GEOLOGIC FORMATIONS OF NEW YOKE 



145 



In the northern part of Lewis county the Potsdam sandstone, 

 in a few small exposures, rests unconformably upon the Arch- 

 aean terrane, and passes above into the Calciferous formation. 

 It extends almost continuously through Jefferson, St Lawrence, 

 Franklin and Clinton counties, and appears southward in the 

 Cbamplain valley in irregular outcrops. 



The Potsdam, though not seen distinctly in the Mohawk val- 

 ley (where its place between the Archaean and the Calciferous 

 sand rock appears to be vacant) is a thick mass in Pennsylvania, 

 and is known northeastward and northwestward over a great 



area. 



The base of the Potsdam at a few places in New York is a 

 coarse conglomerate which gradually passes upward into the 

 typical sandstone. 



Near Whitehall, Saratoga and Poughkeepsie, the Potsdam 

 horizon is represented by a limestone and at the two former 

 localities it passes upward into the Calciferous formation with- 

 out marked change except in fauna. 



The characteristics of the Cambrian strata lead to the con- 

 clusion that the sediments were accumulated in shallow seas 

 near the shore of a slowly sinking land. As the water slowly 

 encroached upon the land in late Middle or early Upper Cam- 

 brian time, deeper water gradually covered the earlier long- 

 shore deposits, and finer sediments were deposited upon them. 

 Toward the close of Cambrian time (Potsdam) only the higher 

 parts of the continent were above the sea. At this time the 

 Potsdam sandstone was deposited along the shores, while in 

 the deeper water the conditions were becoming favorable for 

 the formation of the great beds of Silurian limestone. The 

 conglomerate at the base of the Potsdam, grading upwards into 

 the finer sediments of the sandstone, indicates the deepening of 

 the water along the shore line of the Cambrian ocean. 



At their greatest development in Washington county, the 

 Cambrian formations have a total thickness of 10,000 or 12,000 

 feet. 



