﻿348 W. M. Davis— Structure of the 



represented on Percival's map may be reduced to system when 

 their many forms are interpreted as slight variations on one or 

 two simple types of structure. The trap ridges of the little 

 Woodbury-Southbury Triassic area in Western Connecticut 

 present the same types of structure and much was learned from 

 their examination. The explanation *of the Triassic disturb- 

 ance by external forces must for these reasons devise a mech- 

 anism for the production of a faulted monoclinal, in which the 

 faults will have their upthrow of varying value on the side 

 toward which the beds dip, and in which a more or less pro- 

 nounced "dishing" of the beds may appear on a larger or 

 smaller scale. 



A few years ago, it was suggested* that certain disconnected 

 Triassic areas with opposite monoclinal dips might be regarded 

 as the buttress-like remnants of great anticlinal arches, the 

 larger part of which had been consumed by erosion. From 

 what has already been said, it is apparent that a simple, un- 

 faulted anticlinal cannot be accepted, at least for the Connecti- 

 cut buttress of the Connecticut-New Jersey arch; and the 

 former continuity of the Triassic beds over the crown of the 

 arch, so as to cover all the surface between the present limits 

 of formation, is not only unproved by independent evidence, 

 but is unnecessary to the hypothesis ; for it is clear that if the 

 foundation, on which the Triassic strata rested, were greatly 

 elevated in the Western Connecticut region, the strata would 

 be tilted in opposite directions on the two sides of the eleva- 

 tion, whether they ever stretched across it or not. But what- 

 ever was the former area of the Triassic deposits, the suggestion 

 that their disturbance was physically connected, and that it in- 

 volved the disturbance of a much larger area of the earth's 

 crust than that which they now cover, is a point of funda- 

 mental importance. It has been too much neglected because 

 the rocks about the Triassic areas are so generally crystalline 

 schists and gneisses, whose disturbance is not easily recognized. 

 The absence of tilted sedimentary rocks all over Southwestern 

 New England cannot be taken as evidence that no post-Trias- 

 sic tilting occurred there ; on the other hand, the little Wood- 

 bury- Southbury Triassic area gives direct evidence — not of the 

 former continuity of the Triassic strata from Connecticut to 

 New Jersey — but of the continuity of the disturbing action all 

 across this district. So wide a disturbance must have had a 

 deep penetration ; and the faults in the Triassic areas give suffi- 

 cient evidence that the dislocations were not limited to the 

 relatively superficial coating of Triassic strata. Some of the 

 faults have a throw of several thousand feet — for example, the 



* Independently by Kerr (1814) for North Carolina, and by Bradley (1816) and 

 Russell for Connecticut and New Jersey. 



