Rocks in Southern Connecticut. 235 



that lava once covered a portion of the crystalline rocks 

 east of the great fanlt. Basalt dikes cutting those rocks 

 are not uncommon, and it is quite conceivable that erup- 

 tions occurred there contemporaneously with the flows in 

 the Lowland. It is also possible that the main sheet 

 formerly extended farther eastward. The view is held 

 that recurrent faulting maintained steep topography east 

 of the area of sedimentation during much of Triassic 

 time ; but the scarp may have receded considerably to 

 the east during intervals of quiet, and it is not at all 

 unlikely that the heaviest flows, aggregating hundreds of 

 feet in thickness, encroached somewhat on the crystalline 

 area even if the eruptions occurred only in the Lowland. 

 With later displacement on the fault the lava on the 

 upthrown block would be stripped away, furnishing part 

 of the material for the burial of the main body of the 

 sheet. Davis's diagrams 10 express the conception that 

 the entire Triassic section once extended considerably 

 east of the present boundary, with later faulting and 

 complete removal of the sediments and lavas from the 

 area of the Eastern Highland. There is good suggestive 

 evidence, however, that recurrent faulting maintained the 

 eastern border of the Triassic area near its present posi- 

 tion during a considerable part of the period of sedi- 

 mentation. In addition to the testimony of the fanglom- 

 erate, discussed here, features and relationships observed 

 by Mr. W. L. Russell during recent work along the fault 

 zone testify to recurrent faulting and renewal of the scarp 

 through the period represented by the section including 

 the three trap sheets. 



The essential uniformity of dips in all parts of the 

 Saltonstall basin is noteworthy. In the tunnel section, 

 discussed above, the dip is 35°, and this figure may be 

 taken as the average for the main sheet throughout the 

 length of Saltonstall Eidge. The fanglomerate beds, 

 which lie more than 1000 feet higher in the stratigraphic 

 section, are also inclined toward the boundary fault at 

 an angle of about 35 c , although at the time of their deposi- 

 tion these coarse deposits must have had a slight inclina- 

 tion in the opposite direction. It appears, therefore, that 



10 The Triassic Formation of Connecticut, U. S. Geol. Survey, Ann. Eept 

 18, part 2. See especially Plate 20. 



