﻿GEOLOGY OF THOUSAND ISLANDS REGION 95 



and the rocks of this group are known as the Stones River forma- 

 tion. During Chazy time the depression in which Stones River rocks 

 were forming was encroaching upon northern New York from the 

 south and west, and by the close of the middle Chazy this depression 

 had become sufficiently extensive to involve our district here, and the 

 deposition of the Pamelia formation commenced, the Pamelia being 

 the local New York facies of the Stones River formation, and repre- 

 senting only a portion of its upper division. The tilting of our dis- 

 trict necessary to permit of this invasion from the southwest, 

 changed its former easterly inclination to a southwesterly one, over 

 most of the district; but apparently this change of slope died out on 

 the eastern edge of the Alexandria sheet, east of which lay the land 

 area which separated the Pamelia basin from the Chazy basin ; and 

 this received no westerly tilt, but chiefly retained its old slope to the 

 east. This in our view is the origin of the Frontenac axis, as the 

 narrow isthmus of Precambric rocks which connects the main Adi- 

 rondack Precambric mass with the great Canadian area of these 

 rocks, and which passes through our district here, is called. It 

 simply represents an axis of the old Precambric floor which became 

 less depressed than the portions of the floor east and west from it. 

 The Potsdam-Beekmantown-Chazy depressions sagged the district to 

 the east, covering it with steadily increasing thickness of their de- 

 posits in that direction; the Pamelia depression sagged the district 

 to the west, and in that direction the overlying deposits steadily in- 

 crease in thickness. The Frontenac axis is the pivotal district be- 

 tween the two, where sagging was least and deposit thinnest. Sub- 

 sequent erosion could thus wear away this thin cover and bring the 

 Precambric back to daylight, along this line, as it has done, while yet 

 the thicker cover, east and west, in part remains. 



According to Ulrich the Pamelia formation is of age intermediate 

 between the middle and upper Chazy of the Champlain valley, but 

 little sedimentation having taken place there in Pamelia time; in 

 other words while this region was subsiding and accumulating de- 

 posit, that ceased to subside. With the cessation of Pamelia deposi- 

 tion on the west, resulting in the unconformity between the Pamelia 

 and Lowville, deposition was renewed on the east and the upper 

 Chazy was laid down. In like manner the Lowville formation is 

 but slightly represented in . the Champlain valley, though well de- 

 veloped here, as if, with renewed subsidence here it again ceased 

 there. Toward the close of the Lowville, uplift occurred on the 

 northwest giving rise to the unconformity between the main mass of 

 the Lowville and the Leray limestone. At the same time depression 



