Page:Popular Science Monthly Volume 45.djvu/53

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THE ICE AGE AND ITS WORK. 41

fully as tlie space at my command will allow, the general evidence in favor of the ice origin of certain classes of lakes, and the special conditions requisite for the production of lakes by this agency. The objections of the best authorities will then be considered and replied to, and the extreme difficulties of the alternative theories will be pointed out. I shall then describe certain peculiarities, hitherto unnoticed, which clearly point to erosion, as opposed to any form of subsidence and upheaval, in the formation of the lakes in question. Lastly, the special case of the Lake of Geneva will be discussed, as affording a battle ground that will be ad- mitted to be highly favorable to the anti-glacialists, since most of them have adduced it as being entirely beyond the powers of the ancient glaciers to have produced.

The Different Kinds of Lakes and their Distribution. To clear the ground at the outset, it may be well to state that the great plateau lakes of various parts of the world have no doubt been formed by some kind of earth movements occurring subsequent to the upheaval and partial denudation of the country. It is uni- versally admitted that existing lakes can not be very ancient, geologically speaking, since they would inevitably be filled up by the sediment carried into them by the streams and by the wind. Our lakes must, therefore, be quite modern features of the earth's surface. A considerable proportion of these plateau lakes are in regions of little rainfall, and many of them have no outlet. The latter circumstance is a consequence of the former, since it indi- cates that evaporation balances the inflow. This would have favored the formation of such lakes, since it would have pre- vented the overflow of the water from the slight hollow first formed, and the cutting of an outlet gorge which would empty the incipient lake. Captain Dutton, in his account of the geology of the Grand Canon district, lays stress on this fact, " that the eleva- tion of a platform across the track of a river rarely diverts it from its course, for the stream saws its bed into the rocks as fast as the obstacle rises." Scanty rainfall and great evaporation seem there- fore to be almost essential to the formation of the larger plateau lakes. Rarely, such lakes may have been formed in comparatively well-watered districts, but the earth movements must in these cases have been exceptionally rapid and extensive, and they are accordingly found most often in countries subject to volcanic dis- turbances. Such are the lakes of southern Italy, of Macedonia, of Asia Minor, and perhaps those of Central Africa.

Quite distinct from these are the subalpine lakes of those mountain groups which have been subject to extreme glaciation. These are characteristically valley lakes, occurring in the lower portions of the valleys which have been the beds of enormous glaciers, their frequency, their size, and their depth bearing some

VOL. XLV. 4

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