﻿62 NEW YORK STATE MUSEUM 



calcareous in all cases. The angular form of the pebbles is due 

 to the close jointing of the quartzite bands and granite dikes in 

 the limestone, and the trifling amount of wear exhibited points 

 to residual accumulation in the hollows, whereby they were pro- 

 tected from abrasion. The very small supply of such material, 

 taken in conjunction with the small amount of decay shown by the 

 underlying rocks, is a factor of much significance. 



On the Alexandria quadrangle, both on the mainland and on 

 Wellesley and Grindstone islands, a more extensive and bulky con- 

 glomerate occurs, which has already been described by Smyth [see 

 pi. 14] . 1 The most impressive display of this conglomerate known 

 to us is that in the cliff along the St Lawrence in the extreme north- 

 east corner of the Alexandria sheet where, rising sharply from the 

 river level it reaches a hight of 20 feet above it. Here, as usual, 

 the deposit has a calcareous cement which dissolves away, loosening 

 the cobbles, and giving an exterior resemblance to a cobbly mo- 

 raine, while the adjacent river bottom is solidly paved with the 

 material which has already weathered out. The deposit is every- 

 where very coarse, a cobble deposit rather than a gravel. In the 

 exposure here the cobbles run up to a foot in diameter, and average 

 probably 3 inches. They are round to subangular and consist ex- 

 clusively of Grenville quartzite. Smyth notes the presence of a 

 few small pebbles from 1 the tourmalin contact zones, but agrees 

 in asserting the entire absence of granite and schist material, 

 though several of the conglomerate outcrops rest on these rocks. 



In addition many of the exposures show that the conglomerate 

 is not strictly basal, but has pebbleless sandstone beneath, up to a 

 thickness of at least 10 feet; and in all cases the abrupt transition 

 from sand to coarse cobble at both upper and lower contacts is one 

 of the most interesting features of the deposit. Its coarseness, its 

 abruptness, its horizon, and the lack of variety in material of the 

 cobbles render it an exceedingly difficult deposit to explain. 



There occur, in a few localities on the Theresa quadrangle, small 

 patches of a dark red, very thoroughly indurated and vitreous 

 sandstone which thus differs from the general run of the sand- 

 stone of the district, though similar rock occurs in the formation 

 elsewhere, as in the Clarkson quarry at Potsdam. As it occurs here 

 it seems to be distinctly older than the general formation. All seen 

 of it was absolutely basal, nowhere was the thickness as great as 1 

 foot and it is only visible at actual exposures of the Potsdam 

 contact on the Precambric. But all the sand-filled cracks seen in 



1 op. cit. p. rgg. 



