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 Sieg 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 = — 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 ue 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 
they 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 
erust of ice which formerly enveloped the northern parts of the globe. 
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 n 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 
= 
. Agassiz thinks that a similar great and sudden depression of temper- 
ature probably served the same purpose at earlier periods, by clearing 
the Blobe of one zoological group, to make room for another. 
. of whose rocks fragments are found transported to a dis- 
tance, in different directions, are considered as centres of dispersion, 
by Agassiz. Thus, the Alpe, 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- 
it the same pliesomens" ona prmeecbeer ti sl The at 
