1882.] 29 [Davis. 



that glacial action, if continued long enough, would produce great 

 results. The question then becomes, have the old glaciers of the 

 Alps, for example, had time enough to bring about the results 

 claimed for them. It will be shown farther on by evidence based 

 upon the form of glaciated valleys (C. 4), that they decidedly 

 have not. 



A. Favre says that however long glaciers act, they cannot cut out lake- 

 basins, any more than sand-dunes can grow to equal the Himalaya, or than 

 mud-volcanoes can approach Chimborazo. (On the Origin of the Alpine 

 lakes and valleys; Phil. Mag., xxix, 1865, 207; Recherches Geol. 1867, 

 i, 202.) 



Ramsay says great depth of basin does not militate against his theory, 

 for depth is a "mere indicator of time and vertical pressure in a narrow 

 space." " Given sufficient time," the Great Lakes of North America might 

 thus be formed. (Geol. Soc. Journ. xvin, 1862, 199,202.) He sets no 

 limit to glacial time, but compares it to " eternity" (Phil. Mag. xxvin 

 1864, 303), although he had before stated that Alpine valleys must be 

 essentially of preglacial formation, because in .the quaternary period there 

 has not been time enough for much change. (Phil. Mag. xxiv, 1862, 378.) 



J. Ball and E. Whymper, 11. c, consider the glacial period an insuffi- 

 cient time for great results. 



A. 12. Several Glacial Periods. Now that Croll's writings 

 have popularized what had been before suggested — the recur- 

 rence of glacial periods — those who accept astronomical causes 

 as sufficient to explain our glacial invasions, look upon them as 

 rather frequent, geologically, and if lake-basins cannot have been 

 produced by the last one, they throw the burden of the work 

 upon the ice of earlier epochs. But this is going very far into 

 unproven hypotheses. 



N. S. Shaler admits that the last ice period found the surface of our country 

 pretty much as it now is, and refers the more important changes of form 

 to earlier invasions, which took place " again and again." (These Proceed- 

 ings, x, 1866, 363; xvm, 1876, 126.) 



B. Concerning the amount and distribution of glacial drift. 

 B. 1. Preglacial Conditions. The argument from the amount 



of glacial drift * is the strongest that can be made in favor of gla- 



!The drift must include all the boulder-clay or till, all the kames, and the greatest 

 part of the stratified sands and clays of glaciated regions; for the small amount of 

 postglacial rock-weathering shows that very nearly all this detritus has been in the 

 power of the ice and its accompanying streams. Allowance should be made, if it 

 were possible, for the fine rock-flour lost by washing into the sea. 



