﻿GEOLOGY OF THOUSAND ISLANDS REGION 65 



ing is due to the presence of basal layers which disappear to the 

 east and west as the Theresa thickens. 



The field work in the district was completed, and this report 

 written under the impression that this comparatively thin forma- 

 tion was a unit and all of the same age. In its lower portion L i n - 

 gulepis acuminata is abundant ; above, specimens of a 

 rather large, flat-coiled gastropod occur abundantly in places ; 

 occasional cystid plates are found, and unrecognizable traces of 

 other forms. The horizon seemed the same as, and the beds 

 identical with beds which directly overlie the Potsdam sandstone 

 all across northern New York, a length of outcrop of 150 miles, 

 and which have heretofore been called " passage beds " between 

 the Potsdam and the Beekmantown, the Beekmantown being the 

 formation which overlies the Potsdam for much of this distance. 

 In the belief that no Beekmantown was present here, and yet 

 that there was here a formation which required mapping separate 

 from the Potsdam, the name Theresa was proposed for this 

 magnesian limestone formation, it being well exposed in the 

 township of that name. 1 Recent work by Ulrich, Ruedemann 

 and myself in the Mohawk valley has, however, tended to throw 

 much doubt upon the entire correctness of this position. We 

 find that the formation in the Mohawk valley known as the 

 Little Falls dolomite, and heretofore regarded as of Beekman- 

 town age, is made up of two unconformable formations, the 

 uppermost of which is of lower Beekmantown age, and is a quite 

 fossiliferous limestone which we are proposing to separate and 

 call the Tribes Hill formation; while the underlying dolomite 

 formation is of Upper Cambric (Ozarkic) age. Now the Tribes 

 Hill formation contains, as one of its fossils, a gastropod (named 

 Pleurotomaria hunterensis by Cleland) which 

 Ulrich regards as identical with the gastropod from the Theresa 

 formation ; it also contains numerous cystid plates, and these 

 he also regards as identical with those from the Theresa. The 

 Lingula, however, occurs in the Little Falls dolomite, instead of 

 the Tribes Hill formation, and is in fact a characteristic Ozarkic 

 fossil all around the Adirondack region. Ulrich's present view 

 is therefore that the Theresa formation, as here mapped, is in 

 part of Ozarkic, and in part of Tribes Hill (lower Beekmantown) 

 age. If this be true there must be an undetected unconformity 

 between the two portions. In the field the only lithologic 

 difference noted between the upper and lower portions of the 



1 Geol. Soc. Am. Bui. 19:155-76. 



