96 STUDIES, SCIENTIFIC AND SOCIAL chap 



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 conse- 

 quence of the former, since it indicates that evaporation 

 either balances or exceeds the inflow. This would have 

 favoured the formation of such lakes, since it w^ould have 

 prevented 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 elevation of a platform across the 

 tract 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 w^ell-watered districts, but the earth-move- 

 ments must in these cases have been exceptionally rapid 

 and extensive, and they are accordingly found most often 

 in countries subject to volcanic disturbances. Such are the 

 lakes of Southern Italy, of Macedonia, of Asia Minor, and 

 perhaps those of Central Africa. 



Quite distinct from these are the sub-alpine lakes of 

 those mountain groups which have been subject to extreme 

 glaciation. These are characteristically valley-lakes, occurr- 

 ing in the lower portions of the valleys which have been the 

 beds of enormous glaciers, their frequency, their size, and 

 their depth bearing some relation to the form and slope of 

 the valleys and the intensity of the glaciation to which they 

 have been subject. In our own country we have in Wales 

 a small number of valley-lakes ; in the Lake District, where 

 the ice sheet can be proved to have been much thicker and 

 to have lasted longer, we have more numerous, larger, and 

 deeper lakes ; and in Scotland, still more severely glaciated, 

 the lakes are yet more numerous, many of those in the west 

 opening out to the sea and forming the lochs and sounds 

 of the Western Highlands. Coming to Switzerland which, 

 as we have seen, bears indications of glaciation on a most 



