﻿66 NEW YORK STATE MUSEUM 



formation was the absence above of the sandstone beds which 

 are interstratified with the limestones in the lower division. 

 Otherwise the formation constitutes an apparent lithologic unit 

 and appears as such on the maps; and it seems better to leave 

 it as such instead of attempting to subdivide it at this juncture. 

 If, however, it does consist of these two separate formations the 

 necessity for the name largely disappears, and it is rather a pity 

 that it was ever suggested. It is likely, however, to prove use- 

 ful as a name for the considerable thickness of alternating beds 

 of sandstone and magnesian limestone which everywhere im- 

 mediately overlie the Potsdam sandstone in northern New York, 

 and which should be mapped separately. 



There is then some reason to believe that there is present in 

 this district a thickness of from 20 to 30 feet of limestone of 

 lower Beekmantown age, quite like a similar thickness of rock 

 at the summit of the Little Falls dolomite at Little Falls (where 

 an unquestioned unconformity exists between the two), and hold- 

 ing the same fauna. This is to be separated from the Little 

 Falls dolomite under the name of the Tribes Hill limestone, and 

 the same separation needs to be made in this district. The 

 Theresa formation is to be restricted to the alternations of sand- 

 stone and magnesian limestone which constitute the lower half 

 of the formation as mapped. 



Age of the Potsdam and Theresa formations. These two 

 formations, with a maximum thickness in this district of from 125 

 to 150 feet only, represent the attenuated western edge of forma- 

 tions which, in the Champlain valley, have tenfold that thickness. 

 Their distribution shows that they were deposited in a subsid- 

 ing trough along the present St Lawrence valley line, and that 

 their deposit commenced at the east and worked westward. 

 Everywhere along this line we find a sandstone beneath, grading 

 upward into an overlying dolomite, and everywhere the horizon 

 is characterized by the presence of the fossil L i n g u 1 e p i s 

 acuminata. Everywhere along this line too there seems to 

 be a break between these formations and the next formation 

 above. The two formations seem then to be indissolubly bound 

 together, to rest unconformably on the Precambric, and to be 

 separated by an unconformity from the overlying formation. 

 Since the formations are thin in the immediate district, and are 

 thinning to the west and south, it follows that we are here in the 

 vicinity of the western edge of the subsiding trough. Just how 

 far west its deposits extended can not be told. According to 



