100 STUDIES. SCIENTIFIC AND SOCIAL cttAf. 



Pyrenees were far less severely glaciated than the Alps ; 

 they consequently possess no large valley-lakes, but 

 numerous small high lakes and tarns. As we go eastward 

 in the Alps, the diminished rain and snow-fall led to less 

 severe glaciation, and we find the valley-lakes diminish in 

 size and numbers till far east we have only tarns. The 

 Carpathians have no valley-lakes, but many tarns. The 

 Caucasus has no lakes, and very few tarns, and this may 

 be partly due to the steepness of the valleys, a feature 

 which is, as we shall see, unfavourable to lake formation. 

 In the South Island of New Zealand the lakes are small 

 in the north, but increase in size and number as we go 

 south, where the glaciation was more intense. These 

 numerous facts, derived from a survey of the chief moun- 

 tains of the world, are amply sufficient to show that there 

 must be some causal connection between glaciation and 

 these special types of lakes. What the connection is we 

 shall inquire later on. 



2. The Conditions that favour the Prcduclion of Lakes by 

 lee-erosion. — Those who oppose the production of lake- 

 basins by ice-erosion often argue as if the size of the 

 glacier was the only factor, and urge that, because there 

 are no lake-basins in one valley where large glaciers have 

 been at work, those which exist in another valley where 

 the glaciers were no larger, could not have been produced 

 by them. But this by no means follows, because the pro- 

 duction of a lake-basin depends on a combination of 

 favourable conditions. In the first place it is evident that 

 ice-erosion to some extent must have taken place along 

 the whole length of the glacier's course, and that in many 

 cases the result might be simply to deepen the valley all 

 along, not quite equally, perhaps, but with no such ex- 

 treme differences as to produce a lake-basin. This would 

 especially be the case if a valley had a considerable down- 

 ward slope, and was not very unccpial in width or in the 

 nature of the rocks forming its fioor. The first essential 

 to lake-erosion is, therefore, a differential action, caused 

 locally either by increased thickness of the ice, a more 

 open and level valley-floor, or a more easily eroded rock, 

 or by any combination of these. • 



