
- 
~ Parr Il. Scr. ii. §5.] GLACIER EROSION. ~~ 413 
glaciers, it was believed by many geologists that the erratics stranded 
along the flanks of the Jura Mountains had been transported on 
floating ice, and that Central Europe was then in great part sub- 
merged beneath an icy sea. It is now universally admitted, however, 
that the transport has been entirely the work of glaciers. Instead of 
being confined as at present to the higher parts of their valleys, 
the glaciers extended down into the plains. As already stated, they 
filled the great depression between the Oberland and the Jura, and 
rising high upon the flanks of the latter chain, actually overrode 






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a aa Wiese Rae ee rb << 
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Fig. 147.—SrcTioy To sHow THE EXTENSION OF THE ALPINE GLACIERS (a) ACROSS 
- THE PLAIN OF SWITZERLAND, AND THE TRANSPORT OF BLOCKS TO THE SIDES OF 
THE JURA (m) (B.). 


some of its ridges. Similar evidence abounds in the hilly parts of 
Britain, as well as in other parts of Europe and America, no longer 
the abode of glaciers, that a great extension of snow and ice at a 
recent geolozical period prevailed in the northern hemisphere, as 
will be described in the account of the Glacial Period in Book VI. 
There is proof also that the glaciers of New Zealand were formerly 
much larger. 
- As De la Beche has well pointed out, the student must be on his 
guard, however, lest he be led to mistake for true erratics mere 
weathered blocks belonging to a rock that has disintegrated in situ. 
If, for example, he should encounter a block like that represented in 
Fig. 148, he would properly conclude that it had travelled because it 
did not belong to the rock on which it 
lay. But he would require to prove 
further that there was no rock in the 
immediate neighbourhood from which it 
could have fallen as the result of mere Soi Ns! 
weathering. The granite (c) shown in_ iN ) 
Fig, 149, disintegrates at the summit, and 
the blocks into which it splits find their pig 448 Brock or GRANITE 
way by gravitation down the slope.* RESTING ON INCLINED STRATA (B.). 
(6) Hroston—The manner and the 
results of erosion in the channel of a glacier differ from. those 
associated with other geological agents, and form therefore distin- 
guishing features of ice-action. This erosion is effected not by the 
mere contact and pressure of the ice upon the rocks (though un- 
doubtedly fragments of rock must now and then be detached from 
this cause), but by means of the fine sand, stones, and blocks of rock, 
that fall between the ice and the rocks on which it moves. The 
1 De la Beche, Geological Observer, p. 257. 





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