﻿8 NEW YORK STATE MUSEUM 



single area of the sort appears in the southeast part of the 

 Theresa sheet, on which is found the western portion of the 

 " sand plains," the great Pleistocene delta of the Black river. 



Interesting historically from having been the scene of exploita- 

 tion and settlement by French immigrants of high class, during 

 the early part of the nineteenth century, the district preserves 

 many traces of this immigration, especially in the matter of 

 geographic nomenclature. 



SUMMARY OF GEOLOGIC HISTORY 1 



The rocks of the region are readily separable into two great 

 groups, the one of older crystalline rocks, and the other ot 

 younger sandstones, limestones and shales which rest upon the 

 older group. The rocks of the older group are of Precambric 

 age, are among the most ancient rocks of which we anywhere 

 have knowledge, and are in most respects identical with the 

 crystalline rocks which compose the great central region of 

 northern New York, the Adirondack region, and with those of 

 the much more extensive area which lies to the northward in 

 Canada. These rocks, in the district here reported upon, form 

 a narrow connecting link, or isthmus, between the exposures of 

 these two areas, which otherwise are completely separated 

 from one another by a belt of country of considerable width in 

 which the surface rocks belong to the younger group. It is 

 only in the immediate region therefore that direct connection 

 can be traced between the old rocks of Canada and of New 

 York, and this fact gives added interest to the study of these 

 rocks here. 



These Precambric recks furnish us with our most ancient direct 

 records of the history of the earth, but like most ancient records 

 they are fragmentary and difficult to decipher. Nevertheless they 

 plainly indicate that Precambric time was of enormous duration, 

 involving many millions of years. 



Here, as elsewhere in northern New York, these rocks consist 

 of but a single series of water-deposited rocks, so far as our 

 knowledge goes. This is known as the Grenville series, and 

 comprises rocks which, originally deposited as shales, limestones, and 

 sandstones, are now greatly changed in character and have become 

 white, coarsely crystalline limestones, glassy quartzites, and schists 

 and gneisses of many varieties. Curiously we have not as yet, in 



1 By H. P. Cushing. This is a simple statement of _ the outlines of the 

 history of the region as disclosed by the study of the district. The detailed 

 evidence upon which these statements are based, will follow later. 



