376 THE JOURNAL OF GEOLOGY. 



a difference of time, but to the difference in the relative hard- 

 ness of the rocks. 



On the basis of this principle the age of certain river gorges 

 to which reference will be made later can be fixed. The narrow 

 passage of the Quinnipiac through a sandstone ridge southwest 

 of Meriden cannot belong to the same cycle of erosion as the 

 broad sandstone lowland on either side of it, but manifestly must 

 be much younger. So, also, the narrow passage of the Farm- 

 ington at Tariffville, where it crosses the trap ridge through a 

 gorge free from drift, is of much later date than the broader vdiWcy 

 more or less encumbered with drift which the upper part of the 

 same river has cut in the hard crystalline schists. Cook's Gap in 

 the trap sheet west of New Britain is much broader than either of 

 the above, and belongs to the Tertiary cycle of erosion, although 

 as I shall endeavor to show later, it was probably not occupied 

 by a stream during the whole cycle. In marked contrast, also, 

 with the Tariffville gorge is the gap by which the Westfield river 

 in Massachusetts cuts the trap ridge. This gap was formerly 

 broad and open — the result of Tertiary erosion — but is now filled 

 with drift, in which the river is at present working. Since these 

 two rivers are essentially the same in size, are now at the same 

 level, and the rock is the same in both cases, the only explana- 

 tion for the difference in the two passages is that they belong 

 to different cycles. 



To recapitulate, the results of the post-Cretaceous uplift are 

 seen in the valleys which have been cut in the peneplain. The 

 narrow valleys in the gneisses and schists, the upland valleys in 

 the limestones, the wide open, drift encumbered gaps in the trap 

 ridge, — Cook's and the Westfield river gaps, — the broad open 

 lowland on the sandstones, are all the result of erosion in this 

 cycle. The Quinnipiac gorge in the sandstone, and the Tariff- 

 ville gorge in the trap are just as surely of a later date. They 

 do not at all accord with the work of the earlier cycle either in 

 size, angle of slope, or depth. 



This conclusion is somewhat at variance with an opinion 

 expressed by Professor J. D. Dana,' but it seems justifiable in 



^Amer. Jour, of Sci., vol. x, 3d ser., 1875, p. 506. 



