206 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Thence it extends to its abrupt termination at the Neponset Valley 

 along the western base of the Great Blue Hill. The Fore River as 

 well as the Neponset Valley probably mark great north and south 

 faults. On the north, along a line crossing northern Quincy and south- 

 ern Milton, the alkaline rocks are bounded by an east-west fault 

 contact with the carboniferous sediments of the Boston Basin. At the 

 eastern end on the southern side, in northern Weymouth, the alkaline 

 rocks are probably in fault contact (not actually exposed) with an essen- 

 tially sub-alkaline granite, then for a short distance in fault contact 

 with Cambrian slates or granite through northern Braintree. Further 

 west they are in conformable contact with the coarse (basal) carbonif- 

 erous conglomerate of the Norfolk Basin. On all sides, then, the 

 boundaries are practically great major faults, and the alkaline rocks 

 comprise essentially a great fault block, or more correctly two, an east- 

 ern and western member, and these appear to have been elevated, 

 particularly the western and larger block, which was also tilted up at 

 the north more sharply than the eastern member. ^° The eastern 

 block is crossed by numerous minor, chiefly north and south, faults. 

 These as well as the major boundary faults, and their dissection of 

 the area, have been most fully and carefully worked out and described 

 by Crosby. 



The alkaline rocks are intrusive into Cambrian (certainly as late as 

 middle Cambrian) sediments, consisting essentially of slate with 

 quartzitic and limy bands. They are certainly earlier than the 

 adjoining sediments in the Norfolk Basin, an arm of the Narragansett 

 Basin, which are carboniferous, but at present their exact age cannot 

 be stated. In the immediate neighborhood of the granite, the slates 

 have been indurated and somewhat metamorphosed, but extreme 

 metamorphism is not, it is important to note, a characteristic of such 

 of the sediments as now remain. 



Although all of the rocks of the series were undoubtedly intruded 

 during a single great period of intrusion, one member, the aporhyolite, 

 has been held by Crosby to be the youngest of the alkaline-rocks. 

 As will be shown beyond, there is reason to believe that it was earlier 

 than the other rocks of the series. 



With the exception of the large number of small dikelets of micro- 

 granite that are found cutting the slate at the contacts, and of which 

 Professor Crosby has gi^'en us a very detailed description, of the dikes 

 of granite cutting the slate in the Pine Tree Brook Reservation and 



10 See Crosby, loc. cit., p. 534 et seq. 



