﻿GEOLOGY OF THOUSAND ISLANDS REGION 1 9 



an abundant marine fauna, the large cephalopods being especially 

 conspicuous. 



The Watertown limestone is unconformably overlain by the thin 

 bedded limestones of the Trenton. The time interval between the 

 Lowville and the Trenton was a considerable one, but the surface 

 exposures of these rocks in New York are so near the old shore 

 lines of the time, that the deposits exposed represent the interval 

 very imperfectly. The shore line was one of many and frequent 

 local oscillations, and the rocks which have, of late years, been 

 classified as Black River limestone, represent very different parts 

 of this general interval. 



The Trenton limestone is abundantly fossilif erous and has a thick- 

 ness of 400 feet or more in the immediate region, exceeding the 

 combined thickness of the Potsdam, Theresa, Pamelia, Lowville and 

 Black River together. Found on all sides of the Adirondacks, and 

 with large thickness everywhere, the Mohawk valley excepted, large 

 subsidence is shown, with probable great encroachment of the 

 waters upon the Adirondack island, much diminishing its size. 



As Trenton time drew to a close fine muds commenced to appear 

 in the waters, brought in by currents from the northeast, and in 

 slowly increasing amount. Hence the limestones become impure 

 and grade upward into black shales, at first strongly calcareous, 

 later on lacking lime. This change came on the region from the 

 eastward^ hence shales were forming there while limestone was 

 still being deposited on the west. But the change to mud deposit 

 spread slowly over the whole region and the Trenton is found 

 everywhere to be overlaid by the black Utica shales. This Utica 

 submergence seems to have been the most extensive in the State's 

 geologic past, and it is quite possible that the entire Adirondack 

 island was submerged. If so it seems to have been the last time 

 that such was the case, as it was the first. 



Above the Utica lie the lighter colored shales and shaly sand- 

 stones of the Lorraine formation, the combined thickness of the two 

 shale series being several hundred feet. While neither formation 

 is found within the limits of the area mapped, in which the lower 

 Trenton is the youngest rock found, yet they outcrop in great thick- 

 ness on the Watertown quadrangle and reach to within 6 miles of 

 the south margin of the Theresa sheet, and it seems quite certain 

 that they were originally deposited over part, and likely all, of the 

 district mapped, and are now absent from it because of subsequent 

 erosion. It is even probable that the Oswego and Medina sand- 

 stones thick sand formations which overlie the Lorraine shales, 



