123 
 A7 
 
 UC-NRLF 
 
 B M 170 420 
 
GIFT OF 
 
SCIENCES 
 LIBRARY 
 
 REPORT 
 
 ON 
 
 LJ I 13 li A 
 
 ; 
 CALIFORNIA 
 
 THE GEOLOGICAL MAP 
 
 OF 
 
 MASSACHUSETTS, 
 
 PREPARED BY 
 
 W. 0. CROSBY, 
 
 ASSISTANT IN THE LABORATORY OF THE BOSTON SOCIETY OF 
 NATURAL HISTORY. 
 
 PROF. ALPHEUS HYATT IN CHARGE. 
 
 UNDER THE DIRECTION OF THE MASSACHUSETTS COMMISSION 
 TO THE CENTENNIAl EXPOSITION. 
 
 BOSTON: 
 
 PRESS OF A. A. KINGMAN. 
 1876. 
 
REPORT 
 
 ON 
 
 THE GEOLOGICAL MAP 
 
 OP 
 
 MASSACHUSETTS, 
 
 PBEPABED BY 
 
 W. 0. CROSBY, 
 
 ASSISTANT IN THE LABORATOBY OP THE BOSTON SOCIETY OP 
 NATUBAL HISTOBY, PBOF. ALPHEUS HYATT 
 
 UNIVERSITY OF 
 
 CALIFORNIA. 
 
 UNDER THE DIRECTION OF THE MASSACHUSETTS COMMISSION 
 CENTENNIAL EXPOSITION. 
 
 Distributed by the Boston Society of Natural History at the 
 request of the Centennial Commissioner for Massachusetts. 
 
 BOSTON: 
 
 PRESS OF A. A. KINGMAN. 
 1876. 
 

 
 ,< :-\Vf K. 
 
if 
 X 
 
 LI BEAR Y 
 
 UNIVERSITY OF 
 
 CALIFORNIA. 
 
 
 HON. LEVERETT SALTONSTALL, 
 
 Massachusetts Commissioner to the Centennial Exposition : 
 
 SIR : I have the honor to present herewith the Report on 
 the Geological Map of Massachusetts, prepared under the di- 
 rection of the Commission. 
 
 My effort has been to unite the latest and most reliable 
 information, whether published or unpublished, and, with 
 these results, to give such a view of the Geology of the State 
 as would represent the advances made since the Wall Map 
 of Prof. Edward Hitchcock was published in 1844. 
 
 Over and above the function of directing the work, and 
 securing the execution of this plan, I do not assume any 
 credit for the collection of the materials, or the results reached 
 by this means. Most of the outlines of the formations remain 
 substantially as they appear on Prof. Hitchcock's map; though 
 many of the details, and almost the entire eastern portion of 
 the State, will be found to have been much changed, when 
 compared with any previous map. For these and other mat- 
 ters of original and unpublished interest, the Commission is 
 indebted to the voluntary labors of Mr. W. O. Crosby, Assist- 
 ant in the Laboratory of the Boston Society of Natural His- 
 tory, Mr. L. S. Burbank, of Woburn, Mr. T. T. Bouve, Presi- 
 dent of the Boston Society of Natural History, Prof. N". S. 
 Shaler, of Harvard University, Prof. W. H. Niles, of the 
 Massachusetts Institute of Technology, Prof. B. K. Emerson, 
 of Amherst College, Mr. W. W. Dodge, of Cambridge, Mr. 
 F. W. Very, of Dedham, Mr. W. W. Jacques and Mr. C. W. 
 Kempton, both of Newburyport, Mr. Mathew Anderson, of 
 
 332696 
 
Cambridge, and others. The particular facts which they 
 have contributed will be found in the body of the text. In- 
 formation with regard to the eastern boundary of the Miocene 
 on the island of Martha's Vineyard, and some other points, 
 were contributed by Prof. N. S. Shaler, from maps prepared 
 by him for the United States Coast Survey, and our acknowl- 
 edgments are also due to them. 
 
 Valuable and efficient aid, without which much of the work 
 would necessarily have remained undone, has been received 
 from the Presidents and Directors of the following Railroad 
 Companies, in the form of free passes over their respective 
 lines : the Eastern, Fitchburg, Old Colony, Boston and Provi- 
 dence, .and Connecticut River. The Boston Society of Nat- 
 ural History has also, in the loss of the services of the 
 Custodian and his Assistant, while engaged in the construc- 
 tion of the Map, contributed largely to its successful comple- 
 tion. 
 
 Mr. Crosby has been much in the field, the weather having 
 been very favorable during the past winter. Mr. Burbank 
 and others have also spent considerable time in this way In 
 fact, everything has been done to make the original work 
 accurate, which the time and the necessarily limited appro- 
 priation would permit. 
 
 With great respect, 
 
 Your Ob't Serv't, 
 
 ALPHEUS HYATT, 
 Custodian Boston Soc. JVat. Hist. 
 
EEPOET 
 
 ON THE 
 
 GEOLOGICAL MAP OF MASSACHUSETTS. 
 
 BY W. 0. CROSBY. 1 
 
 In 1830 the Government of Massachusetts instituted, at the 
 public expense, a Geological Survey of the Commonwealth, 
 and Prof. Edward Hitchcock, Sen., of Amherst College, was 
 charged with the work of exploration. His final report, ac- 
 companied by a Geological Map of the State, was published 
 eleven years later, in 1841. This map, enlarged, and revised 
 by Prof. Hitchcock, was republished in 1844, in connection 
 with the Topographical Wall Map of Simeon Borden. No 
 Geological Map of Massachusetts materially differing from 
 this has since appeared. The work of Prof. Edward Hitch- 
 cock, Sen., remains the last general contribution to the geol- 
 ogy of the State ; and the map to which the present report 
 relates which, since it has been prepared under the direction 
 of the Massachusetts Commission to the Centennial Exposi- 
 tion, may, for the sake of a convenient designation, be called 
 the Centennial Map is based primarily, as all subsequent 
 maps must necessarily be, upon those published by him. It 
 should be stated in this connection, however, that some as- 
 
 1 Those parts of this report which relate to that portion of the State lying east 
 of Worcester and north of Rhode Island have been taken from my thesis for 
 graduation in the Mass. Institute of Technology, on the " Geology of Eastern 
 Massachusetts," which will be published during the ensuing summer in the Pro- 
 ceedings of the Boston Society of Natural History. 
 
G 
 
 sistance has been derived from the geological map of Prof. 
 C. H. Hitchcock, which was published in 1871 in Walling's 
 Atlas of Massachusetts, and differs in some points of minor 
 importance from the earlier maps referred to. 
 
 It is proposed to embody in this report a brief account of 
 all the observations, by whomsoever made, upon which the 
 changes appearing in this Centennial Map, as compared with 
 previous maps, are based; with the exception of the data fur- 
 nished by Mr. L. S. Burbank, whose researches in the Nashua 
 and Merrimac Valleys have been of very material assistance, 
 and will be found fully described in his report which accom- 
 panies this. 
 
 In some instances, as in the eastern part of Plymouth 
 county, the alterations of boundaries are based upon the ob- 
 servations of Prof. Edward Hitchcock, published in his final 
 report of 1841, but of which different views have been taken. 
 In such, as in all other cases, the sources of the data used 
 will be clearly indicated. 
 
 My own contributions to the map are based upon observa- 
 tions extending over a period of four years, and during the 
 last year I have been in the field almost constantly. It is 
 only justice to Dr. T. Sterry Hunt to express here a recog- 
 nition of my indebtedness to him for much valuable instruc- 
 tion and advice, while a student in the Massachusetts Insti- 
 tute of Technology, without which the performance of this 
 work by me had been impossible. But, 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 en- 
 tered the field a year ago with a general disbelief in the 
 applicability of these ideas to the rocks of this region, which 
 my first observations only strengthened ; and any confirma- 
 tions of these views which may be found in this report, have 
 been reached in opposition to preconceived ideas wholly at 
 variance with them. 
 
That enormous period in the history of the earth, commonly 
 known as geological time, is usually divided by geologists 
 into four great eras, which, in their order of sequence, are 
 the Eozoic, Paleozoic, Mesozoic, and Cenozoic ; and in Mass- 
 achusetts rocks are found belonging to each of these grand 
 divisions of past time. The rocks of Massachusetts are mainly 
 crystallines, which are believed to belong wholly to the Eozoic 
 era ; and it is a noteworthy fact, that the rocks belonging to 
 the successive eras in this State steadily diminish in amount 
 from the Eozoic to the Cenozoic, so that? Massachusetts is, in 
 this respect, an epitome of the world. The oldest rocks in 
 Massachusetts, as will be presently shown, are on its eastern 
 border, facing the Atlantic; and proceeding westward across 
 the State the crystallines become gradually newer, until, in 
 Berkshire county, we find the semi-crystalline beds believed 
 by an increasing number of geologists to represent the close 
 of Eozoic time. 
 
 The Eozoic rocks of Massachusetts may, to a large extent 
 at least, be divided lithologically and chronologically, into 
 three divisions, which, stated in their order of sequence, are 
 the Norian, the Huronian, and the Mont Alban. I weigh my 
 words well when I describe these divisions as both litholog- 
 ical and chronological; for, although, as already stated, I 
 began my study of the crystallines of this State with a whole- 
 some distrust of the value of lithological data in establishing 
 chronological divisions, and am not yet wholly prepared to 
 apply this principle to wider regions, I do not hesitate to 
 affirm that the lithological characters of the divisions which 
 have been worked out among the crystallines of this region 
 the chronological and geographical distinctness of which I 
 cannot doubt are as unlike as the faunae of any two succes- 
 sive geological formations. 
 
 NOBIAN. 
 
 But two small areas of the rocks of this age have thus far 
 been found in Massachusetts. The largest and most important 
 
8 
 
 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. The second area includes all but 
 the seaward end of large Nahant. The rocks of this forma- 
 tion, though frequently stratified, seem in general to have 
 been somewhat fluent, and usually exhibit more or less ex- 
 travasation; but doubtless in some cases the metamorphic 
 action has stopped Ishort of this extreme term, though de- 
 stroying all traces of bedding. In many places, as notably 
 on Winter Island and Great and Little Misery, the entire 
 formation seems to have been fluent, and the extravasation 
 has been so extensive that the character of the rock changes 
 nearly every rod. One important fact should be noted here, 
 viz. : nowhere in this region does the Norian series appear to 
 be cut by eruptives belonging to another formation, for all 
 the extravasated rocks of this system may be easily referred 
 to, or shown to be derived from, its stratified members. 
 
 The stratified rocks occur chiefly on Marblehead, and on 
 the Beverly shore, west of Curtis Point. On 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 
 30 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 JSTahant. 
 
 The rocks of this formation are composed chiefly of feld- 
 spar and hornblende, or pyroxene. These minerals occur 
 mixed in very various proportions. It has been proved by 
 analysis that some of the feldspar is not labradorite, though 
 closely resembling that species in some of its physical char- 
 acters ; and it yet remains to be proved that there is any 
 labradorite in the formation. But its occurrence here is 
 deemed probable, and as only one analysis has been made, 
 
9 
 
 the question cannot be regarded as settled. It should be 
 said, however, that all the feldspars of this formation agree 
 very closely in their general physical properties with the 
 specimen analyzed. This was placed in the hands of Mrs. 
 Richards, of the Massachusetts Institute of Technology, who 
 made the following report : 
 
 SiG 2 . . . ... . . 
 
 A1 9 O 3 . . . V. ; -. * -. 
 
 K 2 b &i . . . '* . v . 
 
 Na 2 O , 
 
 CaO -_V * .,.: > 
 
 H 2 , * . >,, ... 
 
 100.525 
 
 Such a chemical constitution belongs to no known species 
 or variety of feldspar ; and fearing there had been an error 
 in the first analysis, a second was made from the same piece, 
 a very clear unweathered specimen, with a like result. 
 
 A careful inspection of a large number of specimens ex- 
 plained the anomaly, by showing that the feldspar, although 
 coarsely and perfectly crystalline, is a mixture ; the crystals 
 being formed of thin interlaminated plates of at least two 
 different feldspars. This can be seen only on slightly weath- 
 ered surfaces; one of the feldspars decomposing more read- 
 ily than the other. The constituent feldspars were not 
 analyzed, as it seemed impracticable to separate them. But 
 if, as appearances indicate, it is a mixture of two species 
 only, they are probably an orthoclase rich in soda, and an 
 oligoclase poor in lime, the latter predominating. The color 
 of this feldspar is bluish gray, weathering white, and its sp. 
 gr. varies from 2.55 to 2.60. Disseminated through this feld- 
 spar, are numerous small crystalline grains of magnetite. 
 Biotite is not uncommon in the rocks of this formation, 
 though seldom abundant. Pyroxene is abundant, and the 
 varieties hypersthene and bronzite are believed to occur. A 
 massive, coarsely crystalline diorite, at Nahant, is decidedly 
 epidotic. The most striking and probably the most impor- 
 2 
 
10 
 
 tant mineralogical character of this series of rocks is that all 
 members of it are absolutely destitute of quartz. 
 
 Perhaps the most characteristic rock of this formation is 
 one composed almost entirely of feldspar, and containing 
 little, frequently no hornblendic material. It is usually coarse- 
 ly crystalline, this variety predominating along the Beverly 
 shore, east of Curtis' Point ; and the crystallization is some- 
 times extremely coarse, as, notably, on Marblehead, especially 
 about Dolliber's Point, where it is interstratified with fine 
 grained pyroxenic rocks; but it is frequently fine grained, 
 resembling felsite, and presents a very uniform texture and 
 appearance. This fine grained variety 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 diorite, varying 
 greatly in composition, and frequently occurring as a pure 
 hornblende rock, is the most abundant of our Norian rocks. 
 It is the prevailing rock on Nahant, where it is frequently 
 very coarse and hornblendic, with a high specific gravity. A 
 more feldspathic variety of this rock exhibits traces of bed- 
 ding on the north-east shore of Nahant, as noticed, ante p. 8. 
 This diorite, 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 are mainly fine grained, 
 and consist usually of interstratified, frequently alternating, 
 pyroxenic and feldspathic beds. Among the stratified rocks 
 are found those of all textures, from the finest to the 
 coarsest; the stratified pyroxenic rocks are, however, usually 
 fine grained and very distinctly bedded. 
 
 That this series of pyroxenic and feldspathic rocks, with 
 its associated minerals, which is sometimes stratified, oftener 
 eruptive, frequently very coarsely crystalline, and always 
 quarteless, agrees better, even in the absence of labradorite, 
 with the Norian system than any other that has been de- 
 
11 
 
 scribed, can scarcely be doubted. Other considerations con- 
 firm this view. The great disturbance which it shows, and 
 its thoroughly crystalline appearance, stamp it as older than 
 the Huronian and Mont Alban, and it also clearly underlies 
 these formations, and has been frequently extravasated 
 through them. This is well shown in the cliffs on the shores 
 of Beverly and Manchester, where the Huronian granites are 
 cut extensively by great dykes and eruptive masses of No- 
 rian rocks, both hornblendic and feldspathic ; the feldspathic 
 dykes appearing, usually, to be older and larger than the 
 hornblendic. The Norian eruptives and Huronian diorites 
 are similarly related on Marblehead. At Nahant we find 
 slates, believed to be of Primordial age, resting upon the 
 Norian diorites, which have been extravasated through the 
 slates, producing extensive alterations. The coarse grained, 
 readily disintegrating, exotic diorites, so extensively quarried 
 in Medford, and also occurring in Somerville and Brookline, 
 are, doubtless, extruded portions of this same series, which is 
 the probable seat of many of the eruptive rocks, especially 
 diorites, cutting the newer formations. We thus find this 
 series underlying everything newer than the ETorian ; and, 
 since the Laurentian is not known to occur in Massachusetts, 
 we are, I believe, fully-justified in regarding this series, not 
 only as the Norian, but also as the oldest series in the State. 
 The student of Massachusetts geology, may find here a 
 chronological and stratigraphical starting point. Here is the 
 real base of our geological column. 
 
 It should be stated that Prof. Hyatt, long ago, recognized 
 the rocks about the city of Salem, as probably older than the 
 felsites of Marblehead Neck ; but did not separate them from 
 the Huronian diorites of Salem Swampscot and Marblehead. 
 Dr. T. Sterry Hunt, also, in 1869, identified, as of Norian 
 age, a boulder found near Salem and probably derived from 
 the Norian rocks of that vicinity. 
 
 '"LIBRARY 
 
 UNIVERSITY OF 
 
 CALIFORNIA. 
 
12 
 
 HURONIAN. 
 
 The rocks of this age, like those of the Norian, occur only 
 in the eastern portion of the State, where they cover a wide 
 area, forming the seashore from the New Hampshire line to 
 Plymouth. They are bounded on the west by a line running 
 south-westerly from Salisbury through Essex and Middlesex 
 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. The Huroniaii 
 area has an extreme length, measured from the New Hamp- 
 shire line in Salisbury, to Manomet Hill in Plymouth, of 
 sixty-five miles, and an extreme breadth, measured across 
 the southern end, not counting the narrow band stretching 
 from Concord to Westborough, of about forty miles. It is 
 partially interrupted, near the middle, by an area of Prim- 
 ordial, and more recent rocks, which lie about the shores of 
 Boston Harbor, extending westerly to Natick, and south- 
 westerly to Rhode Island. The Huronian on this map 
 comprises the areas marked as "syenite," "porphyry," and 
 " hornblende slate," on the geological map of Prof. Edward 
 Hitchcock. In the State Cabinet, collected by Prof. 
 Edward Hitchcock, are numerous specimens of hornblendic 
 granite (" syenite " ) from Kingston, and Manomet Hill in 
 Plymouth, and from the latter locality, also, several speci- 
 mens of felsite ("porphyry"). These are typical Huronian 
 rocks and I have, in consequence of their occurrence at the 
 localities named, ventured to extend the Huronian formation, 
 on the map, as far south as a line running westerly from the 
 southern end of Manomet Hill ; although on all previous 
 maps the towns of Plymouth and Kingston are represented 
 as entirely covered with "granite" (micaceous granite, of 
 Mont Alban age). 
 
13 
 
 Prof. C. H. Hitchcock, in his recent report (1875) on the 
 Geology of New Hampshire, has applied the term Labrador 
 to the rocks of this broad Huronian area ; although these 
 rocks iiave scarcely a single character in common with the 
 Labrador or Nofian series as defined by Dr. T. Sterry Hunt 
 and the Canadian Geological Survey. Dr. Hunt, also, in 
 1870, in consequence of finding the Eozoon Canadense in 
 the limestone at Newbury, referred a portion of this series to 
 the Laurentian age. But it will be shown in the sequel, that 
 all the rocks within the area described belong to one and the 
 same lithological and stratigraphical series, the characters of 
 which stamp it as undoubtedly Huronian. 
 
 A glance at the map will show that the attempt to map 
 this formation lithologically, i. e., to show the distribution of 
 its various lithological members, has been attended by mod- 
 erate 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 districts, concealed by superficial deposits. A 
 special color has been used to represent the general Huron- 
 ian formation, wherever the particular lithological represent- 
 ative is not known, but the probabilities are great that the 
 rock, whatever it is, belongs to this age. 
 
 The Huronian series in Massachusetts is composed of the 
 following rocks, or groups of rocks : hornblendic granite, 
 felsite, diorite, stratified rocks, and limestone. 
 
 HORNBLENDIC GRANITE. The typical hornblendic granite 
 of this region, as shown at the quarries in Quincy, Rockport, 
 and other places, is a coarsely crystalline aggregate of ortho- 
 clase, quartz, and hornblende. The hornblende is usually 
 small in amount, and the rock frequently passes, through the 
 disappearance of hornblende, into binary granite. The feld- 
 spar is usually grayish or bluish, though red and green tints 
 are frequently met with. It is worthy of note that this rock 
 is destitute of mica, or at least its presence is a very rare 
 occurrence. This typical granite frequently passes into finer 
 
14 
 
 grained varieties, which, when hornblende is absent, pass 
 through eurite into felsite: The more hornblendic varieties 
 are usually fine grained ; and the increase of hornblende is 
 attended by a diminution of quartz, so that the rock exhibits, 
 through the entire absence of quartz, frequent passages into 
 diorite. That these granites are mainly exotic, can scarcely 
 be questioned, for we have seldom far to look, to find, in the 
 form of enclosed angular fragments of clearly stratified rocks, 
 evidence of their extravasation ; and near the boundaries 
 of the granites we usually find them cutting the adjoining 
 rock, especially if that is stratified, in a manner incompatible 
 with any theory that would regard them, in their present 
 condition, as indigenous or endogenous. Nevertheless it is 
 doubtless true, as suggested by Prof. N. S. Shaler, and later 
 by Mr. T. T. Bouve, that these granites have been derived 
 from sedimentary rocks, and have simply reached the final 
 term in the metamorphic process] igneous, or more prob- 
 ably, igneo-aqueous fusion. A glance at the Centennial 
 Map suffices to show, that the geographical distribution of 
 these rocks is in harmony with the theory that they are 
 extravasated masses, for they are seen to occur among the 
 other members of the Huronian series, in a manner wholly 
 irregular. They do not occur in continuous, well-defined 
 areas, but in isolated patches with boundaries difficult to 
 define, and such as could be accounted for only on the sup- 
 position that the rock has been more or less fluent, and has 
 experienced some extravasation. The large area colored as 
 granite in southern Norfolk and northern Plymouth counties, 
 would appear to be an exception to the above statement; 
 but it should be said of this area that it is largely covered 
 by drift material, especially toward the southern border, and 
 comparatively few observations of the rocks have been made. 
 Slates are said to occur in Abington, and, although, with this 
 exception, no rock has been observed in this area, within 
 the writer's knowledge, but granite, it is quite possible that 
 this broad area includes more or less extensive patches of 
 
15 
 
 other rocks. A more detailed map, such as it is hoped the 
 student of the near future will construct, would show the 
 boundaries of these areas of granite to be much more com- 
 plex and irregular than they are here represented. 
 
 A further inspection of the map will make it evident that 
 these granites are not without some system in their madness, 
 for the most of the granitic areas north of a line running 
 from Natick to tf eponset, will be seen to be arranged along 
 a line having a direction about N. 50 E., forming a discon- 
 tinuous band of variable width, which extends from Natick 
 to Rockport on Cape Ann ; and we readily find in this wall 
 of granite, probably the most resistant rock of this region, a 
 sufficient cause for the existence of this prominent headland. 
 The slates and conglomerates, in Newton and Watertown, 
 are probably underlaid by granite belonging to this range, 
 which accounts for the discontinuity of the range here. 
 
 Between this belt of granite and the large granitic area 
 on the south, are several large, irregular areas of granite, 
 including the Blue Hill range; and it is probable that part 
 at least of the large area marked on the map as general 
 Huronian, in Medway, Medfield and Franklin, now exten 
 sively drift-covered, is underlaid by granite. I have tried in 
 vain to detect any system or order in the arrangement of 
 these masses, and can only say that their present complicated 
 and apparently systemless disposition, is probably due to two 
 forces or tendencies operating to arrange the granites in two 
 sets of parallel bands ; one set coinciding in direction with 
 the prevailing strike of the region, and the Natick and Rock- 
 port belt, north-east and south-west; and the other set hav- 
 ing an east and west trend. 1 
 
 On the map, in the northern half of Essex county, I have 
 delineated three areas of granite, in Rowley, Ipswich and 
 
 1 The outlines of the granites in Dedham, Dover, Sherburne, etc., as well as of 
 the other rocks of that region, are largely due to the observations of Mr. F. W. 
 Very. Valuable data concerning the distribution of these rocks near Boston, ha 
 alsojseen received from Mr.^W. W. Dodge. 
 
16 
 
 Topsfield. The Topsfield rock disintegrates readily, and is 
 known as the " rotten rock." These three patches are widely 
 separated, yet, inasmuch as no other rocks have, within my 
 knowledge, been observed between them, and since the char- 
 acter of the drift indicates the existence of considerable 
 granite in this region, I feel justified in concluding that we 
 have here a broad belt of granite, beginning at the shore in 
 Ipswich and extending thence south-westerly, gradually dis- 
 appearing among the diorites in Boxford and Middleton ; 
 but I have deemed it best not to map beyond the known 
 facts. Toward the northern end of Essex county, in Newbury 
 and Salisbury, are other areas of granite, which, for the most 
 part, are small and are known to lie in ranges parallel with 
 the stratified rocks of that district, i.e., north-east and south- 
 west. But, since they have been only imperfectly worked 
 out, and their number and boundaries remain to be deter- 
 mined, I have not delineated them on the map. To avoid 
 confusion in the mind of the reader, it should be stated that, 
 in the preceding account of the distribution of the granite, 
 no notice has been taken of the very hornblendic, fine 
 grained granites, which frequently pass into diorites, and are 
 so involved with the diorites in their origin and distribution, 
 as to be inseparable from them. They are in fact more 
 truly diorites than granites. 
 
 FELSITE. Porphyry is a much abused word, which, like 
 syenite (by syenite is not meant the aggregate of orthoclase 
 and hornblende to which this term has recently been ap- 
 plied), has outlived its usefulness, and should, in the opinion 
 of the best geologists, be allowed to become obsolete in its 
 substantive use, as a geological term. Much of the rock in 
 this region to which the term porphyry is applied, is not 
 even porphyritic; showing into what logical errors we are 
 led when we choose for the basis of a lithological name a 
 property common to many rocks, and which cannot be 
 correlated with any particular chemical constitution. It is 
 not intended to include under the term felsite, either in this 
 
17 
 
 description or on the map, the felsite which is clearly inter- 
 stratified with quartzite, hornblende slate, &c. This division 
 is entirely arbitrary, but is warranted by the impracticability 
 of separating the different members of what may be called 
 the stratified group. To avoid confusion I will speak of the 
 felsite of the stratified group as stratified felsite. It is not 
 meant, as will be seen farther on, that all the stratified 
 felsites belong to the stratified group of rocks, but only such 
 as are clearly intercalated with that group. The term felsite, 
 even as thus limited, includes in this region numerous vari- 
 eties, which will be briefly described. 
 
 (a) The most abundant variety is the porphyritic felsite of 
 Lynn, Saugus, Maiden and Melrose. It consists of a quartzo- 
 feldspathic base, with interspersed crystals of feldspar. It is 
 usually more or less, but seldom very porphyritic. Its colors 
 are brownish purple, reddish brown, gray, varying through 
 different shades of drab to black. In general it is a structure- 
 less rock, showing no traces of bedding ; but at Dungeon 
 Rock in Lynn, it is distinctly stratified, a dense, black vari- 
 ety being interstratified with a crystalline dioritic variety;* 
 and going northward in Melrose, the porphyritic character 
 gradually disappears, the felsite becomes more siliceous, and 
 gradually becomes interstratified with quartzite and horn- 
 blende slates. The transition is so gradual that it is impossi- 
 ble to define the boundary between the stratified and unstrat- 
 ified felsites, which proves there is no break, no natural 
 division here. In Melrose and Maiden, and at other points 
 the porphyritic felsites exhibit frequent local passages into 
 granite and diorite. It seems probable that considerable 
 portions of this rock have been in a more or less fluent state > 
 this can scarcely be doubted on the west shore of Wenuchus 
 Lake in Lynn, where the tongue of granite penetrates the 
 felsite ; and on Marblehead Neck and the neighboring 
 islands, there is abundant evidence of the softening and 
 extravasation of portions of the rock. Notably on Marble- 
 head Neck, als^ on Red Rock in Lynn, and at the Pirate's 
 3 
 
18 
 
 Glen in Saugus, and, perhaps, at other points, this felsite 
 exhibits traces of a conglomerate origin. This variety of 
 felsite is seldom found beyond the localities mentioned; it 
 has, however, been observed in Hyde Park, near the corner 
 of Huntington Avenue and River Street, and at some points 
 in West Dedham and Dover. 
 
 (b) Passing westward from Maiden, the porphyritic felsite, 
 (a), graduates into a non-porphyritic, homogeneous, struc- 
 tureless variety, which includes all the felsite of Medford, 
 the felsite of West Needham, most of the felsite of Natick, 
 Dover, and Dedham, and a small portion of that occurring in 
 Hyde Park and Mattapan. This typical felsite is usually of 
 a grayish color, varying from white to black, but greenish 
 shades are not uncommon. As we should expect, it frequently 
 passes into the porphyritic variety, and, like that, also proves, 
 by its frequent transitions to granite and diorite, the intimate 
 causal connection of these various rocks. This variety in- 
 cludes the so-called " Saugus Jasper," a bright red, homo- 
 geneous felsite. 
 
 There are in this region two principal varieties of "banded" 
 or laminated felsite, which differ widely in their origins: first, 
 that in which the banding is due to a conglomerate origin, 
 having been produced by a flattening of the pebbles of the 
 conglomerate, and which is described farther on ; and second, 
 
 (c) The much more abundant and widely distributed 
 variety in which the banding represents the original bed- 
 ding of the rock. In this variety, which is usually of a 
 reddish brown color, though frequently of different shades 
 of gray and sometimes white, the banded structure is usually, 
 where the rock has not been greatly disturbed, very uniform, 
 regular and continuous. The banding commonly results from 
 the interlamination of thin layers of qunrtzose and fcld- 
 spathic materials. The thickness of the lamina? usually 
 varies from a mere line to one-sixteenth of an inch, and 
 seldom exceeds one-eighth of an inch. That this banding 
 really represents stratification is proved by tlxe regularity and 
 
19 
 
 continuity of the bands, since a banded structure due to the 
 flattening of the pebbles of a conglomerate would necessarily 
 exhibit little uniformity in the thickness of the lamina, and 
 I find it difficult to conceive of pebbles flattened to such an 
 extent as to produce continuous layers of uniform thickness 
 and yards in extent ; and it is also proved by the constancy 
 of their strike and dip. The felsite at Kent's Island and 
 other points in Newbury, is mainly of the banded variety. 
 It has a reddish color, and, though showing great disturb- 
 ance, the bands dip with considerable regularity to the 
 north and north-west ; it seems to pass into the non-porphy- 
 ritic felsite (b). The banded felsite is occasionally slightly 
 porphyritic, as on the north-west side of Marblehead Neck, 
 where it is of a grayish color with bands from one-sixteenth 
 to one-eighth of an inch thick, and dipping steeply to the 
 south-east. A small amount of banded felsite, similar to 
 that in Newbury, occurs in West Dedham, intercalated with 
 the felsitic breccia, soon to be described. Banded felsites also 
 occur in Hyde Park, east of the Clarendon Hills Station, in 
 Mattapan, and in Milton immediately south of the Neponset. 
 In Mattapan it passes into the closely related variety, not 
 logically separable from it, in which we seem to have the 
 stratification without the banding ; there appear to be no 
 quartzose layers, but this may due to the extreme thinness 
 of the bedding, which can be detected only on weathered 
 surfaces. The strike is K E.-S. W. Mr. W. W. Jacques 
 reports the occurrence of banded felsite on the west end 
 of the large hill in Quincy, west of Willard St., and one and 
 a half miles south-west of West Quincy Village. 
 
 (d) One important variety of felsite yet remains unde- 
 scribed. It is porphyritic with feldspar crystals and contains, 
 in addition, numerous grains of translucent quartz, which 
 cause some portions of it, where the quartz and feldspar 
 crystals are abundant, to resemble granite ; and it may be 
 properly called the granitoid felsite. It occurs extensively in 
 Needham, and the southern half of the Blue Hill region* 
 
20 
 
 including all the higher summits, is composed of it. Through- 
 out the latter district the base is of a dull brown color, and 
 the rock is of a very uniform character, but toward the east, 
 north and west, it undoubtedly becomes more crystalline, 
 more granitic, and passes into the Quincy granite, and the 
 granite of Dedham. This is the opinion expressed by Prof. 
 Edward Hitchcock nearly forty years ago, and most observers 
 since then have taken the same view of the matter. In 
 Keedham the base is of a white and greenish white color, 
 and contains more crystalline quartz than feldspar. It 
 passes into fine grained granite toward the east and south. 
 Good examples of the granitoid felsite, of small extent, 
 occur on Lowell's Island and the north-west shore of 
 Marblehead. 
 
 (e) The felsitic breccia, which occurs at many points in 
 this regii n, should be mentioned in this connection. It 
 occurs in Hyde Park and Mattapan, where its colors are 
 white, purplish, gray, pinkish and bright red. It varies 
 greatly in texture here, sometimes containing angular masses 
 several feet in extent; and also exhibits extensive flattening 
 of the pebbles, producing a sort of pseudo-stratification. 
 The breccia occurs in West Dedham, near the Oven Mouth. 
 Here the predominating colors are red and black ; and the 
 pebbles do not show alteration ; but the breccia appears to 
 pass into felsite through a gradual diminution of the sizes of 
 the pebbles, just as conglomerate passes into slate. 
 
 On Marblehead Neck the breccia, which is here more 
 properly a conglomerate, becomes at some points, epecially 
 on Lowell's Island, a coarse, gritty, feld spathic sandstone, 
 and both the conglomerate and sandstone pass into compact 
 felsite, the former in two distinct ways, which, although 
 observable at several points in this region, are best exempli- 
 fied here. These two modes of metamorphism are : (1) By 
 a blending together of the pebbles and paste, whereby the 
 outlines of the former are lost, or, when the process is not 
 complete, can only be seen on weathered surfaces. (2) By 
 
21 
 
 a flattening or drawing out of the pebbles into thin lentic- 
 ular laminae, which, more or less coalescing at their edges 
 and lying in parallel planes, produce a stratified appearance 
 in the rock, and give rise to a laminated or banded structure 
 closely resembling that already described, due to original 
 sedimentation. 
 
 That compact felsites have been derived from genuine 
 conglomerate in the ways just indicated, has been -fully dem- 
 onstrated by Mr. T. T. Bouve, in Hingham, and Prof. 
 Alpheus Hyatt, on Marblehead Neck. Their views on the 
 subject, together with the main facts, have been published in 
 the Proceedings of the Boston Society of Natural History, 
 vol. XVIII, p. 217. A discussion of .this subject may also 
 be found in the paper referred to in the foot note on p. 5, in 
 which it is further shown that the massive, structureless 
 felsites, (a) and (), have probably been largely derived from 
 massive, obscurely stratified, feldspathic slates, while the 
 normally banded felsite, (c), represents a finely and dis- 
 tinctly stratified slate. 
 
 An inspection of the map shows, that the felsite on 
 Marblehead Neck, and the adjacent islands, forms an area 
 five miles long in a north-east and south-west direction ; a 
 broad, continuous belt of felsite extending, in the same direc- 
 tion, from Red Rock in Lynn, to Arlington; and a nar- 
 row band of the same rock stretching south-westerly from 
 the north-east corner of Needham to South Natick. Thus we 
 have an interrupted belt of felsite reaching from Half Way 
 Rock to South Natick, a distance of 33 miles. An out- 
 lying patch of felsite is said to occur on the Manchester 
 shore, on the line of this belt. It is believed that this belt 
 of felsites is really much wider than here indicated ; for all 
 the crystallines occurring among the newer rocks between 
 this belt on the north of Boston, and the Quincy granite 
 range on the south, excepting such as are clearly eruptive, 
 are felsites. There is an isolated area of felsite in Chelsea, 
 and felsite is believed to occur on the South Shore, in 
 
Hingham. So that, if the newer formations and water were 
 removed from Boston Harbor, and vicinity, we should prob- 
 ably have a large, unbroken area of felsites, trending N". E. 
 -S. W., and vaiying from eight to twelve miles in breadth. 
 Toward the south-west, in Dedham, Dover, &c., the felsites 
 partake of the complexity which appears to reign in that 
 region. 
 
 DIOBIT-E. It is intended to include here only the eruptive 
 diorites, using the term eruptive in a general sense that will 
 include all rocks that have been fluent, whether extravasated 
 or not. There are clearly stratified diorites in this region, 
 but they are inseparable from the other stratified members of 
 the Huronian series, and will be considered as forming part 
 of the stratified group. The diorite, like the granite, varies 
 greatly in texture and composition. As a rule it is quite 
 fine grained, often impalpably fine, and never attains the 
 degree of coarseness common with the granites. In compo- 
 sition it has a wider range ; as already stated, it passes, by 
 an admixture of quartz, into fine grained hornblendic gran- 
 ite ; and it is no less prone, by losing hornblende, to pass 
 into felsite. Fine examples of the transition between diorite 
 and felsite may be seen in Greenwood and Stoneham. Near 
 the Greenwood station on the Boston and, Maine R. R., the 
 diorite contains little or no hornblende, and, but for its more 
 crystalline appearance, would be indistinguishable from fel- 
 site. The diorites are never very micaceous, though fre- 
 quently containing some biotite. The biotite usually occurs 
 in the more crystalline varieties of the diorite. The areas 
 colored as diorite on the map, though mainly diorite, yet em- 
 brace a great amount of fine grained hornblendic granite, 
 which, however, is frequently little more than a quartzose 
 diorite. Any observer of these two rocks will agree with 
 me that they admit of neither a lithological nor a geographical 
 separation. As a rule they are both eruptive, and over 
 large areas they have been extravasated through each other 
 so extensively, and the action has been so mutual, that the 
 
23 
 
 complication is complete; and I have long been accustomed 
 to speak of them as "mixed rocks"; and I know now of no 
 term that will better express their relations, lithologically or 
 petrologically. 
 
 The large area in Sharon, Stoughton and Foxborough, 
 colored as diorite, may include some small areas of coarse 
 granite, but is mainly of the dioritic or "mixed" series. 
 The diorite of this area is chiefly very fine grained, but some 
 quite coarse varieties occur in the hills of Sharon, and Mr. 
 F. W. Very has observed the same in Foxborough. The 
 diorites in other parts of Norfolk county are mainly fine 
 grained and exhibit frequent passages into horablendic 
 granite and felsite. The diorites seem to obey, in their 
 distribution, the same general law as the granites and fel- 
 sites; and the diorite areas north of Boston form a nearly 
 continuous, but very irregular belt, lying parallel with and 
 north of the north-east and south-west granite range, and 
 extending from Beverly to Wayland. * In the north part 
 of Essex county, north and west of the Newbury belt of 
 felsite, there is much diorite occurring among the stratified 
 rocks of that region ; but too few observations have been 
 made for its delineation on the map. 
 
 STRATIFIED ROCKS. While describing the preceding mem- 
 bers of this formation, it has been pointed out that they 
 frequently pass into each other ; and it would seem to be 
 nearly an established fact that they all belong to one and the 
 same series; but a study of the stratified group greatly 
 strengthens the evidence in favor of the oneness of their 
 origin; for here we find them all combined and blended 
 together. As already stated, we find, on going northward 
 through Melrose, the porphyritic felsite, (a), gradually be- 
 coming less porphyritic and assuming a stratified appearance] 
 North of Howard Street traces of stratification are common, 
 though porphyritic felsite occurs as far north as Greenwood. 
 North of Central Brook, in Saugus, the felsites are chiefly 
 stratified, the bedding increasing northward, are largely 
 
24 
 
 quartzose, passing into quartzite, and are frequently inter- 
 stratified with hornblende slate and stratified diorite. No 
 observer, who has been over this ground, can doubt that these 
 different rocks are stratigraphically inseparable. Along the 
 eastern border of this area of stratified rocks, one can find 
 beautiful examples of the passage of stratified diorite and 
 hornblende slate into the eruptive diorite and fine grained 
 hornblendic granite. There is nothing abrupt about these 
 transitions, the gradation is perfect. The stratified rocks 
 strike N. E.-S. V., and dip very steeply to the N. W. 
 About four miles farther north-west, in Reading, is another 
 area of stratified rocks, which includes the varieties pre- 
 viously mentioned, but with relatively more hornblende slate 
 and diorite ; and some of the felsite is slightly micaceous. 
 Here, also, we find the rocks shading insensibly into eruptive 
 diorite and granite. The strike and dip are the same as in 
 Saugus and Melros^e. 
 
 On the Boston and Lowell R. R., near the North Woburn 
 line, is a distinct argillite conformably interstratified with 
 rocks similar to those just described. The narrow Huronian 
 band, extending from Concord to Westborough, lies on, and 
 parallel with, the line of strike of the stratified rocks in 
 Reading and North Woburn, and consists of stratified 
 diorite and hornblende slate, with a small amount of inter- 
 calated mica schist. The strike is parallel with the trend of 
 the band. Between Concord and the Boston and Lowell 
 R. R., in Burlington, North Lexington, and Lincoln, there 
 are few outcrops, but it is probable the rocks are chiefly 
 stratified, and they have been so represented on previous 
 maps. At Putnamville in Danvers, and Locustdale in Pea- 
 body, are stratified diorite, hornblende slate, and a well 
 marked feldspathic gneiss containing some mica, all dipping 
 steeply to the N. W. The character of the drift in Wen- 
 ham indicates a wide area of stratified rocks in this region ; 
 but the limits are not known. In southern Middleton the 
 drift is composed largely of imperfect gneisses, passing into 
 
25 
 
 hornblendic granite, which must occur here in situ. The 
 large area of diorite stretching from Stonehara to West on 
 includes numerous small patches of stratified rocks horn- 
 blende slate and petrosilex. A few of these patches have 
 been represented on the map, but many more probably re- 
 main undiscovered. They are usually narrow, parallel with 
 the strike, and vary in length from a few rods to a mile or 
 more. Strike, KE.-S.W. ; dip, steep to K W. They 
 pass frequently into the enclosing rock, showing that they 
 are mere remnants of the stratified group, which yet pre- 
 serve traces of the structure once possessed by the whole 
 mass of the rock. Large masses of the hornblende slate, &c. , 
 occur enclosed in granite at Hospital Point on the Beverly 
 shore, and in Norian rocks on the north side of Great Misery . 
 Irregular fragments and masses of the micaceous petrosilex , 
 hornblende slate, and other stratified rocks, are occasionally 
 found enclosed in the diorite of Swampscot, Marblehead and 
 Salem; and the diorite itself sometimes appears stratified' 
 It thus becomes evident that this diorite, the so called 
 " Salem syenite," has, like the diorites elsewhere, resulted 
 from the extreme metamorphism of the stratified group. 
 The mica slate, so called, enclosed in exotic granite on 
 Marblehead Neck, belongs to this stratified group. 1 The 
 area of stratified rocks in Waylaud is probably much larger 
 than represented on the map. They are distinct gneisses 
 which belong lithologically to the Mont Alban rather than 
 the Huronian series ; and it is possible the Mont Alban bound- 
 ary is deflected this far to the east, but more probable that 
 the rocks are Huronian. Their strike is N.-S. and dip . 
 vertical. The stratified rocks in Sherburne are imperfect 
 gneisses and stratified diorites. The area in Dover is less, 
 gneissic and more felsitic. It is deemed probable, judging from 
 the character of the drift, that the Sherburne area should 
 be extended southwesterly into Holliston. Stratified Huron- 
 
 1 Much assistance in mapping the rocks in this portion of Essex county has been 
 derived from the notes of Prof. A. Hyatt. 
 4 
 
ian rocks, other than felsite, are of rare occurrence soutli of 
 the Charles River, and, besides the area in Dover, I know of 
 but one locality where they occur; this is on the Cohasset 
 shore, between Black Rock and Little Harbor, where there is 
 an indistinct feldspathic gneiss, poor in quartz. In the north- 
 ern part of Essex county, in Newbury, Newburyport and West 
 Newbury, the stratified rocks, other than felsite and lime- 
 stone, are well developed. They resemble the indigenous 
 rocks of Reading, but are less felsitic, and, according to the 
 observations of Mr. C. W. Kernpton, more calcareous and 
 magnesian. They strike K E.-S. W. and dip to the N. W. 
 The principal ore deposits of that district occur in these 
 rocks. Traces of bedding appear in some portions of the 
 granite in Rowley. 
 
 The stratified group and its distribution have been thus 
 minutely described for the purpose of showing its intimate 
 and complex petrological relations to the other members of 
 the series, and how completely it binds them together as a 
 lithological unit. I have already indicated that much of the 
 hornblendic granite has been derived by metamorphism from 
 felsite, and, in the paper referred to on p. 5, I have shown 
 that this is the probable origin of all the Huronian granites. 
 In the same paper, it is demonstrated that the most, and 
 probably all, of the felsites of this region, are now, or were 
 originally, stratified, and may, therefore, be logically included 
 in the stratified group. 
 
 I have now described all the rocks occurring within the 
 area designated as Huronian, except the limestone, which is 
 small in amount and may be neglected here : and, making a 
 resume of the whole, we are brought to the conclusion, 
 that the rocks of this area consisted once, speaking gen- 
 erally, of stratified felsite, hornblende slate and stratified 
 diorite only, which were then, and are even now, so related 
 stratigraphically, and exhibit such frequent lithological trans- 
 itions, as to prove them to be members of one and the same 
 unbroken series ; and that the felsites, by metamorphism, have 
 given rise to granite, the hornblende slates, in like manner, 
 
27 
 
 to fine grained hornblendic granite approaching diorite, and 
 the stratified diorite to eruptive diorite. 
 
 LIMESTONE. The limestones, or more correctly dolomites, 
 of this formation are not abundant in this region. They are 
 usually serpentinic (ophiolite), arid are seldom, if ever, 
 coarsely crystalline. The principal deposits occur in N"ew- 
 bury and Lynnfield, and are distinctly stratified, having a 
 N". E.-S. W. strike, and dipping to the N". W. ; but small 
 patches are also known in Stoneham, Natick and Walpole. 
 In Newbury the ophiolites are enclosed in diorite and gran- 
 ite, and in Lynnfield, though the relations of the rocks are 
 much obscured by drift, the association appears to be the 
 same. As already stated, the Eozoon Canadense has been 
 found in this rock, in Newbury. 
 
 All the stratified rocks of this formation, north and west 
 of Boston, including, besides the so called stratified group, 
 the dolomites and stratified felsites, dip, with few and unim- 
 portant exceptions, to the north-west. And, summarizing 
 what has been said concerning the distribution of these 
 rocks, we find, proceeding from southeast to irorth-west, the 
 following general geographical arrangement. Underlying 
 Boston and its environs, is the broad belt of felsite already 
 described ; and to the north-west occur in succession, petro- 
 silex, hornblende slate, stratified diorite, and rocks more 
 or less micaceous, to the limits of the formation. And, 
 since the limits of the different rocks are marked by grad- 
 ual transitions, which precludes the existence of faults of 
 any great extent ; and since there are no apparent repeti- 
 tions of the different rocks, which precludes the exist- 
 ence of any considerable folds of the strata ; we are forced 
 to the conclusion that the geographical arrangement cor- 
 responds with the stratigraphical succession ; and the rocks 
 to the south-east must underlie those to the north-west the 
 felsites must be older than the hornblende slates and diorites. 
 South of the felsite is the broad area of granite in Norfolk 
 and Plymouth counties; and since the granites are more 
 intimately associated with the felsites than any other of the 
 
28 
 
 stratified rocks, and are probably derived from them; and 
 since the exotic granites occur as eruptives through all the 
 stratified rocks ; the inference is plain that the granites be- 
 long normally at the bottom of the series, and may be taken 
 to represent the lower portions of the felsite. 
 
 The evidence supporting these conclusions is given in 
 detail, accompanied by a general section illustrating the 
 stratigraphy of this region, in the paper referred to on p. 5 ; 
 and, in the same paper, attention is called to the interesting 
 fact that in the northern part of Essex county, we find sub- 
 stantially the same geographical arrangement, and corre- 
 sponding stratigraphical succession, as that just described, but 
 on a smaller scale. There, as farther south, the stratified 
 crystallines all dip to the north-west; and we have, on the 
 south-east, the granite of Ipswich and Rowley, succeeded by 
 the felsite of Kent's Island and the River Parker, and this 
 by the eruptive diorite, hornblende slate, and stratified dior- 
 ite, between the River Parker and the Merrimac. In the 
 paper so often referred to, it is shown, after a consideration 
 of many details omitted here, that this grand repetition of 
 the strata necessitates the existence of a gigantic fault, ex- 
 tending south-westerly from the shore in Ipswich, and having 
 the down-throw on the south-east. The distribution of the 
 rocks toward the south-west indicates that the fault wanes in 
 that direction, the vertical displacement being insufficient to 
 bring up the granite beyond Middleton ; but it is conceived 
 that the line of fracture extends along the southern border 
 of the Mont Alban formation, through Concord, and does 
 not finally die out before reaching Westborough. And it is 
 further believed, that we may find in this great dislocation a 
 sufficient cause for the existence of the peculiar Huronian 
 peninsula, which, as already stated, extends south-westerly 
 from Concord, penetrating the Mont Alban formation. This 
 is the principal fault of this region ; but many minor fractures 
 exist, some of which are minutely described in the paper 
 referred to. 
 
The oneness of this formation has been established ; it is 
 proved by its petrological, its lithological, and, as I have else- 
 where shown, its chemical characters. I have pointed out, 
 in the preceding pages, that it distinctly overlies the Norian 
 rocks ; and it no less clearly underlies the Mont Alban. It 
 exhibits much greater disturbance than the Mont Alban, but 
 less than the Norian; and near the common boundaries of 
 this series and the Mont Alban, the gneiss of the latter 
 is cut by the eruptive granite and diorite of the former. 
 And since this series so closely resembles, in its internal 
 characters and its external relations, the Huronian of other 
 regions, we are bound to conclude that it is the Huronian. 
 
 MONT ALBAN. 
 
 The rocks referred to the Mont Alban age cover a wide 
 area in Massachusetts, forming the greater portion of the 
 State. They extend uninterruptedly from the western 
 boundary of the Huronian formation to the Triassic sand- 
 stones of the Connecticut Valley, and spread over most of 
 the area between these Mesozoic rocks and the Hoosuc 
 Mountains. They are also wide-spread in the southern por- 
 tions of Bristol and Plymouth counties, about the shores of 
 Buzzard's Bay. The Mont Alban areas on the Centennial 
 Map comprise, with few exceptions, the areas marked as 
 "granite," "gneiss," "mica slate," "argillite," "metamorphic 
 slate," and " quartzite," on the geological map of Prof. Ed- 
 ward Hitchcock. They also exhibit a general correspond- 
 ence with the areas marked as "White Mountain group," 
 " granite," " Merrimac schists," " calciferous mica schist," and 
 " St. John's group," on the map of Prof. C. H. Hitchcock, in 
 Walling's Atlas of Massachusetts, 1871. The prevailing 
 strike of the rocks of this formation is, stated generally, 
 N.-S.; but, east of Worcester, it approximates to N.E.- 
 S.W., showing a tendency to parallelism with the strike of 
 the Huronian beds. 
 
 The principal lithological constituents of this formation 
 :ire granite, gneiss, mica slate, argillite, and limestone. In 
 
30 
 
 the western portion of the State, there are bands of horn 
 blende slate and stratified diorite included in the Mont Al- 
 ban series, which, though usually delineated on geological 
 maps, are so intimately associated with the gneiss and mica 
 slate, that they are regarded as logically inseparable from 
 them. 
 
 GRANITE. The granites are of two, probably three, dis- 
 tinct kinds, distinct in origin, but often difficult to distin- 
 guish with the eye. They are exotic, indigenous, and en- 
 dogenous. The exotic or eruptive granite is of most import-, 
 ance. It is usually whitish or gray, seldom red or greenishr 
 as the Huronian granite frequently is ; and it differs furthe- 
 from the Huronian granite, in that it is always more or less 
 micaceous, and seldom contains any hornblende. It fre- 
 quently passes into the indigenous variety, which is nothing 
 more than a coarse, granitoid, apparently structureless gneiss. 
 There can be no doubt that the exotic granite has resulted 
 from the extreme metamorphism of portions of the gneiss ; 
 and hence the indigenous and exotic varieties are essentially 
 the same, and I shall not attempt to distinguish them here. 
 Both these varieties are sometimes porphyritic with crystals 
 of feldspar, which are occasionally very large. 
 
 The distribution of these granites is well shown on the 
 map. In the western half of the State, the larger areas are 
 found along the borders of the Triassic sandstone, where the 
 granites occur cutting mica slate and gneiss. The area in 
 Williamsburg, Goshen, Westhampton, and Northampton, 
 and also that east of the Connecticut, in Amherst, though 
 marked as wholly granitic, include, according to the obser- 
 vations of Dr. Hitchcock, much mica slate, which is associ- 
 ated with the granite in such a manner as to prove the ex- 
 otic nature of the latter. On the Centennial Map, I have 
 colored as Mont Alban granite the two areas one in Lud- 
 low and Belchertown, and the other in Northampton and 
 Hatfield marked on all previous maps as "syenite" (horn- 
 blendic granite). My reasons for this course are : (1) These 
 
syenites, so called, frequently contain nearly or quite as much 
 mica as hornblende, and closely resemble portions of the ad- 
 joining granites. They are really hornblendo-micaceous or 
 quarfenary granites, and bear but little likeness to the horn- 
 blendic granites of the Huronian formation. (2) Prof. 
 Edward Hitchcock, on his revised map of 1844, represented 
 a small patch of granite enclosed in the Ludlow and Belcher- 
 town area of syenite. (3) Hornblendic gneisses, hornblende 
 slates, and even pure hornblende rock, certainly occur in the 
 Mont Alban formation, and form a part of the same ; and, 
 since the granites have resulted from the metamorphism of 
 the gneisses, I can see no reason why the Mont Alban gran- 
 ites may not sometime be hornblendic. 
 
 In the eastern portion of the State, the most interesting 
 area of granite is the long but narrow belt which extends 
 from Worcester to Dracut. It will be found fully described 
 in the report of Mr. L. S. Burbank, which accompanies this. 
 The extended and little known area of granite lying north 
 of the gneiss in Plymouth and Bristol counties is referred to 
 this age, on account of its intimate relations to the gneiss, 
 and its unlikeness to the Huronian granites. The granites 
 cutting the Carboniferous and Primordial slates near New- 
 port are also referred to this horizon. They resemble, in 
 some respects, the quaternary granites, just described, of the 
 Connecticut valley. 
 
 The endogenous granites, which have not a distinctive 
 color on the map, are usually coarser than the preceding, and 
 more distinctly crystalline; never exhibiting traces of schist- 
 osity, or genuine stratification. They, however, sometimes 
 possess the banded appearance due to their mode of deposi- 
 tion, which may be readily mistaken for stratification, since 
 the veins are frequently of a lenticular form, usually occur 
 in gneisses of similar composition, and conform in trend and 
 dip with the bedding of the enclosing rock. These gpanites 
 are, as Dr. T. Sterry Hunt has shown to be the case with en- 
 dogenous Mont Alban granites generally, characterized by 
 
32 
 
 the abundance and variety of the accessory minerals which 
 they contain, though appearing to be poorer in this respect 
 than the endogenous granites of some other regions. 
 
 GNEISS. This is the most abundant rock in Massachu- 
 setts, forming nearly one-half the State. Accurate and com- 
 prehensive descriptions of it may be found in Prof. Edward 
 Hitchcock's final report on the Geology of the State; and I 
 will only add here, that its general lithological and petro- 
 logical characters stamp it as unquestionably Mont Alban, 
 rather than Laurentian, gneiss. Its distribution, as shown oti 
 the Centennial Map, corresponds very closely with previous 
 maps, and there is only one important exception to note. 
 This is in the eastern part of Plymouth county, where, it 
 will be observed, the bands of gneiss and granite have been 
 extended eastward to the shore. This disposition of the 
 rocks seems justified by the E.-W. strike of the gneiss in 
 this region, and by the following extract from the final report 
 of Prof. Hitchcock, just referred to: " On my former map I 
 colored a deposit of granite, connected on the north with 
 that just described, and extending to Brewster on Cape Cod. 
 I did this because a ridge of considerable elevation extends 
 down the Cape to Brewster, and many boulders of granite 
 are found of great size upon its hills. But re-examination 
 renders it probable that the largest and most abundant of 
 these boulders are granitic gneiss, approaching so near to real 
 granite, as easily to be mistaken for it. A somewhat similar 
 rock occurs in place in Rochester and New Bedford ; and my 
 present impression is, that probably a . ridge of this gneiss, 
 and not granite, forms the axis of the Cape." 
 
 The band of altered slate and conglomerate occurring in 
 Bellingham, and marked on the map of Prof. Edward 
 Hitchcock as "metamorphic slate," has, on the Centennial 
 Map, been merged with the gneiss of that region. The rea- 
 sons for this are, that it is distinctly interstratified with the 
 gneiss, and exhibits frequent and beautiful passages into 
 gneiss and mica slate, indistinguishable from those forming a 
 
83 
 
 large part of the great Mont Alban formation. I am inclined 
 to regard it as essentially a part of the gneiss, and to see in 
 the more conglomeritic portions an indication of the con- 
 dition of a large part, perhaps all, of our schistose gneisses at 
 some time in the remote past a small sample arrested in its 
 development, and now available as a key to the origin of 
 thousands of square miles of crystalline rocks. 
 
 The conglomerate in Harvard, which has been carefully 
 studied by Mr. L. S. Burbank, and will be found described 
 in his report, though placed nearer the top of the series, and 
 having, HS I conceive, no stratigraphical or genetic connec- 
 tion with the great body of the gneisses, exhibits similar 
 metamorphoses. 
 
 MICA SLATE. Mica slate is, next to gneiss, the most abun- 
 dant rock in the State. Its characters and distribution have 
 been well described by Prof. Edward Hitchcock. Some 
 alterations of the boundaries have been made in the Nashua 
 and Menimac valleys, which are explained in Mr. Burbank's 
 report. This rock is believed to generally overlie, conform- 
 ably, the gneiss. Throughout the State there are numerous 
 bands and patches of mica slate, which are regarded as local 
 variations of the gneiss, and have not been delineated on the 
 map. 
 
 ARGILLITE. Argil lites of several distinct ages occur in 
 Massachusetts. To the Mont Alban age are referred the ar- 
 gillites of the Nashua and Connecticut valleys only. The 
 boundaries of this rock remain substantially as they appear 
 on Prof. Hitchcock's map. It unquestionably overlies the 
 mica slate ; and its lithological and petrol ogical characters, as 
 described by Prof. Edward Hitchcock, make its Mont Alban 
 age extremely probable. The same is true of the mica slate. 
 
 The narrow strip of conglomerate in Harvard, already re- 
 ferred to, although regarded as logically inseparable from the 
 argillite, has received a distinctive color on the map, on ac- 
 count of the scientific interest which attaches to it, and the 
 accuracy with which its boundaries have been worked out. 
 5 
 
LIMESTONE. The Mont Alban limestones are less mag- 
 nesian than those of the Huronian age, and are rarely ser- 
 pentinic. They occur in numerous small lenticular beds, 
 usually interstratified with gneiss and mica slate. The de- 
 posits east of the Nashua valley are regarded by Mr, Bur- 
 bank as partaking mainly of the nature of veins, rather than 
 stratified beds; but, west of the Connecticut, as Prof. 
 Hitchcock has pointed out, some at least of the masses 
 are distinctly and regularly stratified. The limestone in 
 Chelmsford has afforded specimens of the Eozoon Cana- 
 dense ; and, since this peculiar structure has been found in 
 Laurentian, Huronian, and Mont Alban limestones, those 
 believing in its organic nature must admit that the animal 
 producing it had a very long range in time. 
 
 Accompanying the report, so often referred to, of Prof. 
 Edward Hitchcock, are several E.-W. sections of the State, 
 which, though exhibiting correctly the more general strati- 
 graphical features of the Mont Alban formation, leave sev- 
 eral important points in great obscurity. Between the 
 western boundary of the Huronian formation and the mica 
 slates of the Nashua valley, in the northern part of Middlesex 
 county, the gneiss all dips to W.-N. W. ; the amount of the 
 dip varying from 50 to vertical. There are, as is clearly 
 shown on the map, three N.-S. bands of mica slate in the 
 Nashua valley, each of which dips steeply to the west. Be- 
 tween the first and second of these belts, reckoning from east 
 to west, is a long and narrow range of granite and granitoid 
 gneiss ; while a broad band of argillite, which has also 
 steep westerly dips, separates the second and third belts of 
 , mica slate. Between the first range of mica slate and the 
 granite, in Harvard and Bolton, lies the narrow strip of con- 
 glomerate and argillite, conglomerate on the east and 
 argillite on the west, already referred to as the Harvard 
 conglomerate. This, like all the preceding, dips to the west. 
 West of the third range of mica slate is a broad expanse of 
 gneiss, which, along its eastern border, dips gently, 15-25, 
 
35 
 
 to the east; and resembles, in its essential lithological fea- 
 tures, the gneiss of Middlesex county. 
 
 Such is the general surface arrangement of the rocks of this 
 region ; and this arrangement necessitates, as I conceive, the 
 following structure below the surface: The gneisses on the 
 eastern and western sides of the valley are stratigraphically 
 the same ; the three belts of mica slate are parts of one and 
 the same stratum; and the strip of argillite and conglom^ 
 erate in Harvard and Bolton is an isolated portion of the 
 great area of argillite forming the centre of the valley. 
 This valley is a vast synclinal, with its axial plane dipping 
 to the west, and having a grand fault on either side. The 
 broad band of argillite is a stratum not less than 10,000 feet 
 thick folded sharply upon itself, and forms the centre of the 
 synclinal. Enclosing the argillite is the stratum of mica 
 slate, forming the second and third bands of that rock. Be- 
 tween the third band of mica slate and the gneiss on the 
 west, there is, as already indicated, an abrupt break in the 
 bedding : the slate dips steeply to the west, while the gneiss 
 has gentle eastward dips. This stratigraphical break proves 
 the existence of a fault on the west side of the synclinal. 
 The down-throw was to the east; and the high range of hills, 
 of which Wachusett forms the culminating point, is prob- 
 ably a remnant of the grand escarpment produced by this 
 dislocation of the strata, and marks the point where the 
 down-throw was greatest. This fault, it is believed, extends 
 nearly or quite across the State. The common bound- 
 ary of the first band of mica slate and the narrow band of 
 granite and gneiss, marks the position of the eastern of the 
 two faults mentioned. Here, also, the down-throw was to the 
 east, and has been sufficient to cut off very obliquely 4,000 or 
 5,000 feet of mica slate, and bring the underlying gneiss up 
 to the present plane of denudation. The occurrence of ar- 
 gillite in Harvard and Bolton, east of the granite, proves that 
 the lateral displacement produced by the fault has not been 
 less than a mile. This fault probably extends as far south as 
 
36 
 
 Worcester. It accounts perfectly for the present relative 
 positions of the granite, mica slate, and conglomerate in this 
 part of the valley ; and we are able to find in the great dis- 
 turbance, of which this fracture is an index, a cause for the 
 metamorphism of the conglomerate and the conversion of 
 much of the gneiss along the line of fracture into granite. 1 
 
 Beyond the northern boundary of Bolton, the second or 
 middle band of mica slate disappears, and reappears five 
 miles farther north near the common boundary of Harvard 
 and Ayer. Here, also, the granite widens rapidly, and is 
 brought, by the disappearance of the mica slate, in contact 
 with the argillite. In the paper referred on p. 5, I have 
 shown that this disposition of the rocks necessitates, for its 
 explanation, a second fault, which begins in the northern part 
 of Bolton, and extends, nearly parallel with the fault just 
 described, along the western border of the granite. The 
 down-throw of this fault is on the west, increasing toward the 
 north ; and only after the fault enters Harvard does the 
 down-throw become sufficient to throw all the mica slate 
 below the present surface. 
 
 . Between Ayer and the Merrimac, there is, as Mr. Burbank 
 has shown, a synclinal of mica slate ; and the mica slate is 
 conformably underlaid on either side by gneiss. This Merri- 
 mac synclinal is really a branch of the main Nashua synclinal^ 
 but contains no argillite ; and it embraces a broad belt of 
 granite and granitoid gneiss, which is an expansion of the 
 narrow band extending southward through Harvard and 
 Bolton. To raise this immense mass of the underlying 
 gneisses to their present position, and leave the bands of 
 mica slate on either side unremoved by denudation, requires 
 two faults; one on each side of the valley, between the 
 
 1 Since writing this sketch of the stratigraphy of the Nashua valley, I have 
 learned that, concerning the structure of the region about the Harvard conglom- 
 erate, my views bear some resemblance to those previously advanced by Mr. Bur- 
 bank. He, however, regards the complication of this region as due to ill-defined, 
 vertical movements of partially plastic materials, rather than to well marked 
 faults in rockg essentially rigid. Wherever the two views coincide his is entitled 
 to priority. 
 
37 
 
 granite and the mica slate, with the downthrow in each case 
 on the side toward the mica slate. But, in Harvard, we al- 
 ready have two such faults bounding tie granite; and we 
 have only to conceive these to extend north-easterly along 
 the margins of the Merrimac synclinal, and the structure of 
 the whole region becomes clear immediately. 
 
 The broad, unbroken expanse of gneiss between Wachu- 
 sett and the Connecticut valley exhibits several prominent 
 anticlinals prominent geologically, but without definite 
 topographical representation; the corresponding synclinals 
 appear, however, to be shallow, as they contain no mica slate 
 or argillite. In the Connecticut valley the crystallines are 
 pretty effectually concealed by the Triassic beds; but there 
 is, according to Prof. Hitchcock, much mica slate along the 
 eastern side of the valley, in Amhcrst, Wilbraham, and other 
 towns, which, for the most part, dips to the west, and appears 
 to overlie conformably the gneisses to the eastward. The 
 well known argillite in Bernardston covers an area three to 
 four miles broad, and has very steep easterly dips. On the 
 south, it appears to pass under the Mesozoic sandstone, which 
 is unbroken in that direction and, south of Northampton, 
 fills the entire valley, making observations of the underlying 
 rocks impossible. A small patch of argillite, however, appears 
 in Whately immediately west of the sandstone, and north of 
 the granite ; and, since the argillite in Bernardston extends 
 northward along the Connecticut for many miles, we may, I 
 think, safely conclude that it extends nearly or quite across 
 the State in a southerly direction. West of the argillite and 
 the overlying sandstone is a broad area of mica slate, which 
 stretches, in a N.-S. direction, quite across the State, and, 
 like the argillite, extends a long distance beyond the State 
 limits. In the eastern half of this belt of mica slate, the 
 dips are very steep to the east. Thus we find the valley of 
 the Connecticut underlaid by a grand synclinal of Mont 
 Alban rocks. This synclinal embraces the same rocks as the 
 Nashua synclinal, and they are arranged in the same order. 
 
38 
 
 The small area of gneiss which crosses the Deerfield River 
 at Shelburne Falls, and exhibits a distinct anticlinal structure^ 
 bears the same relation to the Connecticut synclinal that the 
 broad gneissic area of which Wachusett is the highest point 
 does to the Nashua synclinal. The anticlinal structure of 
 the Shelburne gneiss and the overlying mica slate, is shown 
 in the section from Greenfield to Williamstown, which ap- 
 pears in the Report of Prof. Hitchcock on the Geology of 
 Vermont. A glance at the map will show that the granitic 
 area in Williamsburg and Westhampton lies on the line of 
 strike of the Shelburne gneiss, and is elongated in a north 
 and south direction. It represents, as I conceive,. the exten- 
 sion southward of the Shelburne anticlinal. And the great 
 disturbance displayed by the gneiss in the river at Shelburne 
 Falls has only been carried a step further to produce the ex- 
 travasated granite of William sb'urg. West of the axis of this 
 anticlinal, the mica slate exhibits steep westerly dips, which 
 continue to the band marked on the map as talcoid slate. 
 This extends nearly across the State, and has very steep, 
 usually vertical, dips. 1 Beyond this band, to the west, the 
 mica slate and the underlying gneiss dip steeply to the east. 
 These facts prove the existence along this line of a syncli- 
 nal similar to the others, but in which the argillite is re- 
 placed by the talcoid slate. The probable unconformability 
 of the mica slate and talcoid slate is shown by the fact that 
 the latter, along its western boundary, in the southern half 
 of its extent, is underlaid, not by mica slate, but by gneiss. 
 In the gneiss of the Hoosac Mountains, we have another 
 great anticlinal, which, in the northern half of the State, 
 is flanked on the east by mica slate, but bears upon its 
 western slopes the semi-crystalline rocks of Berkshire county, 
 already referred to. 
 
 il am indebted to Prof. B. K. Emerson of Amherst College for data relative to 
 the boundaries of the talcoid slate in Blaiidford, and also, for information concern- 
 ing the occurrence of emery in situ three miles south of Chester on the line of 
 strike of the emery deposits at that place. 
 
39 
 
 This brief and imperfect sketch of the stratigraphy of 
 the region between the western boundary of the Huronian 
 formation and the Hoosac Mountains, makes it probable 
 that the strata are essentially continuous and conformable 
 throughout, and belong to one and the same series. And 
 this view is confirmed when lithological comparisons are 
 instituted between the different portions of this great area. 
 No important differences of a general nature can be shown to 
 exist between the gneisses of the various regions named. 
 The gneiss west of the Connecticut closely resembles much 
 that occurs east of Worcester; and it is believed the same 
 may be said of the argillites and mica slates. The gneiss and 
 mica slate east of the Connecticut are usually regarded as 
 essentially identical with the rocks of the White Mountains 
 as typical Mont Alban; and it seems necessary, in the 
 light of the facts now known, to conclude, provisionally at 
 least, that the rocks of Wachusett and the Hoosac range are 
 synchronous. And, since the rocks west of the Connecticut 
 lie in N.-S. bands, continuous with similar bands in Vermont, 
 the inference seems probable that the gneiss of the Green 
 Mountains is also of Mont Alban age. 
 
 The limestone, quartzite, talcoid slate, and clay slate, in 
 Berkshire county, appear on the Centennial Map substan- 
 tially as on previous maps. In the region of Great Barring- 
 ton and Stockbridge, however, some important alterations of 
 boundaries will be observed, which are based principally 
 upon the published observations of Prof. J. D. Dana. Prof. 
 Dana's views concerning the order of succession of these 
 various rocks and their lithological relations are, in the main, 
 adopted here. All the rocks named seem to belong to one 
 and the same series, which is essentially conformable through- 
 out. The limestone lies principally at the base of the series, 
 and is overlaid on the east by the quartzite, which is fre- 
 quently in terst ratified with and passes into mica slate and 
 gneiss. Toward the west, the quartzite is mainly replaced 
 by mica slate ; and this, farther west, gradually becomes the 
 
40 
 
 so-called talcoid slate, which is essentially hydro-micaceous. 
 Thus the quartzite, mica slate, and talcoid slate appear to be 
 stratigraphically the same. And above these, it is believed, 
 belongs the clay slate, of which only a small portion lies 
 within the limits of Massachusetts. 
 
 The age of these rocks is regarded as yet an open ques- 
 tion. This series is, however, undoubtedly newer than the 
 gneiss of the Hoosac Mountains ; and it seems probable that 
 it will ultimately prove to be, as an increasing number of 
 geologists are inclined even now to regard it, older than the 
 Primordial ; i. e^ may be taken, as suggested farther back, to 
 represent the close of Eozoic time. But, in view of the 
 great uncertainty in which this question is shrouded, I have 
 on the map applied to the different members of what may 
 be* called the Berkshire county series no terms having chron- 
 ological significance. The names employed are, Eolian lime- 
 stone, quartzite, Taconic slate, and clay slate. With the 
 quartzite are included the gneiss and mica slate with which 
 it is associated. Taconic slate has no reference to the form- 
 ation so named by Emmons, but means simply the slate of 
 the Taconic Mountains; the object in using this term being 
 to distinguish this talcoid slate from the talcoid slate farther 
 east. Though having, as already stated, no settled opinion 
 relative to the age of these rocks, I have, however, in the ex- 
 planatory column connected with the map, grouped together^ 
 partly for the sake of convenience, the different members ot 
 .this series, and the talcoid slates to the eastward, and placed 
 the whole at the top of the Eozoic. 
 
 PALEOZOIC. 
 
 Although the fossils characteristic of the Acadian group 
 have been found at only one locality in Massachusetts, viz. ? 
 Hay ward's quarry, in Brain tree, yet most observers agree that 
 the greater portion of the slates in the vicinity of Boston 
 are probably of Primordial age ; and I have so represented 
 them on the map. Argillite occurs on Kent's Island, in 
 
BK . 
 
 OF 
 
 . 
 
 Newbury, apparently folded up in the felsite, though possibly 
 thrown down by a fault, which has been frequently referred 
 to the Primordial ; and I have here taken the same view of 
 the matter. This argillite is associated with conglomerate 
 which resembles some that I have referred to the Primordial 
 in the vicinity of Boston. The hard metamorphic slate in 
 the vicinity of Newport, R. I., has also been referred provis- 
 ionally to this horizon. 
 
 The Norfolk county belt of conglomerate, which stretches, 
 with varying width, through the towns of Braintree, Ran- 
 dolph, Canton, Norwood, Walpole, and Wrentham, into 
 Rhode Island, and which may be traced thence, more or less 
 continuously, to Pawtucket and Providence, has quite dis- 
 tinctive characters. It has been well described by Prof. 
 Edward Hitchcock, and, later, by Mr. W. W. Dodge. This 
 series is probably, as Prof. Hitchcock long ago suggested, 
 older than the Carboniferous ; and yet, on the other hand, it 
 seems to be newer than the Primordial ; and I have, therefore, 
 followed Prof. Hitchcock, and referred it provisionally to the 
 Devonian, assuming it to be equivalent to the Catskill or Old 
 Red Sandstone. 
 
 Gray conglomerates and sandstones, precisely similar to 
 the genuine Carboniferous, have been observed along the 
 middle of this Devonian belt, in Wrentham ; also by Mr. F. 
 W. Very, in Walpole, and Mr. W. W. Dodge, in Canton. 
 These observations seem to indicate the existence of a 
 tongue of Carboniferous rocks extending north-east from the 
 Carboniferous basin and splitting the Devonian band ; and I 
 have ventured to represent it so on the map. 
 
 The conglomerate so well developed about Boston, and 
 widely known as the Roxbury conglomerate, is lithologically 
 identical with the Carboniferous conglomerate of Bristol 
 county and Rhode Island, and seems to be similarly related 
 to the Primordial rocks. It has been frequently referred to 
 the Carboniferous horizon; and, in view of the facts just 
 cited, and the absence of any positive evidence to the con- 
 6 
 
trary, this is certainly the most probable view of its age. 
 Nevertheless, it closely resembles the Primordial conglom- 
 erate of this vicinity ; and, in the absence of paleontological 
 characters, we are obliged to rely upon their stratigraphical 
 relations as a means of determining their relative distribu- 
 tion, and this, on account of the drift-covered nature of the 
 country, seems wholly inadequate. Yet it is probably safe 
 to regard as newer than the Primordial, the great unbroken 
 mass of conglomerate lying between the Boston and Albany 
 Railroad and a line running S. 80 W. from Savin Hill to 
 the Charles River. 
 
 Near the low water line, 011 the north-east shore of Marble- 
 head Neck, unconformably overlying the ' banded felsites, is a 
 hard, whitish, fine grained sandstone, or arenaceous slate. I 
 have shown, in the paper referred to on page 5, that it is 
 probably newer than the Primordial, and have referred it to 
 the Carboniferous. 
 
 The boundaries of the Triassic formation in the Connect- 
 icut valley have not been altered ; but three shades of color 
 have been used to represent the three divisions of this form- 
 ation. 
 
 The eastern boundary of the Miocene on Martha's Vine- 
 yard has been altered in accordance with data furnished by 
 Prof. N. S. Shaler. 
 
 The deposit marked as Champlain, on the eastern end of 
 Nantucket, has been proved to belong to this recent period 
 by the investigations of Mr. S. H. Scudder and Mr. A. E. 
 Verrill. To the same age are referred the fossiliferous de- 
 posits in Truro, on Cape Cod, and on Point Shirley, near 
 Boston. 
 
GEOLOGY OF THE NASHUA VALLEY. 
 
 BY L. S. BURBANK. 
 
 The Wachusett range of highlands forms the western 
 boundary of the valley of the Nashua. There is also, east of 
 the valley, a parallel range of less elevation, extending from 
 Worcester and Shrewsbury to the northern boundary of the 
 State in Tyngsborough. This latter range is a well marked 
 feature in the topography of the region, and forms a water- 
 shed and climatic division of considerable importance. 
 
 A branch of the main valley extends from the north part 
 of Harvard, north-easterly, to the Merrimac River. Two 
 other minor valleys extend westward ; one along the North 
 branch of the Nashua to Fitchburg, another from Groton, 
 through Shirley and Townsend to Ashby. 
 
 The widest part of the valley is near the junction of the 
 north and south branches of the river at Lancaster. Here 
 are the broad and fertile intervales or flood-plains, that at- 
 tracted the attention of the colonists, and led to the early 
 settlement of the valley. Near the northern boundary of the 
 State, the hills on either side, in Pepperell and Dunstable, 
 approach nearer the river, which here has cut its channel 
 through sandy and barren plains of the Terrace Formation 
 to its junction with the Merrimac. In this sketch of the 
 geology of the valley, I include, also, some observations on 
 
 (43) 
 
44 
 
 the region bordering it on the east ; embracing the first range 
 of highlands and a part of the valley of the Merrimac. 
 
 The rock formations of this region may be classified litho- 
 logically, as follows : 
 
 1. Argillite. 
 
 2. Mica Slate and Quartzite. 
 
 3. Granite and granitoid Gneiss. 
 
 There are also beds of conglomerate of small extent associ- 
 ated with the argillite and mica slate; and, enclosed in strata 
 of gneiss, are several nodular masses of crystalline magnesian 
 limestone, that have been regarded by some geologists as 
 parts of true beds contemporaneous with the gneiss. 
 
 The principal argillite beds form a band extending north 
 and north-easterly from Worcester to the northern boundary 
 of the State. The widest part of the band is seen in the 
 towns of Lancaster and Shirley, and here some of the beds 
 are quite regular and the lamination smooth and even, form- 
 ing a good roofing slate. For the most part, however, the 
 strata are much disturbed and contorted, and abound in veins 
 and lenticular masses of quartz. The strike of the strata 
 varies from nearly due north to north-east, averaging, where 
 most regular, N. 15 E. A section across the widest part of 
 the band shows a uniform westerly dip of 45 to 90, and 
 the mica slate on the western border appears to overlie it 
 conformably, as if forming a part of the same series. Some 
 small areas on the eastern side near the granite have an east- 
 erly dip at a small angle, as in Harvard, on the west side of 
 Prospect Hill. The mica slate associated with the argillite 
 extends in a narrow band, southward, from Worcester to 
 the southern boundary of the State, where it nearly dis- 
 appears, passing into gneiss. In its northern and eastern ex- 
 tension it is separated by the granite, and forms two distinct 
 ranges; 1st, that which encloses the argillite and lies mostly 
 in the valley of the Nashua, 2d, a range which, bending 
 eastward, extends in a narrow band along the valley of the 
 Merrimac. 
 
Parallel to this latter range, on the north-west side, and sep- 
 arated from it by the granite, the mica slate extends from the 
 Nashua River in Groton to the Menimae in Tyngsborough, 
 in a band having a uniform southeasterly dip, thus forming a 
 synclinal which includes the granite. Beyond the Merrimac in 
 this direction, the slate disappears, passing into gneiss. That 
 this narrow range of slate belongs to the same system with 
 the Lowell slates is indicated, not only by its position, but 
 by similarity in mineral ogical character. A peculiar, choc- 
 olate-colored, quartzose mica slate, that abounds in the 
 vicinity of Pawtucket Falls, in Lowell, is exactly identical, 
 in appearance, with slates from the steatite quarry in West 
 Groton, and from several other localities in Groton, Duns- 
 table, and TyngsboroKgh. 
 
 The slates on the eastern side of the granite range are very 
 variable in mineralogical character, but all become coarser 
 toward the south-eastern border, and finally pass into gneiss. 
 In the vicinity of Lowell, there are numerous beds of very 
 coarse feldspathic gneiss, interstratified with the mica slate. 
 The same structure is seen, also, throughout the range, as far 
 south as Bolton. Further south, in Clinton and Boylston, 
 numerous bands of quartzite are interstratified with the slate, 
 which here crops out abundantly, forming some of the high- 
 est hills of the range east of the Nashua valley. 
 
 In Harvard and Bolton, east of the granite range and 
 closely connected with the slates just described, beds of con- 
 glomerate occur, interstratified with argillite, which here 
 coincides in strike and dip with the mica slates. (Strike, N. 
 65 to 70 E. Dip, 45 to 80 N. W.) This conglomerate 
 is very interesting from its relations to the surrounding 
 rocks, and the remarkable changes that portions of it have 
 undergone. It appears in a range extending from Pin Hill, 
 in Harvard, to the summit of Wattoquotoc in the south-west 
 part of Bolton, forming several high hills, that are marked 
 features of the landscape. The series of specimens that I 
 have placed in the Cabinet of the Society shows that the 
 
pebbles have, in many cases, been flattened, bent, and even 
 drawn out into layers, giving an agate-like structure to some 
 of the rock. While the specimens collected indicate some of 
 the changes, only a careful study of the localities can show 
 how complete is the transition from a sedimentary to a crys- 
 talline rock. The principal conglomerate beds lie between 
 hills of poryhyiitic granite on the west and north, and mica 
 slate and gneiss on the south-east, yet not a pebble of granite 
 or gneiss, so far as I have seen, enters into its composition, 
 and the slaty pebbles that occur, appear to belong to argillite 
 rather than mica slate. Some ten miles to the south-west, 
 however, are extensive beds of slate, interstratified with a 
 quartz! te closely resembling the pebbles that form the mass 
 of the conglomerate. The conglomerate beds are most abun- 
 dant and least altered in Harvard, at the most northern 
 locality where they appear. On tracing them southward, 
 in the direction of the strike of the strata, they are seen to 
 become more altered and less distinct in character, until they 
 are completely blended with enclosing, conformable strata 
 of mica slate. In a large exposure of the rock, on the sum- 
 mit of Wattoquotoc, in Bolton, only a few small spots can be 
 found where the conglomerate structure can be made out. 
 Further south the rock is entirely crystalline, showing no 
 traces of mechanical origin. 
 
 The rock which immediately encloses the beds of slate and 
 conglomerate at Harvard is of a very peculiar character, as 
 shown by specimens that I have placed in the Society's Col- 
 lection. It appears in unstratified masses, resembling granite, 
 but mixed with fragments of slate, and destitute of mica; 
 and also as a schistose feldspathic gneiss in which slaty part- 
 icles take the place of mica, while the feldspar is in imperfect 
 rounded crystals, giving a porphyritic aspect to the rock. 
 
 The area colored to represent granite and granitoid gneiss 
 is much larger than has hitherto appeared on any geological 
 map of the region. It covers, however, only the region 
 where granite predominates, though there are, especially in 
 
the north-eastern division, near the Merrimac River, some 
 intercalated, beds of well stratified gneiss. From Groton 
 southward to Boylston, this granite is decidedly porphyritic,. 
 and in Clinton and Berlin, where it is largely developed, the 
 crystals of orthoclase are very large and perfect, often two 
 inches or more in diameter. Further south it passes into 
 porphyitic and ordinary gneiss. I have included .the granite 
 of Worcester in the same formation, though it cannot be 
 shown that it is actually connected with the same range. 
 It may, perhaps, be more properly classed with some smaller 
 out-crops of granite that occur among the slates west of the 
 principal range. 
 
 From the facts observed, it seems reasonable to infer that 
 this granite is a metamorphic rock, produced by alteration of 
 deeply buried sediments underlying the slates, together with 
 some portions of the same series of slates and conglomerates 
 that appear in their partly altered condition at the surface. 
 The least altered of the slates and conglomerates probably 
 formed the highest parts of an elevated range, which settled 
 to its present position simultaneously with the elevation of 
 the softened material which formed the granite -range. 
 
 The gneiss on the south-eastern side of the range of slates 
 contains the magnesian limestones to which I have referred. 
 Several of the larger deposits occur in a line coinciding with 
 the strike of the gneiss, which extends through Bolton, Box- 
 boro', Littleton, and Chelmsford. I have elsewhere 1 given 
 reasons for holding that these limestones are not true beds, 
 but segregated masses of a vein-like character, derived by 
 solution from the' enclosing strata of gneiss. This view, 
 which was maintained in opposition to the theory of their 
 organic origin, was not altogether new, as may be seen, by 
 the following quotation from Hitchcock's Geology of Mass. 
 (1841), p. 563. 
 
 " The geological situation of these limestone masses, and their 
 mineral character, are so similar that one description will embrace 
 
 1 See Froc. Boston Soc. of Nat. History, Vol.' xiv, p. 19f>. ' 
 
48 
 
 them all. They all correspond to the description which Dr. Maccul- 
 loch gives of the limestone of Tirey, one of the Western Islands of 
 Scotland ; 'It is,' says he, ' improperly called a bed, as it is only an 
 irregular rock, lying among the gneiss, without stratification or con- 
 tinuity. In this respect, it resembles the greater number of primary 
 limestones found in gneiss and mica slate, and may be considered 
 as a nodule.' " 
 
 It is true that the rock at Chelmsford and Bolton, which con- 
 tains the so-called Eozoon Canadense has some appearance of 
 stratification, but this is for the most part a banded structure, 
 which is seen only on and near the walls of the cavities, 
 while the mass of' the limestone shows no appearance of 
 stratification. It is not improbable, however, that some por- 
 tions of the impure limestone of the walls may be partially 
 altered strata of the gneiss. It is a fact of some significance 
 also, that, while the unstratified mass of the deposits is dolo- 
 mite, the limestone of the walls is nearly pure calcite, filled and 
 penetrated in every direction with crystals of various mag- 
 nesian silicates, and the serpentine takes various concretion- 
 ary forms, or appears as chrysotile veins traversing the ser- 
 pentine masses and granules, and sometimes investing them, 
 forming a fibrous wall on their surfaces, as they occur im- 
 bedded in the calcite. In these localities, the eozoonal forms 
 occur only near the walls of the cavities and associated with 
 other minerals; never in the principal masses of the lime- 
 stone. At Chelmsford, the gneiss enclosing the limestone 
 has clearly an anticlinal position; at the other localities, this 
 structure is not so plainly shown, but I think the same theory 
 will account for all the deposits, viz., that they are vein-like, 
 concretionary masses, filling cavities in the gneiss, and bccur- 
 ing especially along the line of a nearly closed and reversed 
 anticlinal fold. The great number of mineral species associ- 
 ated with this limestone, some of which are characteristic 
 vein minerals, tends to confirm this view. The following is 
 a list of minerals found in the limestone of Bolton, Box- 
 boro', Littleton, and Chelmsford : Scapolite, boltonite, nut- 
 talite, chondrodite, petalite, sphene, pyroxene, diopside, apa- 
 
49 
 
 tite, actinolite, asbestus, augite, allanite, coccolite, pargasite, 
 chromite, pyrite, magnesite, phlogopite, talc, ophite, chrys- 
 otile, satin spar, rhomb spar, allanite, spinel, garnet. 
 
 In the vicinity of Lowell, the slates are intersected by 
 numerous dykes of diorite, which sometimes occur as appar- 
 ent beds included between the layers of mica and hornblende 
 slate. One of these dykes at Pawtucket Falls is interesting 
 as showing fine examples of boulders of disintegration. A 
 dyke which is included between nearly perpendicular strata 
 of slate appears at the surface as if composed of numerous 
 boulders, closely packed, and somewhat regularly arranged. 
 Some of the rounded masses have become detached, and 
 slightly water-worn, and these cannot be distinguished by 
 their appearance from boulders of trap, in the ordinary drift. 
 Whether an original concretionary structure in the rock has 
 determined the size and form of these boulder-like masses ; 
 or whether ordinary decomposition and exfoliation, beginning 
 on the corners and edges of blocks resulting from a jointed 
 structure, has produced them, may be doubtful. In either 
 case, the facts indicate that some of the rounded boulders of 
 the drift may owe their forms less to any mechanical action 
 of wearing and grinding, than to such a process of previous 
 decomposition and exfoliation. 1 
 
 A small trap dyke, three or four inches in width, that tra- 
 verses the gneiss and limestone in one of the quarries at 
 Chelmsford, exhibits an irregular lamination, causing it to 
 separate, when broken, into transverse, flattened, somewhat 
 prismatic layers. As these occur in place in the dyke, they 
 are uniformly bent upward in the centre, showing that the 
 material when forced into the fissure was in a plastic condi- 
 tion and adhered to the walls on each side. The trap rock 
 of the numerous dykes near the Pawtucket Falls, in Lowell, 
 abounds in joints which divide it into small blocks. These 
 blocks are often coated with a black, or dark green mineral, 
 generally striated or semi-fibrous in structure, and having a 
 
 1 See Proc. Boston Soc. of Nat. History, Vol. XVI, p. 150. 
 

 50 
 
 brilliant, polished appearance on the surface, exactly resem- 
 bling, in some specimens, the appearance on the walls of 
 veins, which has been called slicken-sides. At Lowell, such 
 layers occur on all sides of angular blocks, in positions show- 
 ing that their polished appearance cannot be due to any 
 mechanical process of grinding or sliding over another sur- 
 face of rock. These coatings are generally very thin, but 
 sometimes occur in layers of half an inch or more in thick- 
 ness, forming a distinct mineral, which in physical and chem- 
 ical characters closely resembles delessite. 
 
 In regard to the position in the geological scale of the 
 rock formations here described, I have no very positive opin- 
 ions to offer. I have been inclined to regard the gneiss east 
 of the Nashua valley as belonging to a distinct system older 
 than the Wachusett gneiss. The mica slate, also, which is 
 interstratified with it and overlies it conformably, evidently 
 belongs to the same continuous series. On the western bor- 
 der of the valley the mica slate has still a westerly dip, and 
 appears to pass under the nearly horizontal strata of the 
 Wachusett gneiss. The peculiar concretionary veins con- 
 taining beryl and tourmaline crystals, which characterize the 
 Wachusett gneiss, do not occur east of the mica slate. The 
 slates containing andalusite and staurolite are abundant on 
 the western border of the valley, but do not occur east of 
 the argillite. The gneiss east of the valley is generally more 
 compact, heavier and less micaceous than that of the Wachu- 
 sett range. 
 
 The argillite beds also appear, for the most part, to occur 
 in strata conformable with the mica slate, and, on the west- 
 em border especially, they are so blended that no well 
 marked boundary line between them can be laid down. The 
 andalusite crystals characteristic of this part of the forma- 
 tion occur abundantly in both the mica slate and argillite. 
 The argillite beds are generally highly inclined, and in many 
 places nearly in a vertical position. They are also much 
 
51 
 
 contorted, and filled with veins, layers, and lenticular masses 
 of quartz. 
 
 For reasons, some of which I have stated above, I have 
 heretofore regarded the gneiss, mica slate, and argillite, as 
 forming a continuous series, all older than the gneiss of the 
 Wachusett range. Mr. Crosby has proposed a different view, 
 which will explain most of the facts quite satisfactorily, on 
 the theory that the strata have been brought into their pres- 
 ent relations by the occurrence of several faults, by which 
 the slates have been brought in contact with the granite and 
 gneiss. While I have no doubt that vertical movements to 
 a considerable extent have occurred, especially among the 
 rocks east of the valley in Harvard and Bolton, I think there 
 is evidence showing that these movements took place when 
 some of the rocks were in a highly plastic, and almost fluid 
 condition, so that the porphyritic granite of Harvard, for 
 example, though undoubtedly metamorphic, may also be re- 
 garded as an eruptive mass. 
 
 With the exception of the granite and gneiss used for 
 building, few valuable products are obtained from the rock 
 of this region. The argillite of Lancaster was formerly 
 quarried for roofing slate, and recently the debris from the 
 old quarries has been ground into " slate flour" to be used as 
 paint, and in compositions for roofing. The metamorphic 
 slate of Harvard was formerly sawed into slabs and used for 
 gravestones and other purposes, but these quarries have not 
 been worked for many years. 
 
 In Leominster, a fine arenaceous mica slate has been man- 
 ufactured into scythe stones, which have acquired a consider- 
 able local reputation, the farmers of the neighboring towns 
 generally preferring them to all other kinds. 
 
 The manufacture of lime, which was carried on more than 
 fifty years ago, at Bolton, Chelmsford, Boxboro' and Littleton, 
 was continued at Bolton till about fifteen years ago. The 
 other quarries were abandoned many years before. I have 
 learned that the work was abandoned, in most cases, on ac- 
 
52 
 
 count of the exhaustion of the limestone, or the increased 
 difficulty in quarrying it, rather than from any lack of demand 
 for the lime. 
 
 There are numerous veins and nodules, and sometimes 
 intercalated beds of ferruginous quartz associated with the 
 argillites and mica slates of the Nashua Valley. This quartz 
 is of a vitreous character, and entirely distinct from the 
 quartzite layers that are abundant in the older slates. 
 
 The close resemblance of these rocks to the gold-bearing 
 quartz of North Carolina and Virginia, is certainly very 
 striking. Many excavations have been made in them in 
 search of gold. I do not know that any proper test has 
 been made to determine the question whether any gold is 
 really to be found in them. I have examined many speci- 
 mens from different localities, with the microscope, but find 
 no gold visible. These veins are especially numerous near 
 the eastern border of the argillite in Harvard and Boylston. 
 At the latter place I have examined several openings made 
 by returned Californian miners, where the decomposing sul- 
 phides are very abundant in the quartz veins, and I am not 
 surprised that the attention of persons familiar with gold- 
 bearing veins of other regions was attracted to these 
 localities. 
 
 New excavations have recently been made in the old lead 
 mine in the south part of Sterling, but nothing of value was 
 obtained. It is not uncommon to find, in the quartz veins 
 and nodules enclosed in the slates, some traces of galena and 
 copper pyrites, as well as of the ores of iron, but no well- 
 marked veins of any value have yet been discovered in any 
 part of the region that I have explored. 
 
 The anthracite of Worcester was fully described in Prof. 
 Hitchcock's Report. No other similar beds have been found 
 in the formation, and this has not been found valuable, nor 
 explored to any considerable extent. No fossils having been 
 found in the associated slates, the deposit is not specially im- 
 portant in its geological significance. 
 
 IBlc 
 
14 DAY USE 
 
 RETURN TO DESK FROM WHICH BORROWED 
 
 EARTH SCIENCES LIBRARY 
 
 This book is due on the last date stamped below, or 
 
 on the date to which renewed. 
 Renewed books are subject to immediate recall. 
 
 LD 21-40m-l,'68 
 (H7452slO)476 
 
 General Library 
 
 University of California 
 
 Berkeley 
 
Gaylord Bros. 
 
 Makers 
 Syracuse, N. Y. 
 
 PAT. JAN. 21, 1908