244 ^'^^ POPULAR SCIENCE MONTHLY. 



educators for missionary work in this line. This work our dairy 

 schools, agricultural experiment stations, dairymen's associations, 

 and similar organizations are doing, and American dairying is 

 rapidly progressing toward a higher standard through their 

 agencies. 



THE ICE AGE AND ITS WORK. 



By ALFRED R. WALLACE, F. R. S. 



EROSION OF LAKE BAS>m^. {Continued.) 



IV. 



THERE is really only one alternative theory to that of ice ero- 

 sion for the origin of the class of lakes we have been dis- 

 cussing, viz., that they were formed before the Glacial epoch, by 

 earth movements of the same nature as those which are concerned 

 in mountain formation, that is, by lateral pressure causing folds 

 or flexures of the surface ; and where such flexures occurred 

 across a valley a lake would be the result. This is Prof. Bonney's 

 theory given in his paper in the Geographical Journal, and it is 

 also that of Desor, Forel, Favre, and other eminent geologists. It 

 is explained fully in the work of M. Falsan (already quoted), who 

 also adopts it ; and it may be considered, therefore, that if this the- 

 ory can be shown to be untenable that of glacial erosion will hold 

 the field, since there is no other that can seriously compete with it. 

 Prof. Bonney considers this theory completely satisfactory, and 

 he complains that the advocates of glacial erosion have never dis- 

 cussed it, intimating that they " deemed silence on this topic more 

 prudent than speech." 



As this theory is put forward with so much confidence, and by 

 geologists of such high reputation, I feel bound to devote some 

 space to its consideration, and shall, I think, be able to show that 

 it breaks down on close examination. 



In the first place, it does not attempt to explain that wonderful 

 absence of valley lakes from all the mountain regions of the 

 world, except those which have been highly glaciated. It is, no 

 doubt, true that during the time the lakes were filled with ice 

 instead of water, they would be preserved from filling up by the 

 influx of sediment ; and this may be fairly claimed as a reason 

 why lakes of this class should be somewhat more numerous in 

 glaciated regions, but it does not in any way explain their total 

 absence elsewhere. We are asked to believe that in the period 

 immediately preceding the Glacial epoch say, in the Newer Plio- 

 cene period earth movements of a nature to produce deep lakes 

 occurred in every mountain range without exception that was 



