﻿60 NEW YORK STATE MUSEUM 



away of the sandstone ; in other words that it is the reappearance at 

 the surface of a topography of tremendous antiquity. 



It further shows that this surface was little affected by the ice 

 sheets of Pleistocene times, otherwise this identity of character could 

 not have been so well retained. 



Except for the local accumulation of a very scanty amount ot 

 residual material in small pockets in the depressions (and this almost 

 exclusively quartzose) the Precambric surface, as it passes under 

 the Paleozoics, is remarkably free from signs of surface decay, 

 even the weak rocks being astonishingly fresh. In this respect also 

 the conditions are like those noted in many places in Canada and 

 the United States, as described by numerous observers. 



The relief of the Precambric surface under the Potsdam is much 

 the same in character here as elsewhere along the northern and 

 eastern borders of the Adirondacks, but is apparently less in amount 

 than it is further east, where there are differences in level of three 

 or four hundred feet at least. The evidence there, however, is com- 

 plicated by the presence of numerous large faults and is by no 

 means so well shown as it is here. On the south border, in the Mo- 

 hawk valley region, the surface was much smoother than here, ex- 

 ceedingly smooth in fact. 



Potsdam sandstone 



The first deposits laid down upon the worn Precambric surface 

 consisted of medium grained, quite pure quartz sand, now firmly 

 cemented to sandstone. On the Alexandria quadrangle the forma- 

 tion attains a maximum thickness of about 125 feet. This thickness 

 diminishes both southward and westward, but shows a steady in- 

 crease to the eastward of the area mapped. Within that the thick- 

 ness of the sandstone is not greatly in excess of, or else does not 

 quite equal, the variation of level shown by its floor, so that it is 

 subject to continual variation from place to place, and thins to 20 

 feet or less over the old ridge summits. On the Theresa quad- 

 rangle, and on Wellesley island, it locally failed to overtop the 

 highest of these, and the Theresa dolomite is found resting directly 

 on the Precambric. 



The bulk of the formation consists of a very pure quartz sand, 

 quite thoroughly cemented with a silicious cement. The general 

 color is light gray to buff, weathering white, but in the northern 

 portion of the mapped area there is much red, or banded red and 

 white rock in the lower half of the formation. The bulk of the 

 formation is evenly bedded, and the greater part is thick bedded, 



