396 THE JOURNAL OF GEOLOGY. 



The manner in which the excavation was formed has been 

 explained principally in two ways. One hypothesis is that it 

 owes its origin to a time of subaerial denudation preceding the 

 Glacial epoch, during which a valley, or series of valleys, was 

 worn out by stream erosion; and that the depression thus pro- 

 duced has been but slightly modified by ice action. The closing 

 of the ancient valley has been referred to orographic movements 

 and to the filling of its outlet by glacial debris. Another 

 hypothesis is to the effect that the excavation is mainly due to 

 ice erosion during the Glacial epoch, without special reference to 

 previous topographic relief. A warping of the earth's crust so as 

 to produce a true orographic basin does not seem to require con- 

 sideration, for the same reason as already stated, that the rocks 

 in which the basin lies have been but little disturbed from their 

 original horizontal position. Future study of the region must de- 

 termine which of the two hypotheses outlined above best suits the 

 facts; or if each hyphothesis has something in its favor, what 

 combination of the two may be accepted as the final explanation. 



It is a suggestive fact in connection with the first of these 

 hypotheses, that the youngest rocks in the region antedating the 

 Pleistocene belong to the Carboniferous. This seems to show 

 that the land has not been submerged since at least the close of 

 the Paleozoic. If not a region of sedimentation during this vast 

 interval, it must have been subjected to erosion. The erosion of 

 an ancient land surface might result in the production of topo- 

 graphic forms of diverse character, depending on its altitude, on 

 the length of time it was exposed to atmospheric agencies during 

 various stages of elevation, and on climatic and other conditions. 

 The study of topographic forms is now sufficiently advanced to 

 enable one to predict somewhat definitely what features would 

 appear under certain conditions. We also know the char- 

 acteristics of topographic forms due to glacial erosion. It seems 

 evident, therefore, that a knowledge of the hard -rock topography 

 in the Laurentian basin, would enable one to draw definite con- 

 clusions in reference to the part that ice and water each had in 

 shaping the forms now. found there. 



