﻿GEOLOGY OF THOUSAND ISLANDS REGION 



59 



There is an occasional quartzite pebble along the contact, otherwise 

 the sandstone is normal, and gives no sign of basal conditions. 



Around to the left the slope of the knob steepens. There are 

 occasional bands of coarsely crystalline, purer quartzite in the schists 

 which are far more resistant to weathering. On this steep front one 

 such layer iprojects as a cornice 

 with the sand-filling beneath, as 

 shown in figure 3. Photographic 

 attempts here proved wholly un- 

 satisfactory. 



Besides the contacts on larger 

 slopes, of which the preceding are Fig> 3 A nearer view of a portion of the 



incton^flc fVic>t-/a ira o •nn-miViAt- n-P contact showing a local steep slope of the 



instances, mere are a numoer 01 hm and pro;jecting CO rnice of an extra re- 

 minor examples of the sort, chiefly sistant ^ uartzite la y er - 

 as filled hollows of the limestone surfaces. A sand-filled hollow of 

 the sort appears at the top of the limestone quarry near the Theresa 

 boat landing, and is shown in plate 2. In the section there shown the 

 hollow is about 6 feet deep and with twice that width at the top. 

 Another example may be seen at the quarry just south of the 

 Theresa depot, though the overlying sandstone is gone except for 

 the small residual patch resting in the hollow so that its original size 

 can only be guessed at. A considerable number of other examples 

 have been seen, some merely sand-filled, others containing rock frag- 

 ments as well. In all cases the cement is calcareous and the rock 

 weak and easily removed. 



The above evidence of the character of the surface on which the 

 Potsdam was deposited, is of precisely the sort so convincingly set 

 forth by Wilson in his discussion of similar features in Ontario. 1 

 In New York these features are developed in a belt of considerable 

 breadth across the strike, showing a great number of ridges and 

 valleys, with patches of overlying Potsdam, and with the relief in 

 every case owing to differential, erosion on rocks of varying resist- 

 ance, and in no case to subsequent folding. In this State 

 exposed patches of residual materials resting on the old surface are 

 more numerous than in Ontario, and these are in the depressions in 

 all cases, showing that the depressions were in existence and served 

 as receiving pockets for this material at the commencement of sand- 

 stone deposition. The evidence is abundant, clear and convincing 

 that the Precambric surface underneath the sandstone is precisely 

 like that where the sandstone is absent, and that the present topogra- 

 phy of the Precambric areas is that resulting from recent stripping 



'Wilson, A. W. G., Can. Inst. Trans. 7:146-55. 



