﻿GEOLOGY OF THOUSAND ISLANDS REGION 6l 



with thinner bedded upper portions; where deposited on sloping 

 surfaces the lower portion is often very massive, and quite ir- 

 regularly bedded with a rude tendency to conform to the surface 

 01 deposit [pi. ii, 12]. Cross-bedding is present somewhat but 

 in by no means prominent development. Ripple marks, however, 

 abound. • Much of the silicious cement has been deposited as 

 secondary enlargement of the original quartz grains, the slides fur- 

 nishing some beautiful examples of this. 



Occasional long, cylindrical concretions (?) of a telegraph pole 

 type appear in the sandstone, and are called " tree trunks " by the 

 populace! As seen in cross section on rock surfaces they appear 

 as circular portions of the rock, from 1 to 3 feet in diameter. On 

 cliff sides they are long, vertical cylinders of sandstone. There 

 is no perceptible difference in composition between them and the 

 adjacent rock, but in every case the two are sharply separated by 

 what may be for convenience styled a circular joint. No tendency 

 to taper at the ends was noted, but the actual terminations were 

 in no case seen. They certainly reach a length of 20 feet and may 

 be considerably longer. Unless they represent a type of concre- 

 tionary structure, .we are wholly at a loss to account for them. 

 If so they certainly are an unusual type both because of size and 

 shape, and because of having the same composition as the inclos- 

 ing rock. In plate 13 is shown an excellent example of one of 

 them, in the Potsdam sandstone at Rideau, Ont, seen by us in 

 1908 under Dr Ami's guidance. This has been already described 

 by the Canadian geologists, and is here introduced because, while 

 corresponding precisely to the New York examples, it furnishes a 

 much better illustration than any there seen. 1 



Only at the base and the summit does the sandstone vary from 

 these general characters. Basal conglomerates are present in but 

 scant amount, with small thickness and patchy distribution. The 

 majority of contacts show only a few, scattered quartz or quartzite 

 pebbles in the basal layer of the sandstone. There are, however, 

 frequent patches of coarse, basal conglomerates, especially on the 

 Theresa quadrangle. They seem in all cases to occupy local hol- 

 lows in the limestone valley floors, and to occur only where the 

 limestone contained thin quartzite bands, or granite dikes. The 

 pebbles are all sizes up to that of the fist, and show little or no 

 rounding in most cases, being usually very angular. They con- 

 sist chiefly of quartzite and of white granite, though in some cases 

 pebbles of red, quartzose sandstone also occur. The cement is 



1 Ells, R. W. Roy. Soc. Can. Trans., ser. 2, v. 9, § 4, P- 103- 



