Russell— Great Triassic Fault. 483 



Art. XLII. — The Structural and St rati graphic Relations 

 of the Great Triassic Fault of Southern Connecticut; 

 by William L. Russell. 



General Geologic Relations and Summary. 



The area studied extends from Lighthouse Point, on 

 New Haven Harbor, to the vicinity of Durham, and is 

 over 15 miles in length and several miles broad. The 

 general geologic relations of this tract are quite simple. 

 The Triassic rocks of the Newark series form an east- 

 ward dipping homocline, cut off on its eastern border by 

 the Great Fault, where the Triassic rocks abut against 

 the crystallines. The sedimentary rocks are of consider- 

 able thickness, and contain three intercalated lava 

 flows, — the lower, main and upper sheets. Normal 

 faults, which tend to repeat the strata, are of frequent 

 occurrence. 



The main conclusion of the present paper is in strong 

 support of the hypothesis of Prof. Joseph Barrell 1 that 

 the depression in which the Triassic rocks of Southern 

 Connecticut were laid down was produced by intermit- 

 tent movement along an eastern fault plane during the 

 course of sedimentation. The evidence for this conclu- 

 sion is two-fold. In the first place, fanglomerates, com- 

 posed of huge, angular bowlders, occur at various hori- 

 zons in the Newark series close to the fault plane, and 

 grade abruptly into finer sediments along the strike to 

 the west. The second line of evidence is furnished by a 

 large quartz lode that occurs along the eastern side of 

 the fault zone. This lode appears to have been formed 

 in pre-Triassic time, and therefore movement had taken 

 place along the present fault zone even before the 

 Triassic. Pebbles of this vein are found in several hori- 

 zons in the Triassic, indicating that when the rocks in 

 which they occur were laid down, faulting had already 

 taken place, for the lode was formed along a fault that 

 existed before its formation, and also proving that at 

 that time the Triassic did not extend east of its present 

 eastern boundary. 



Acknowledgment. — The field work upon the problem 

 under discussion was carried on in the fall of 1921 and the 



1 For references see the end of this article. 



