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 BASINS.—(Continued.)
IV.
THERE is really only one alternative theory to that of ice erosion for the origin of the class of lakes we have been discussing, 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 theory 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 discussed 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 Pliocene period—earth movements of a nature to produce deep lakes occurred in every mountain range without exception that was
