1882.] 133 [Wads worth. 



of lava flows, ashes, dikes, bosses, etc., leaving the geology in an 

 intricate state ; the same as it would be anywhere, when eruptive 

 action has prevailed a long time, the sea action going on at the 

 same time, mixed deposits being formed, and the whole exposed 

 during countless ages to denudation and metamorphism. 



Older rocks than the Braintree (Paradoxides) argillites may 

 exist, but of their existence we thus far have no proof. We can 

 only ascertain the age of the other rocks, unless more fossiliferous 

 deposits can be found, by tracing, as the writer has begun to do, 1 

 the relation of the adjacent rocks to the Braintree argillites, the 

 relations of the former to their surrounding rocks, and so on in a 

 constantly widening circle — guided by facts. 



As an illustration of the confusion produced in this region by 

 eruptive action attention may be called to another paper by 

 myself, relating to the Stoneham limestone, in the Harvard Univer- 

 sity Bulletin (1880, n, 359, 360), in which it is shown that the 

 argillite has been cut through and through by dikes and greatly 

 indurated and altered, so that one class of observers claimed that 

 all were of sedimentary origin and another class that all were 

 eruptive, when in truth part were eruptive and part sedimen- 

 tary. 



In this connection it may be pointed out that the rock in the 

 Quincy granite which is marked as amygdaloid on the Geological 

 Map accompanying the third volume of the Occasional Papers of 

 the Boston Society of Natural History, is, in part at least, a well- 

 marked argillite, which the writer has followed westward from the 

 Quincy granite to the Randolph Turnpike. This covers quite a 

 large extent of the district, but how much the writer has not 

 ascertained, since his observations were made some years prior 

 to the publication of the map. 



i These Proc. 1881, xxi, 274-277. 



