﻿54 NEW YORK STATE MUSEUM 



quite so prominently of this type as in the case of the corresponding 

 rocks of the central and eastern Adirondacks. We are forced to 

 argue that, when these rocks were deformed, a considerable thick- 

 ness of other rock overlay them, which thickness was subsequently 

 worn away. This surface wear goes on very slowly at best, and 

 must have been continued through long ages, yet was completed 

 before Potsdam deposition began. The time involved is many mil- 

 lions of years, in all probability a rock thickness of at least a mile 

 or two was removed, and yet at the close the region was pared 

 down to a surface of comparative smoothness. Much Grenville 

 has thus disappeared, the tops of the igneous bathyliths are gone, 

 together with whatever of younger rocks may have been present 

 above them. The diabases were intruded toward the close of this 

 long period, since plainly they solidified not far beneath the surface. 



PALEOZOIC ROCKS 1 



The Paleozoic rocks of the district, for mapping purposes on 

 maps of this scale, are separable into six quite distinct lithologic 

 units, which in large part coincide with the subdivisions of these 

 rocks made long ago by the early geologists of the State. These 

 are, in order of age from below upwards, the Potsdam sandstone, 

 Theresa dolomite, Pamelia limestone, and Lowville, Black River 

 and Trenton limestones. Above the last named the Utica and Lor- 

 raine shales come in, but these nowhere reach the map limits, their 

 northerly boundaries being found on the Watertown and Sacketts 

 Harbor sheets, next south. 



The basal member of this sedimentary series, the Potsdam sand- 

 stone, was deposited upon the worn surface of the Precambric 

 rocks, and in order to properly describe the sandstone it is neces- 

 sary to present in some detail the character of this surface. 



Precambric surface underneath the Potsdam 

 That the surface on which the Potsdam sandstone was laid down 

 was far from being an even one was clearly stated by Smyth, in his 

 report on the district. 2 



That a similar irregular floor is present in many parts of Canada, 

 of the upper lake region and of northern New York, has been shown 

 by many observers. There is therefore nothing novel in the features 

 to be described, but they are worthy of somewhat extended descrip- 



i By H. P. Cushing. 



2 N. Y. State Geol. 19th An. Rep't, p. rioo-i. 



