166 H. p. GUSHING PALEOZOIC IN NORTHWESTERN NEW YORK 



From tlieir areal distribution it is clear that these two formations rep- 

 resent the thinned, shoreward edges of deposits in a basin which chiefly 

 lay to the eastn^ard and which reached the immediate district only at the 

 time of its greatest westward extent. This appears to be the limit of 

 Upper Cambrian subsidence in this direction: a considerable land area to 

 the westward which received no Potsdam and Beekmantown deposits 

 must be postulated, and the fact that true Beekmantown deposits never 

 even reached the district would indicate that the subsidence here per- 

 sisted but a short time and was followed by low uplift, causing the shore- 

 line to work back eastward. This tendency to a contraction of the em- 

 bayment, or at least to a shifting of its shores away from the immediate 

 region, seems to have persisted through all of Beekmantown and much 

 of Chazy time, limiting the deposit of these formations to the east and 

 north sides of the Adirondack region and giving rise to a low land area 

 in the northwest district which underwent some erosion. Eenewed sedi- 

 mentation here was inaugurated by a depression which came in from the 

 southwest, changing the former eastern slope of the surface to a westerly 

 inclination, so that what had been the westerly shoreline of the Potsdam- 

 Theresa bay became the easterly shoreline of this new invasion, the two 

 overlapping slightly, so that we find here the thinned shoreward edges of 

 both sets of formations, one above the other. The Pamelia formation 

 was deposited in this depression, followed by the Lowville, Black Eiver, 

 Trenton, and Utica. Meanwhile the sea was rapidly extending itself 

 over the whole northern Xew York region; so that, beginning with the 

 Black Eiver, we find connecting seas and similar deposits and faunas on 

 all four sides — the first time in its geologic history that this had been 

 the case. 



The Canadian Section 



Across the river in Canada more erosion has taken place than on the 

 ISTew York side, because of which the pre-Cambrian isthmus which con- 

 nects the Adirondack mass with tbe Canadian protaxis is broader, and 

 the deposits of the earlier eastern and later western basins are not found 

 in Juxtaposition as they are in jSTew York. On the east side of the 

 isthmus the Potsdam sandstone is fomid resting on the crystalline rocks, 

 beyond which follow in succession the normal deposits of the eastern 

 basin — Beekmantown, Chazy. Lowville, etcetera — as one passes north- 

 eastward. On the west side of the isthmus, around Kingston, is a basal 

 sandstone with calcareous cement, the Eideau sandstone of Ami, above 

 which is a small thickness of impure, reddish limestones, followed by the 



I 



