378 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



quartz, and continues thus across to the river. The granitic material on 

 the west has been brought from 20 miles south, the slaty material from 

 Vernon and Northfield to the northeast, and the two cuiTents pass each 

 other well established, with plenty of room to move in, and do not show 

 any indication that they are located near the head of a narrow bay. 



The behavior of the great overflow trap sheets is instructive as indicating 

 the character of the bottom over an extended area at a given time. The 

 Deerfield bed is an overflew, as is proved by the beautiful ropy surface at 

 Turners Falls. That it flowed over the muddy bottom of the bay is indi- 

 cated by the kneading together of trap and shale in Greenfield (see p. 419). 

 It rests on the Mount Toby conglomerate from Gill Center nearly to Fall 

 River, then on fucoidal sandstone and shale to Deei-field, then on ai'kose 

 to the Connecticut, and on the ]\Iount Toby conglomerate to the south 

 end of Mount Toby. It had little influence upon the later rocks, and is 

 covered by the same rocks as those which lie beneath it, except that the 

 boundary of the fucoidal sandstone and the jMount Toby conglomerate is 

 shifted to the north by an amount equivalent to the thickness of the trap. 



The same is true of the Holyoke bed. The same buff arkose that pre- 

 cedes its advent also rests upon it, and does not receive the smallest influence 

 from the abundant iron in the trap, as it was immediately covered by the 

 strong currents. It continues to rest on the arkose to Holyoke, and from 

 there to the south line of the State rests upon the fucoidal sandstone and 

 the shale. All these rock types thus formed portions of the bottom of the 

 basin at the same time. 



The shallowing of the basin effected by the outflow of the great mass 

 of trap made itself manifest in the transfer of the boundary of the arkose 

 and sandstone far to the north. That is, it shallowed the waters so that 

 along the central axis of the valley the finer-grained sandstones character- 

 istic of the shallower central area extended mucli farther north. This 

 strengthens the impression that one gets from the signs of repeated 

 emergence from the water, so abundant in the sandstones, and their absence 

 from the arkose, viz, that the sandstone was deposited in shallower water 

 and laid bare at low tide. That the arkose and calcareous shales were 

 deposited at the same time is further shown by the fact that from Titans 

 Pier, where the Holyoke trap sheet crosses the Connecticut, nearly to the 

 Westfield River, about ten miles, the trap, which here everywhere rests 



