﻿346 W. M. Davis — Structure of the 



ridges from Talcott Mountain and the Hanging Hills to Toket 

 and Pond Mountains near Long Island Sound are simply re- 

 peated outcrops of a single heavy sheet of trap, separated by 

 faults; and that these faults and others belong to a system of 

 fractures that divide the formation into a number of long, rela- 

 tively narrow blocks, in every one of which the beds dip as a 

 rule to the east at a moderate and tolerably constant angle. 



The simple monoclinal also fails to explain the crescentic 

 curvature of the trap ridges, that so early awakened interest. 

 Their form cannot be attributed to the curvature of the fissures 

 through which the trap was supplied, for the two finest exam- 

 ples, Pond and Toket Mountains are found to be overflow 

 sheets, not intrusions ; and the sedimentary beds below and 

 above them are curved conformably with them. Among the 

 observations that prove the contemporaneous origin of the 

 trap, the following may be mentioned: the sandstone directly 

 overlying the Toket Mountain trap-sheet contains many frag- 

 ments of trap; and the cavities of its waterworn amygdaloidal 

 surface are found neatly filled with fine, unbaked sediment. A 

 very coarse conglomerate, containing numerous bowlders of 

 trap, is closely associated with the posterior ridge of Pond 

 Mountain. The two mountains must be regarded as the curved 

 outcrops of gently folded or "dished" strata, cut off on the 

 eastern side by faults. The crescentic form of the ridges is in- 

 deed closely correlated with the whole system of faults ; for 

 in several cases their curvature is increased by a succession of 

 small dislocations on the hooks of the crescents. For example, 

 the southern end of Pond Mountain, where crossed by the 

 New Haven and Branford highroad and by the Shore Line 

 Eailroad, is notched by oblique faults, whose eastern upthrows 

 materially add to the departure of the ridge from a straight 

 line. Larger illustration of the same structure is seen in the 

 dislocated members of the Hanging Hills by Meriden, and of 

 the Mount Holyoke range in Massachusetts. Moreover, this 

 correlation of faint folds with faults explains the order of over- 

 lays or offsets in the successive ridges of a curved range ; at the 

 southern end of a crescent, the upthrow on the eastern side of 

 the faults produces the advancing order, seen in miniature at 

 the southern end of Pond Mountain, and in a much larger way 

 along the Hanging Hills and as far north as Talcott Mountain; 

 at the northern end of a crescent, the same upthrow produces 

 the opposite overlap, or receding order, as in the little ridges of 

 the range that crosses the Mattabesick at Beckly station, or 

 again in the peaks of the Holyoke range. 



The topography of Pond and Toket Mountains (Percival's 

 E, I and E, II) is particularly instructive when thus interpre- 

 ted. Pond Mountain is the simpler of the two, and is espe- 



