Rocks in Southern Connecticut. ^9 



expected to exhibit some symmetry with respect to the 

 curve of the ' ' dish, ' ' and not a common parallelism. The 

 faults of the Saltonstall basin have the trend character- 

 istic of the region. Davis pointed out that faults in the 

 southern part of the Connecticut Lowland extend gener- 

 ally from southwest to northeast, 3 and a glance at the 

 geologic map of Connecticut confirms this statement. 

 Davis and other students of the region have also called 

 attention to the predominance of faults with apparent 

 downthrow on the west. Thus the small faults at the 

 tunnel appear to fit into the regional scheme, and we are 

 led to suspect that they are related genetically and chron- 

 ologically to the larger faults nearby; in other words, 

 that they are "sympathetic faults." 



Our knowledge of the existence of large faults affecting 

 the Triassic area depends mainly on the repetition or 

 offsetting of trap ridges, and not on actual observation 

 of fault surfaces. Because of the character of the evi- 

 dence the exact nature of displacements is largely specu- 

 lative, and in this connection two especially obvious 

 questions are presented. 



(1) To what degree was the displacement lateral, 

 parallel to the strike of faults ? 



(2) "What is the direction of dip on the major fault 

 planes i 



Some indirect evidence bearing on the first question is 

 found for certain parts of the Triassic area. Thus the 

 absence of important offsetting in the outcrops of the 

 great dikes in the towns of Hamden and Wallingford 

 may be taken to indicate that the dikes are essentially 

 vertical and that displacement on the large oblique faults 

 which evidently extend from the vicinity of Meriden 

 southwestward had no important lateral component. 4 

 There are grounds for a similar conclusion regarding the 

 faults that affect the Saltonstall basin. The map (fig. 1) 

 shows that in the northern part of the "dish" the trap 

 ridges are offset toward the north on the west side of 

 faults, whereas in the southern part the offset is uni- 

 versally in the opposite sense. This arrangement — the 

 "receding order" and "advancing order" of Percival— 

 is exactly that to be expected if all of the faults occurred 



ri 7th Ann. Kept. U. S. Geol. Survey, p. 474. 



4 Davis, W. M. : 18th Ann. Kept. U. S. Geol. Survey, part 2, p. 102. 



