262 GENERALIZATIONS. 



rich the Limmat breaks through a moraine, and the lakes of 

 Iseo and Garda are kept up by a barrier of ancient glacier- 

 rubbish. Examples of moraine-lakes are shown at Sempach 

 and Baldegg, as well as in the small lakes of Pusiano, Annone, 

 and Alserio in the Brianza, since their outflow. is hemmed in by 

 great masses of glacier-rubbish, which probably reach down to 

 the old river-bed and have stopped it up. 



Drift-glaciers have aided in the preservation of Swiss lakes ; 

 and they have also materially assisted in doing away with the 

 inequalities of the land, since innumerable depressions have 

 been filled up by the enormous masses of rubbish which they 

 carried with them. By the melting-away of the glaciers vast 

 quantities of water must have been produced, which have borne 

 down sand and stones to a distance and have deepened the beds 

 of rivers and brooks, and thus have taken an important part in 

 changing the configuration of the land. During the drift-period 

 a much greater amount of aqueous precipitation probably took 

 place, and the climate was not only colder, but also more moist 

 than at present ; and thus an explanation may be given of the 

 immense accumulation of ice at that epoch. Swiss mountains 

 in those ages must have been worn and weathered on a grand 

 scale, the evidence of which remains in the enormous mass of 

 stones which have fallen down from them and been distributed 

 all over the lowlands. 



During the drift-period, as in our own days, the congelation 

 of water was one of the principal agents in the disintegration of 

 rocks. Water being absorbed by all the fissures of the rocks, 

 and expanding by frost, the rocks split up or the fissures become 

 enlarged ; and the effect will be the greater the more frequently 

 this succession of freezing and thawing takes place. Prof. Heer 

 is convinced that in the High Alps of Switzerland, as well as in 

 the extreme north, this phenomenon has a much more decided 

 action than erosion. 



Section 3. The Climates of the various Geological periods . 



The plants and animals have shown us that the climate of the 

 primaeval world was very different from that in which we live, 

 and that it was subjected to numerous changes. Thus, with re- 



