128 W. B. Dwight — Potsdam strata near Poughke&psie 



meadows of Casper Creek. Immediately north of the Driving 

 Park, in fields on the north side of Hooker avenue (locality O y 

 plate VI), in place of limestone there are hills of Hudson River 

 shale, and this same rock crops out along Hooker avenue to 

 the west all the way to Casper Creek and beyond. From this 

 line these shales continue unbroken many miles to the north 

 beyond Hyde Park. Along this northeasterly and southwest- 

 erly line, the intervening space between the outcrops of shale 

 on the north and of limestones on the south consists of a mostly 

 level strip of drift having bogs and springs along its southern 

 edge. Evidently this is a line of fault, across the strike,, 

 between the Hudson River shales on the north and the Pots- 

 dam, with its associated Calciferous and Trenton, on the south. 

 This trough, cut through the hilly ridges, is a conspicuous 

 feature in the local landscape. It is made available for traffic by 

 the avenue that passes through it obliquely into the city; it 

 furnishes the long and smooth plat (mostly underlain by the 

 shales) which constitutes the Driving Park. 



The stratigraphic relations of the western margin of the belt 

 of Potsdam have proved far less obvious than those of its 

 northern extremity. But after a careful detailed inspection of 

 the ground, a correct solution, as I believe, has at last been 

 reached. During the earlier examinations, Calciferous or 

 Trenton strata were naturally looked for, in intervening folds, 

 between the Potsdam and the Hudson River shales to the 

 west. The western hill of conglomerate above mentioned 

 '(Hill B) and its southern extensions, were at first taken to be 

 the Trenton coralline conglomerate which occurs abundantly 

 in the vicinity. But on further investigation, two facts were 

 developed which opposed this supposition. The first fact was 

 that the lithological constitution differed entirely from that of 

 the Trenton wholly-calcareous conglomerate, in being highly 

 arenaceous, as also in the entire lack' of the microscopic corals 

 which abound in the Trenton. On the other hand, its litho- 

 logical characters are quite in harmony with those of the 

 adjoining ledges known to be Potsdam. 



In the second place, an outcrop of the Hudson River shales 

 was discovered close to the western base of this hill, within a 

 few feet, thus marking the line as the actual western limit of 

 the limestone. This limit is at the most but 300 feet west of 

 the fossiliferous Potsdam strata, a space by no means sufficient 

 to allow the presence of Calciferous and Trenton strata of the 

 usual thickness exhibited in the region. A further study of 

 all the phenomena makes it quite certain that the Potsdam 

 extends to the western margin of this belt of limestone, in its 

 entire southern extension through the district examined. This 

 limestone margin (as traced from the north) is broken by three 



