FORMATION OF THE TEIASSIC BASIN. 



375 



they, with the equally peculiar rocks which are disclosed by erosion in 

 the midst of the conglomerates of Mount Toby, are carried soutli over the 

 granites of the eastern shore, and the arkose derived from these granites at 

 last takes their place and is itself continued south at the foot of the slate 

 bluffs of Wilbrahani, where it slowly gives place to a slate-conglomerate. 

 Where the basin is narrow these two rocks — the arkose on the west and the 

 conglomerate on the east — meet and blend in an interdigitating boundary. 

 When the basin widens they separate to include broad areas of sandstones 

 and shales, representing the sand and mud flats which intervened between 

 the strong curi-ent which moved up the west side and that which passed 

 down the east side of the basin. 



I have elsewhere (p. 353) described the Mount Toby conglomerate as 

 resting upon a pedestal of crystalline rocks whose surface is nearly 400 

 feet above the sea. This pedestal is continued south as the great ridge upon 

 which Amherst is built. The presence of this ridge and the consequent 

 shallowness of the waters explain the fact that the arkose extending south 

 from the ridge expands entirely across the valley and contains from its 

 eastern border clear to the Mount Tom station in the center of the basin 

 angular pebbles of granite, often as large as one's fist. 



Wherever I have examined the cross-bedding it tells the same story 

 as to the direction of the cun-ents; as in the bluff's of Mount Tom, and 

 especially in a fine island of arkose in the northwest bend of the Deerfield 

 River (which is interesting as having more than sixty potholes cut in its 

 surface by the strong high-water current of the present river, which passes 

 over it), where the beautiful cross-bedding is plainly directed northerly. 



President Hitchcock presents the matured results of his long studies 

 of the Trias in the introduction to the Ichnology of Massachusetts (1857), 

 wherein he gives the details of four sections across the sandstones, in which 

 he obtained the following thickness: 



Thickness of the Triassic sandstones at different localities. 



