﻿24 NEW YORK STATE MUSEUM 



Gilbert gulf. 1 The shores of the glacial and sea level waters 

 are conspicuously preserved in many places, and specially in 

 Jefferson county immediately south of the area; while their sedi- 

 ments occupy the valleys [see pi. 29]. 



The slow tilting uplift of this part of the continent finally 

 raised the Thousand Islands district above the ocean level and 

 then Lake Ontario was initiated. The uplifting has continued 

 until the outlet and lake are now 246 feet above tide. 



As the lake and marine waters were slowly drained away from 

 the gently sloping surface of the area the storms and streams 

 resumed their briefly interrupted work, and for a few thousand 

 years they have again been gnawing 2X the rocks and land surface 

 with important effects. 



THE ROCKS* 

 Precambric rocks 



The Precambric rocks of northern New York, as at present 

 known, may be most conveniently classed in four groups, (a) 

 a series of old sediments or rocks laid down under water, the 

 Grenville series; (&) a series of granitic gneisses of igneous 

 origin, which cut the Grenville sediments intrusively and hold 

 abundant inclusions of them and which, in so far at least as the 

 immediate region is concerned, are correlated quite confidently 

 with the Laurentian granite-gneisses of Canada; (c) a series of 

 somewhat younger igneous rocks which cut and hold inclusions 

 of both the preceding groups, which have a great development 

 in the eastern Adirondacks but occur in less force in the imme- 

 diate region, and which consist of anorthosites, syenites, granites 

 and gabbros, the last three of which occur here in masses of 

 usually small size; and (d) of much younger igneous rocks, of 

 late instead of early Precambric age, which appear as dikes of 

 diabase or trap, and which have some development in the region, 

 though less abundant than in the eastern Adirondacks. 



The Grenville sediments are the oldest known rocks of the 

 region, and the fact that they are water-deposited rocks necessi- 

 tates belief in the existence of a floor of older rocks on which 

 they were laid down. No certain trace of this old floor has ever 

 been discovered in New York, and though it is possible that 

 fragments of it may be contained as inclusions in the granite 

 gneiss, we are as yet unable to distinguish such, if present, from 



Gilbert Gulf (Marine waters in Ontario basin). Geol. Soc. Am. Bui. 

 17:712-18. 

 2 By H. P. Cushimr. 



