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Ireland, and Wicklow, to be other points from which blocks and 
gravel have been dispersed, each district having its peculiar debris, 
traceable in many instances to the parent rock, at the head of the 
valleys. Hence, observes M. Agassiz, it is plain the cause of the 
transport must be sought for in the centre of the mountain ranges, 
and not from a point without the district. The Swedish blocks on 
the coast of England do not, he conceives, contradict this position, 
as he adopts the opimion that they may have been transported on 
floating ice. 
In describing the phenomena sitooenbea by erratic blocks and 
gravel, M. Agassiz first insists upon the necessity of distinguishing 
between stratified gravel and mud containing fossils, which could 
not have been accumulated by true glaciers, although the materials 
may have often been derived from them; and unstratified masses, 
composed of blocks, pebbles, and clay. These stratified deposits 
he considers to be of posterior origin to the glacier epoch. | ‘The till 
of Scotland, or the great unstratified accumulation of mud and 
gravel, containing blocks of different) size heaped together without 
order, and containing no organic remains but bones of Mammalia 
and insignificant fragments of shells, he is of opinion was also not 
produced by true glaciers, although intimately connected with the 
phenomena of ice. The polished and striated surfaces of the blocks 
leave no doubt on M. Agassiz’s mind that these masses have been 
acted upon by ice in the same manner as the blocks which are ob- 
served under existing | glaciers, and which are more or less re- 
arranged by water derived from the melting of the glaciers. 
Similar detritus fills the bottom of all the Alpine valleys, as that 
of the Rhone from its mouth to its junction with the Lake of Geneva, 
and the valley of Chamounix: it is found between the Hospice de 
Grimsel and the borders of the lower glacier of the Aar ; thence to 
the neighbourhood of Goutharen in the valley of Oberhasli, at Im 
Grund, in the plains of Meiringen, and in Interlasken ; also between 
Thun and Berne. At all these localities, M. Agassiz considers, the 
‘blocks were left, when the glaciers extended to them. © — 
With respect to the valley of the Aar, M. Agassiz says it is easy 
to prove that the rounded pebbles of Alpine rocks spread along its 
whole course, were not transported to their present position by that 
river, because between the glacier from which it.issues and Berne, 
the flowing of the stream is interrupted by the barrier of Kirchet, 
the Lake of Brienz, and the Lake of Thun; and because between 
these lakes its velocity is so small, that it transports only mud and 
very fine gravel, and that the pebbles over which the river flows 
below Thun do not issue from the lake. Supposing that the vo- 
lume of the Aar was formerly greater, why, asks M. Agassiz, are 
not the lakes of Brienz and Thun filled in the same manner as the 
plain of Meiringen and the bottom of the valley which separates the 
two lakes? (All difficulties, however, he is of opinion, vanish, if 
the pebbles be considered the detritus of retreating glaciers, and: 
that the hollows occupied by the a of Brienz and Thun were. 
filled with glaciers. . 16.9 
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