SOME UNSOLVED PROBLEMS IN GEOLOGY. 71 



east was wafted southwestward by the Arctic currents, while lighter 

 ice from the Rocky Mountains was being borne eastward from these 

 mountains by the prevailing westerly winds. We thus have in the 

 West, on a very wide scale, the same phenomena of varying submer- 

 gence, cold currents, great ice-floes, and local glaciers producing ice- 

 bergs, to which I have attributed the bowlder clay and upper bowl- 

 der drift of Eastern Canada. 



A few subsidiary points I may be pardoned for mentioning here. 

 The rival theories of the glacial period are often characterized as those 

 of laud glaciation and sea-borne icebergs. But it must be remem- 

 bered that those who reject the idea of a continental glacier hold to 

 the existence of local glaciers on the highlands more or less exten- 

 sive during different portions of the great pleistocene submergence. 

 They also believe in the extension of these glaciers seaward and 

 partly water-borne, in the manner so well explained by Mattieu Will- 

 iams ; in the existence of those vast floes and fields of current-and 

 tide-borne ice whose powers of transport and erosion we now know to 

 be 80 great ; and in a great submergence and re-elevation of the land, 

 bringing all parts of it and all elevations up to five thousand feet suc- 

 cessively under the influence of these various agencies, along with those 

 of the ocean-currents. They also hold that, at the beginning of the 

 glacial submergence, the land was deeply covered by decomposed 

 rock, similar to that which still exists on the hills of the Southern 

 States, and which, as Dr. Hunt has shown, would afford not only 

 earthy debris, but large quantities of bowlders ready for transporta- 

 tion by ice. 



I would also remark that there has been the greatest possible ex- 

 aggeration as to the erosive action of land-ice. In 1865, after a visit 

 to the Alpine glaciers, I maintained that in these mountains glaciers 

 are relatively protective rather than erosive agencies, and that the 

 detritus which the glacier streams deliver is derived mostly from the 

 atmospherically wasted peaks and cliffs that project above them. 

 Since that time many other observers have maintained like views, and 

 very recently Mr. Davis, of Cambridge, and Mr. A. Irving have ably 

 treated this subject.* Smoothing and striation of rocks are undoubt- 

 edly important effects, both of land-glaciers and heavy sea-borne ice ; 

 but the leveling and filling agency of these is much greater than the 

 erosive. As a matter of fact, as Newberry, Hunt, Belt, Spencer, and 

 others have shown, the glacial age has dammed up vast numbers of 

 old channels which it has been left for modern streams partially to 

 excavate. 



The till, or bowlder clay, has been called a " ground moraine," 

 but there are really no Alpine moraines at all corresponding to it. On 

 the other hand, it is more or less stratified, often rests on soft materi- 



* " Proceedings of the Boston Society of Natural llistory," xxii ; " Journal of the 

 Geological Society of London," February, 1883. 



