﻿342 W. M. Davis — Structure of the 



The probability of my being unable to continue this investi- 

 gation, for some time at least, has induced me to publish these 

 very incomplete results in the hope that the beginning I have 

 made will be of use to others who may wish to follow this sub- 

 ject further. 



Sheffield Laboratory, April 10, 1886. 



Art. XXXIX. — The structure of the Triassic formation of the 

 Connecticut Valley ; by William Morris Davis. 



[Published by permission of the Director, U. S. Geological Survey.] 



Contexts: The physical problem — Limitation of acceptable hypothesis — Disturb- 

 ance has taken place — Its date is after the period of deposition — It was not 

 caused by overflow or intrusion of trap-sheets — It was not a simple monoclinal 

 tilting — Kvidence for occurrence of unseen faults — Their systematic position — 

 Their control of the topography — Crescentic ridges — Area and depth of disturb- 

 ance — Probable character of disturbing force — Its action on the fundamental 

 schists — Consequent monoclinal faulting of overlying Triassic strata — Strike of 

 faults determined by strike of schists. 



The trap-ridges in several parts of the Triassic formation of 

 the Connecticut Valley were examined in the summer of 1885, 

 with the aid of Mr. C. L. Whittle, volunteer assistant, and the 

 results obtained fully confirm the suggestion made several 

 years ago (this Journal, 1882, xxiv, 345; Bull. Museum Comp. 

 Zool., vii, 1883, 249 ; Proc. Boston Soc. Nat. Hist, xxii, 1882, 

 116), to the effect that nearly all these ridges are the outcrop- 

 ping edges of contemporaneous lava overflows, and that the 

 outcrop of a single sheet is repeated several times by faults 

 nearly parallel to the strike of the beds. The evidence on 

 which these conclusions rest will be presented more fully in 

 the Seventh Annual Eeport, U. S. Geological Survey. The 

 only considerable ridges formed by intrusive trap sheets seem 

 to be those in the lower part of the formation included in Per- 

 cival's western group ; but these have not yet been sufficiently 

 examined to make it clear that they are all of this origin. 



On the basis of these results, it is desired to present here in 

 brief form an hypothesis that may account for the peculiar 

 eastward monoclinal structure of the formation. It will be best 

 understood by approaching it through a review of the sug- 

 gestions that have heretofore been made to the same end. 



It was early suggested* that the monoclinal attitude of the 

 formation might result from original oblique deposition or cross- 

 bedding on a large scale. Apart from the improbability of 



* H. D. Rogers, Third Ann. Rep. Geol. Surv. Pa., 1839; Geol. Xew Jersey, 

 1840, 166. 



