270 THE CAMBRIAN. [bull. 81. 



rocks, belong to the same series as the East Point slate and limestone 

 the entire series must be referred to the Cambrian. 1 



In his report of 1880 Prof. Crosby considers that the strata of the 

 Boston Basin were deposited in a depression excavated in the crystal- 

 line formations and that there are several formations belonging to the 

 Cambrian. First, a conglomerate, represented by the Eoxbury pudding- 

 stone. Second, a slate series which incloses the Paradoxides zone in 

 South Braintree. He assigns to the conglomerate a maximum volume 

 not exceeding 1,000 feet, and considers that the greatest thickness of 

 the slate can not be much less than that of the conglomerate, although 

 in some cases certainly falling below 500 feet. 2 At the trilobite quarry 

 in Braintree, the known locality of Primordial fossils, the slate is de- 

 scribed as of a greenish gray color, " somewhat siliceous, fine-grained, 

 and remarkably uniform. Minute grains of pyrite are pretty generally 

 diffused. In the quarry the rock is apparently only very coarsely 

 jointed ; but the weathered surface reveals much finer division by this 

 means. The stratification is very massive and can hardly be detected, 

 except by means of the trilobite remains ; these may be supposed to 

 lie in the plane of the bedding, and according to this indication the 

 strike is E.-W., and the dip S. 80°-85°. Across the strike, southerly, 

 the slate, still apparently maintaining a high dip in tbat direction, is 

 met at a distance of perhaps 300 feet by fine-grained grayish and red- 

 dish Huronian granite, while on the west the same granite cuts off the 

 slate at about twice the above distance from the trilobite quarry; that 

 is, the contact of the two formations, as shown on the map, is oblique 

 to the strike of the slate." 3 On the map the slate series throughout 

 the Boston Basin is correlated with the slates at Braintree, and repre- 

 sented by a common color. 



• Iua letter received from Prof. Crosby, dated January 27, 1891, he says: "The Nahant slate is 

 entirely isolated, and concerning its relations to the conglemerate we know nothing. I have supposed, 

 however, that it should be referred to the newer series, above the conglomerate." Of the general 

 geology of the Boston Basin he says : 



" Assuming that the slates of the Boston Basin now known to be Cambrian are overlain in succes- 

 sion by the conglomerates and a newer slate series (my present working hypothesis), it is still pos- 

 sible, I suppose, that all these rocks, in the Boston Basin, may be Cambrian; notwithstanding the 

 marked uniformity between the conglomerate and lower slates. 



•' The weak points in the stratigraphy are (1) the general (not entire) absence of slate pobbles in the 

 conglomerate; (2) the absence of clear sections showing the conglomerate resting upon a lower slate 

 series. Wherever the base of the conglomerate is seen, it reposes upon the granite or felsite. Some 

 of the sections are very clear and unmistakable in this respect; and apparently the same granite and 

 felsite are eruptive through the Braiutree slates. It would, of course, be di Hi cult to prove that the 

 granites and felsites are all of the same age ; but it seems best to assume that until the contrary clearly 

 appears. 



" The best general statement of the stratigraphy of this region appears therefore to be this: A 

 lower slate, known to be Cambrian ; cut in succession by diorite, granite and felsite; all these rocks 

 overlain ttnconformably by the conglomerate of unknown age; and the conglomerate in turn overlain 

 conformably by a newer slate of unknown age. 



" The chief problems are, then, to determiue which are older and which are newer slates, and to de- 

 termine the age of the conglomerate and upper slate. 



"As an indication of progress, we have the discovery by Mr. Woodworth, just reported, of fossils 

 in the slate of both Somerville and Newton." 



2 Contributions to the geology of Eastern Massachusetts. Boston Soc. Nat. Hist. Occasional 

 papers, No. 3, 1880, p. 266. 



3 0p.cit,p.l92. 



