﻿GEOLOGY OF THOUSAND ISLANDS REGION 55 



tion since it is very exceptional to have the evidence as abundant 

 and as clearly shown as it is here. 



The evidence of surface irregularity is of threefold nature, (a) 

 that given by exposures of direct contacts, (b) that given by the 

 tracing of the lines of contact of the Potsdam and Precambric, even 

 without exposures of the actual contact, and (c) that given by the 

 topography of the present Precambric surfaces, since it can be 

 shown that these surfaces are substantially those upon which the 

 Potsdam was originally deposited ; in other words that the Potsdam 

 is just being pared away from the Precambric over part of the 

 district, its numerous outliers testifying to its former presence over 

 the whole and to the recency of its removal where now absent. 



Where the Potsdam has been removed the Precambric surface 

 disclosed is one of low ridges and valleys, with general northeast- 

 southwest trend. The ridges are low and with hummocky surface, 

 and the valleys are broad and shallow, and developed on the weak 

 rocks (such as the limestones and weak schists) or on lines of 

 structural weakness (as along lines of sheared and shattered gran- 

 ite). The extreme of relief does not much exceed ioo feet, and is 

 generally less. The quartzites, resistant gneisses and some long 

 and wide granite dikes constitute the ridges. In the relatively ele- 

 vated country occupied by the igneous bathyliths the surface is of 

 the knob and basin type [pi. 6 and 7]. The numerous granite 

 dikes and small bosses which cut the limestone and are resistant to 

 weathering, diversify the valley bottoms. Hence a large part of the 

 area consists of slopes, and extensive flats do not appear. 



The surface underneath the Potsdam is precisely of this sort. 

 The smaller Potsdam outliers are usually mere remnants remaining 

 in places where the floor was lowest and the sandstone thickest. 

 The larger outliers cover both high and low ground. The Pots- 

 dam resists wear, and hence usually presents cliff fronts at its mar- 

 gins, showing thicknesses of from 20 to more than 60 feet of sand- 

 stone, yet even with these thicknesses the summits of the outliers 

 are often overtopped by neighboring' granite knobs. The evidence 

 of the occasional inliers of Precambric. rocks in the Potsdam is 

 even more obvious. The two small inliers east of Goose bay (Alex- 

 andria quadrangle) along the road from Alexandria Bay to Chip- 

 pewa Bay, have their tops at the same level as that of the sandstone 

 plain in which they lie, yet a 20 foot thickness of sandstone shows 

 at the Potsdam margin, just to the northward. This line of evi- 

 dence might be pursued at great length but since it is less conclusive 

 than are the other lines the above will suffice. 



