﻿GEOLOGY OF THOUSAND ISLANDS REGION 117 



the limestone folds in this district, as well as others in shales in 

 western New York and Ohio, demonstrated that they were super- 

 ficial and postglacial, and attributed them to " horizontal expansion 

 of superficial strata, consequent on postglacial amelioration of cli- 

 mate." 1 The writer does not question the correctness of this ex- 

 planation as applied to the folds in shales and shaly rocks, which 

 Gilbert describes, but is not so sure as to its adequacy in the case of 

 quite massive, rigid limestones such as the Lowville, and is especially 

 doubtful of it as applied to a well cemented, massive sandstone like 

 the Potsdam, which is an exceedingly rigid and resistant rock. Post- 

 glacial climate is no warmer than was preglacial climate. Unless 

 therefore the weight of the overlying ice was sufficient to cause some 

 lateral spreading of the rocks, at the same time that it was producing 

 contraction in them by lowering of their temperature, postglacial 

 warming would merely reexpand them to their preglacial condition. 

 There is no question as to the competency of the ice weight to pro- 

 duce lateral spread in shales and shaly rocks. Many shales are 

 known to spread and to give rise to buckles under much smaller 

 pressures, hence the cause suggested by Gilbert would seem ample 

 to account for the results. But the pressure necessary to produce 

 spread in a massive, rigid limestone is quite another matter, and that 

 required in the case of such a rock as the Potsdam sandstone is of 

 a still higher order. The weight of an ice sheet I mile thick would 

 be equal to that of from 1700 to 1800 feet of average sedimentary 

 rock. We do not know the thickness which the ice attained over this 

 region but even the supposition that it was much more than a mile 

 thick does not greatly enhance our figures of rock thickness. Are 

 such pressures, even if applied continuously for a long time, suffi- 

 cient to bring about lateral spreading in such a rock as the Potsdam ? 

 So far as known to the writer there are no direct, positive data 

 which warrant a definite answer to this question. It is certain, how- 

 ever, that at such depths below the surface such rocks are abundantly 

 fissured, are often porous, and permit free passage of fluids. This 

 certainly suggests that they are not under sufficient weight to close 

 up cracks. 



If. however, this pressure due to the ice load could be rein- 

 forced by pressure from some other source in sufficient amount, 

 the necessary lateral spreading could be brought about. A 



1 Gilbert, G. K. Am. Ass'n Adv. Sci. Proc. 35 -.227 ; 40 :24Q. 



Am. Jour. Sol. ser. 3, 32:324. 



The writer is under great obligations to Dr G. K. Gilbert, J. C.Branner 

 and H. F. Reid for references to the literature and for personal discussion 

 of these folds. 



