﻿GEOLOGY OF THOUSAND ISLANDS REGION 6j 



Ells the Theresa formation outcrops on Howe island, and on the 

 Canadian mainland to a point midway between Gananoque and 

 Kingston. 3 In the district about Kingston, as seen by us in 

 1908 under Dr Ami's guidance, the Potsdam is certainly present, 

 though no Theresa was seen. The Potsdam is in patchy distri- 

 bution, in depressions of the old Precambric surface, and is still 

 thinner than it is at Clayton. The Theresa may never have 

 been deposited here, or it may have been thinly laid down and 

 then eroded, prior to Pamelia deposition. But it certainly seems 

 as if, here at Kingston, we are very near the westerly end of this 

 old, St Lawrence, Upper Cambric trough. 



On the basis of its fauna and position the Potsdam sandstone 

 of northern New York was classified as of Upper Cambric age by 

 Walcott, and in this he has been followed by practically all 

 geologists. One can start on the formation at Lake Champlain 

 and follow it without a break to Clayton, as a single continuous 

 sandstone formation. Unquestionably its deposition commenced 

 first at the east, and gradually extended westward; unquestion- 

 ably the basal portion in the western sections is younger than the 

 base in the east. But, so far as known to us, there is not a scrap 

 of evidence to show that the deposition of sand had ceased, and 

 that of dolomite begun on the east, before sand deposit had 

 even commenced on the west. And even were this true, as is 

 quite possible, there is certainly no evidence of such considerable 

 age difference between the eastern and western ends of the 

 formation, as to warrant their classification in two entirely dif- 

 ferent geologic periods, the one end Cambric, the other Ordovicic, 

 as has been recently done by Professor Grabau, who classes the 

 Potsdam here as of Beekmantown age. 2 That seems to us a 

 stretching of facts to fit theory that is certainly not permissible. 

 It is quite possibly true that the sandstone deposition slowly 

 worked its way westward by progressive overlap, as the trough 

 continued to subside ; but the evidence seems to us to indicate 

 clearly that the length of time consumed in the process is far less 

 than Grabau would have us believe. We have now gathered 

 evidence from many points in New York indicating that every- 

 where the Beekmantown formation is unconformable on what 

 lies beneath. Detailed study of section after section has shown 

 the presence of the unconformity in every case; and though the 



1 Roy. Soc. Can. Trans., ser. 2, v. 9, § 4, p. 97-108. 

 2 Science, n. s. 29 1356-58. 



