lO NEW YORK STATE MUSEUM 



is one where the early Cambric sediments overlapped the crystal- 

 lines and have been scraped off therefrom in highly irregular fashion 

 exposing the old rock floor, but leaving the sedimentary Potsdam 

 sandstone in patches and outliers. The mapping here involves some- 

 what more cautious procedure than is required in the regions pre- 

 viously mentioned where it is known that the outcrops run in ap- 

 proximately east and west lines. The larger part of the Theresa 

 quadrangle is occupied by the Paleozoic rocks but in the northeast 

 portion the Potsdam sandstone is in the process of being stripped 

 away from the underlying crystallines, thus affording opportunity 

 for studying the topography of the ancient rock surface on which 

 the Potsdam sediments were laid down. This old sea floor was of 

 the ridge and valley type, a form derived from erosion before its 

 partial submersion beneath the sea. The ridges trend northeast 

 and have altitudes of lOo' to 200' above the valleys. The Potsdam 

 sandstone has a maximum thickness of 200' over the old valleys but 

 on the higher ridges it was never deposited at all. 



The Precambric or crystalline rocks are the Granville series, 

 recognized to be of sedimentary origin, and an amphibolite series, 

 the latter badly cut into by a granite batholite. 



The Granville sediments disclose two main groups, a thick and 

 pure limestone formation and a formation of thinner beds con- 

 sisting of sedimentary gneisses, quartzites and limestone. All are 

 repeatedly cut by dikes of igneous rocks chiefly of granite. 



The Potsdam sandstone has its usual quartzose character and is 

 overlain by a thickness of about 100' of thin bedded sandy dolomites 

 with interstratified layers of coarse, brown and mottled sandstone. 

 Both types of rock contain the characteristic brachiopod L i n - 

 g u 1 e p i s acuminata which is recognized as one of the index 

 species of the Potsdam sandstone. The upper formation is, however,, 

 clearly differentiated from the typical Potsdam below and the Beek- 

 mantown limestone above and it is therefore being mapped as a 

 separate formation. 



The later and overlying limestone occurs in the southern portion 

 of the quadrangle but the boundaries have not yet been fully 

 determined. 



Long Lake quadrangle. The survey of this area in the 

 heart of the Adirondack mountains has been completed and the 

 report and map are in course of publication. 



Valcour island. In the report of last year there was reference 

 to the progress made by Prof. G. H. Hudson in the survey of 



