﻿GEOLOGY OF THOUSAND ISLANDS REGION 1 5 



granite dikes such as are found nearly everywhere in the Grenville 

 limestones, sometimes containing- only sand. There is comparatively 

 little basal conglomerate in the district back from the river, but 

 there, both on the mainland of the Alexandria quadrangle and on 

 Wellesley and Grindstone islands is an exceedingly coarse conglom- 

 erate, from 10 to 20 feet thick, full of coarse cobbles derived from 

 the ponderous and resistant Grenville quartzite of the vicinity. 



Except for these conglomerates the formation is everywhere a 

 sandstone and mostly pretty thoroughly cemented, the cement being 

 chiefly of silica. Its colors are red, brown, yellow, white, and rarely 

 black. Its thickness over the immediate district will scarcely exceed 

 ioo feet, and it thins out toward the west and south. The deposits 

 of sand began forming first in the Champlain region and gradually 

 worked their way westward, being deposited in a shallow trough or 

 basin whose axis roughly coincided with the modern St Lawrence 

 axis, so that hereabouts we find simply the thinned western edge of 

 the formation. As its thickness here is substantially equal to the 

 difference in altitude between the ridge crests and valley bottoms 

 of the old erosion surface upon which it was deposited, it follows 

 that it varies rapidly in thickness from place to place and was but 

 scantily deposited upon the elevations, some of which it utterly 

 failed to overtop. 



It is not known whether or not the formation in its entirety is a 

 marine formation. The sparse fossils indicate such origin for the 

 upper beds with comparative certainty, but many things about the 

 remainder of the formation suggest a land surface and an arid cli- 

 mate as the conditions under which the accumulation took place. 



Theresa dolomite. A change in conditions ensued and de- 

 posit of dolomite began. Some sand was still supplied from the 

 neighboring land however, as the dolomite is everywhere sandy, 

 and at first the supply was from time to time in excess, so that layers 

 of coarse weak sandstone alternate with those of dolomite. Hence 

 there is a gradation from one rock to the other instead of a sharp 

 boundary between the two. The greatest thickness of the 

 formation within the area mapped does not exceed 35 feet, 

 though its original thickness may have been somewhat greater. 

 The thickness increases eastward and diminishes to the west and 

 south as was the case with the underlying sandstone. The 

 waters were more fitted for the existence of life and the fossils 

 are more abundant than in the sandstone, but unfortunately 

 conditions for their preservation have not been favorable. 



