﻿GEOLOGY OF THOUSAND ISLANDS REGION 79 



represented in the Champlain area, Ulrich correlating the dove, 

 reef limestone, only a few feet in thickness, which forms the 

 basal member of the upper Chazy there, with the Pamelia hori- 

 zon. In the much more complete sections about Chambersburg, 

 Pa., a 200 foot thickness of limestone with an upper Chazy fauna, 

 separates the Pamelia horizon from the Lowville. Subsidence 

 apparently ceased in the Champlain basin during the time of 

 Pamelia depression and deposit in this district, and as this ceased 

 here, upper Chazy depression was renewed there, the uncon- 

 formity between the Pamelia and Lowville representing this 

 upper Chazy interval. Knowledge of this led Ulrich to predict 

 the unconformity and induced the search for it. Otherwise it 

 might easily have escaped our notice. 



Mohawkian series 1 



The Mohawkian series comprises the Black River and Trenton 

 groups. The Black River group is composed of the Lowville beds 

 including the Leray limestone, and the Watertown limestone. In 

 giving to the Black River group this larger scope, we return to' the 

 original conception of several of the geologists of the First Geo- 

 logical Survey of New York, i. e. Hall, Vanuxem and Mather, with 

 the exception that the Black River then also included the Chazy 

 limestone. Emmons, however, to whose district the Black River 

 region belonged, did not use the term " Black River." He dis- 

 tinguished the " Birdseye limestone " and the " Isle La Motte 

 marble " employing the latter term 1 for a bed locally the main 

 object of the quarrying industry, and known as the " Seven foot 

 tier." Hall, in the first volume of the Palaeontology of New York, 

 restricted the term Black River to this " Seven foot tier " and 

 through his influence and the description of a very striking 

 cephalopod fauna from the bed, the term " Black River " was 

 quite generally accepted for the "Seven foot tier." Since, how- 

 ever, mainly through the investigations of Dr Ulrich, the fact has 

 become apparent that beds which in the Mohawk valley and the 

 Lake Champlain region have been referred to the Black River 

 limestone, are both older and younger than the Black River as re- 

 stricted by Hall, but fall within the limits of the original concep- 

 tion of Black River, it has become advisable to revive this original 

 usage of the term to avoid confusion. The " Seven foot tier " or 

 Black River limestone of Hall has then to be renamed and the term 

 "Watertown" is here used for this formation [see p. 84], 



1 By R. Ruedemann. 



