﻿92 NEW YORK STATE MUSEUM 



This black basal limestone of the Trenton contrasts strongly 

 with the equally thick underlying Seven foot tier in being a most 

 inconspicuous element in the physiography of the region. In fact 

 its presence is hardly suspected over the greater part of the area, 

 since it is nearly always hidden at the base of the rounded Trenton 

 hills. Only where the formations are planed to one level, as about 

 Rosiere, is it observed to outcrop as a recognizable belt. 



The remainder of the Trenton, as far as the area of the map is 

 concerned, consists then of about 50-60 feet of thin slabby lime- 

 stones, with shaly intercalations. The limestones are partly gray 

 and crystalline with many crinoid joints and partly fine grained, 

 dark gray to black. The latter limestone swells sometimes into 

 thicker beds (1 foot thick and more) of black limestone which is 

 either quite barren of fossils save worm tubes, or as on Carleton 

 island, almost entirely composed of the shells of Plectambo- 

 nites sericeus. 



Plate 25 shows the general aspect of the thin bedded limestones 

 in the creek bed, at Threemile Bay and plate 24 which gives a 

 closer view of the rocks in the same locality, illustrates the regular 

 alternations of limestones and shales in the formation. 



The greater middle and upper part of the Trenton is found in 

 the region south of the map, on the other side of Black River bay. 



The fauna of the Trenton has the general aspect of that of the 

 formation in other parts of the State. Its details have not yet 

 been studied. 



SUMMARY OF PALEOZOIC OSCILLATIONS OF LEVEL* 



It has been shown that the Potsdam and Theresa formations were 

 deposited in the west end of a sagging basin or trough which occu- 

 pied the general line of the present St Lawrence valley; that the 

 deposition began at the east and worked westward, involving our 

 region here only in its later stage ; and that the depressed trough was 

 a westward extension from a similar subsiding trough along the 

 Champlain valley line. There the Potsdam is very thick, is followed 

 by beds similar to those here called Theresa, and these are overlaid 

 by nearly 400 feet of dolomites which have been heretofore classed 

 with the Beekmantown formation, as Division A of that formation. 

 No -such beds as these last appear in our district here, though the 

 Potsdam and Theresa may be equivalent to them in time. In the 

 Champlain valley also appear four other divisions of the Beekman- 

 town, with an aggregate thickness in the neighborhood of 1400 feet. 



1 By H. P. Cushing. 



