareous belt in the Hopper, have rendered necessary the peculiar construction seen in tlie section. Bald mountain thus consists on the east of a sharp anticline turned over to the east, followed on the west by a syncline which probably consists of minor folds. West of Bald mountain, along the spur between the line of the strike of locality 242, on the east, and localities 106 and 645 (Hopper), on the west, there is an anticline corresponding to the one at the top of Mount Prospect followed westerly, between localities 218 and 217, by a syncline corresponding to that on the west face of Prospect. West of this again, between localities 117 and 217, such a succession of westerly dips occurs that it has been necessary to insert a conjectural compressed syncline and anticline in order to explain the dips as well as the absence of the lower limestone. From the dips in the limestone and schist in the Hopper on the northern side of the spur it is probable that another small anticline occurs 168 GREEN MOUNTAINS IN MASSACHUSETTS. between localities 1 15 and 117 on the spur. In fact, judging- from the many alternations in the dip and the absence of the lower limestone, the whole spur west of Bald mountain seems to consist of a series of minor folds whose number probably varies but slightly from that represented in the section. Fig. 56, p. 155, represents a specimen from locality 115 on this portion of the section. In constructing the section the depth of the limestone has been governed by the angle of pitch along the spur and the relations of the Hop- per and Mount Prospect (Section G) to Groodell hollow (Section J). About three-quarters of a mile east of that arm of the Grreen river known as Ashford brook the section crosses a hill known locally as " Pine Cobble." On tlie west side of it is a small limestone area cut off by schist: on the north, from the Hopper limestone area, on the south from the New Ashford limestone area, and on the west from the South Williams- town, limestone area. On the east this limestone underlies the Bald mountain schists conformably, l)ut on the west side it is unconformably underlaid by schist, owing to a fault, the character of which has been partially described under Case x, Fig. 55. There would seem to have been a sharp ruptured anticline here, the eastern limb of which, consisting of the upper 400 feet of the lower limestone, with the overlying schist, was thrown up, while the western part slid under the limestone, the break having occurred along the eastern limb of the anticline in the upper part of the limestone bed. This fault strikes with the fault along the eastern side of Deer hill and at Sawmill hill, already described, and with the one referred to by Emmons. The displacement here can not well be less than 500 to 600 feet. The structure of the entire spur also indicates a great deal of compression. ' West of the fault the schist dips high west, or 80°, and on the west side of Deer hill, a little north of this section, the limestone of the South Williamstown A'alley occurs in contact with and under the schist, both rocks dipping east. On the east side of Deef hill the dips are 90°, or west, indicating a synclinal structure for the central portion of that hill. A small ravine skirts the west brow of Deer liill, the east side of which is formed by a cliff of schist, the west by a low ridge of limestone. At ' At locality 331, on west side of Sugarloaf, about 3^ miles south of this part of Section I, there is au auticline turned over to the west, bringing the schists under the limestone; and there are some indications of a fault between them, but the evidence is not conclusive. MOUNT GllEYLOCK. 169 locality 3'J, a little south of east from South Williamstowu village, the struc- ture of the schist is fiuely exposed (see Fig. 58), the coarse stratification foliation (plications) dipping about 45° east with a southerly pitch, associ- ated with a cleavage foliation dipping 35° east. Following this ravine southerly, its sides gradually approach each other until the two rocks are finally found in superposition with a westerly dip. The chief jioints of interest in the remaining sections will be only briefly referred to. TRANSVERSE SECTIONS A-F, J-O. Section A, PI. xviii, crosses the uortheromost portion of the range at North Adams, aud shows a comiiressed syncliue turned over westward.' The actnal contact may be seen about a thousand feet west of the North Adams railroad depot, the limestone overlying the schist, both rocks striking north 22° east, and dipping 45° southeast. I failed witli careful search to find any (juartzite outcrops in this part of Greylock, ulthough there are numerous bowlders of it which have probably been brought from Clarksburg mountain or beyond.' There is room, between the lowest outcrop of quartzite on Clarksburg mountain and the western side of the steep portion of the Greylock mass traversed by this section, for a bed of limestone 1,400 feet thick dipping at an angle of 50°, which is the dip of the schist at locality fll6 (see map); and none of the measurements of the thickness of the lower limestone obtainable on Greylock Indicate a greater thi(ikness than that. Section B, PL xviii, about a mile and a half south of North Adams. The limestone of the Hoosac valley and the schist of Mount Greylock appear here in their normal relations. The syncline which farther south consti- ^ tutes the central portion of Eagged mountain appears ; and there is a second syncline west of it, identical with the one on Section A, but open, aud also with that on the east side of the N"otch. In the western portion of the section two synclines and an anticline have been conjectured from observations farther south. It will be observed that this section crosses the Greylock ^'"^' 65— Cross-sections a, b. mass below the horizon of the upiier limestone and calcareous schist. Section C, PI. xviii, about 2 mi es south of North Adams. The calcareous bench ' See J. D. Dana, Tacouic Rocks and Stratigraphy, Sec. 47, p. 405. Also, On the Quartzite, Lime- stone, etc., of Great Harrington, p- 273. - J. D. Dana, On the Taconic Rocks aud Stratigraphy, p. 406: ■' A prolongation of it [the Clarks- burg mountain quartzite] appears to esteud south of Braytonville into the north end of the Greylock mass, along the ascending road (but chiefly on its eastern side) for a mile.'' 170 GREEN MOUNTAINS IN MASSACHUSETTS. on the east side of Rigj,'ed mountain appears with minor undulations. A well-marked syncline forms the top of that mountain ; on its west side the calcareous belt is crossed twice with an intervening tongue (jf the underlying schist, necessarily anticlinal in structure. At the west end of the section there is what might easily be mhtalien for imconformnbility between the limestone and schist, a foliation in the limestone (at localities 1035, 1036). striking north 77O-80^ east and dipping 250-50° south, while the plications in the schist close by and higher up (locality 1038) dip westerly with a southerly cleavage, conformable to the foliation in the limestone. It is highly prob- able that the fi)liation in the limestone is • cleavage, and that a stratification dipj)ing west conformably to the plication in the schist has been obliterated. This would make a syncline with the limestone underlying, as in the section. Section D, PI. xviii, nearly h:ilf a mile farther south. The Ragged mountain syn- cline continues with the upper limestone dipping under it. On the north side of Mount Williams there is a bench circling around from " Wilbur's pasture" (the saddle of this Saddle mountain), at the south end of the north to south part of the Hopper, and con- tinuing into the Notch. The eastern part of this bench is visible from tlie north end of Ragged mountain. Along this bench probably passes Formation Sbp — heie, however, without any outcrops that are calcareous, exceiJt at 641 and 645 on the north-northwest side of Mount Williams. The presence of masses of non-calcareous schist measuring from a quarter to three-quarters of a mile in length and several hundred feet wide on the northeast side of Ragged mountain and on the southwest side of Saddle Ball in the upper limestone and calcareous schist, and the fact that in the Hopper the strata of this horizon are much less calcareous and more micaceous than at the south end of Saddle Ball or in the Notch, and, finally, the iiresence of noncalcareous quartzite as well as limestone in the same horizon in the Notch, all indicate the very changeable lithologic character of this horizon. Furthermore, the general synclinal structure of the central ridge, the presence of a calcareous belt on both sides of it, and the similar constitution of Ragged mountain, together with the fact that at both ends of that mountain the calcareous belts are connected, and the greater difficulties involved in any other construction of the central crest, all lead to the interpretation given in the map, and in this, as well as the other sections. Section D traverses Mount Williams a little south of this belt of Formation Sbp. The ba^is for the remaining features of this section will be found largely in the next one. Section U, PI. xix, crosses Ragged mountain, Mount Williams, and the north end of Symond's peak (Prospect mountain). The Ragged mountain syncline passes east of the top of that ridge. Along the east base of Mount Williams a long ledge of schist shows iilications dipping 40O-15o west, crossed by an easterly cleavage dii^ping 60°. These west dips on the east side and the higher westerly dips- on the west side of Mount W^illiams indicate the character of the syncline of the central ridge seen farther south on Section G. The high westerly dips on the top of Mount Prospect (north end, or MOUNT GREYLOCK. 171 Saddle mouutaiii, Icjcalities G19, 621,) are coustrued as indicating a structure similar to that on Section Gr on the same mountain, but more compressed. The presence of an area of level arable land measuring about 1,000 feet square — "Wilbur's pasture" — at an altitude of 2,200 feet above sea level between the schist masses of Symonds peak on the west and of Mount Williams on the east, forjning the saddle of this Saddle mountain, and corresponding, as it does, to the similar area, "Shattuck flats," about 2J miles south, between Bald mountain and the central crest, at an altitude of 2,500 feet, and also to the calcareous bench on the western and southern side of Saddle Ball between the 2,200 and 2,500 feet contours, together with the occurrence of the calcareous belt between Wilbur's pasture and Shattuck flats in the Hopper ravines, all point to a structural if not to a lithologic similarity. (See PI. xvii.) Section F, PI. xix, is confined to Ragged mountain. The syncline and anticline observed about the limestone quarries between Zylonite (Howlands) and Renfrew, on the mountain side, appear here. The lower schists measure only about 1,000 feet on the east side of Ragged mountain at this point. In the centre of the Notch, locality 032, highly contorted strata of a feldspathic quartzite with a low southerly pitch occur. The occurrence of a similar fig. oe.-Crosssection f. rock is so frequent in this belt that it may be said in part to characterize the horizon.' Section J, PI. xxi, south of Section I, from a point a quarter of a mile south of Maple Grove station (South Adams), crosses a lens-shaped compressed syncline of the lower schist, which is here very graphitic, as it is frequently near the lower limestone. At the contact, on the east siile, the schist seems to inclose large lenticular blocks of the underlying limestone. West of the main belt of the lower schist is an area, nearly 2,000 feet wide, of a rock resembling the feldspathic quartzite of the Notch, referred to under Section F, but so micaceous as to constitute a fine-grained gneiss.'* The strata dip west, and appear to overlie the adjoining schists. For these reasons this area has been considered as forming part of the ui)per limestone belt. The observa- tions at the west end of this section in Goodell hollow on the south side of Bald moun- tain have already been referred to under Section I. Dip observations taken at different elevations indicate that the folds become more acute in the lower as well as the higher parts of the mass. Sections K, L, PL xxi, commence north and south of Cheshire harbor. The schist mass east of Cheshire harbor on Section K, which sends out a tongue southwards, crossed also by Sections L and M, is that represented in Emmons's section as under- lying the Hoosic valley limestone, and corresponding to the schist of Sawmill hi)' near Sweet's corners. But observations made by other members of this division of the geological survey along the base of Hoosac mountain show that this schist mass ' Mr. Wolff's determiiiiitions of this rock are given ou p. 185 (locality 345). •^ See p. 186, specimen from locality 616. 172 GREEN MOUNTAINS IN MASSACHUSETTS. probably overlies the Hoosic valley limestone. Along set-tious K and L there is dif- ficulty in tracing the connection of the upper calcareous belts of both sides of the central ridge, owing to the absence of outcrops on the west side of Saddle Ball. The central ridge (Saddle Ball) there slopes ott" to the east at an angle of about 10^, form- ing a bench which is even less inclined than that on the west flank of the mountain. See the view fi-om Lenox mountain, PI. xv. The conjectural track of Horizon Sbp. which on the map joins the outcrop of micaceous limestone at the south end of Sad- dle Ball ("Jones's Nose") with those in Peck's brook, Section J, has been drawn to conform to the strike and trend of the central ridge, and to those of the calcareous belt on its west side. It is based on both structural and topographic considerations. (Compare the remarks on Section D.) On the west of the mountain and about Gulf brook there are calcareous schists separated from the upper calcareous belt by non- calcareous schists. These have been thrown into the lower schists as probably repre- FiG. S7.— Cross sections J, K, L. senting mere transitions from the lower limestone to the lower schist, such as were ob- served at several localities over small areas (Deer hill, 630; Lanesboro, 365; New Ashford, 530), and are thus regarded as only indicative of the proximity of the lower limestone. In Section L the opening out of the compressed and overturned fold of the central ridge into a very broad and open syncline is seen. The calcareous belt of the Hopper becomes here a gently sloping bench of arable land nearly a quarter of a mile wide, once dotted with farms, and still used for pasturage. (See PI. xiv.) The rock becomes much more calcareous, aud dips east at a low angle under the upper schists of the central ridge, and bends around eastwardly between Saddle Ball and Eound rocks, the former consisting of the upper and the latter of the lower schists. The upper schists form a cliff on the south side of Saddle Ball at the incision in the central ridge, which is seen so plainly from the Tacomic range (PI. xiii), and MOUNT GRETLOCK. 173 from East mountain (Fig;. 74, p. 194). Here the strata are horizontal or dip very low east, and are crossed by a cleavage- foliation, as shown in Fig. 68. The section passes along the foot of these cliffs. The upper bench of Saddle Ball, shown in Section L in the upper schist, and also in the views (PI. XIV and Fig. 74), does not correspond to any calcareous horizon. A quarter of a mile north it measures about 800 feet in width. Section L has been extended through East mountain, where the strike changes to north 40° to 50° east, crossing the trend of the hill, and a sharp syncline occurs in the schist with the limestone of the Hancock valley dipping under it on the west. This schist is continuous with the lower schist of the Greylock mass, but the outcrops did not yield further structural data. East mountain seems to be one of the subordinate folds of the Greylock synclinorium which would thus measure here nearly seven miles in width. M L,oc.442. .^^ -_^ ^ - — 2 ft E. ^^^^^^^a ^^S^ -A^ \\ v'^'A'x ^ \\~ _<..3«, \ ' Cleayoffe. dzf) -SSE . WHH. Fig. 68. — Structure in schist in cliffs on south .side of Saddle Ball above the Bellowspipe limestone. Fia. 60.— Cross-sectious M, N, 0. Section 31, PI. xxi, begins about midway between Cheshire and Cheshire Harbor. The axis of the central syncline .seems to continue in the lower schists across Round I'ocks, where a cliff about 1,000 feet long from east to west and 150 feet high shows low east dips at its west end and low west dips at its east end. (See Fig. 74.) East of this point observations were few and unsatisfactory. Farther west the sec- tion crosses Sugarloaf mountain, which is a small open syncline. (See Appendix B.) West of it a number of minor folds produce the frequent alternations of schist and limestone about New Ashford. The entire synclinorium here consists of a greater number of smaller folds. The section is below the horizon of the upper limestone. w. lotJI5. Quart? lamina Stratif. 174 GREEN MOUNTAINS IN MASSACHUSETTS. Section N, PI. xxii, begins at Cheshire and shows a synchne in the schist north of the Farnham's quarry limestone area. This syncline appears to be continuous with that of Sections K and L, and is also on the line of the Bagged mountain syncline. North of the Lanesbort) limestone area there are indications of an anticline in the schist; and between this and the syncline on the east the numerous easterly dips are interpreted as indicating a compressed fold, inclined westward, between the central syncline and the eastern one. Between East mountain and the central Greylock ridge, in the western part of the section, minor undulations yield alternating areas of schist and limestone as on Section M. Both this and the following section indicate an increasing compression, the folds becoming more numerous, relatively to the distance, less open and more inclined than on Section L. Section 0, PI. xxii, starting from Cheshire reservoir, crosses the Farnham's quarry limestone area. At the east foot of the high schist ridge, which presents its precipitous side to the Hoosic valley (compare PI. xv with this section), the limestone evidently dips under the schist. At the south end and east side of this ridge the schist has a high westerly cleavage, and very low westerly or horizontal plications (localities 315, 427, 325J), together with a northerly pitch (locality 325). Toward the limestone on the west the westerly dip ajipears still ^ ,„ „. , ,., ^, to continue (localities 325,410,411). The structure at Fig. 70. — Structure in schist ou the ^ 77/ ridge west of Cheshire reservoir. locality 315 is represented approximately in Fig. 70 ; that at locality 325 in Fig. .59, p. 156. From the syncline north of the Farnham's quarry limestone area (Section N), from the northerly pitch south of it on Savage mountain, from the westerly dip in the schist east of that area, and the easterly dip of the same rock west of it (Section O), from the character of the dips in the limestone itself, as well as from the isolation of this limestone from that of the Hoosic valley, it has been inferred that a schist syn- cline underlies the Farnham's quarry limestone, and, therefore, that, although litho- logically identical with the lower limestone, it belongs stratigraphically with the up- per. We have here, apparently, asmall limestone basin similar in structure and position to the larger one which surrounds and underlies Ragged mountain. The difference in the limestone of these two areas is mainly in degree of metamorphism. But in several places the limestone of Hoosic valley resembles that of the Notch. About half a mile SSW of the west end of this section (O), at the east foot of East mountain (locality 749, back of Mr. Pine's house), the schist apparently dips east, as does also the lime stone. No plications are discernible. If this be the correct dip it indicates an over- turn, the dips corresponding to those on the east side of Potter m'ountain (locality 984) and on the road from Pittslield to Lebanon (locality 1020). Cleavage dip 15°w. MOUNT GREY LOCK. 175 General pitch of the folds. — The observations of pitch are recorded on the map by a special symbol. It will be noticed that the direction of the pitch through the northern part of the central ridge is south, while at its southern extremity, west of Cheshire reservoir, it is north. Sugarloaf mountain, New Ashford, has a northerly pitch at its south end, and a south- erly pitch at its north end. Ragged mountain has a southerly pitch at its north end, and the succession of the horizons at the surface and other facts indicate a northerly pitch at its south end. From the "Bellowspipe" the pitch is probably both north and south. In places a similar pitch seems to prevail along parallel lines across the central ridge as well as the subordi- nate folds; thus the southerly pitches on the Bald mountain spur, the north- erly pitches on Potter mountain. Constitution hill, and the Noppet, and on Savage mountain in Lanesboro; again, the northerly pitch at Cheshire Harbor is xuidoubtedly repeated at Round rocks, although not observed there in the plications. LONGITUDINAL SECTIONS. The facts stated above are shown on the four longitudinal sections appearing on PI. xxiii. Three of these, on a reduced scale, are given in Fig. 71. The north is at the right. Fig. 71.— Longitudinal .sections P, Q, R. Section P follows for 12 miles the axis of the eastern or Rag-gred mountain S5mcline, beginning at the Hoosic river a little south of North Adams, between Cross-sections A and B. At the north end of Ragged mountain the upper limestone and the upper schist horizons are shown with the steep southerly jjitch which marks the whole northern end of the Greylock mass (compare the symbols on the map). On Cross-section F there is a thinning of the lower schist. There are some indications of a 176 GREEN MOUNTAINS IN MASSACHUSETTS. northerly pitch on the east flank of Ragged mountain west of Rowland's, between Sections F and E ; but along the Notch brook the pitch is south like that on the Central crest (Section Q). The deeper part of the syn- cline is about under the center of Ragged mountaiii. Tlie upper lime- stone rises to the surface about a mile soutli of the south end of this moun- tain with a gentle northerly pitch, ^ and about IJ miles farther south the underlying schists also rise to the surface, forming the pinnacle and the neighboring' schist masses which hedge in on the south the northern area of the upper limestone. South of this is the Farnham's quarrj' limestone area, with well observed opposite pitches north and south of it, forming- a shorter and shallower troug-h in the same axis. The section ends on the east of Savage mountain. The length of tlie Ragged mountain trough is about Zi miles, and the entii-e length of the Farnham's quarry trough, extending beyond the limit of the section, would be about 6 miles Section Q follows for 14i miles the axis of the central or Clreylock syncline, begimaing at the foot of Clarksburg mountain, a little north of Cross-section A. From observations made hj other members of this di- vision the quartzite of Clarksburg mountain is known to have a southerly pitch. The lower limestone is, for topogiviphic reasons, supposed to pass completel}^ around between the Clarksljurg and Greylock masses, and thus, of course, to conform in pitch to the horizons below and above it. A steep southerly pitch is observed at the north end of the central crest, Mount Williams. This section shows a deep trough corresponding to that on Sec- tion P, but with its center about 2 miles farther soutli, at Cross-section I, in the saddle between Greylock and Saddle Ball. The south end or edge of this trough is at Round rocks, almost in a line with the south end of the great trough in the eastern syncline. This trough is a little longer, meas- uring 8J miles. In the incision between Round rocks and Saddle Ball the upper limestone and calcareous schists come to the surface. South of this ' North of thi.s part of the syncline, at the south end of Ragged mountain, tlie vertical distance between the top of the upper limestone horizon, where it is overlaid hy the smaller mass of the upper schist, and the lowest contour, where the upper limestone occurs, together with the slight thickness of the deposit necessitate a southerly pitch. Thus also south of the saddle (the Bellows- pipe) ; and for similar reasons a northerly jjitch is supposed between that saddle (Section G) and lo- cality 632 in the Notch (Section F). MOUNT GREYLOCK. 177 is a shallower trough analogous aud parallel to the minor one shown oii Section P. Sections B' and JR" pass through two of the minor synclines on the west flank of the Grreylock mass; R' through Stone hill and Deer hill, the syn- clinal axis of which probably continues southward through East mountain (Section L) and Potter mountain. At the north end (see Appendix A, Stone hill) the north pitch is not directly observable, but is pai'tially indicated by an observation of Mr. Hobbs in one of the ravines of the Taconic range. The relations between Stone and Deer hills are a repetition of those which have been inferred between Clarksburg mountain and the Greylock mass, the quartzite of Stone hill pitching under the limestone of Green river, and that under the schists of Deer hill. Section B" passes through Sugarloaf mountain (see Appendix B), one of the smaller lateral synclinal axes, which, farther north, appear in Bald mountain and Symond's peak. (Sections G and I). In this part of the syn- cline, which .measures only about 6 miles in length, there are two well marked troughs, one underlying Sugarloaf, and the other the high schist ridge south of it. RESUME, STRUCTURAL. Mount Greylock, with its subordinate ridges, is a synclinorium consisting in its broadest portion, of ten or eleven synclines alternating with as many anticlines. While the nu nber of these minor synclines is so considerable at the surface, it is found, in carrying the sections downwards, that they resolve themselves chiefly into two gi'eat synclines with several lateral and smaller ones. The larger of these two forms the central ridge of the mass ; the smaller one, east of it, forms Ragged mountain and an inner line of foothills farther south. The anticline between these coincides with the Bellowspipe; that on the west of the central syncline is a little west of the north and south part of the Hopper. The major central syncline is so compressed east of Syinonds peak (Mount Prospect) and Bald mountain, and its axial plane is so inclined to the east that the calcareous strata, which underlie the cen- tral ridge, have on its west side a westeiV dijj (Sections G and I). Far- ther south this syncline opens out (Section K), and all tlie relations become moi-e normal. But between the villages oi Cheshire nnd Lanesboro the MON xxui 13 178 GREEN MOUNTAINS IN MASSACHUSETTS. folds become sharper again and more compressed, and the schist area rapidly narrows (Sections N and O), and the structure continues much com- f^ pressed to the extremity of the mass. On either side of these p two main synclines the subordi- c nate folds are more or less open, and have their axial planes ver- tical or else inclined east or west. The continuity of the folds and ^ their nuitual relations are shown in Fig. 72. Longitudinal sec- tions along the two main syncli- nal axes (P and Q ) show that the trough bottom deepens at two points. In the eastern syncline H (P) the deeper part of the north- ern trough is shown to be about under the center of Ragfo-ed ' mountain, while in the central one (Q) it is about 2 miles far- , ther south between Grevlock and Saddle Ball (Section I); and this also would seem to be 1/ the deepest part of the entire . synclinorium. The northern edge of both of these troughs is - M at the exti'eme north end of the N Grevlock mass, and their south- Q eru edge 7^ to 8i miles distant, near Round rocks and the soiith- east spur of Saddle Ball. South- of these main troughs are two shallower parallel ones, the centers of which lie west of Cheshire reservoir (P, Q). To the west of these two long axes the mountain mass is made up of Flc. 72.— Diagram showing the continuity of the main folds in the Grcyhick synelinoriiun. Keducetl from the large sections, Pis. XVHI-XXII. MOUNT GREYLOCK. 179 numerous minor folds which do not show the continuity seen in P and Q. It will be observed that the direction of these two main synclines represented by P and Q is north-northeast to south-southwest, thus nearly parallel with the direction of the valley lying between the Clarksburg- granitoid mass and Hoosac mountain, and that at the south end they converge, and perhaps unite in the narrow schist ridge between Berkshire and Lanesboro vil- lages. Traversing the folds of this canoe-like complex synclinorium is a cleavage-foliation, sometimes microscopically minute, dipping almost uni- formly east. This cleavage-foliation is distinct from the "slaty-cleavage" early described by Sedgwick, Sharpe, and Sorby and reproduced experi- mentally by Tyndall and Jannetaz, but consists sometimes of a minute, abrupt, joint-like fracturing of the stratification laminae, but more gener- ally of a faulting of these laminae as the result of their extreme plication — a mode of cleavage "Ausweichungsclivage" (slip cleavage) so well described by Heim and recently reproduced in part by CadelP by a slight modification of the experiments made by Pi'of. Alphonse Favre, of Geneva, in 1878.^ This fault-cleavage, when carried to its extreme, results in a form of cleavage very nearly approaching, although not identical with, slaty-cleavage. To the unaided eye all traces of stratification-foliation are lost, and even under the microscope they are so nearly lost as to be of no avail in determining the dip. LITHOLOGIC STRATIGRAPHY. As may be inferred from the descriptions of the sections, there are five more or less clearly defined horizons in the Glreylock mass. These are described below, beginning with the lowest. The Vermont formation.— T\\Q iQ[(\B\)&\\\\(i quartzite of the northwest end of Deer hill, which corresponds to the quartzite of Clarksburg and Hoosac mountains and of Stone hill, will be noticed more particularly in Appendix A, on Stone hill. This is Emmons's "Granular Quartz," and has recently been shown to be of Lower Cambrian age. The Stockbridge limestone.^The crystalline limestone of the Hoosac and Green river valleys, which has long been known to constitute the base of Mount Greylock, is the Stockbridge limestone of Emmons, and extends ' Op. cit. (see p. 137), third series of experiments. » Alphonse Fnvre ; The foruiatiou of uioiintains. Nature, \ol. 19, 1878, p, 103, 180 GKEEN MOUNTAINS IN MASSACHUSETTS. tlii'ough Berkshire up into Vermont. It has been shown to be of Cambro- Sihirian age. The Berkshire schist. — An overlying- mass of schist forms the lower, steeper slopes of the mountains on all sides. This is a part of the magne- sian or talcose slate of Emmons, Dana's hydro-mica schist, and has come to be regarded as of Lower Silurian ag-e. The BeUowspipe Umestone. — A series of limestone strata and calcareous (sometimes noncalcareous) schists constitutes the higher benches, the Notch, and the Farnham's Quarry area. In places the rock is quartzite. This horizon seems to have been overlooked by previous geologists on Greylock. In 1888 Mount Everett, near Sheffield, in southern Berkshire county; Mount , Anthony, near Bennington, Vermont; Mount Equinox, near Manchester, Vermont, and Mount Dorset (Eolus), near Dorset, Vermont, were visited by the wriier in the hope of finding again on some of these higher summits of the Taconic range the upper limestone and calcareous schist of Greylock, but a careful exploration of them all failed to yield any trace of this horizon, excepting on Mount Anthony. A bench of calcareous schist occurs there in the mass of schist above the limestone, but the relations are not suf- ficiently clear to enable one to determine whether these calcareous layers form part of the Berkshire schist or Bellowspipe limestone formations. Dur- ing the year 1889, however, quartzites were found on Monument mountain, in southern Berkshii'e, which appear to overlie the Berkshire schist, and thus seem to belong to the Bellowspipe limestone formation.^ The Grei/Iock schist. — A second series of schists similar to the lower ones constitutes all the higher summits of the central ridge and the top of Ragged mountain. This forms part of Emmons's magnesian or Talcose slate and, together with the Berkshire schist, has been regarded by Hall and Walcott as of Hudson River age, and by Dana as representing some member of the Lower Silurian. All these groups of strata succeed each other conformably. • The theory aflvanced by Mr. W. H. Hobbs duriug the printing of this monograph (see Journal of Geology, vol 1, Xo. 7, Chicago, October-November, 1893, p. 72.T), that the limestone along the east- ern foot of Mount Everett corresponds to the Bellowspipe limestone and the schists which overlie it to the Greylock schist recjuires verificeitiott to accord -with lesults farther north. MOUNT GREYLOCK. 181 PETROGRAPHY. The petrographic character of the beds of these formations will now be described with tlie aid of Mr. J. E. Wolff's notes on the microscopic sections, which have been briefly summarized. XnE VEUMONT I'OUMATION. As the beds of this formation are only represented by one or two out- crops in the Grreylock area they will only be described in connection with Stone hill in Appendix A. tHK STOCKBRIDGE LIMESTONE. The lower limestone is a coarsely or finely crystalline limestone or mar- ble, usually white, but often banded or mottled, and in places entirely dark grey, and there argillaceous. South of and near the South Adams quar- ries it is very quartzose, and at the south end of Stone hill there are grad- ual passages from limestone to quartzite, the rock consisting of an "aggre- gate of calcite grains with rarely a small grain of feldspar and of quartz." About Williamstown and along the Grreen river north of Sweet's corners, the limestone is very fme grained, and has a hardness intermediate between that of quartzite and limestone, and contains occasional quartz grains. This fine-grained quartzose limestone may be more characteristic of the base of the horizon, but pure quartzite occurs near the top. The coarse crystalline limestone is often so micaceous as to resemble a gneiss.^ A specimen from a point a little southeast of the North Adams reservoir was found to consist of "coarse grains of calcite interbanded with muscovite and biotite, and containing occasional porphyritic crystals of feldspar. The feldspars contain inclusions of muscovite, rutile, pyrite, etc. There are occasional grains of quartz. Some fragments of feldspar are microcline, and the calcite cuts across these grains," indicating the possibility of replacement by calcite. The limestone about Sugarloaf mountain is also quite micaceous. Prof Dewey speaks of the flexibility of this micaceous limestone from New Ashford.^ Lenses and seams of quartz are not infre- quent. Prof. Emmons noticed the occurrence of albite in the limestone of ' See E. Hitchcock. Final Report Geology of Massachusetts, p. 569. ^ "Notice of the flexible or elastic marble of Berkshire county." Am. .Jour. .Sci., 1st ser., vol. 9, 1825, p. 241. 182 GEEEN MOUNTAINS IN MASSACHUSETTS. Williamstown,^ also the presence of galena and zinc blende here and there in small quantities. Prof. E. Hitchcock gave five analyses of the limestone of this horizon, which show it to be in places a dolomite.^ In the upper part, near the overlying schist, occur iiregulai" deposits of limonite, as at Cheshire, and along the north side of Mount Prospect, and on the east side of Potter mountain.^ Prof Dana has fully explained the origin of these ii-ou-ore beds.* Towards the upper part of the limestone occur also sti-ata of quartzite; thus on the east side of the extreme end of the Greylock schist mass near Pittsfield, and also near the Adams quan-ies. The fossils foimd by Mr. Walcott, and ah-eady referred to, came from this horizon, but fossils seem to be exceedingly rare.^ The structural peculiarities of the rock are its almost universal flexure into minor pitching folds, and, as already explained (p. 157), its not infrequent mini;te plications, and also its cleavage sometimes obliterating all ti-ace of stratification. THE BERKSHIRE SCHIST. This consists of the lower sericite-schists. The groundmass of these schists is made up of interlacing fibers of muscovite (sericite) and folia ' Geology Second District, New York, 1842, p. 158. ^Fiual Report Geology of Massachusetts, 1841, p. 80, 81. 'At tlie latter place (Lanesboro Iron company's ore bed) the ore occurs in two positions. In one place, owing to an overturn, it lies below the limestone and above the schist. In another it lies on the npper side of a small limestone anticline, the schist capping having been eroded. In another place a reddish, partially decomposed schist overlies the limestone, the ore probably occurring between. The stratigraphic position of the ore is identical in all these cases, however. On the Rchist side of the ore there is usually a mass of mottled clay, probably originating in the decomposi- tion of the schist, and on the limestone side a yellowish ochre. Manganese ore (pyrolusite) occurs here associated with the iron ore (limonite). ^ Am. Jour. Sci., 3d ser., vol. L4, 1877, p. 132. Berkshire geology in ''Four papers of the Berkshire Historical and Scientific Society,'' published by the society, Pittsfield, June 1, 1886, p. 19. Much of interest in reference to these Silurian limonites will be found also in vol. 15 of the Tenth Census (1880), Washington, 1886, especially in the introductory chapter by Prof. Raphael Pumpelly on the geographical and geological distribution of the iron ores of the United States (p. 10, on the limonites), aud also in Mr. Bayard T. Putnam's notes on the samples of iron ore collected in Connecticut and Massachusetts, p, 87. ■■•Since the completion of the manuscript the writer has found crinoid stems in the upper part of the limestone on Quarry hiU, New Ashford. MOUNT GKEYLOCg. l83 of chlorite and grains of quartz. Whether the hydrous character of the rock proceeds from the chlorite or from some other hydrous mica can hardly be determined, as the two minerals are intimately interlaced. The talcose appearance and touch of much of the Greylock schist, which have given it the names of talcoid-schist, hydro-mica schist, magnesian slate, is due largely to the presence, almost if not quite universal, of these exceed- ingly minute folia of chloi'ite;^ and the variable proportions of the chlorite and the muscovite in different localities explain the difference in the chem- ical analyses of it as well as the variety of names geologists have given it. The color of these schists varies with the varying proportions of its prin- cipal ingredients — muscovite, chlorite, and quartz. Often it is black from the presence of graphite, or porphyritic from the presence of feldspar, or spangled from the presence of other minerals. Quartz lenses and seams are almost universal. There are also great variations in the texture of these rocks. Their structural peculiarities have been described at length on pages 138-157, and constitute one of their chief characteristics The following is a brief summary of Mr. Wolff's microscopic analy- ses of the typical specimens collected : Among the minerals of most fre- quent occurrence are black tabular rhomboidal crystals or lenticular plates of ilmenite and chlorite, a plate of ilmenite being interleaved between two of chlorite. "Similar forms have been described by Renard from the met- amorphic rocks of the Ardennes, but they are surrounded by sericite layers and not by those of chlorite." He also describes large plates of chlorite inclosing small octahedra of magnetite, which also occur on Greylock.^ Very minute bluish green crystals resembling the ottrelite of the Rhode Island Coal-measui'es are found.' Perhaps fully as common, if not more so, is albite, which occurs in simple twins or untwinned, sometimes with a rim of clear feldspar separated or not from the central crystal by a rim of quartz, and surrounded by fibers of muscovite and chlorite. (Thus specimens from locality 458, south 'See E. Hitchcock, Report Geol. of Vermont, 1861, vol. 1, p. 501. James U. Dana, Am. .Jour. Sci., 3d ser., vol. 4, p. 366, and vol. 14, p. 139. " See A. Renard, Recherches siir la composition et la structare des phyllades ardennais. Bulletin Mus. Roy. Belg., vol. 2, 1883, p. 127-152, and vol. 3, 1885, p. 230-268. 'See .J. E. Wolff: Ou some occurrences of ottrelite and ilmenite schist in New England. Bull. Mus. Comp. Zool., Geological Series, vol. 2, p. 159, 1890. Cambridge, Mass. 184 GREEN MOUNTAINS IN MASSACHUSETTS. of Sugarloaf mountain; 494, between that mountain and Round rocks; 324, on the line of contact between the Stockbridge hmestone and the small mass of the Berkshire schist south of Sugarloaf; 474, in the deep cut between east and Potter mountains ; 475, at the southwest end and foot of East mountain in Hancock; and 703, at the triangulation point on the north summit of East mountain. More rarely garnets occur, giving rising to chlorite. Thus at locality 40, on the tongue of schist north of the Adams quarries. Garnets occur also in the small isolated schist mass west of Lanesboro village. The graphitic schist of this horizon was early noticed by Emmons^ and Hitchcock.'^ It generally occurs near the underlying limestone, as about New Ashford, at locality 274, and near Maple Grove station,^ locality 139, on the east side of Greylock. The graphite is in microscopic, irregular layers, or in masses, surrounded by even sized quartz grains and scales of gi-aphite and muscovite. Octahedral crystals of magnetite are in many places scattered through the schist,^ but the most characteristic minerals are albite, interleaved ilmenite and chlorite, and graphite. The rock is sometimes calcareous, but not continuously so. Rarely veins of calcite and chlorite traverse it. Between New Ashford and Lanes- boro a graphitic limestone occurs in the schist, containing angular, often rhombohedral, crystals of albite partially replaced by calcite. THE BELLOWSPIPE LIMESTONE. For structural reasons the Farnham's quarry limestone has been placed here. That limestone is generally white (though sometimes gray) and highly crystalline, like the Stockbridge limestone; but in the other areas of this formation the limestone is finer grained, less often white, frequently argilla- ceous, micaceous, or pyi'itiferous. Frequently the micaceous element pre- dominates and the rock is a calcareous schist, and in several localities the cal- careous element disappears altogether. Galena, zinc blende, and siderite occur along with pyrite in the limestone of the Bellowspipe. Associated with these limestone and calcareous schists are beds of slightly micaceous ' Geology of Second District, New York, p. 153. ' Final Report on the Geology of Massachusetts, p. 581. 'Emmons, Geol. Second District, New York, p. 141. MOUNT GKBYLOCK. 185 graiiulite or fine grained gneiss, lliese do not seem to be confined to any- particular portion of the horizon, nor are they persistent where they do occur. The seams and lenses of quartz in the calcareous schist are calcareous, and the rock itself is often calcareous where it looks least so, and vice versa. In structure it shows the same peculiarities as the limestone and schist of the lower horizons. No fossils have yet been found in this formation on Greylock, although the rock in many places is sufficiently fine grained and not too metamor- phic for their preservation. The only reason for the entire omission of this horizon from Emmons's section seems to be that his section traversed the mountain in one of the few places where there are no outcrops on the calcareous belts.^ The following is a summary of Mr. Wolff's report on these rocks. A bluish gray, finely crystalline limestone composed of calcite grains and quartz grains, with occasional flakes of muscovite and considerable pyrite scattered tlu"ough the calcite.^ (Thus a specimen from locality 212 on Peck's brook, about 2 miles south of the Bellowspipe.) Traversing the limestone are thin beds of graphitic, pyritiferous quartzite composed of quartz, feldspar, pyrite, graphite, and muscovite. (Thus locality 704 in the Notch about three-quarters of a mile south of its highest point.) The calcareous schist is composed of large grains of calcite mixed with stringers of muscovite and graphite containing inclusions of mica, graphite, calcite, and quartz. Pyrite and small fragments of microcline also occur in it. (Thus a specimen from locality 712 on the west side of Ragged mountain near its south end.) The feldspathic quartzite so often associated with or replacing the cal- careous schist of this horizon consists of an interlocking aggregate of grains of qviartz and feldspar with rare flakes of muscovite, small crystals of rutile, and specks of limonite (thus at locality 345 in the Notch, west of the center of Ragged mountain); and the gneiss, which seems intimately related to the above, is a mixture of quartz with a large amount of feldspar, twinned and untwinned j)lagioclase, with occasional grains of microcline and muscovite ' See Emmons's American Geology, part 2,p. 18. " From the termination of the limestone [i.e., the Stockbriilge limestone] to the top of Greylock the talcoso slate is uninterrupted." ^Recent assays of a similar specimen of this horizon are said to have shown the pyrite to be aurif- erous, but not sufficiently so to give the rock any metallurgical value. 186 GREEN MOUNTAINS IN MASSACHUSETTS. plates, magnetite, zircon, rutile, etc. (Thus at locality 616, in the gneiss area west of King Cole mountain and Maple Grove station.) THE GREYLOCK SCHIST. This also consists of schists resembling in their petrographic character, appearance, and structure those of the Berkshire schist formation. If there be any difference between them it consists in that the upper schists are more chloritic and albitic, and less frequently calcareous or plumbaginous than the lower ones, but all the minerals occux-ring in the Berkshire schist recur in the Greylock schist. The interleaved plates of ilmenite and chlorite are the same as in the Berkshire schist. (Thus specimens from locality 1,076 in the most southerly of the Hopper ravines, about 1,300 feet below Grreylock summit.) The magnetite octahedra are also frequently met. (Thus at locality 449 in the cliffs on the south side of Saddle Ball, and again west of the top of Grey lock about a quarter of a mile east of locality 1,076.) The feldspathic schists of this formation are characterized here and there by large crystals of albite. At locality 709, on the west side of the Notch, east of Mount Fitch near section F, the rock might be called an albite-gneiss. It consists of "numerous squarish albite crystals, rarely in simple twins, crowded closely together," but surrounded by "interlacing fibers of muscovite, chlorite, and biotite with magnetite grains and many tourmaline needles. Quartz occurs rarely, in little grains or aggregates. The biotite and chlorite are often in separate masses, but often pass into one another in the same piece. Some of the chlorite may result from the hydration of biotite. The feldspars contain inclusions of muscovite, chlo- rite, biotite, magnetite, tourmaline, etc." Mr. Wolff separated the feldspar of this rock by the use of the Thoulet solution, and a double analysis of it was made at the chemical laboratory of the U. S. Geological Survey in Washington by Mr. R. B. Riggs (F. W. Clarke, chief chemist). The result shows the feldspar to be an almost pure albite. MOUNT GREYLOCK. Analysis No. 567. Feldspar from specimen 709a, D. I. 1886, 187 SiO, Al,03+(Fe.03<5%) MnO CaO MgO Na^O KjO Ignition I. II. 68.08 67.83 20.11 19.92 trace trace trace trace ? ? 11.00 11.65 .36 .25 .31 .12 99.86 99.77 Dried at 105 C. Sliecific gravity slightly above 2. 6545, between 2. 6.545 ami 2. fil.' At the south eiid of the top of Ragged mountain in the small isolated schist area (locality 764), the albite gneiss is "coarsely foliated with a wavy sti'ucture composed of bands of dark mica, alternating with irregular layers of calcite mixed with quartz and large rounded feldspar crystals. Needles of tourmaline occur occasionally. The albite crystals are not twinned, have a rounded outline, often lie with their longer axes across the foliation of the rock, and contain inclusions of calcite, quartz grains, and flakes of both micas. The groundmass consists of interlacing fibrous layers of muscovite and biotite, little grains of quartz and great quantities of calcite, not in grains but in masses. The calcite sometimes penetrates a large feldspar, breaking it up into isolated cores of feldspar, surrounded by calcite. It is difficult to say with certainty whether the calcite was formed later than the quartz and mica or contemporaneously with them. It occurs in vein-like masses, not in grains ; when it has encroached on the feldspar it does so irregularly and not parallel to the schistosity of the rock, as the quartz and mica do; rarely tongues of calcite cut in two inclusions of quartz in the feldspars — it seems rather therefore to be pseudomorphous — replacing quartz as well as feldspar." Towards the top of Greylock and along the central ridge the feldspar crystals are very minute and are not rounded. (Thus at locality 861 on the Greylock road.) ' Mr. Wolfi' adds for comparison the analysis of a colorless all)ite from Kiriibinsk, Urals. SiOj 68.45, AljOa 18.71, FeO 0.27, NaOi! 11.24, K^O 0.6.5, CaO 0.50, MgO 0.18. Total 100. Spec. grav. 2.624. 188 GREEN MOUNTAINS IN MASSACHUSETTS. Fig. 73 represents a slightly enlarged section of a specimen of the felds- pathic schists, which may be regarded as petrographically and structurally typical of this formation. From all the foregoing the transitional lithologic character of the for- mations is manifest.' In the Stockbridge limestone there are passages from limestone to qiiartzite and to schist. In the Berkshire schist the rock is often calcareous. In the Bellowspipe limestone there are transitions from limestone to calcareous schist, and from these to noucalcareous schists and to quartzite and gneiss. Mr. AVolff's microscopic examinations indicate that this feature is due in part to vainous replacements and other Fig. 73.— Thin section of albitic seritite-schist fiom locality 542, between Greylock summit ami SadiUe Ball, enlarged IJ diameters. A typical specimen of the Greylock schist, showins the minute plications, the (|uart7, lamina?, the slip cleavage with the alhite interspersed. {From a jihotograph.) chemical changes at the time of or subsequent to metaraorphi.sm, as well as in part to variations in tlie character of the original sediments. THICKNESS-. The numerous folds, and the fact that they are sometimes compressed and overturned, not to mention the difficulties arising from cleavage, render exact measurements of thickness very difficult, if not impossible, in the Greylock area, but approximations can be obtained. The figures appended to the following table are given only as estimates based upon the sections. The difference in the estimates arises in pai-t fi-om the varying amount of thickening in plication (Stauung). As thickening in consequence of plica- ' Prof. J D- Daua refers to this in several of his papers on the Taconic rocks. MOUNT GREYLOCK. 189 tion g-enerally occurs in the Greylock mass the actual tliickuess is probably less than the minimum figures given in the table, and may possibly be considerably less. It will be observed, however, that the maximum thick- ness of the entire series does not exceed the minimum thickness attributed to the Lower Silurian rocks in the Appalachian region.^ GEOLOGIC AGE. The question of the age of the beds of Gfreylock, and the treatment of the whole subject from the standpoint of historic geology are beyond the province of this report, but the various conclusions which have been reached and are being reached in regard to the geologic age of these formations are added in separate columns for convenience of reference. ' See J. D. Uana, Manual of Geology, third edition, jiji. Itt2, 210. 190 GREEN MOUNTAINS IN MASSACHUSETTS. KESUME, LITHOLOGIC STRATIGRAPHY. The (jentrul lilholugic character, order, and estimated thickness of the strata of Mount Greylock, East mountain, and Stone Mil. Formatious, natural order. / Age.' Lithologic character. Thickness. Emmons, 1855. Hall, 1839-1844. Dana, 1882-1887. Walcott, 1888. Feet Greylock Muscovite (aericite), chlorite, and 1,500-2,200 Pre-Potsdam. Trenton. Lower Si- Trenton. schist. quartz schist, with or without bi- Lower Taconic (Hudson lurian. (Hudson Sg- otite, albite, magnetite, tabular crystals or lenticular plates of interleaved llmenite and cldurite. ottrelite, inicroscopic rutile and tourmaline. Tliese schists are rarely calcareous or graphitic. No. 3. "Talcose or magnesian slate." river.) river.) Bel 1 0 w s ]> i p ti Linieatone, more or less crystalline, 600-700 Pre-Potsdam, Trenton. Lower Si- Trenton. liiuestone. generally micaceous or pyritifer- LowerTaconic (Hudson lurian. (Hudson Sbp. ous, pnssing into a calcareous schist, or a feldspathic quartz- ite, or a fine-grained gneiss with zircon and microcline, or a schist like Sb. The more common minerals are: Graphite, pyrite, albite, and mi- croscopic rutile and tourmaline. More rare: Galena, zinc blende, No. 3. Included in "Talcose or mag- nesian slate." river.) river.) siderite. i Berkshiro Muscovite (sericite), chlorite, and 1,000-2,000 Pre-Potadam. Teenton. Lower Si- Trenton. schist. quartz schist, with or without Lower Taconic (Hudson lurian. (Hudson Sb. biotite, albite, graphite, magne- tite; frequently with tabular crystals or lenticular jdates of in- terleaved ilmenite and chlorite. Garnet, ottrelite. Microscopic ru- tile and tourmaline. These schists are in places cal- careous, especially towards the underlying limestone, where they No. 3. "Talcose or magnesian slate." river.) river.) are often graphitic. Stockbridge Limestone, crystalline, coarse or 1,200-1,400 Pre-Potsdam. Lower Si- Lower Si- Trenton. limestone. tine; in places a dolomite, some- Lower Taconic lurian. lurian. (Trenton.) ess. times quart zose, or micaceous, No. 2. "Stock- (Trenton Canadian. more rarely feldspathic, very rare- bridge lime- stone.^ and lower.) (Chazv, ly foasiliferous. Galena and zinc Califer- blende rare. Irregular masses of ous.) iron ore (limonite) associated sometimes with siderite, often with manganese ore (pyrolusite). Some quartzite. Vermont for- Quartzite, fine grained, alternating 800-900 Pie-Potsdam. Cambrian. Lower mation. with a thin- bedded, micaceous, Lower Taconic (Potsdam.) Cambrian. ev. and feldspatbic quartzite. (The latter with calcite, pyrite, tour- maline.) Associated with these quartzites, and probably at the base of this horizim. is a coarse- grained micaceous quartzite (tour- maline) passing, in places, into a No. 1. "Granu- lar quartz." (Olenel- las.) cony:lomerate. and containing blue quartz, feldspar (plagioclase, microcline) and zircon, all of clastic origin. Total thickness: Minimum 5,000 7,200 Maximum ' For Prof. E. Emmons's views see his works already referred to, especially his American Geology, Part 2, pp. 10-18, 48, 128. For Prof. James Hall's views, announced as early as 1839-1844, but not then published, see American Journal of Science. 3d ser., vol. 28, October, 1884. p. 311 : " Prof. James Hall on the Hudson river age of the Taconic slates." Also Jules Maroon : "On two plates of stratigraphical sections of Taconic ranges by Prof. James Hall," Science, vol. 7, 1886, p. 393. New York. For Prof. Dana's views see his papers: " (xeological Age of the Taconic System," Quarterly Journal of the Geolog- ical Sociely of L(mdon, vol. 38, 188"J. p. 3'.)7 ; "On Taconic Rocks and Stratigraphy," American Journal of Science. 3d ser., vol. 33, May, 1887, p. 410. and also 'On the Hudson river Age of the Taconic Sc-hists," etc., ibid, vol 17, 1879. p. 375. For Mr. diaries D. Walcott's views see the map and section appended to bis paper, " The Taconic system of Emmons, and the use uf the name Taconic in geologic nomenclature," American Journal of Science. 3fl ser., vol. 25. April, Ma.y, 1888, pp. 307, 394, pi. 3, also "The Stratigraphicai auccessioa of the Cambrian Faunas iii North America" (abstract of hie MOUNT GREYLOCK. 191 AREAL AND STRUCTURAL. The geologic map of the Greylock, East and Potter mountain masses, presents a great body of" the schists of the Berkshire schist formation, sur- rounded by the underlying Stockbridge limestone. It is probable, although not demonstrable, that this limestone passes around the north end of the Greylock mass, between the schist on the south and the quartzite (Vermont formation) of Clarksburg mountain on the north. It is also probable that that quartzite underlies the entire Greylock synclinorium, for it occurs on the north on Clarksburg- mountain, on the east on Hoosac mountain, and on the west on Stone liill, and is also brought up again by a fault on the east side of Deer hill. The Bei'kshire schist sends out tongues corresponding structurally to syuclines into the lower limestone area, as west of Zylonite on the east side of the range, and at Deer hill on the west side ; also at Constitution hill, west of Lanesboro. There are also reentering angles of limestone in the schist area, corresponding to anticlines, as nortli of Lanesboro, and about New Ashford. There are isolated schist areas, generally lenticular in form, corre- sponding to more or less open synclines, as a little southwest of South Adams, and south of Constitution hill, in Lanesboro and about New Ash- ford. The most interesting of these is Sugarloaf mountain, which is a canoe-shaped open syncline. (See Fig. 74, Appendix B, and Sections M, R.") There are also isolated limestone areas, corresponding to compressed anticlines, projecting through the overlying schists, exposed by their erosion. Two of these occur between New Ashford and Lanesboro, and a smaller one is described in Appendix B, at Quarry hill, New Ashford. (Figs. remarks before tho lutornational Geological Cougress. Lourtou, September, 1888). in Natiu'e, vol. 38, No. 23, October 4, 1888, p. .551; also Ills paper, "Straligrapbic Position of tbe Olenellus Fauna in North America and Europe," American Journal of Science. 3(1 ser., vol. 37. May, 1889, p. 374. For a defense of Emiuou.s's classilicatiou see "Pal:i?ontologic and Stratigraphic Principles of tbe adversaries of the Taconic," by Jules Marcou, American Geologist, July, 1888; and for Mr. Marcou's own classification of the Taconic rocks see his memoir, "The Taconic system and its position in stratigraphic geology,"' Proceedings of tho American Academy of Arts and Sciences, vol. 12. Cambridge, ISi^-S, p. 174. For a summary of the dilfercnt phases of opinion in regard to the age of the Taconic rocks see "A brief history of Taconic itleas,"' by J. D. Dana, Am. Jour. Sci., 3d ser., vol, 36. December, 1888, p. 40. For tbe literature and a 8y.=demati<-. presentation of the Taconic question .lee Bulletin 81, U. S. Geol. Survey, Correla- tion Papers — Cambrian, by Chas". D. Walcott. For evidence of the Loweri Cambrian age of the lower part of the Stockbridge limestone, see article by J. E. Welti", "' On tlje Lower Cambrian age of the Stockbridgelimestone," Bull. Geol. Soc. Am., vol. 2, 1890, p. 331. Also paper by T. Nelson Dale, " On the structure and age of the btookbridge limestone in the "Vermont Talley,''Bull. Geol, Soc. Am., vol. 3, 1891, p. 514, 192 GEEEN MOUNTAINS IN MASSACHUSETTS. 78, 79.) The limestone area iu the western part of the Bald mountain spur is anticlinal in structure, but faulted. The relations which have been described above as existing between the lower limestone (Stockbridge limestone) and the lower schist (Berk- shire schist) are repeated at a higher level between the upper limestone (Bellowspipe limestone) and the upper schist (Greylock schist). Ragged mountain and the higher portions of the central ridge (Saddle Ball, Grey- lock, Fitch, Williams) are synclinoria of the upper schist resting upon and surrounded by the upper limestone. The tongues and reentering angles and isolated schist areas occur here, as well as in the lower formations. But the isolated limestone area southwest of Cheshire, instead of being an anticline of the Stockbridge limestone projecting through the Berkshire schist, seems to be a syncline of the Bellowspipe limestone resting upon the Berkshire schist, homologous to that which encircles and underlies Ragged mountain, but without any similar mass of schist on it. The relative height of the surface of the Farnham's quarry limestone, as shown in Section P, accords well with this interpretation.^ RELATIONS OF GEOLOGY TO TOPOGRAPHY. It remains now to show the relations of the structural, lithologic, and areal geology to the surface features. We fhid here evidence of the opera- tion of several causes : First. The mineralogic character of the rock, presenting minerals more or less easily disintegrated by physical or chemical agencies. Second. The internal structure and position of the strata, forming ele- vations and depressions in the mass and determining the surface relations of the different kinds of rocks. Third. Erosion, glacial, as well as pre-glacial and post-glacial, bringing physical and chemical agencies to bear upon those irregularities in the form and composition of the surface. ' These facts are brought out on the accompanying map and the sections (Pis. i, xvm-xxili). Besides the usual dip and strike symbols there have been added on the map symbols indicating the direction and angle of jiitch, and also the symbols proposed by Dr. H. Reusch (op. cit.) to indicate the cleavage dip and strike, aud IJually, uumbers of the important localities referred to in ttis report. 'HfW^F^'^T'^'T^^, "J 7 fV ^i' K 'f^^' *,'r '-jlMCKm '/ 'A. -«--xJ.:^- »" .e--& s * ? . = * .S- OQ o +- 0) 2 "* 5 = ? S «" ■I § S u Q. ^ E 5 S § £ S o 2 -C c -o" ■*- O C t- ^ o m n m (0 "^^ » « § I ■j= -S -c J^ 01 41 o. " 5 *; t/i ^ CN CD :£ tn i: r % t) 0) E E 5 -S USGEOLOSICAL SURVEY MONOGRAPH XXIII PL XVIII MT. nnKYLOCK \'crli(;tl .'MIorizDiilril Scale linnri orli iiichcs-l mil p. SYMBOLS- CHLOR-MICA ISERICITCISCHIST_ CALCAREOUS MICA-SCHIST & LIMESTONE ifi placesllieScl'fstisnorCalcareous.in ptacesQuartrite _ _ LIMESTONE _ _ (fUA RTZITE j\(j/^*.- TUe r/ra.\'ru/p fafitrtfnii in oii(v shinvTi frusstiu/ t/n- slnitifitdliini -/hiififioii n-/iric r/ren-ru/r r/if> m-iix t/rtrriiiuicr/ hut. il Irin-pr.si'.v f/ic i/tr/ilnprirf o/'tJif iiiti.ss O/j.-itnii/ J(7favtt>/r (/)fjs /StraftY'.' L/000 i.50J S£A LEVEL .500 -—1500 -220U Oh.seri'rfl Sf/tiO/'" f/f/j so 60' ■*S" Af ADAMS ZEE HOOSIC R S£A L EyiL . '-_ TOO — 2O0 Sh - £S£. S>rf/o// A NOTCH 3 ROOK E Sr{//o// /? j (7< f/rr//fi- t/i/K^ Observe (/ ' \Sfrfffff'" LONG 73 -iO ' 35 "E- ■36°-- 46'E ■fO-SO'S 25" S NOTCH BROOy. S3'W HOOSIC R es€ f'f'/U^// ( ^ '//■ ^5 Ci S Si. ' ;; I; ^ s t i^ ^ <: -i^ .'^ J^ .•n, ^ *v to V oO'C SO'W HOOPER BROOK SOUTH FORK 35'E 60:E. S0-SS''£. 'central crest e high 0° 6' long ts'io'. PECKS BROOK HOOSIC RIVER SecWon I. us GEOLOGICAL SURVEY MONOGRAPH XXin PLXXI MT. (;nKVF.()('K Verliral Fi-Horizv)iil;iI ScnlciTHSB oris iiulic.s=l niilc SYMBOLS CHLOR-MIC^ IS€RICITEISCHIST ._ . CALCAREOUS MICA-SCHIST & UMESTONE in places the Scfiisris notCalcareous. m pieces (fuartzite LIMESTONE . qUARTZITE. Oh.svn Tf/ I _ 3200 f' . 2500 . 2000 -. 1000 soo ^SEA LEVEL ^ssiiuf tJir sf rtiti/irft fun t -/alia fioii ir/zz/y vh'fi\'(u/r (h'f./ kv/.s- f/c/i'minir*/ hiif if /rft I 'rr.st'.s //tr ' v {W 40'SL EAST MT ROAD B/t00H3!,'C Snar. 60-£. SOe 55 £ S Y/SPUROF SADDLE BALL 30' IS-25' 20 E O'O" E E E LOW lowC £ 40°-4S 35-'tsE. ^*-£S£ £ BASSEn BROOK /It, J I Oefiva/rf dips WESTROAD70 3Q' LANESBORO € Senile .2700 F' .2000 .1500 ,1000 -600 -SEA LEVEL ,500 .900 45" ^5-50° 15-20' EC C 40'SO'IS' 45"'. 60"40^ SUGAR w W VIVtE.EW EEC LOAF MX HIGH C M CENTRAL CREST "ROUNO ROC Kfi" E O'EW SC SPUR or S0'30'iLn£L.fiac/ts Fig. 75.— Sketch of the Greylock m,iss from the .southwest (locality 1008, on north Potter mount.iiii) showing the .surfiice of the Bald nimmtain spur anil of Round rock.s pitching toward each other owing to the pitch of the .syurlinoi-ial axis. from Clarksburg mountain and the Stamford valley are due to the presence of the two iDelts of the upper limestone and calcareous schist on either side of the central ridge (Berkshire schist) one forming the Notch, the other "Wilbur's pasture," and the north to south part of the Hopper. The gentle northerly slope of the surface from Hound rocks to Jones's Nose (see Fig. 74, and Section Q), and the similar southerly slope of the top of the Bald mountain spur, as seen from North Potter mountain on the southwest (Fig. 75), are probably due to the trough structure of the entire mass, the former constituting a part of the northern trough of the great cen- tral syncline. To this structure are probably also due the long, steep south- 196 GREEN MOUNTAINS IN MASSACHUSETTS. ern face of Round rocks and the steep south side of Saddle Ball. The former is a very striking object in the landscape both from the east and west. (Compare Section Q with PL xiii and Fig. 74). An east to west system of joints and fractures growing out of the pitch may have aided glacial and other erosion at these points. The great west spurs which characterize the west side of the range (PI. xiii) are portions of the mass left by the erosion which chiseled out the Hopper and the hollows farther south, while the pleasing variety of surface features seen on the east side from Hoosac mountain (PI. xii) is the result of the Berkshire schist forming a sei'ies of foothills between the upper and the lower limestone.. Some of these are also shown in PI. xv, the view from Lenox mountain. East of the summit, however, these schists have been eroded almost down to the level of the Stockbridge limestone, thus enabling one to look over from Hoosac mountain into the area of the Bellowspipe limestone and southwards for 2 miles to a point where the Berkshire schist rises from under the Bellowspipe limestone and hedges it in ("The Canoe" PI. xii), forming several considerable masses, the pinnacle and the southeast spur of Bald mountain. These constitute the ridge between the northern and the southern trough of the eastern syncline and shut in the view. (Com- pare PI. XII and Section P.) A careful comparison of the topography and geology of the map, with the transverse and longitudinal sections, and the general views (Pis. xii, XIII, XV, and Figs. 30, 75) will show more clearly than words can the general structural relations of the Grreylock mass to its surface features. APPENDIX A. STONE HILL, NEAR WILLIAMSTOWN. This oft-studied and problematic locality has not yielded anything very remark- able.' Observations of strike and dip were made, typical rock specimens were col- lected and submitted to Mr. Wolff for microscopic examination. Three cross sections have been constructed, S, T, U (Fig. 76), and one longitudinal one, E' (PL xxiii). The dififlculties at Stone hill arise from the small number of outcrops and their entire absence at critical points. The areal geology of the hill is indicated on the geologic map. The first question which arises is whether the mass of quartzite along the east side of West brook val- ley, apparently overlying the .limestone, forms a part of the quartzite at the top of the hill. There is a gentle slope of arable land between the two, and a small lime- stone outcrop on the east side, at the north end of the westerly mass, has a foliation which strikes with the trend of this strip of cultivated land. It has therefore been conjectured that the two masses are separated by limestone, but the other supposition would be tenable. The dips in the main mass of quartzite, on both sides and in the center, are easterly; but at the south end the dip (pitch?) is south, and a well marked southerly pitch occurs in the quartzose limestone at the southwest end of the hill (localities 1103-1105). A very high southerly pitch occurs also iu the limestone a little farther south (locality 62) on the north side of the Green river bridge crossed by the road from Sweefs corners to South Williamstown. Here there is a small, sharp anticline with an almost vertical pitch. A southerly pitch occurs again iu the schists at the north end of Deer hill. High up on the southeast side of Stone hill (locality 1106) an outcrop of quartzite with limestone north of it shows a southeast pitch. This, however, has been regarded as a quartzose part of the limestone. Formation €3Ss. From all these facts the quartzite at the top of the south end of Stone hill appears to pitch under the limestone farther south and down the hill, and that limestone to i>itch under the Berkshire schist of Deer hill. We thus have here in their normal succes- sion, the Vermont formation, the Stockbridge limestone, and the Berkshire schist, and the relations which seem to exist between Clarksburg mountain (Oak hill) and the north end of the Greylock mass are repeated here between Stone hill and Deer ' See Emmons: Geology of the Second District of New York, pp. 145, 156, 159; Report on agricul- ture pp. 83-86. James D. Dana: Taconic rocks and stratigraphy, p. 406; Geology of Vermont and Berkshire, p. 206. 197 198 GEEEN MOUNTAINS IN MASSACHUSETTS. bill. (See sections Q and R')- These relations at the south end of the hill, together with the structure of Buxton hill and the northerly pitch observed by Mi-. Hobbs at locality 2005, a little west of Buxton hill, lead to the supposition that the quartzite of the top of the hill, at the north end, pitches under the limestone at Williamstowu. The correctness of this conclusion is also rendered probable by the petrographic character of the Stone hill beds, which is similar to that of the Oak hill beds. On the east of Stone hill strata of micaceous feldspathic quartzite occur between those of massive quartzite (locality 627). In three localities a flue schist or phyllite of Observed Stratify dips . lOOOFl . 700 .500 TooLof TaconicRanae roaa l5'-2Q3tf 5S6OWMt,S0° 50" North part of 45° Probable Green E.El E.prookE E. Stone Mill t. Fault R. _Sea level Izl888 660 Ffc Observed StratiFC dips EioooFd. 700 500 300 Sea level West Brpoh 4d'S0°3Z42°65° Probable roadEX. f. E- Fault, -*-E Green R. •CSS. 700 Ft, Observed StratiFC dips i ioooFl 700 500 West Brook I I V >E Stone Hrll 75^55?60' Green roao South part LE. PtDbatJieFault. R 720 Fb. •C55 Sea level "df:'') Fig. 76.— Cross-sectious S, T, U, Stone Hill. inconsiderable thickness appears. Towards the north end of the east side of the hill a blue ijuartz conglomerate, and a quartzite containing blue quartz and detrital feldspar occur.' Mr. Wolff's descriptions of these rocks are given beyond. ' Dewey : "Granitell of Kirwan, quartz, and feldspar. This aggregate forms extensive strata at the east base of Stone hill, the feldspar is ditt'iised in grains through the quartz, and sometimes crys- talline, forming porphyritic fjuartz. This aggregate is often compact and very hard, hut frequently it is porous and hard, forming good millstones. Sometimes the quartz appears in such fragments that the stone resembles breccia." Am. Journal of Science, ser. i, vol. I, 1819, p. 343. See also Emmons, American Geology, p. 16, on the conglomerate of the granular quartz at Oak hill. STONE HILL. " ]iJ9 In constructing transverse sections of Stone hill several difficulties present them- selves. The quartzite with detrital blue quartz and feldspar, which may naturally be supposed to occur near the base of the quartzite and towards some underlying gneissoid rock, and which Emmons places at the base of his "granular quartz," occurs only on the east side of the hill dipping toward the limestone outcrops of Green river and Williainstovvn (Formation €!Ss). On the west side of the western mass of quartzite the rock is massive, and seems to be conformably underlain by the limestone of Formation €Ss, but that quartzite we should expect to reiiresent the upper part of the quartzite (Formation €v). One explanation of these facts would be that on the west the apparent superpo- sition of the quartzite upou the limestone is the result of an overturn, while on the east the two rocks are separated, as Emmons supposed, by a fault.' Such a fault would be nearly, if not quite, on the line of the fault on the east side of Deer hill (Sec- tion G), and with that farther south near the west end of the Bald mountain spur (Section I) and also on a line with faults in southern Vermont at East Pownal. The highly contorted character of the limestone strata along Green river east of Stone hill, and in the village of Williamstown^ also leiul probability to such a hypothesis. Upou this basis of fact and probability the folds in the Stone hill sections have been constructed. On the east side of Stone hill a fault is represented; the central portion of the hill consists of a syncline followed on tlie west by an anticline over- turned to the west; the outlying masses of quartzite on the southeast and northwest sides of the hill involve two minor anticlines. All the folds have a southerly pitch at the south end of the hill and a northerly one at the north end. The entire thickness of the Stone hill (juartzite and its associated micaceous feld- spathic rocks would thus measure between 800 and 900 feet. If a simple anticline be supposed it would measure about 1,300 feet, and if a monocline, as represented by E. Hitchcock in his Massachusetts section, about 2,600 feet.^ The rocks of Stone hill are frequently jointed; one of the systems of joints may possibly be connected with the pitch, as may also the occasional east to west joints and some of the secondary cleavage planes on Greylock. On the east side of the southern portion of the hill the massive quartzite is traversed by joints striking north 65° east, and dipping 05° to 75° northwesterly. On the east side of the central part the micaceous quartzite has a set of joints striking north 80^ east and dipi)ing 80° southerly, and another set striking north L'Oo west, and dipping 45° easterly. The dark pyritiferous quartzite (locality 18) near the top and center has joints striking north 72° east, and dipjting 65° north-northwest. 'See his Section 46 (Geology second fUstriet, New York, p. 145), in wUicli lie repi'csents a I'ault immediately eiist of Stone hill, and another farther east along the western foot of the Greylock range. ^ Dewey refers to the contortions here: Am. Jour, of Sci., ser. i, vol. 9, 1825, p. 18. ' Report Gaol, of Vermont, vol. 2, pi. xv, fig. 5. 200 GREEN MOUNTAINS IN MASSACHUSETTS. The following is Mr. J. E. Wolflf's summary of his notes on the Stone hill micro- scopic sections: STONE HILL ROCKS. "We have in the quartzite series of Stone hill an interesting illustration of the share that the original detritus and the modification produced by mechanical and chemical agencies take in producing certain rocks. "The quartzite varies microscopically from a fine-grained rock, composed to the eye of quartz grains and more or less mica to a coarse fragmental quartzite or fine- grained conglomerate (locality 628) in which angular fragments of feldspar and rather rounded masses or pebbles of blue quartz are visible; the latter grade insensibly into the granular white quartz forming the rest of the rock. "Studied in the thin section the structure of the rocks is as follows: The large masses of blue quartz show in polarized light that they have been subjected to great pressiu'e and strain, which has resulted in a partial or total breaking up of the original homogeneous quartz into a 'groundmass' or mosaic composed of extremely small particles of quartz in which are contained cores of cracked and strained quartz which are remnants of the original masses.' The comparatively large fragments of feld- spar are seen to be in most cases microcline or a plagioclase feldspar, but sometimes without evidence of multiple twinning, and in that case probably orthoclase. The substance of the feldspar is cloudy, owing to kaolinization. The forms are sharj)ly angular and evidently detrital. The remainder of the rock is a very fine-grained aggre- gate of little grains of quartz and rarer ones of feldspar, the latter being similar in character to the larger fragments of the same mineral. Irregular and interrupted layers of a colorless muscovite, which has the wavy 'interwoven' structural form characteristic of sericite, give the rock a lamination, the plane of which is parallel to the planes of crashing in the quartz, that is, at right angles to the pressure. When one of these layers of mica touches one of the large clastic feldspars, it often forks and completely siu-rounds the feldspar, the two parts joining again on the other side; accompanying this there is a thickening of the layer of mica around and near the feld- spar, and sometimes litttle tongues of the mica, branching from the main mass outside, penetrate the feldspar, especially along cleavage cracks. It is therefore evident that the clastic feldspar exercised an influence on the formation of the mica and probably gave up part of its substance to form the latter. These large feldspars, like the quartz, are fractured and broken, the quartz aggregate of the 'groundmass' filling the fissures. "The small feldspars of the 'groundmass' have in part the same characters as the large detrital ones, and in fact are often evidently derived from an adjacent large ' See PI. X in Part il for an enlarged photograph of a thin section of this cruehed blue quartz from Stone hill. > STONE HILL. 201 grain, but in part they have a more rounded form and show little trace of decompo- sition. In some of these grains there is a central core which is opaque owing to kaolinization (as is the case with the whole grain in the case of the large fragments) but surrounded by an outer rim of clear fresh feldspar material, which has the same crystallographic orientation as the inner core, the two forming one grain. If these grains are detrital, as they seem to be, there must have been a recrystallization of the old feldspar or a deposition of new feldspar around the old grain. ^ "In certain finegrained varieties of these Stone hill quartzites the amount of feldspar is very large, and it is difficult to say whether these small grains are in their original detrital shape or are metamorphic. " In sojae cases the large clastic feldspar masses are aggregates of several individ- ual grains of feldspar, forming thus a rock fragment which resembles closely the coarse granitoid gneiss found on Clarksburg mountain to the northeast and Hoosac mountain to the east, which undei'lies the whole Taconic series. Hence there is a pos- sible derivation for the material of the quartzite. " Prisms of tourmaline are common in the rock, and there are occasional rounded grains of zircon. Secondary limonite often stains the rock yellow. Grains of pyrite are abundant in some specimens (locality IS, near top of hill), and in one there is a large amount of calcite present in small grains and irregular masses. "These quartzites seem to derive their present materials from two sources, the original detrital material and the material produced from this, at least in part, by mechanical and chemical agencies. The blue quartz 'pebbles' (locality 628, east side) may be regarded as pebbles whose original outlines have been largely obliterated by mechanical deformation ; the large feldspar fragments are undoubtedly detrital and so is the zircon. The cement or 'groundmass' is composed of detrital quartz and leldsijar mixed with an unknown amount of the same minei-als formed in situ and by muscovite in large part and tourmaline produced by metamorphism. "The distinction made here between clastic and metamorphic feldspar is well marked in the extremes as found on the clastic side in these rocks; on the metamor- phic side in the albite of the schists of Greylock and Hoosac mountains, and analo- gous feldspars of the gneisses of Hoosac mountain." 'Cf. Irving and Van Hise, Bull. U. S. Geol. Survey No. 8, p. 44. APPENDIX B. NEW ASHFORD. Fig. 77.— Apex of the main anticline of Stockbridge limestone protruding ttimugli the Berkshire schist at Quarry hill, New A&hford. Height, 6 feet. Southern side. See locality 296, Fig. 78. There is an area of between 3 and 4 square miles south and east of the village of New Ashford, within which nearly all the structural and areal features that char- acterize the Greylock mass are repeated on a small scale and within easy reach. PI. i shows the geology of this tract. Section M traverses it. Fig. 74 gives a view of the greater portion of it and of Sugarloaf mountain which covers a large part 'of the area. This little schist mountain, the synclinal struc';urD of which has already been alluded to, is entirely sur- rounded as well as underlain by limestone. It forms a conspicuous object in the land- scape, views of it from the north (Fig. 30) and the south (PL xv) showing the depression on either side of it corresponding to the limestone. A line of cliffs, masked, however, by foliage, traverses its south end from east to west, rising above the limestone which pitches under it. On the west side of Sugarloaf the synclinal structure is concealed in most of the limestone out-cro]is by cleavage foliation. (See Fig. 37.) A northerly pitch is well observed at the south end in some of the minor folds (see Fig. 60), as well as a southerly pitch in the schist at the north end. Section II", which follows the synclinal axis of Sugarloaf, shows the trough structure of that mountain. Another trough exists in the schist mass south of it. Several isolated schist masses cap the limestone folds along the foot of the mountain on the south. The phenomena of cleavage and stratification in one of these have been shown in Fig. 35. On Quarry hill the converse of the structure presented by Sugarloaf mountain appears. A limestone anticline with subordinate folds protrudes through the schist. 202 • Fig. 78 Geologic map of Quarry hill. New Ashford. NEW ASHFOUl). 203 The diaigraiiis (Figs. 77, 78, 7'.») leprcseut the area, size, and structm-e of this anticline, and Figs. 32, 33, 3i show the cleavage phenomena iu it. The schists at the foot of the hill toward the village form part of those of East mountain (Beach hill). The easterly cleavage would easily mislead oue here into a wrong interpretation of the relations. The broad area of limestone in which the old Ashford Schisl Sb 3S'S-£ SChtst +5" S £ SchiStSb ''■'"" r' QUARRT Hiltl :7"''12;^4s.s/i^^^;^!'!^i Fig. 70. — Section through Quarry hill, Xt;\v Ashlbrrt, showing the structural relations of the Stockbridge limestone and Berkshire Hchist. quarries lie, forms an anticline, and the schists referred to overlie its base with a westerly dip. It is uncertain whether the section given by Emmons (Geology of sec- ond district, p. 155), through the New Ashford marble quarry, relates to this quarry or to one of several others iu the vicinity. INDEX Page. Adam9,Mas9., exposures of gneiss near 84 Ampbibolites, areas and character of 65-66 Anthonys creek, exposures on s4 Ausweichungsclivage of Hoim 139 Bald mountain, figured specimen of schist from. ... 144 Baltzer, A., cited 143 "Bellowspipe," location of 160 Bellowspipe limestone, thickness of 20 age and character of 180, 184-186 Berkshire schist, exposures of 08 syncline of, in Cheshire 15, 16 thickness of 20 age and character of 179, 182-184 Berkshire valley described 6 Bowens creek, section on 85 Burlingames hill, exposures at 85 " Buttress," location and structure of 22 exposures of gneiss at 83 Cadell, H. M., cited, 179 Cambrian quartzite correlated with Hoosac conglom- erate 28.29 Cambrian and pre-Cambrian rocks not easily distin- guished 25 Cambrian rocks, varying character of 31 Cheshire, schist and limestone in 16 ideal section near 17 Cheshire hills, transition from limestone to schist at south end of 16, 17 Chester ampbibolites, geologic place of 30 Clarksburg mountain, structure of 8-9, 10, 27, 176 rocks of 26,27 exposures at 99 Cleavage, list of works on 137, 138 nature of. Mount Greylock 158 Cleavage and stratification foliations, relations of. . 136-137, 139, 140, 141, 144-155, 155-157, 161, 173 Connecticut valley described 6 Conway schist, place of 30 Cook, George H., cited 157 Correlation of Green mountain rocks 9, 35 Dale, T. Nelson, work of xiv, 12, 19-20 paper on Mount Greylock by 119-203 cited 191 Dalton, exposures in 96 Dalton-Windsor hills, structure of 16 Dana, J. D., cited 9, 50, 1 07, 131, 132, 155, 157, 159, 163, 169, 182, 188, 189, 190, 193 Darwin, C, cited 143 Deer hill, location of 135 Dewey, Chester, cited 131 , 163, 181, 198 Dry brook, exposures near 91, 93, 94, 96, 98 East mountain, specimen of limestone from 142 figured specimen of schist from 146 Eaton, Amos, cited 131 Emerson, B. K., work of 10, 26, 30 Emmons, B., cited 14, 1U7, 131, 132, 159, 163, 164, 181, 184, 185, 190, 197. 198, 199 Page. Favre, A Iphonse, cited 179 Feldspar, analysis of 187 Formations, table of 190 Geology and topography, relations of 192-196 Greylock schist, thickness of 20 age and character of 180,186 Greylock and Hoosac rocks correlated 13-20 Hall, James, cited 108,132,159,190 Hawes, G. W., cited 67 Heim, A., cited 106,139 Hitchcock, C. H , cited 7,58,100,107 Hitchcock, E., cited 67,107,131, 132, 159, 164, 181, 182, 183, 184, 199 Hobbs, W. H., work of xvi, 131 aid by 12 cited 180 Hoosac conglomerate correlated with Cambrian quartzite 28, 29 Hoosac mountain, structure of 8, 20, 21, 80, 104-106 rocks of 26,44-69,102 report of J. E. Wolfi' on 35-118 general topography of 41-44 Hoosac tunnel, geologic value of -■ 8 described 8.9,42,69-72 Hoosac schist, exposures of, in tunnel 23,69,72 described 59 exposures of 72,80,81,82,87,88 Hoosac schist and Stockbridge limestone, zone of lateral transition between 15, 17 Hoosac and Greylock rocks correlated 13-20 Hoosic river, exposures on 87 Hoosic valley schist 97-98 *' Hopper," location of 134 Hunt, T. Sterry, cited 108 Irving and Van Hise, cited 201 Jukes, J. B., cited 143 Lachines creek, contact of Stockbridge limestone and Cambrian quartzite on 12 Logan, "W. E., cited '. 7 Marcou, Jules, cited 191 Metamorphism, phases of 32, 33, 34 Mount Greylock, structure of 21, 125-127, 177-1 79 structure of rocks of 102, 127-128, 136-155, 181 paper by T. Nelson Dale on 119-203 description of 125,133-136 areal geology of 128 relations of geology to topography on 128, 129 figured specimens of schist from 145,147 pitch of folds of 175 table of formation s of 190 Mount Prospect (Symonds peak), location of 134,135 figured specimens of schist from 144, 145 structure of 162-163 New Ashford, geology of area near 202-203 North Adams, exposures at 87-88, 98 205 20*) INDEX. "Notch," location of ( ileuelhis casts tniiml Pierce, Josiali, ;iid by Pitili, methoils of dtterioininji, general. Mount (Ireylock riaiiilieUl schist, geoloj;ic phice *if- Page. 100 10, 29 siv 157 175 :jo PniigiiqiiaK, N. Y., contact of old gneiss and Cam- brian quartzite at 21i PunipeUy, K., cited 182 Putnam. B. T., work of xiii-xiv. 8, 10,21 (Quarry bill. New Asbford, exposures at 138, 139, UO Ragged mountain, location of 135 topography and structure of iSO, 170, 171.175,170 Renard, A., cited 183 Reusch, H., cited 143, 102 Richthofen, F. von, cited 27 Riggs, R. B., analysis of felds]>ar by GO, 186-187 Rosenbusch, H., cited 63 Rowe schist, doubtful age of ■ 29 place of 30 area of 65 Saddle Ball, location and height of 135 Savoy Center, outcrop of white gneiss conglomerate near 79 Savoy Hollow, outcrops near 78, 79 Southwick creek, stratigraphy on 77 Spruce hill, exposures near 86-87,88 Stamford, Vt., expi>sures near 98-102 contact of Stamford gneiss and Vermont quartz- ite at 100, 101 dike in 11 Stamford gneiss, description of 45-48 fossils of 51 exposures of, in Eoosac tunnel 69,72 contact of, with conglomerate of Vermont for- mation 73, 100 Stockbridge limestone, syncline of, in Cheshire 15 thickness of 20 description of 64-65 Page. Stockbridge limestone— Continued. exposures of, in Hoosac tunnel 69.72 exposures of 84, 87, 89, 98 age and character of 179, 181-182 Stockbridge limestone and Hoosac schist, zone of ''lateral transition between 15, 17 Stone bill, geology and topography of 197-201 Strata of Mount Greylock. table 190 Stratiiicalion and cleavage foliations, relations of. at Mount Greylock 136,137 Stratification foliation, character of 158 Sugar loaf mountain, strata at 140, 141 structure of 173 Symonds peak (Mount Prospect), structure of 162-163 Tacnnic mountains described 6 Topbet creek, exposures of Hoosac schist on. 81-82 exposures of rocks of Vermont formaticui on 84. H5 Topographic work on Hoosac mountain, methods of 41 Topography and geology, relations of 192-196 Vermont formation described 48-59 exposures of, in Hoosac^ tunnel 69, 72 exposures of 72, 82-88, 88-90, 94 contact of, with Stamfoi'fl gneiss 73 age and character of , 179, 200-201 Vermont quartzite and Stockbridge limestone, fea- tures of contact of 95-96 Walcott, C. D., aid by xiv. 10, 29 fossils found in Stamford gneiss by 51 cited 163. 190, 191 Whittle, C. L., aid by xiv Winchell. A .. cited 58 Windsor, Vt .exposures at 96 Windsor hill, exposures at 88 Wolfl; J. E , wtuk of xiii. 8, 10, 11, 14, 28, 125 cited 164, 171. 183, 187, 191 quoted on microscopic sections of Stone bill rocks 200 Vokura, Mr., aid by xiv