314 LECTURE XI. 



belong to living species^ must therefore be separated from other 

 diluvial formations^ in so far as it contains along with extinct, 

 many of the existing species, and the plants, at all events, 

 are of the same kind as are found in the later deposits above 

 the glacial loam. The spread of the glaciers, therefore, does 

 not, as hitherto believed, mark a new epoch, a new section in 

 the history of the earth ; it only changed temporarily the as- 

 pect of the earth, of the Fauna and Flora, where it occurred. 

 After the retirement of the glaciers and the Arctic sea to its 

 present northern confines, the antecedent condition was re- 

 established ; the Fauna and Flora returned to their native 

 district, excepting some species which became extinct. We 

 are, however, far from asserting that no new species arose 

 after the retirement of the glaciers. Desor has with great 

 acumen shown the fallacy of such an assumption, and if we 

 accept the theory of the transformation of sjDecies, there is no 

 reason why this metamorphosis should not proceed as well in 

 the present as it did in periods gone by. 



Let us now examine the various deposits, which have occurred 

 since the glacial period, and let us commence with Switzerland, 

 where the comparison with the process still in action in the 

 Alpine chain gives a clue to the origin of the above phenomena. 

 The glacial loom is, in that country, a more or less grey or 

 bluish clay, without any trace of stratification, and in which, 

 near the Alps and almost on the whole plains of Switzerland, 

 are found pohshed and furrowed stone boulders. This forma- 

 tion is obviously connected with the large angular erratic blocks 

 on the slopes of the Jura, which, on the Chasseron, in Waad- 

 tand, rise to 1,600 meters above the level of the sea, or 1,000 

 meters above that of the lakes. It is now generally admitted 

 that these blocks have been deposited by the glaciers, which 

 have spread over almost the whole plain of Switzerland, and Swiss 

 geologists have succeeded in nearly determining the limits of 

 these old glaciers which reached far up the Jura. I must refer 

 you to the map of Escher von der Linth, which you will find in 

 my text-book as well as in my Principles of Geology, and in 

 which the limits, which these glaciers reached at the time of 

 their greatest expansion, are marked. 



