4^8 
margin of the glacier, and as they exactly resemble the rocks 
which the glacier is converting into precisely similar forms, 
there cannot be a doubt that they owe this striking peculiarity 
of form — the roundness, smoothness, and grooving — to pre- 
cisely the same cause. The conclusion is indeed so obvious, 
that it would scarcely seem to require any comment, were it 
not that it forms one of the strongest links in the chain of 
evidence for believing that similar causes operated to produce 
similar results, which, as will be noticed in the sequel, are 
plainly to be recognised in the elevated valleys of Great 
Britain, and especially in the mountainous region of Snow- 
donia, in North Wales. 
When the ice of the glacier is wholly dissolved, the rocks 
which it carried on its surface are deposited across the valley, 
and unite the lateral moraines. The water rushes over the 
barrier or terminal moraine thus formed, and its torrent is 
not without a peculiai^ agency in wearing the rocks, near the 
margin of the stream ; but this agency is altogether dif- 
ferent from that of ice — so much so, that a mere glance is 
sufficient to distinguish the water-worn rocks from those which 
have been rounded, polished, and scratched by ice. 
Simple glaciers, derived from one and not from various 
sources^ like those already noticed, have their lateral moraines 
parallel and concentric. 
An example is given by Agassiz of the formation of a 
medial moraine from the junction of two glaciers, and also 
of the movement of the entire mass ; a cabin, constructed in 
1827, at the base of the mountain, having since moved, with 
the glacier, a distance of about 4,600 feet. 
Pohshed and striated rocks are found in localities where 
glaciers no longer exist, but where, from the inferences 
already alluded to, Agassiz concludes that glaciers did exist ; 
and we thus advance another step to the evidence which is 
derivable from such records. 
