56 NEW YORK STATE MUSEUM 



chips brought up from this stratum, from which its Upper Cam- 

 brian age was determined. 



These records seem to demonstrate the presence of deposits of 

 Upper Cambrian age in Oswego county, under cover of the newer 

 rocks and at a distance of some 20 miles from the nearest sur- 

 face pre-Cambrian outcrops to the eastward. They have not been 

 reported in surface outcrops along the pre-Cambrian boundary, 

 the overlying Silurian limestones seeming to lie directly on the 

 pre-Cambrian there. Apparently we have here direct evi- 

 dence of overlap, and also evidence of a considerable change in 

 the lithologic character of the Upper Cambrian, it being no longer 

 t3'pical Potsdam sandstone. Nor is the formation here of any 

 great thickness, as compared Avith the Potsdam sandstone of the 

 St LaAvrence and Champlain valleys. But in answer to this last, 

 it might be argued that the diminished thickness here was due 

 to overlap, and that suflflciently deep wells located some few miles 

 to the westward of the Oswego county wells might show a greatly 

 increased thickness of Upper Cambrian rocks, and with our 

 present knowledge this argument could not be gainsaid. * 



In summing up, it may be said that we have in Oswego county 

 good evidence of the presence of the Upper Cambrian horizon, 

 and that it is absent along the pre-Cambrian boundary because 

 of overlap. Along the line from Kome to Little Falls the evidence 

 is not decisive as to the pres"ence of the Upper Cambrian, nor is 

 there any special indication of thickening in the whole series of 

 deposits between the Trenton and the pre-Cambrian, as ^hould 

 be the case were the conditions those of overlap. But there is 

 the possibility that the Potsdam is represented in the Rome and 

 Utica wells, though it is only a possibility and does not enable 

 us to decide definitely whether the Upper Cambrian was ever 

 deposited anywhere within the limits of the upper Mohawk valley 

 or not. 



Beekmantown overlap. The uncertainty which exists in regard 

 to conditions hereabout in Potsdam times, ceases with the begin- 

 ning of Beekmantown deposition. The evidence of Beekmantown 

 overlap is clear and decisive. 



