542 NEW YORK STATE MUSEUM 



dicate the lines along which future investigation should be car- 

 ried on. It may be stated that a good beginning has been made 

 toward the acquirement of a more precise knowledge of the 

 Potsdam sandstone. The results are arranged in sequence under 

 the numbers of the questions on p. 530 and p. 531 at the begin- 

 ning of the report. 



1 The base of the Potsdam has been seen to rest on the ir- 

 regular surface of the pre-Cambric rocks, with a nonconformable 

 contact, the nature of the contact being such as to indicate a 

 period of long continued erosion. It is of interest to note that 

 in some cases the materials composing the basal conglomerate 

 have been transported from considerable distances and contain 

 no fragments of the subjacent rock of the immediate vicinity. 

 Such an instance is afforded by our basal sandstone, 150-E2, west 

 of Keeseville, a good description of which has been already 

 published by Cushing. 



No formations of Cambric age have been found below the Pots- 

 dam in the area covered by this report. Certain hypotheses sug- 

 gested by megascopic examination of the pebbles contained in 

 many layers of the Potsdam can not be discussed till careful 

 microscopic examinations have been made. 



2 A possible indication of Ordovician relationship may be 

 afforded by the presence of the genus Ophileta with Platyceras 

 in the upper part of the Kent Falls section. More extensive 

 collections should be made at this point. 



In the area under discussion the transition beds between the 

 Potsdam and Beekmantown are absent, both by cutting out 

 along fault lines and by glacial erosion, drift-filled valleys ex- 

 tending parallel to the strike of the formation at those horizons 

 where should be found the transition beds. Farther north in 

 the towns of Champlain and Chazy where the strike is at right 

 angles to the direction of movement of the continental glacier 

 these softer beds of the uppermost Potsdam are exposed to 

 view. They prove to change by slow gradations from sand- 

 stones with thin intercalated dolomites, through sandstones 

 with thicker dolomites, finally merging into dolomites with 



