﻿GEOLOGY OF THOUSAND ISLANDS REGION 99 



limestones' at Adams just about compensates for the disappearance 

 oi the Potsdam and Theresa formations there, and that the dip is 

 substantially the same as the fall of the rock floor, or 24 feet per 

 mile. At most there is a deduction of but 100 feet to be made, 

 amounting to 3 feet a mile in 30 miles, and reducing the total to 21 

 feet per mile. If, as is likely, this is still not the direction of true 

 dip, being too nearly due south, the figure must be somewhat en- 

 larged, and in all likelihood it amounts to from 25 to 30 feet per 

 mile, certainly not exceeding 35 feet. 



It is of interest to note that this dip, and this slope of the 

 Precambric floor, are much less than those worked out in the 

 upper Mohawk valley by Miller and myself (Remsen and Little 

 Falls quadrangles) where the dips approach 100 feet per mile to 

 the southwest, and the Precambric floor underneath has a slope 

 exceeding that of the dip by some 30 feet. The matter of the 

 present dips is simply the sum total of tipping given to the rocks 

 since they were deposited, by the various oscillatory movements 

 to which each region has been subjected since; showing that the 

 Mohawk rocks have been somewhat more tipped than those 

 here. The matter of floor slope however shows clearly that the 

 shore line in the Mohawk region had a somewhat greater cant 

 than was the case here, producing more rapid overlap of the 

 rocks there. 



In the northeast portion of the Alexandria sheet the dip has 

 flattened out to practical horizontality, Potsdam with overly- 

 ing Theresa forming the river bluffs. Going east, down the 

 river, the dip soon changes to the northeast, carrying these 

 formations beneath the water and the westerly edge of the Beek- 

 mantown becomes the surface rock, beyond which, for many 

 miles, the river flows through Beekmantown rocks, all with 

 slight northerly dip. These are the deposits of the eastern basin, 

 and received no tilt to the west. 



ROCK STRUCTURES 1 

 Foliation 



Foliation is the name applied to the species of cleavage de- 

 veloped in rocks which, under compression, have wholly or 

 largely recrystallized. The cleavage is chiefly due to the ar- 

 rangement which the compression enforces on many of the re- 

 crystallizing minerals, which tend to develop in the shape of 



1 By H. P. Cushing. 

 4 



