CAMBEIAN AND LOWEE ORDOVICIAN. 



115 



Brain tree, where we first met these rocks, they are either typical clay slates or shglitly calcareous; 

 but along the northern base of the Blue Hills occasional layers are distinctly sihceous. They 

 probably underlie a large part of the Boston Basin, being covered by the conglomerate and the 

 newer slate; and north of the basin they occur in isolated areas among the eruptive rocks. In 

 some of these areas, especially in the Middlesex Fells and Melrose, and in Woburn, clay slate 

 similar to that in Quincy and Brain tree is repeatedly interstratified with quartzite; while 

 toward the southwest, in Natick, and also in Reading and Lynnfield, there are extensive devel- 

 opments of quartzite with httle or no slate. It is very clear that the quartzite north and west 

 of the Boston Basin is the source of the quartzite pebbles which play such a prominent part ia 

 the composition of the conglomerate, especially in the central and northwestern sections of the 

 basin. In general, the quartzite is more and the slate less abundant northwestward, indicating 

 that the ancient shore line along which these strata were deposited lay in that direction; and 

 originally the Primordial strata were probably spread continuously over all the region to the 

 southeast of that line. 



Crosby ^^° in his earlier work included in the Cambrian the Roxbury conglom- 

 erate, which he ^^^ and others now assign to the Carboniferous. 



The fossiliferous slates of Braintree are referred to in the literatui-e on the 

 geology of eastern Massachusetts since 1818. Fossils were described by W. B. 

 Rogers, who stated : ^''^ 



The rock in which these fossils occur is a compact, dense, rather fine-grained siHco-argilla- 

 ceous slate or slaty sandstone, containing little or no carbonate of hme. * * * One of 

 the most curious facts relating to the trilobite of the Quincy and Braintree belt is its seeming 

 identity with Paradoxides harlani, described by Green in his monograph of North American 

 trilobite s. 



Recent finds of fossils belonging to the Braintree slate are reported by Shimer,^^^ 

 who says: 



Some time ago while having a driveway excavated at his home on Quincy Avenue, in East 

 Braintree, Mass., Mr. Thomas A. Watson found a rather angular slate bowlder, about 2 feet 

 in diameter. He kindly turned it over to the Massachusetts Institute of Technology. 



The slate is quite similar in appearance to that of the celebrated Paradoxides quarry 

 on Hayward Creek; it is similarly metamorphosed but is lighter gray in color and lacks the 

 pecuhar purplish tinge of the Hayward Creek slate. 



In it the following fauna was found : 



a c=very common; c=common; r=rare; R=very rare. 



This fauna includes five species of the Middle Cambrian, two of which are very abundant, 

 and two of the Lower Cambrian. There is thus a great predominance of the Middle Cambrian 

 element, though it indicates a persistence of the Lower Cambrian element into Middle Cambrian 

 times. So while we have here a transition fauna, the rock must be assigned to the Middle 



