76 NEW YORK STATE MUSEUM 



dam sandstone and Calciferous sandroek) which have been so 

 overspread with glacial deposits, mostly to considerable depth,, 

 that the underlying rock but seldom protrudes through them, and 

 exposures are rare except along the larger streams. 



The boundary of this plain against the hilly tract is a some- 

 what irregular one. The ridges rise rather abruptly above the 

 level of the plain, but the valleys between the ridges have the 

 same level as the plain itself, the rather even surface of the 

 glacial deposits extending unbroken up into the valleys, and the 

 Potsdam sandstone frequently doing the same thing. Though! 

 the boundary is quite abrupt, the hills are low and well rounded 

 and apparently pass under the Potsdam with the same character. 



The present evenness of the plain is almost wholly due to the 

 deposition of glacial materials on its surface. Prior to the Glacial 

 period it was also a plain, though considerably more dissected by 

 the streams than at present, that is the main stream valleys were 

 wider and deeper, and tributaries were, more abundant than now. 

 Apparently at that time also the level of the plain corresponded 

 with that of the valley bottoms, along its contact with the hilly 

 tract. From the contact the plain slopes away to the north and 

 northwest to the St Lawrence valley. The contact itself drops, 

 strongly in altitude going westward. 



Hilly tract. The hills are composed of crystalline rocks whicli 

 are much older than the Potsdam sandstone, and which formed 

 the floor on which that rock was deposited. At that time the 

 region consisted of low hills whose rocks were deeply weathered. 

 It sank slowly beneath the sea, the depression commencing at the 

 northeast and progressing slowly toward the south and west, as 

 indicated by the decreasing thickness of the Potsdam in those 

 directions, specially the former. It can not yet be certainly stated 

 whether the paleozoic rocks were deposited over the whole of the 

 Adirondack region or not, but there can be no question that they 

 once extended far beyond their present limits. Their thickness* 

 in the district is unknown, but at the northern end of the Cham- 

 plain valley, where they are thickest, the minimum value that can, 

 be assigned to them is 4000 feet, with the strong probability that 



