364 The Glacial Theory of Prof. Agassiz. • 



the bursting of glacier lakes, occasionally formed in the upper parts of 

 valleys by barriers of ice. Hence the origin of a second portion of 

 the existing alluvial cover. 



The deposits of clay and gravel spread over the great Swiss valley, 

 must be due to floods arising from both the causes just mentioned. 

 These floods, Agassiz thinks, must have had a depth of not less than 

 300 feet, for the sand and fine gravel found on the higher parts of Jura 

 have been washed off from the lower to this height. Masses of ice, 

 forming icebergs, would occasionally float in them, and carry boulders 

 from one place to another. 



Sheets of ice occupied the lakes of Geneva, Neuchatel, and others, 

 at this time, and prevented them from being filled up by the dispersion 

 of the alluvial matter. 



The clay containing the bones of fossil elephants on the sides of the 

 Alps, he considers as contemporaneous with the deposits entombing 

 similar remains on the northern shores of Siberia, and he infers that 

 one and the same catastrophe had enveloped these districts, and all 

 the northern parts of both continents, in ice. The catastrophe had 

 arrived suddenly ; for, as Cuvier remarks, the Siberian fossils show by 

 their numbers that the animals had lived where their remains are found, 

 and by the actual preservation of the flesh and skin in some cases, that 

 tkej had rested but a short time on the ground before the ice covered 

 them. The retreat of the ice, however, had been slow, as demonstrated 

 by the moraines forming a series in some valleys, with a gradually 

 decreasing range, both in extent and elevation. The present glaciers 

 may be considered as the puny and feeble representatives of that vast 

 crust of ice which formerly enveloped the northern parts of the globe. 



The great incrustment of ice necessarily extinguished organic life, 

 so far as its domain extended. The animal tribes which then perished 

 — ^the mastodon, Elephas primigenius, rhinoceros, and others, — have left 

 their remains in the alluvium, and are found closely to resemble the 

 existing races, which were of course introduced after the ice disap- 

 peared, and the region acquired the temperature necessary for their 

 support. 



Agassiz thinks that a similar great and sudden depression of temper- 

 ature probably served the same purpose at earlier periods, by clearing 

 the globe of one zoological group, to make room for another. 



Mountains, of whose rocks fragments are found transported to a dis- 

 tance, in different directions, are considered as centres of dispersion, 

 by Agassiz. Thus, the Alps, whose boulders strew the plains of Swit- 

 zerland, Italy, Austria, and France, form one centre of dispersion, 

 embracing Jura within its range. The Vosges (in Alsace), which ex- 

 hibit the same phenomena on a smaller scale, are another. The Ce- 



