56 NEW YORK STATE MUSEUM 



the initial region, up the Champlain and the St Lawrence troughs. 

 And these upper beds 'of the formation are marine in origin, as 

 their fossils show. In the Ogdensburg region only a thin, upper 

 portion of the formation was deposited, its thickness barely equal- 

 ling the irregularity of the floor on which it was laid down, so 

 that is does not appear at all on the higher parts of the old, Pre- 

 cambrian surface. 



The passage to the Theresa was a gradual one, the sand supply 

 lessening, and calcareous matter increasing. The fauna remained 

 substantially the same. 



The Theresa seems to graduate upward into the Heuvelton with- 

 out any break; at least we have, so far, failed to detect one. But 

 there is a prominent change in the fauna, which Ulrich compares to 

 that of the uppermost portion of the Little Falls dolomite of the 

 Mohawk valley. We are still in doubt as to just what the history 

 was at this stage. The fauna is a comparatively unknown one in 

 New York, and there is nothing in the St Lawrence region which 

 can be directly correlated with the thick Little Falls dolomite of 

 the Mohawk and Champlain valleys, which there follows the 

 Theresa without a break. Whether the Heuvelton is a thinned 

 representative of the Little Falls, of quite different lithologic 

 character because of deposit in a separate basin, whether it is 

 younger than most or all of the Little Falls and represents a deposit 

 of an age hitherto unknown in the New York section with a break 

 between it and the Theresa representing Little Falls time, or 

 whether there is no break, and the Potsdam and Theresa of this 

 region were being deposited at the same time that the dolomite was 

 forming in the Mohawk region, can not yet be told. One or the 

 other of the three no doubt represents the true condition of affairs. 



ORDOVICIAN TIME 



There is a distinct break between the Heuvelton and the overlying 

 Tribes Hill formation in the Ogdensburg region, and such a break 

 seems everywhere to mark the close of the Cambrian in New York. 

 We are therefore justified in concluding that marine waters were 

 completely withdrawn from the region for a time. Slight warping 

 of the surface took place and a very moderate amount of wear 

 occurred, so moderate as to indicate that the land was of low 

 altitude. Then the sea returned and Tribes Hill deposition com- 

 menced. This formation runs all through the Mohawk valley with 

 considerable prominence. It is absent in the Black river valley, but 

 reappears as a thin wedge in the Theresa region, is absent on Brier 



