622 G. M. DAWSON ON THE SUPERFICIAL GEOLOGY OF 



have taken directions caused by surface- currents from the south and 

 west, or even been impelled by the prevailing winds. Some of the 

 Laurentian debris, as we have seen, reached almost to the mountains, 

 while some of the quartzite drift can be distinguished far out to- 

 wards the Laurentian axis. 



The occurrence of Laurentian fragments at a stage in the sub- 

 sidence when, making every allowance for subsequent degradation, 

 the Laurentian axis must have been far below water, would tend 

 to show that the weight and mass of the ice-cap was such as to en- 

 able it to remain as a glacier till submergence was very deep. 



The emergence of the land would seem to have been more rapid ; 

 or at least I do not find any phenomena requiring long action at 

 this period. The water in retreat must have rearranged to some 

 extent a part of the surface-materials. The quartzite drift of the 

 third steppe was probably more uniformly spread at this time, and 

 a part of the surface- sculpture of the drift-deposits of the second 

 plateau may have been produced. It seems certain, however, that 

 the Rocky Mountains still held comparatively small glaciers, and 

 that the Laurentian region on its emergence was again clad to some 

 extent with ice, for at least a short time. The closing episode of the 

 Glacial period in this region was the formation of the great fresh- 

 water lake of the Red-River valley, or first prairie-level (which 

 was only gradually drained), and the reexcavation of the river- 

 courses. 



It must not be concealed that there are difficulties yet unaccounted 

 for by the theory of the glaciation and deposit of drift on the plains 

 by icebergs ; and chief among these is the absence, wherever I have 

 examined the deposits and elsewhere over the West, of the remains 

 of marine Mollusca or other forms of marine life. With a submer- 

 gence as great as that necessitated by the facts it is impossible to 

 explain the exclusion of the sea ; for, besides the evidence of the 

 higher western plains and Rocky Mountains, there are terraces be- 

 tween the Lake of the Woods and Lake Superior nearly to the 

 summit of the Laurentian axis, and corresponding beach-marks on 

 the face of the northern part of the second prairie escarpment. 



Mr. Belt, in an interesting paper (Quart. Journ. Geol. Soc. Nov.1874), 

 deals with similar difficulties in explaining the glaciation of Siberia. 

 The northern part of Asia appears in many ways to resemble that 

 of America ; surrounded by mountain -chains on all sides save the 

 north, it is a sort of interior continental basin covered with " vast 

 level sheets of sand and loam." As in the interior regions of Ame- 

 rica, marine shells are absent, or are only found along the low 

 ground of the northern coast. To account for these facts, Mr. Belt 

 resorts to a theory first suggested by him eight years ago, by which 

 he supposes the existence of a polar ice- sheet capable of blocking up 

 the entire northern front of the country, and damming back its 

 waters to form an immense freshwater lake. The outfall of this lake, 

 during its highest stage, he supposes to have been through the de- 

 pression between the southern termination of the Ourals and the 

 western end of the Altai to the Aral and Caspian Seas. 



