28 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull. 



had been developed by the end of that period. This is shown by 

 the fact that the Potomac deposits of the late Jurassic or early 

 Comanche are laid down on a gently hilly surface which was 

 eroded across the Triassic and all the older rocks. As measured 

 by the work of erosion, Jurassic time, the noon-time of the reign 

 of reptiles, must have been long. 



But the larger crust movements are slow, though marked by 

 the spasmodic violence of earthquakes. Erosion begins at the 

 same moment as uplift and its rapidity is increased with the height 

 of the mountain growth. Therefore in reconstructing the re- 

 gional landscape at the close of the tilting and faulting movement, 

 the upturned sides of the crust blocks must be shown as already 

 partly destroyed, though the mountains still hold through the 

 early Jurassic considerable relation to the tilting character of the 

 movement as well as to the position of the more resistant rocks. 

 The magnitude of the fault movements suggests that some of the 

 block mountains of Connecticut may have even reached the 

 clouds. Some of the uplifted blocks, however, mantled by soft 

 rocks, were rapidly sapped by erosion and could never have 

 attained much of the height suggested by the structure. Other 

 uplifted portions, composed of hard and massive rocks, must have 

 required the whole of Jurassic time for their degradation. 1 



Close of the Triassic Sedimentation, Figure /. Still another 

 flight backward in time, and the tilted and faulted structure of 

 the Triassic strata has not yet come to be. The nature of the 

 Triassic sediments and the geographic conditions under which 

 they accumulated, rather than the structure imposed later by 

 crustal forces, now engage our attention. The shales and sand- 

 stones below the lowest lava flow show a thickness of 5,000 to 

 6,500 feet where exposed over the western half of the Central 

 Lowland. A small remnant of the same beds occurs some fifteen 

 miles west of the Central Lowland, existing because protected 

 from erosion as part of a down-sunken crust block within the 

 Western Highland. The thickness of these lower beds, as shown 

 by a boring, is here but 1,200 feet, proving a rapid thinning from 

 east to west. Studies in New Jersey by Kummel indicate that 



1 The problems connected with Mesozoic erosion and the evidence which bears 

 on some of these conclusions have been, more fully discussed by the writer on 

 pages 96-109, Vol. xxxvii, American Journal of Science, 1914. 



