ii j } ; , , , ; F : ‘ r Stat hg mr 4 “e ' ‘ j i Ja ! a ani Aa i L ; penren ee : F : : ; aad r ‘ > a \ ‘ wi y ORY. =, ‘HIST CROSBY NATURAL ILLIAM. ETY 0 soc W $$ ON” — BOS’ OCCASIONAL PAPERS OF THE BOSTON SOCIETY OF NATURAL HISTORY. LEE, BOSTON: Pos LEO. POR. THE. SOc LET Y¥: 1880. Digitized by the Internet Archive in 2011 with funding from California Academy of Sciences Library http://www.archive.org/details/occasionalpapers31880bost CONTRIBUTIONS TO THE GEOLOGY OF HASTERN MASSACHUSETTS. BY WILLIAM -O. CROSBY BOSTON: BOSTON SOCIETY OF NATURAL HISTORY. 1880. PUBLISHING COMMITTEE. : Spire ScupDER, | . ALtpHEUs Hyatt, | S. L. Aszor, M.D., J, AP tains Epwarp BurRGEss. Press of Rockwell and Churchill, 39 Arch Street, Boston. TABLE OF CONTENTS. Page INTRODUCTION . 4 : F : : - : : Z 1 Topography and Gore Outlines . - : F : : ‘ : 8 Eozoic ForMATIONS ‘ , ‘ ‘ . ; é ; . F : 13 Naugus Head Series . ; F : ; ‘ - : : ; , 14 Huronian . : é : ; . ’ . : 2 = : 24 Granite ; ; ; ; ; ; ; : : ; j ‘ 27 Petrosilex . : ; : 45 Relations of the Dbieiites to the ete Gratn F : . 47 Details of the Petrosilex . : ph \ Ste , . : a 55 Petrosilex in Newbury . : : . j : ; ‘ 55 Concretionary Structure ‘ : 57 Petrosilex on Marblehead Neck tel the maihase ‘Taide : 63 Petrosilex of the Lynn and Medford Area . ‘ : ; 74 Petrosilex in Needham 5 ‘ : : i 79 Petrosilex in Dover, Medfield, aa Deibad: F : 80 Petrosilex in West Roxbury, Hyde Park, Milton, ae the Blue Hill Region A é é 5 : 82 Petrosilex in Hingham and Pima 5 E . . : 91 Relations of the Petrosilex and Granite ; ? : i P 95 Diorite 3 3 Z - = is F 101 Hornblendic Gneiss, ie »— “Stratified ‘Geous ; : é : " 105 Limestone . F ; ;: : 5 é ; : 112 General Relations of the Pavenaas Rocks*. ; . . é : 116 Eozoic FoRMATIONS,—continued . : . : : ; : : , 123 Montalban : : ; : . : ; p : , , 123 Gneiss 3 : - F - x Z ‘ F , - 5 135 Mica Slate . 5 : . - : ‘ S p z : ; 136 Argillite . ; : . : : : . : : . : 137 Limestone . : : ; ; : ‘: 139 General Relations of the Wotiattan ieee : : : p ; 140 Stratigraphy of the Nashua and Merrimac Valleys . : . 154 General Relations of the Older Crystalline Formations of Eastern Wastes chusetts : ° ee : : : : : 5 . : 161 Shawmut Group : : : : : : : : : 163 Details of the er ane : 5 , ; : é ; é 166 iii 1V TABLE OF CONTENTS. Page Shawmut Group in Newbury : : ; : : : 166 Shawmut Group in the Marblehead Repion : : : ; 167 Shawmut Group in the Lynn and Medford Region : : 169 Shawmut Group in Brighton, Newton, and Needham . . 170 Shawmut Group in Dedham, Hyde Park, Dorchester, and Milton, 174 Shawmut Group in the South Shore District : : : 5 175 General Relations of the Shawmut Group . ; - : - : 177 PALEOZOIC FoRMATIONS . - : ; : ; ° : : : . 181 Primordial : : : : : . , : . : ; : 183 Boston Basin . - ‘: “ A 4 183 Stratigraphy of ihe Boston aa : “ ‘ ; : 195 South Shore District . ; . : : : : : 197 Quincy and Milton : : - : 208 The Hyde Park, Mattapan, anid Bataabiira Belt : 4 : 216 The West Roxbury and Dorchester Synclinal : is j 223 The Brookline and Roxbury Conglomerate Belt . ‘ ° 229 The Upper Falls, Chestnut Hill, and Boston Slate Belt . : 235 The Conglomerate Bordering the Newton Lower Falls and Brighton Band of Amygdaloid . : - ° 245 Conglomerate and Slate in Needham and South Natick : 250 The Broad Slate Belt, between the Boston & Albany Railroad and the Crystallines of Waltham, Arlington, Medford, and Malden. “ : : 5 ; ‘ : : : 253 Nahant : . : 5 : = . : . : 261 Marblehead Neck . : : : : : ; ° : 263 Volume of the Conglomerate and Slate 5 : - 265 Relations of the Conglomerate and Slate to the Ghillie: : 266 Basin of the River Parker . ; 5 : : j : ‘ - 267 Mineralogical Notes . : : : . 5 ° : . : 269 Pinite . : . ; A - : : - : : : 269 Kaolinite . : - : : : : ‘ : : 3 272 Devonian ? ; - ; : : 3 : : : ‘ : : 273 Carboniferous . : : : : : : : - : 275 Supplementary Note. : : i : : é 5 ; : : 276 PREFACE, EaRLy in the summer of 1875 I began a systematic study of the rocks in this vicinity, with the view of collecting such data as, added to my previous knowledge, would enable me to prepare a thesis for graduation in the department of Natural History of the Massachusetts Institute of Technology, on the ‘¢ Geology of the Environs of Boston.” In the beginning of the following winter I was charged by Prof. A. Hyatt, under the direction of the Massachusetts Commission to the Centen- nial Exhibition, with the task of preparing a geological map of the State, for exhibition at Philadelphia. Opportunity was afforded me by the Commission to visit all parts of the State. The Boston Society of Natural History generously permitted the use of my time as assistant in their Museum for this purpose, and, as that winter was unusually favorable for ex- ploration, I was in the field almost constantly. The Centen- nial map and the report on the same were completed in the following spring. The wider range of observation which this work for the State had afforded seemed to justify me in ex- (v) vl PREFACE. tending the scope of my thesis so as to include the whole of Eastern Massachusetts, and it was accordingly finally written, under the title of this paper. The thesis was accepted for publication by this Society, and the Government of the Massa- chusetts Institute of Technology made an appropriation to pay for printing the map. I resolved, however, partly on the suggestion of Prof. Hyatt, to test my conclusions by another year of field work, and various circumstances have conspired to extend this time to nearly two years. By this means my original observations have been doubled, anid, although the general plan of the paper remains the same, it far exceeds its former limits, and many of the geological boundaries on the map have been brought nearer to an expression of the truth. In deference to the view of Dr. T. Sterry Hunt, expressed to me personally, and before this Society, I have given the rocks, which in the original paper were designated as the Norian system, a name having no chronological signification. The Huronian petrosilex and felsite are no longer regarded as synchronous with the breccias; but the latter rocks, together with the amygdaloids, which appear to belong to the same horizon, are set apart as a distinct formation under the name of the Shawmut group. The conglomerates about Boston have been found to underlie the slates, and hence are now regarded as Primordial instead of Carboniferous. For assistance received my acknowledgments are due, first of all, to Prof. Alpheus Hyatt, who generously placed at my disposal his original notes, sketches, and collection, represent- ing observations made during several years on the geology of Essex County; and I am also indebted to him for many opportunities for investigation in the field and laboratory. @ PREFACE. Vil To Dr. T. Sterry Hunt I am indebted for much valuable instruction, advice, and criticism, while a student in the Mas- sachusetts Institute of Technology and since, without which the performance of this work by me would have been impossible. Though placed in a position most favorable to a thorough acquaintance with Dr. Hunt’s views concerning the origin of crystalline rocks and their relations to the more recent, fossiliferous rocks, I entered the field with a general disbelief in their applicability to the rocks of this region, which my first observations only strengthened; and any conclusions in har- mony with these views which may be found in this paper have been reached in opposition to preconceived ideas wholly at variance with them. | For valuable data, freely contributed, my acknowledgments are also due to Mr. L. S. Burbank, of Woburn; Mr.-f271. Bouvé, President of the Boston Society of Natural History ; Mr. F. W. Very, of Hyde Park; Prof. W. H. Niles, of the Massachusetts Institute of Technology; Mr. W. W. Dodge, of Cambridge, and others. The chemical analyses recorded in this paper were made in the Woman’s Laboratory of the Massachusetts Institute of Technology, and for these I am under special obligations to Mrs. Alice B. Crosby, Mrs. Ellen H. Richards, Miss E. M. Walton, Miss H. A. Walker, and Miss Jennie M. Arms. The President and Directors of the Eastern Railroad Com- pany have assisted very materially in the exploration of Essex County by granting free passes over their line. Boston, August, 1878. Ale ‘ae ; TORS AME tino hae te 4 tas pate 1 » ’ ‘ My % it t i he ts tb voy : ) Le k ‘ oe 4 Vit a) Bw hMity ry 7 Ly baa i, ° : os es: eae e ‘Th Peedi ty 8 eo var TH ( ( © 7 Hh f ney ‘ re he Pye eee . bs ‘ ayy ‘SU 4 ae f i 4 t aul 7 | Md 1 j y ‘ Pel iM ry £4 D any : GLE OO LEP egrne CEP NTA ES Cae ey ‘ aaa eS ect | f F 3 F » EP CLR aae A <3 eter ty } Peg thy . Vy ae? . \ & ; Pigs, ee te iy i} ' is ; 1 r i 4 att 4 (tA F q Pee whe RA be - } ny r a), n pA CN i Dd COHAN OMS bah oth EP Pe ecg ‘ H), } \ 1 neq ed Fa ; ‘ y oA 4 he yi + AL : : id ad " | 4] call ; : , a eee Ay j ©) LON PY ah re ae Loa Many h : 7 ny ¢ ‘a ‘ 4 ; f wit ‘ ta . hil ti Pye Ag Teregu ys ee hb ee py 7 ae ’ ha es me , ‘ Ley ‘ f Py , Nat si « J oe ‘ ‘wie 1s ; vay POR g weld . ' + i , "ek Py ae “oe bs Ce os eee nN Tae bee eyes 1.) Tee A ve ‘ ’ ; : es Py A te ba SHR ORL ieee ri : : Ree aes nea me te i . ¥ z : 7 ‘ et yh j Pah 4 #4) ih e P . ‘ 1 f * a i Re he WAS) Wie ift< : ‘ . . iy pss . ~ - LJ * a - m . . ~ ‘ A ’ pn ge om : , INTRODUCTION. THE vast region extending from Long Island Sound, north- easterly, and nearly insulated by the St. Lawrence, Richelieu, and Hudson Rivers, and Lake Champlain, is well-marked off, geographically, from the rest of the continent. Dr. Ezekiel Holmes,! recognizing the geographic distinctness of this most eastern member of the continent, and its almost com- plete isolation by water, an isolation that appears to have been perfect during the first part of the Paleozoic era, has correctly termed it a peninsula. Newfoundland is a detached portion of this geographic unit. In its geologic relations, also, the region indicated is readily separable from the adjoining terri- tory. The rocks are mainly crystallines, and, except in the lower part of the valley of the Hudson, cannot be connected, along their strikes, with the rocks of other regions. The ? Ann. Report on the Agriculture and Geology of Maine, 1861, p. 100. * The Paleozoic sediments of this region, it is true, are continuous through the broad interval between the Adirondacks and the Highlands of the Hudson, and were probably connected, at one time, through the valley of the St. Lawrence, with the more exten- sive deposits of the same age to the westward. Yet the crystallines are to so great an extent the predominant rocks of this large territory, and, as the writer conceives, have had, as a whole, an origin so vastly more remote than the most ancient of the uncrys- talline sediments (these newer formations being regarded as merely superficial deposits, often obscuring, but not essentially altering, the main structural features of the region), as to warrant leaving the latter out of view in these general remarks. OCCAS. PAPERS B. 8S. N. H.—III. 1 2 north-easterly strike prevalent north-east of a line extending, in a general way, from Boston, via the White Mountains, to the northern end of Lake Champlain, would, if continued south-westerly, carry the crystallines of New England over New York, and the Middle States generally. But beyond the line indicated the formations bend to the south, and through- out south-western New England are characterized by approxi- mately north and south strikes, being cut off at right angles by the waters of Long Island Sound. Along the New York boundary, in southern Connecticut, however, the north-east strike is resumed, and the rocks of that district sweep across Manhattan Island and the Hudson. The breadth of New England strata subjected to this westerly deflection is -prob- ably not so great as at first sight appears. ‘The crystallines of the Highlands of the Hudson are Laurentian, and are rec- ognized by all geologists as older than those farther south, in the vicinity of New York. It is the latter only that can be traced easterly into New England, or that are here considered as really belonging to this great geological province. The Laurentian strata of the Highlands have a north-north-east strike, varying toward north and south; and it is probable that in their northerly extension, although soon dipping beneath more recent formations, they sweep around to a course parallel with the one hundred and fifty miles of newer crystallines on the east, and reappear in Washington and Saratoga Counties in New York; thus connecting the Lauren- tian of the Highlands and New Jersey on the south with the great Adirondack area on the north, and constituting at all points a barrier against which abut the western edges of the crystalline series proper to New England. The complete absence, so far as has been definitely ascertained, of Lauren- tian rocks from western New England corroborates this view. In short, the crystalline formations of the northern half of the Atlantic seaboard exhibit, in their strikes, a grand double curve. Beginning in the north-east with a trend parallel with the St. Lawrence and the Laurentide 3 Mountains, they bend to the south in southern New Eng- land, and conform with the Laurentian barrier just pointed out; while the remnant that is not cut off in the southern extension, by the Atlantic, finally regains the former strike, and spreads across southern New York and New Jersey. This great Laurentian wall, which is doubtless continuous, under the Paleozoic deposits, from Labrador to Virginia, or beyond, formed the western shore of the ocean, in which the newer crystallines of the Atlantic slope were deposited ; and, subsequently, in its southern half, it became the eastern border of the great Paleozoic sea. It has been a primary axis of continental oscillations, a centre of disturbance — itself undis- turbed. The importance of this ancient Laurentian axis, as a structural feature of the continent, makes still clearer the isolation and unity of what may be called the New England or North-Eastern geological province. It is not intended to deny the existence in New England of the older crystalline formations, for Hitchcock has shown that they certainly occur in the White Mountain region; but I may safely assert that their occurrence in the western and central portions of this province is always something exceptional. They are not characteristic of any considerable areas. This north-eastern province is naturally divisible, in a geo- graphical sense, at least, into two parts, —a south-eastern and a north-western. The division. is marked by a line of valleys parallel with, but subordinate to and less distinct than, that isolating the province on the north and west. ‘This axis of depression follows the valley of the Connecticut, from Long Island Sound, to near its source ; then, curving north-easterly, and skirting the northern base of the White Mountains, it is marked by the valley of Lake Umbagog, and the great lake region of Northern Maine, passing to the northward of the Katahdin Range. Entering New Brunswick, it is continued, with the same trend, through the valley of the Restigouché River and Bay of Chaleur, to the Gulf of St. Lawrence. This secondary line of depression, like the first, is marked + throughout the greater part of its extent by considerable deposits of uncrystalline sediments. These are, however, almost wholly of early Paleozoic age, and show that these valleys are of very great antiquity. The rocks forming the subject of this paper occur wholly in the more south-eastern of the two divisions, or sub-provinces, above indicated. ‘This division embraces ‘that great deflection of our Atlantic coast-line known as the Gulf of Maine. The head of the Gulf of Maine is at Portland; and its apparently but slightly contracted mouth is guarded by the two salient angles, Cape Sable and Cape Cod; while the Provinces of New Brunswick and Nova Scotia enclose its north-eastern end as Eastern Massachusetts does its south-western. The line joining the head of the Bay of Fundy and Plymouth, in Massachusetts, may be regarded as the axis of the gulf, and is about three hundred and. seventy-five miles long, with a trend approximating north-east and south-west. This line is of fundamental importance in the structure of this region, a base line for both geography and geology. The maximum breadth of the gulf at Portland is one-third of its length, or one hundred and twenty-five miles. The prevailing and all but universal line of strike throughout this entire region, from Rhode Island to the Gulf of St. Lawrence, as already stated, is north-east and south-west, — parallel with the axis of the Gulf of Maine. The rocks which meet the water of this large eulf are, for the most part, ancient crystallines ; and, where- ever these are exposed to observation, the indications are plain that the gulf is the product of erosion, — has been carved from the ancient strata by which it is bordered, — and is not the result of a synclinal or other depression of this portion of the earth’s surface. The agent of erosion appears, at first sight, to have — always been the waves of the Atlantic, and to have acted uniformly from the east toward the west. A study of the true contours of this body of water, however, brings to light facts which seem to militate against this view. An examination of a Coast Survey chart of this region shows that the water does 4) not become constantly deeper as we proceed seaward from the shore, anywhere between Portsmouth, N.H., and Eastport, Maine; but, on the contrary, the water is usually as deep at twenty to fifty miles from shore as at any greater distance less than two hundred and fifty miles, beyond which the abysses of the ocean begin, and the bottom falls away rapidly to twelve hundred and fourteen hundred fathoms; and not un- frequently the depth is actually greater at the lesser distance from shore, so that the water shallows seaward. Thus, the greatest depth recorded in the region of the Gulf of Maine, one hundred and sixty fathoms, is only fifty miles from Ports- mouth; and thirty-five miles from Mt. Desert gives us one hundred and forty-five fathoms: while beyond this the water shallows sensibly, and an equal depth is not found nearer than one hundred and fifty miles from shore. Extending easterly from Cape Cod, for nearly two hundred miles, is a broad expanse of shallow water, with depths rang- ing from five to fifty fathoms, and including George’s Bank, which reaches the surface one hundred and twenty miles from land. A similar but less distinctly marked area of shallow water stretches southward from Nova Scotia, with a breadth of sixty miles, and a maximum depth of seventy fathoms. Sepa- rating these two areas of moderate depths is a strait of deeper water, eighty to one hundred and fifty-five fathoms. Following the fifty-fathom line, this strait is, perhaps, fifty miles wide ; while for the one-hundred-fathom line it is reduced to thirty- five miles. It is plain that a broad submarine ridge or plateau extends over nearly nine-tenths of the distance between Nova Scotia and South-eastern Massachusetts, forming a nearly com- plete barrier between the comparatively deep water of the Gulf of Maine and the greater depths of the ocean beyond. If the sea bottom were elevated fifty fathoms, the Gulf of Maine, although still three hundred miles long, and having a maximum depth of one hundred and ten fathoms, would be changed from a broad-mouthed bay to an almost completely land-locked gulf. The glaciating agent operated powerfully-to obliterate that 6 feature of the Gulf of Maine, upon which I am here insisting, viz., its nearly continuous eastern rim; which, though no less real than the western border of the gulf, escapes recognition through its submergence beneath the waters of the Atlantic. One of the most important effects of glacial action was the transportation of immense quantities of detrital materials from the north and north-west toward the south and south-east. According to Prof. Hitchcock, the sheet of drift covering South-eastern Massachusetts is at least three hundred feet thick ; and any one familiar with the geology of this region, who notes the characters of the pebbles and boulders which this drift contains, cannot doubt that it has been largely derived from the land to the northward; nor will it seem improbable that Northern Massachusetts, and even New Hamp- shire, may have contributed something to the result. We have no reason to think the action of the ice-cap in this imstance was anything exceptional ; it probably swept Northern as well as Southern New England, and in even a more thorough manner. Where now is the vast accumulation of debris that, according to this supposition, must have been removed from portions of Maine and New Hampshire? I believe it has been shoved, in large part, beyond the present limits of the land, and is now spread over the bottom of the Gulf of Maine; contributing largely to the elevation of this bottom to the level of the eastern rim, and thereby tending to obliterate the latter as such. Jeffrey's Bank and Cashe’s Ledge, lying off the mouths of the Kennebec and Penobscot Rivers, sixty to seventy miles long, two-thirds as broad, rising from a depth of one hundred and sixty fathoms, and approaching at many points within five to fifty fathoms of the surface, should, apparently, be regarded as a huge pile of glacial detritus, which, although everywhere distinct from the eastern rim of the gulf, greatly obscures its basin-like character. That the glacial action, while tending to fill up the gulf, has contributed little or nothing to the formation of the ridge or plateau constituting its eastern border becomes sufficiently obvious when we note 7 the remoteness of this barrier from the land, and observe that a considerable breadth of deep water intervenes at all points between it and Jeffrey’s Bank. If, then, the debris were removed from the bottom of the Gulf of Maine, it would doubtless appear much more clearly than now to be a genuine depression, or basin, shut off from the outside ocean by a broad barrier ; which, whether we consider it to have been, at the time when this gulf was eroded, above the sea, and backed, may be, by an Atlantis, or as having been always submerged, constitutes a serious objection to the view that the erosion of the Gulf of Maine has been effected wholly or mainly by the waves of the Atlantic. Without intending or desiring to raise the question of the existence,in past geologic time of an Atlan- tic continent, it will be shown in the sequel that there are other reasons than those here advanced for believing that the Gulf of Maine was, at least during the period of its formation, land- bordered on the east as well as on the west; that it was. eroded mainly, if not entirely, before the beginning of Pale- zoic time; and that the rocks bordering this gulf, in Eastern Massachusetts at least, are so disposed about the line which I have denominated the axis of the gulf as to indicate that this line is really the axis of a great anticlinal or series of anti- clinals, the erosion of which, after the usual manner of anti- clinal erosion, has, I conceive, produced the depression in question. It has long been known that the rocks bordering the Gulf - of Maine exhibit a general uniformity along the strike. This likeness is usually believed to extend not only to the Paleozoic and more recent sediments and their relations to the crystal- lines, but also to the crystallines themselves, which are con- ceived to be arranged in broad irregular belts, parallel with the axis of the Gulf of Maine, and each of which is, in a general way, of the same age and composition throughout its extent. Careful comparisons of the rocks, especially the crystallines, of these various districts, have never been instituted; and the prevalent opinion here alluded to, though probably correct, is 8 based upon general impressions gathered from hasty, bird’s-eye- view surveys, rather than detailed and accurate knowledge. During the last decade, various members of the Canadian Geological Survey, and especially Mr. G. F. Matthew and Prof. L. W. Bailey, have greatly extended our knowledge of Acadian geology. Their labors, however, have been mainly confined to New Brunswick; and Nova Scotia is yet, as regards an accurate knowledge of the composition and relations of the crystalline formations of the province, well-nigh an unknown land. Our knowledge of the crystallines of Maine, also, is wholly inadequate for the purposes of comparative geographic study. In the study of the rocks of Eastern Massachusetts presented here, a detailed comparison of this region with those to the north-east has not been attempted ; our knowledge, as just stated, not being considered ripe for accurate generalization over so wide an area. Yet, although the end kept steadily in view has been merely to advance in some degree our knowledge of the character, distribution, and relations of the various rocks occurring in a limited district, it is hoped the way has been, in a measure, prepared for the higher study hinted at. TOPOGRAPHY AND GENERAL OUTLINES. The extent of territory coming within the scope of this paper is intended to be shown by the smaller of the two accompany- ing maps. It includes in a general way all that part of the State east of the north-south range of highlands of which Mt. Wachusett is the culminating point, and north of the Rhode Island boundary ; comprising the whole of Essex, Middlesex, Norfolk, and Suffolk Counties, the eastern half of Worcester County, and the northern portions of Bristol and Plymouth _ Counties. As shown by the map, the length and breadth of the district are approximately equal, and the area is not far from three thousand square miles, or about three-sevenths of the State. 3 The topographic descriptions given us by the elder Hitchcock and other writers, and the general evenness of the surface, obviate the necessity for extended remarks under this head here ; and, furthermore, the trends of the principal water-courses and shore lines are sufficiently obvious on the map. I would call attention, however, to the general relation of the topographic features to the geologic structure, —a point likely to be over- looked in a level and drift-covered region like this. The prevy- alent line of strike in Massachusetts, as is well known, is north and south, and in the western half of the State there are no exceptions of importance to be noted. East of the Nashua valley, however, a north-east and south-west strike prevails, especially in Essex and Middlesex Counties ; a comparatively limited area in the south-eastern part of Worcester County and the adjacent portion of Rhode Island exhibits a strike at right angles to this, or north-west and south-east ; while among the Paleozoic strata a nearly east and west strike is most com- mon. ‘That ‘‘ geology is revealed in topography ” is a trite axiom of the science, which is well exemplified, even in this extensively glaciated region. In a recent paper in the Ameri- can Naturalist,! ‘‘On the Surface Geology of Eastern Massa- chusetts,” I have discussed at some length the relations of the topography of this district to the glaciating agent and to the geologic structure. The general conclusion reached is, ‘‘ that, comparatively speaking, the ice-cap rested lightly upon the land, and that the topographic features having a skeleton or framework of rock are, as a rule, of pre-glacial origin.” In other words, to quote further, ‘‘ it appears probable that if the present mantle of drift were entirely removed from the face of the country, leaving a surface of naked rock, we should have in all important respects a restoration of the" pre-glacial contours. And this ancient topography having been, as I conceive, fashioned mainly by agents more subtle than an ice-cap, and hence taking a deeper hold on geologic structure, would, if thus undisguised, reveal a closer correspondence with the structure ? Vol. x1., pp. 577, et seq. 10 lines of the subjacent rocks than we are able to detect in the existing hills and valleys considered as a whole.” One of the most remarkable facts in the distribution of glacial detritus, or drift, in Massachusetts, as I have already observed, is the com- paratively great depth to which it has been accumulated . over the south-eastern portion of the State. There is a marked paucity of rock outcrops in the southern half of Plymouth County ; south of Plymouth and east of Middleborough they are rarely met with; and Barnstable County is absolutely des- titute of them. It is not improbable that the solid rocks in this region are so deeply buried by the unconsolidated superficial deposits, that, if the latter were removed, the whole of Barn- stable County and a considerable part of Plymouth County would be invaded and covered by the sea. Certainly here, if anywhere, we may expect lake basins and river valleys to ex- hibit in their forms and trends a complete independence of the underlying rocks; and, as I have shown in the paper above cited, this expectation is fully justified by the facts. In Worcester County, and north of this region of excessive drift, however, the dependence of the surface lineaments on the geo- logic structure is very marked. This is especially noticeable in ‘the case of the larger features, such as the Blackstone, Nashua, Merrimac, Parker, Ipswich, Charles, Neponset, and other rivers; the Wachusett range of highlands; the parallel range forming the eastern rim of the Nashua valley ; the somewhat irregular belt of hills extending from Cape Ann to Beverly ; the well-known range sweeping with bold front from Swampscott to Waltham; and the Blue Hill range in Milton and Quincy. The last three lines of hills, being principally composed of un- stratified rocks, are less regular and distinct than the others ; yet they no less clearly reveal the structure of the rocks com- posing them ; for exotic rocks, being, in a certain sense, struc- tureless, only conform with the general law in giving rise to a systemless topography. The rocks of Eastern Massachusetts admit of a convenient, and, it is believed, a chronologic, division into two great 11 groups, — the crystallines and the uncrystallines. The former predominate, constituting the surface rocks over fully nine- tenths of the area to which this paper particularly relates. The chronologic distinctness of these groups will probably not pass unquestioned. Yet it is true that, with the exception of the argillites of the Nashua valley, which Prof. C. H. Hitchcock ! has correlated with the Primordial slates about Boston, none of the crystallines of the eastern part of the State have, of late years, been regarded as of Paleozoic or more recent age, by any geologist familiar with the rocks of this region. I have studied the relations of our Primordial strata to the erystallines at every accessible point within my knowledge, and in all cases, where the evidence is of a decisive nature, it affords unequivocal support to the view, that between the crystallines and the oldest uncrystallines there is a great chronologic break, a ‘‘lost interval” of immense duration; for the unconform- ability is perfect, and the imperfect lithologic resemblance sometimes observable is due in every case to derivation or to very local alteration. Furthermore, it will be shown in the sequel, mainly on stratigraphic grounds, that the oldest rocks in the State are on its eastern border, facing the Atlantic, and that the various crystalline series appear to be so arranged geographically as to become successively newer as we proceed from Massachusetts Bay to the Berkshire Hills; so that, even if there were crystallines of Paleozoic age in Massachusetts, they would not probably be found in the eastern portion of the State. I cannot but regard the distinctness of these two great classes of rocks in this region as the most certain of all the conclusions reached in this paper. Considering the Primordial horizon as the base of the Paleozoic system, then it is true that, in this region at least, the profound break between the crys- tallines and the uncrystallines is coincident with that recognized generally between the two grandest chronologic divisions, the 1 Geological Map of Massachusetts, in Walling’s Atlas, 1871. Subsequently, in the final report upon the Geology of New Hampshire, Prof. Hitchcock has referred these argillites to the ancient Huronian system. 12 Eozoic and the three succeeding eras, — Paleozoic, Mesozoic, and Cenozoic, —for which, taken collectively, geologists have failed to provide a name. Several divisions have been established among the crystallines of Eastern Massachusetts, based, for the most part, on litho- logic and stratigraphic characters. The groups thus formed are believed to have chronologic value, to be chronologically dis- tinct. It is not without hesitation that, in describing these series as both lithological and chronological, I raise the vexed question of the value of purely physical characters as tests of age among crystalline rocks; for I began my study of the erystallines of this State with a wholesome distrust of the value of lithological data in establishing chronological divisions, and am not yet wholly prepared to apply this principle to wider regions. Within my limited field of observation, however, I am satisfied that its application is safe. It adapts itself so per- fectly to all the facts, so far as I know them, that I do not hesitate to assert that the lithological characters of the divisions which have been worked out among the crystallines of Eastern Mas- sachusetts — the chronological and geographical distinctness of which can scarcely be doubted — are as unlike as the faune of any two successive geological formations. 13 EOZOIC FORMATIONS. The oldest of these crystalline series is the smallest in extent. I propose to designate it, provisionally, as the Naugus Head series, —in allusion to the promontory of this name on the Marblehead shore, where the rocks of this series are best developed and were first observed. Succeeding the Naugus Head series, in the order stated, are the Huronian and Montal- ban systems, and a series of semi-crystalline rocks, of very dissimilar composition, but appearing to be identical in age and geognostic relations, which, having their greatest development in the vicinity of Boston, I have named collectively the Shaw- mut group,— Shawmut being the ancient Indian name of Boston. In employing the names ‘‘ Huronian,” ‘‘ Montalban,” etc., terms having chronological signification, to designate the crys- talline divisions of Eastern Massachusetts, I mean to assert the general lithological and stratigraphical resemblance, but not necessarily the chronological equivalence, of the groups thus designated to the great and generally recognized Eozoic divisions of other regions. My views on this subject cannot be justly characterized as merely speculative ; for at all impor- tant points they rest on the solid ground of facts. My position with respect to our crystalline or Eozoic formations, briefly stated, is this: There exists in Eastern Massachusetts a great series of crystalline schists agreeing in all important respects with the Montalban system as defined by Dr. T. Sterry Hunt. Farther east is another vast crystalline formation, differing widely from the first, and possessing all the essential char- acters of the Huronian. ‘This second series is plainly older than, and underlies, the first ; and it is underlaid in its turn by a still more ancient-looking terrane which, consisting prin- cipally of coarsely crystalline, and frequently exotic, basic rocks, presents a general resemblance to the Upper Laurentian or Norian series of the Canadian geologists. This, as above . 14 stated, I have named, provisionally, the Naugus Head series. In other words, the crystalline divisions of Eastern Massachu- setts agree in composition and sequence with those established in neighboring regions ; that this implies chronological equiva- lence might fairly be questioned, and I do not assert it. NAUGUS HEAD SERIES. Several small areas of the rocks referred to this series have been marked on the accompanying map.' Of these, two are of especial importance; the first includes the city of Salem, Salem Neck, the islands of Great Misery and Little Misery, Baker’s Island, Naugus Head on the north end of Marblehead, various small islands between Marblehead and Great Misery, and several narrow strips along the Beverly shore; while the second area embraces all but the seaward end of Large Nahant. A glance at the map will show that these areas lie contiguous to the sea-shore; and this fact — since these shores are for the most part rocky —has enabled me to trace their boundaries with considerable accuracy. In Salem and Peabody, however, the rocks marked as belonging to this series seem to have been more or less fluent, and are blended with the diorites of the Huronian formation in inextricable confusion ; which, to- gether with the absence of outcrops north of North river, has rendered the determination of the western boundary of this area difficult ; and the line as laid down on the map is partly hypothetical. The rocks of this series, though frequently stratified, appear in general to have been somewhat fluent, and usually exhibit more or less extravasation; but doubtless in some cases the metamorphic action has stopped short of this extreme term, though destroying all traces of bedding. In many places, as notably on Winter Island and Great and Little Misery, the entire formation seems to have been plastic, and the extravasa- tion has been so extensive that the character of the rock 1 See post., pe 23. 15 changes at nearly every rod. One important fact should be noted here, viz., nowhere in this region does the Naugus-Head series appear to be cut by eruptives belonging to another for- mation ; for all the exotic rocks of this group may be easily re- ferred to, or shown to be derived from its stratified members. The stratified rocks occur chiefly in Marblehead, and on the Beverly shore west of Curtis Point. In Marblehead the strike is E.-W., with a vertical dip; while on the Beverly shore the strike varies from N.-S. to N.E.-S.W., and the dip is thirty degrees to vertical to the N.W. The average strike of the whole system is N. E.-S.W. More or less distinct bedding has also been observed on the north side of Great Misery, at several points on Salem Neck, on Coney Island, and at one point on the north-east shore of Nahant. The rocks of this series are composed chiefly of feldspar and pyroxene. ‘These minerals occur mixed in very various pro- portions. Perhaps the most characteristic rock is one composed almost entirely of feldspar, containing little pyroxenic or hornblendic material, and frequently destitute of it. It is usually coarsely crystalline, this variety prodominating along the Beverly shore east of Curtis Point; and the crystallization is sometimes extremely coarse, as, notably, on the Beverly shore, near the western end of West Beach, and in Marble- head, especially about Dolliber’s Point. At the former locality the rock is evidently exotic, and some of the feldspar crystals are of immense size, ranging from three to six inches in length, and one to two inches in breadth; but at Dolliber’s Point it is distinctly bedded, and is interstratified with fine-grained pyrox- enicrocks. Another, but less abundant, variety, is fine-grained, presenting a very uniform texture and appearance, and ap- proaching the compactness of felsite. This occurs at many points, but is most largely developed on Baker’s Island, which is principally formed of it. A dark, heavy, usually coarse- grained diabase or norite, varying greatly in composition, and frequently occurring as a nearly pure pyroxene rock, is the most abundant rock of this series. It is the prevailing rock on 16 Nahant, where it is frequently very coarse and pyroxenic, with a high specific gravity. A more feldspathic variety exhibits traces of bedding on the north-east shore of this peninsula, as already noticed. This stratified rock is about midway between the Spouting Horn and Maolis Garden, and is of very limited extent. The lines of bedding trend E.-W. for one hundred feet or more, and then toward the west curve abruptly, but smoothly, around to the south, forming a right angle.. Dip vertical. This pyroxenic rock, in its several varieties, and the feldspathic rock just noticed, occurring chiefly as eruptives, underlie the city of Salem, and form the peninsula of Salem Neck, and the islands of Great Misery, Little Misery, Pope’s Head, and Eagle Island. The stratified rocks of this series consist usually of interstratified, frequently alternating, pyrox- enic and feldspathic beds. They are of all textures, from the finest to the coarsest; the stratified pyroxenic rocks, however, are generally fine-grained, schistose, and very distinctly bedded. These last are well developed on Marblehead, west of Dolliber’s Point, and especially about Naugus Head. It has been suggested that some of the coarse feldspar, so abundant in the rocks of this series, may be labradorite ; but the analyses thus far made do not support such a view. Yet, as only two specimens have been submitted to analysis, the question cannot be regarded as settled. The first specimen analyzed was taken from the very coarsely crystalline, stratified, feldspar rock on Dolliber’s Point, Marblehead, and was placed in the hands of Mrs. E. H. Richards, of the Massachusetts Institute of Technology, who made the following report : — BIO} «> ; 3 : : ; : : 7 66.639 ALO... : : : ; : : : : 19.375 KO): : ; : : ‘ : : ; 4.500 Na,O . : : : : : : 3 ; 10.011 CaO. . ; : ; : : : ‘ : traces ie be Ge ; : : ; : : traces 100.525 ie Such a chemical constitution belongs to no known species or variety of feldspar, though coming very near orthoclase ; and, fearing there had been an error in the first analysis, a second was made from the same piece,—a clear, apparently un- weathered specimen, — with a like result. A careful inspection of a large number of specimens seemed to explain the anomaly, by showing that the feldspar, although coarsely and perfectly crystalline, is probably a mixture ; the crystals appearing to be formed of thm interlaminated plates of at least two different feldspars. This can be seen only on slightly weathered sur- faces, one of the feldspars decomposing more readily than the other. The constituent layers were not analyzed, as it seemed impracticable to separate them. But if, as appearances indicate, it is a mixture, and a mixture of two species only, they are probably orthoclase, and albite or oligoclase, the former pre- dominating. ‘The color of this feldspar is bluish-gray, weather- ing white, and its sp. gr. varies from 2.55 to 2.60. Dissemi- nated through it are numerous small crystalline grains of magnetite. The coarse feldspar of the Beverly shore, similar in physical characters to the preceding, was analyzed by Mrs. Crosby, and found to have substantially the same chemical con- stitution, containing one-half of one per cent. of calcium oxide. The specimens just described, from the coarsely crystalline, nearly pure, feldspar rock, are certainly not labradorite, and yet that this species exists in some of the rocks of this series does not, in my mind, admit of reasonable doubt. Much of the labradorite in the norite of Essex County, New York, and probably of other localities, is of a pale-green color and waxy lustre, and seemingly compact. In the coarse pyroxenic rock of Nahant, the feldspar is frequently physically identical with this, and - yields by analysis a larger percentage of lime. The silica and calcium oxide only were determined, with the following result : — SiO, CaO Labradorite from Westport, N.Y. . - 54.60 4.85 Feldspar from Nahant E : ; (48.71 8.70 OCCAS. PAPERS B. 8S. N. H.—III. y) 18 The same feldspar, apparently, occurs on Salem Neck, and at several other points in that region. In a formation so desti- tute of free silica as the Naugus Head series, it were natural to expect to find the feldspar mainly triclinic; and this expecta- tion is fully realized, for, save in the very coarse feldspar rock of the preceding paragraph, I have rarely failed to observe the striz indicative of plagioclase ; and in not a few instances, judg- ing by physical characters alone, this plagioclastic feldspar is most probably labradorite. A triclinic feldspar from the west end of Salem Neck, apparently little altered, afforded Mr. Geo. H. Barton fifty-seven per cent. of silica. Dark-colored mica, probably biotite, is common in the rocks of this formation, though seldom abundant. Pyroxene appears as a principal constituent, and hypersthene is believed to occur. The massive, coarsely crystalline diabase, or norite, at Nahant is often decidedly epidotic; and the epidote is particularly abundant on the south shore, east of the steamboat wharf. The most striking, and probably the most important minera- logical character of this series of rocks is, that all members of it are absolutely destitute of quartz. Lithologists will recognize, in the complete absence of quartz from this formation, a strong indication that the principal constituent, after feldspar, is pyroxene, and not hornblende ; and it is probable that every basic rock of this series may be properly classified, in a gen- eral way at least, ‘as either diabase, norite, or pyroxenite. The coarse feldspar rock, mentioned above, and consisting mainly of orthoclase, is probably often a true syenite. That this series of pyroxenic and feldspathic rocks, with its associated minerals, — which is sometimes stratified, oftener eruptive, frequently very coarsely crystalline, and always quartzless, — is distinct from anything observed elsewhere in Massachusetts, cannot be doubted. Lithologically at least, it may be said to be sw? generis in Massachusetts geology. The great disturbance which the Naugus Head series everywhere exhibits, and its thoroughly crystalline appearance, stamp it as older than the Huronian and Montalban formations ; and other 19 and more. important considerations confirm this view. ‘The northern or Salem area of this series is bounded everywhere either by the sea or by rocks belonging to the Huronian system. On the north, in Beverly, we have coarse Huronian granite, such as is quarried in Quincy and Rockport; while to the west and south, in Peabody, Salem, and Marblehead, are dioriteand fine-grained hornblendic granite, also Huronian. What, now, are the geognostical relations of the Naugus Head series to these Huronian terranes? It underlies them. Everywhere, along the boundaries of the Naugus Head areas, we find the various members of this series penetrating and cutting through the Huronian rocks. But the converse of this is never ob- served. Nowhere, so far as my observations extend, does the Naugus Head series appear to be cut by the adjoining Huronian rocks; nor by any member of the Huronian system; nor, in fact, by any rocks not easily referable, as already stated, to the stratified portions of this series itself. In short, the Naugus Head series appears to be, as it were, at the bottom; and, while it has been extravasated extensively through superjacent formations, it is penetrated by nothing foreign to itself. The relations of the Naugus Head series and the Huronian forma- tion are best displayed in the cliffs along the shores of Beverly and Manchester. Here the Huronian granite, already men- tioned, is cut extensively by great dykes and eruptive masses of rocks, both pyroxenic and feldspathic, clearly belonging to the Naugus Head series; the feldspathic dykes appearing, usually, to be older and larger than the pyroxenic. But this granite, although it has evidently been more or less fluent, is never found cutting any member of the Naugus Head series ; and we are thus forced to the conclusion that this series is older than the granite. | ~The Norian beds of Canada and Labrador were formerly regarded as forming part of the Laurentian, bearing the name ‘¢ Upper Laurentian”; and, wherever their relations to the un- derlying terranes have been observed, they lie directly upon the Laurentian, never upon the Huronian or Montalban. The 20 Huronian, also, has heretofore, wherever its foundations could be clearly made out, been found resting on a Laurentian floor. The Norian and Huronian have never been observed under con- ditions favorable for the accurate determination of their mutual stratigraphic relations, and hence these are not positively known. Prof. Bailey and Mr. Matthew, however (in the Geological Survey of Canada, Report of Progress for 1870-71, p. 41), report the occurrence, seven miles east of St. John’s, in the Province of New Brunswick, of a small area of crystalline, anorthic, and hypersthenic rocks, which are regarded as Norian by Dr. T. Sterry Hunt, after an examination in setu; and oencerning the geognostical relations of which they say, ‘* On the north side these anortholite rocks are met by red gneiss and granite, similar to the Laurentian gneiss at Indiantown, and on the south are covered by conglomerates and diorites of the Huronian series.” This language is explicit, and indicates that here, at least, the normal position of the Norian is be/ow the Huronian. Yet it seems doubtful if this can be regarded as proved by the facts observable at this place; for, according to Dr. Hunt, these Norian rocks are, apparently, devoid of stratification, and may belong to an extravasated mass; while the adjoining Huronian beds are vertical. The mere occurrence, however, of these Norian outcrops between ledges of Laurentian and Huronian rocks, suggests that this is their normal position, and if their geognostical relations to the Laurentian and Huronian beds are regarded as due to the action of eruptive agencies, it is hardly conceivable that they have come from any horizon above the Huronian. Although conclusive proof that the Norian system is older than the Huronian has not been afforded us by a study of their relations at the only poimt where they are known to occur together in s¢tu, yet the bulk of the evidence points in that direction ; and the general facts, that the Norian rocks are usually more crystalline, have in every respect a more ancient aspect, and always exhibit less unconformability with the Laurentian than the Huronian, increase the probability 21 that this is the true sequence of these oreat crystalline forma- tions. This, it may be added, is the sequence deemed most probable by Dr. Hunt, viz., Laurentian, Norian, Huronian. The Naugus Head series is certainly distinct from, and (as I have already shown) probably underlies, the Huronian ; and, since it bears no likeness to the Laurentian system, we are brought to the conclusion, that, if it is to be correlated with any series already described, that series is the Norian. In short, .the Naugus Head series does not resemble the Laurentian, and is, stratigraphically, where we should expect to find the Norian ; and these are, mainly, the considerations which led me to designate this series as the “ Norian,” on the Centennial Geo- logical Map of Massachusetts, and in my report on the same. Having been informed, however, by so good an authority as Dr. T. Sterry Hunt, whose opinion I had no opportunity to obtain before the publication of the report above referred to, that this series, chiefly on account of the supposed absence of labradorite, cannot be regarded as of Norian age, I have em- ployed here, and on the accompanying map, a provisional designation ‘having no chronological signification. I am con- strained to believe, however, that, save in not holding labra- dorite as a principal constituent, if such proves to be the fact, the Naugus Head series presents a fair agreement, lithologically, with the essential characters of the Norian, as the latter has been described by Dr. Hunt. It is proper to state, in this con- nection, that Dr. Hunt, in 1869," identified, as belonging to the Norian formation, a boulder found on Marblehead Neck, and possibly derived from the Naugus Head areas to the northward. I know by personal observation that the Naugus Head rocks are scattered as erratics all over the town of Marblehead, including the Neck. Prof. A. Hyatt long ago recognized the rocks about the city of Salem as probably older than the petrosilex of Mar- blehead Neck ; but he did not separate them from the Huronian diorites of Salem, Swampscott, and Marblehead. 1 Amer. Jour. Sci. (2) xirx., 183, 398. 22 The only rocks in Massachusetts that have been observed passing below the Huronian system, or cutting through its lower members, are those composing the Naugus Head series; and this, together with its crystalline character and immense dis- turbance, convinces me that this series is the oldest in the State. In the light of our present knowledge the conclusion cannot be avoided, that the Naugus Head series is the real base of the geological column of Massachusetts. The coarse-grained, readily disintegrating exotic diabase, so extensively quarried in Medford, —and also occurring in Somer- ville, Brookline, and, probably, other places, — bears a strong resemblance to certain members of the Naugus Head series, es- pecially to the coarse pyroxenic rock of Nahant and Salem Neck; and the idea is natural that they are extruded portions of this series, which may, I think, be regarded as the probable seat of many of the eruptive masses cutting the newer formations of this region. For the accurate identification of these exotics in Medford, Somerville, Brighton, Brookline, etc., we are indebted to Mr. M. E. Wadsworth.'! This observer has found that the principal dark constituent of these rocks, in the unaltered condition, is always pyroxene, never hornblende; a fact which accords well with the general conclusion already stated with respect to the composition of the basic rocks of the Naugus Head series. The forms and general relations of these masses will be more fully described in connection with the uncrystallines which they intersect. The small area of basic exotics exposed near the head of Washington avenue, in Chelsea, has been doubtfully referred to the Naugus Head series. North of Wenuchus Lake, in Lynn, is a hill composed of a nearly pure feldspar rock, — a coarse, whitish fieldspar, apparently the same as that so well developed on the Beverly shore. The rock is massive, having the aspect of an exotic; and around the base of the hill it can 1) Prod. .B..S3) IN. yek Xa 23 be seen penetrating the Huronian diorite. It is undoubtedly safe to regard this as an outlier of the series in question. In what precedes I have described all the areas of the Naugus Head series marked on the map; but recent observations have con- vinced me that others exist. The hills immediately north of Wadsworth’s Station on the New York and New England Rail- road, in Franklin, appear to be entirely composed of a rock very similar to the prevailing type on Salem Neck, — quite destitute of quartz, and consisting chiefly of a coarse, triclinic feldspar, of bluish and grayish colors, with some mica, dark-colored, and often bronze-like, a green mineral that may be hypersthene, and a very little hornblende or pyroxene. No stratification is visible ; and the boundaries of this area are entirely unknown, save that it does not appear to extend much, if any, south of the railroad. The high hills in Sharon, near Sharon Centre, and on either side of the Boston and Providence: Railroad, appear to afford another area of these rocks. These hills are near the centre of the large area marked on the map as Huronian diorite ; and some observations made by Mr. F. W. Very, in Foxboro’, in connec- tion with my own, lead me to suspect that, on the geological map of the future, the Naugus Head series will demand a con- siderable portion of the territory here assigned to a newer for- mation. Among the rocks occurring here I have recognized the most of those found in the Salem and Beverly areas. They are of all textures, and some varieties hold abundant grains of magnetite or menaccanite. In passing over the road leading north-easterly from Reading Village, and about one mile from the Boston and Maine Railroad, I have observed several ledges of a coarse, apparently exotic, dioritic rock, the chief constitu- ent of which is a coarsely crystalline plagioclase, which I am strongly inclined to believe is labradorite; the rock, in that event, probably being a norite. It is in a region where out- crops are few and far between, and I could learn nothing of its extent. 24 HURONIAN. The rocks that may be referred to this system in Massachu- setts, like those of the Naugus Head series, are believed to occur only in the eastern portion of the State. They cover a wide area; and, except where the Naugus Head series, Shaw- mut group, and rocks of Paleozoic age face the Atlantic, they form the seashore from the New Hampshire line to Plymouth. This formation is bounded on the north and north-west by a line running south-westerly from Salisbury through Essex and Mid- dlesex Counties to Concord. Here, after giving off a long and narrow deflection, which continues nearly twenty miles farther to the south-west, the line bends to the south, and continues through Framingham, Holliston, Medway, and Bellingham, to the north-east corner of Rhode Island. On the south, it is met mainly by the carboniferous rocks of Bristol and Plymouth Counties. The Huronian area has an extreme length, measured from the New Hampshire line in Salisltury to Manomet Hill in Plymouth, of sixty-five miles ; and a maximum breadth of forty miles across the southern end, not counting the narrow band stretching from Concord to Westborough. It is almost completely divided near the middle by the Primordial, and, possibly, more recent rocks, which lie about the shores of Boston Harbor, extending westerly to Natick, and south-westerly to Rhode Island. On the accom- panying maps the Huronian series comprises the areas marked as ‘* syenite,” ‘* porphyry,” and ‘‘ hornblende slate,” on the geo- logical map of Massachusetts, prepared by Professor Edward Hitchcock.! The existence in Eastern Massachusetts of rocks of Huro- nian age was first announced by Dr. T. Sterry Hunt, in 1871. In a paper on ‘* Granites and Granitic Vein-stones”? he speaks of the « felsites” and «< felsite porphyries,” or ‘‘ orthophyres,” oc- * Ihave marked the Huronian boundary on the map with a heavy line, for the sake of greater distinctness. * Chemical and Geological Essays, p. 187. 25 curring in Lynn, Saugus, Marblehead, and Newbury ; and says in thisconnection, ‘‘ These rocks are, throughout this region, dis- tinctly stratified, and are closely associated with dioritic, chlo- ritic, and epidotic strata. They appear to belong, like these, to the great Huronian system.” Dr. Hunt has included here all the rocks which it is proposed to refer, in this paper, to the Huronian series, save the binary and hornblendic granites (which are so characteristic of Kastern Massachusetts) and the limestones. In consequence of finding the Hozoon canadense in the serpentinic limestone of Meakin, Dr. Hunt, in 1870, referred this limestone and the associated rocks, as well as the more crystalline and ‘less serpentinic limestones of Chelmsford and Bolton, and the gneiss in which they are included, to the Laurentian system. He made no mention, in this connection, of the serpentinic limestone in Lynnfield, which is probably of the same age as the Newbury deposit, since the associated rocks appear to be the same. Prof. C. H. Hitchcock, also, in 1871,? referred to the Triassic period, certain diorites in the vicinity of Salem and Boston, which are here referred in part to the Nau- gus Head series, and in part regarded as of Huronian age. And more recently, in his late report (1875) on the geology of New Hampshire, he has applied the term ‘‘ Labrador” to this broad Huronian area, although these rocks have scarcely a single character in common with the Labrador or Norian series as defined by Dr. T. Sterry Hunt and the Canadian Geological Survey.? It will be shown in the sequel, however, that all the rocks within the area described probably belong to one and the same lithological and stratigraphical series, the general characters of which stamp it as Huronian. A glance at the maps will show that the attempt to delineate this formation lithologically, 7. e., to show the distribution of its various lithological members, has been attended by moderate 1 Amer. Jour. Sci. (2), xurx., 75. 2 Geological Map in Walling’s Atlas. 8 Still later, in the atlas of the New Hampshire Survey (1878), Prof. Hitchcock recognizes the existence of several limited Huronian areas in Essex County. 26 success. More might have been accomplished with a longer time for exploration; yet much must ever remain undone, on account of the great extent to which the rocks are, in some dis- tricts, concealed by superficial deposits. A special color has been used to represent the general Huronian formation wherever the particular lithological representative is not knewn; but the probabilities are great that the rock, whatever it is, belongs to this age.! . The Huronian system in this region, like the Naugus Head series, though in a somewhat less degree, exhibits great disturb- ance. Distinctly bedded rocks are the exception ; and, although many apparently structureless rocks are probably really strati- fied, it is undoubtedly true that a large part, perhaps the greater part, of the formation has been more or less fluent, and ex- travasation may be set down as a characteristic structural feature. The extent to which some of the rocks of this series, in Eastern Massachusetts, are characterized by a condition ap- proaching chaos, can be fully appreciated only by those’who have studied them somewhat in detail in the field. The strati- fied portions of this series have usually a N. E.-S. W., vary- ing to HK.—W., strike ; and the areas of unstratified and extrava- sated rocks generally exhibit in their trends a tendency to parallelism with the strike of the stratified rocks. The latter usually dip steeply to the north-west. The Huronian series of Eastern Massachusetts is principally composed of the following rocks, or, rather, groups of rocks : — 1. Granite (hornblendic and binary). 2. Petrosilex (passing into felsite and quartzite). 3. Diorite (unstratified and largely exotic). 4. Hornblendic Gneiss, Stratified Diorite, etc. 5. Limestone. 1 In adopting the plan, wherever practicable, throughout this work, of mapping the various formations lithologically rather than geologically, I have done much to keep my facts and theories separate. 27 Although so connected lithologically and stratigraphically as to be clearly members of one great series, yet these various groups are, on the whole, well separated, occurring mainly in large masses. The stratigraphic distinctness would be much more striking but for the wide-spread extravasation which some of the divisions have experienced. Their general separateness implies that they are, for the most part, of different ages, —are chronologically distinct; and may each be regarded as a sort of sub-formation. | Whence it follows that, in all their more special relations, they admit of separate description ; and that is the plan adopted here. The different groups will be taken up in regular order, beginning with the oldest. The true sequence, excluding some of the limestone, is expressed in the foregoing classification. GRANITE. The typical granite of this region, as shown at the quarries in Quincy and other places, is a coarsely crystalline aggregate of orthoclase, quartz, and hornblende. Orthoclase is the pre- dominant mineral, and in its abundance constitutes the leading character of the rock; this is pre-eminently a feldspathic gran- ite. The hornblende is usually small in amount, and the rock frequently passes, through the disappearance of hornblende, into binary granite. All the Huronian granites of this region, and especially this typical variety, are remarkably firm and coherent, being strongly contrasted, in this respect, with the most of the granite of the Montalban and newer formations. In Topsfield, in and near the village, the granite is locally rather loose and friable ; but this is the only instance of the kind within my knowledge which is not clearly the result of atmospheric action. And it may be said that these granites, as a rule, resist chemi- cal quite as well as mechanical forces. The colors of the gran- ite are mainly due to the feldspar, the hornblende seldom being sufficiently abundant to sensibly darken the tint of the aggre- gate. The feldspar is usually grayish or bluish, though differ- 28 ent shades of red and pink are very common, and green and other tints are frequently met with. Feldspars of several dis- tinct colors are sometimes commingled in the same hand speci- men. * os Pe ee oe ee = v 77 purple. It rarely holds pebbles, and might be described as de- void of all traces of sedimentary structure but for the limited patches of banded petrosilex above described, which occur in it, and differ from it only in being banded. We have no reason to doubt that this rock is mainly petrosilex. A characteristic brownish-red specimen from Lynn contains 73.9 per cent. of silica. Towards the north, in Lynn, the rock varies much, is frequently of a gray or drab color; and near the reservoir, on the road to Dungeon Rock, it holds grains of quartz as well as feldspar. Where the tongue of granite penetrates the petro- silex on the west side of Wenuchus Lake, these two rocks appear to have reacted upon each other, producing mutual modifications, so that the contact is now marked by a zone of debatable ground, in which one seems to find all sorts of transi- tions between petrosilex and granite; and it is clear that one, perhaps both, of these rocks must have been fluent. Of the same general character — frequently quite compact, and never very porphyritic —is the most of the petrosilex in the north-west corner of Saugus and the adjacent portions of Wakefield and Melrose; also in Medford, north of the Naugus Head diorite, where it shows frequent local approximations to granite ; in short, this is the prevailing, the characteristic va- riety of petrosilex for the entire Lynn and Medford area. A grayish, non-porphyritic specimen from Maplewood gave 72.6. per cent. of silica. At many points in North Saugus, between Main Street and Saugus River, the rock is more than usually porphyritic, yet it holds numerous pebbles, and may possibly belong with the breccia; this, however, is improbable, since the undoubted breccia contains pebbles of this pebble-bearing petrosilex. Are there two breccias here? I think not, although the appearances are not wholly inconsistent with that view. Along Fulton Street in Medford, between Salem Street and the granite on the north, there is a large area of a dioritic rock which appears to alternate between granite and felsite. It is composed mainly of pinkish and greenish feldspar, with some hornblendic material which, inconspicuous and slaty-looking, as 78 a rule, occasionally assumes the form of slender crystals half an inch long. Grains of quartz are rare, and usually wanting. Some of the feldspar, at least, is triclinic ; and, although marked on the map as petrosilicious, this rock, in the vicinity of Fulton Street, is more properly a diorite. To the eastward, how- ever, its physical characters change; and near the line between Medford and Melrose it appears through the Shawmut brec- cia as a compact gray base, holding conspicuous crystals of plagioclastic feldspar and slender needles of hornblende. Thus it is clear that we here have a felsite passing into diorite. A somewhat similar, but more porphyritic, rock is found still far- ther east, beyond the Boston and Maine Railroad. This closely resembles the crystalline layers in the stratified felsite at Dun- geon Rock in Lynn, and, like that, it has the composition of a true felsite, the percentage of silica being not quite 63. It car- ries a few pebbles, however, and hence I am obliged to regard it as possibly a recomposed rock, related to the diorite only by derivation. Near the northern boundary of Melrose it appears again, decidedly crystalline and free from pebbles; and on the west side of the railroad at Greenwood Station there is an im- mense exposure of a light-colored, feldspathic, crystalline rock resembling that on Fulton Street in Medford, but finer grained, which should be set down as diorite, though probably passing into felsite. It is very basic, yielding only 57 per cent. of silica. Indications are not wanting that all of these dioritic felsites in Melrose and Wakefield may belong to the Shawmut group. Granitoid petrosilex, such as occurs on the south-western end of Marblehead Neck, has been observed at only two points in the Lynn and Medford area, on the Newburyport turnpike in Malden, and in North Saugus, near where Main street crosses the Wakefield line. At both places the rock is very local, and — yet its transition character is plain. This extended area of petrosilicious rocks, although many times larger than the Marblehead Neck region, and fully equal- ling that limited area in structural complexity and the variety of problems which it presents, has been the field of even fewer ee — 79 observations; in an area where the student should traverse nearly every square rod, there are almost whole square miles which I have not seen. ‘The insufficiency of my observations has necessitated the foregoing summary treatment of this interesting region. Any other course would have involved me in serious errors of commission as well as omission. There are petrosilicious rocks in Reading and Woburn, inter- stratified with quartzite, hornblendic gneiss, and other rocks ; but since the amount is small and the close relations to the associated rocks very evident, I will defer further mention of them until that group is taken up. oi Petrosilex in Needham. —Outcrops are very rare in the narrow strip marked as petrosilex south of the Boston and Albany Railroad, in the northern part of Needham, and hence this area is largely conjectural. Near the station in Wellesley the rock is reddish-brown, compact, and has a quartzose appearance. It is probably continuous with the quartzite in Natick, and appears to be everywhere closely associated with the coarse granite. Quite distinct from this is the petrosilex of the large area in -the central and southern portions of Needham. The rock is remarkably uniform over this entire area. It always presents a compact, grayish or greenish-white base,porphyritic with feld- spar crystals, and the most of the rock is elvanite, holding grains of transparent quartz in addition to the crystalline feldspar. The quartz grains are half a line to a line in diame- ter, more conspicuous than the feldspar, and they seldom assume the shape of crystals. Toward Newton Upper Falls, and east of the Charles River, in Newton, the visible quartz is wanting. So far as observed, the rock never holds pebbles nor exhibits any traces of bedding; and yet very commonly it presents a slaty appearance and yields to the knife, raising doubts as to its Huronian age. The base is sometimes absent, or nearly so, the rock being crystalline throughout, and ap- proaching granite. A specimen from the railroad, one mile south of Needham Station, afforded 75.45 per cent. of silica, 80 from which, and its great uniformity, I infer that this rock is all petrosilex. The vertical joint planes in this rock frequently intersect in such a manner as to develop a beautiful and perfect columnar structure. The columns are not uncommonly hex- agonal, four to eight inches in diameter, eight to twelve feet or more in length, and as true and regular as any in basalt. Sey- eral fine examples of this structure are exposed in the cuts on the New York and New England Railroad, north-east of Charles- River Village. Petrosilex in Dover, Medfield, and Dedham. — The two small patches of petrosilex along the railroad in Dover and Medfield exhibit local transitions toward granite and diorite. The rock is of greenish and grayish hues. My data from this region are meagre, and further exploration would probably dis- cover more of these small islands of petrosilex among the granite and diorite. The petrosilex of the large area in Dover and West Dedham presents many varieties. As a rule it becomes more crystalline toward the west, appearing to pass insensibly into the granites which border it in that direction. On the other hand, we are as yet unable, as already stated, to separate the petrosilex from the breccia on the south. The difficulty here arises from the enclosure of pebbles in portions of the petrosilex adjacent to the breccia; and the question is un- settled as to whether the explanation found adequate on Marble- head Neck will fit the similar phenomena in West Dedham. The magnificent exposures in the former locality greatly facili- tated the determination of the relations of the rocks. The main body of the petrosilex in the vicinity of the breccia is of a greenish hue, waxy lustre, usually not very hard, and, save when holding pebbles or an occasional grain of quartz, quite compact. It seems to have the composition of petrosilex, an average speci- men from the hill west of the ‘‘ Oven Mouth,” on Main Street, giving 72.35 per cent. of silica. Yet its inferior hardness, usually yielding to the knife, suggests recomposition, though it may mean simply decomposition. It does not differ in this respect, however, from much of the elvanite in Needham. 81 What appears to be the same rock, minus the pebbles, and sometimes more crystalline, covers a large area to the north and west, being the prevailing variety in this region. The pebbles are very rare, except in the immediate vicinity of the breccia, and here they are never so numerous as to give the rock a conglomerate aspect; ¢.e., as regards texture, there is nothing like a transition from the petrosilex to the breccia. Morover, the breccia holds pebbles of a petrosilex indistinguish- able from that in question. The pebbles are usually petrosilex, | but many granitic pebbles have been observed ; these are fine- grained. The petrosilex pebbles are commonly pinkish, and this is the color of many of the pebbles in the breccia. | At the corner of Main and Pond Streets in West Dedham, and quite near the breccia, there is ‘‘ flattened pebble” or schis- tose petrosilex, identical, save that it has a reddish tinge, with that studied on Marblehead Neck. It bears no resemblance as regards either structure or component materials to the adjoin- ing breccia. More or less distinctly banded petrosilex is known to occur at several points in this Dedham and Dover area, but always in small patches. On Dover Street, in Dover, it holds rounded grains of quartz. Toward the east, on and near Fox Hill, we have the most typical petrosilex which this area affords. It is black, hard, and compact, except at one point where Mr. Very has observed banding. The bands are distinct on the weathered surface, but entirely wanting on the fresh fracture ; and, since this petrosilex is very homogeneous throughout, we conclude that it was probably all banded originally. This rock has contributed many pebbles to the breccia. A specimen from Fox Hillhas been found to contain 72.9 per cent. of silica, and 18.7 ‘per cent. of aluminum and iron sesquioxides.