506 Geological Society. 
glaciers once existed in the British Islands, but that large sheets 
(nappes) of ice covered all the surface, and that the former were the 
remnants of the latter. 
The author then details the proofs that glaciers did not descend 
from the mountain summits into the plains, but are the remaining 
portions of the sheets of ice which at one time covered the flat country. 
It is evident, he says, if the glaciers descended from high mountains, 
and extended forward into the plains, the largest moraines ought to 
be the most distant, and to be formed of the most rounded masses ; 
whereas the actual condition of the detrital accumulations is the re- 
verse, the distant materials being widely spread, and true moraines 
being found only in valleys connected with great chains of lofty 
mountains. 
It must then be admitted, the author argues, that great sheets of 
ice, resembling those now existing in Greenland, once covered all 
the countries in which unstratified gravel is found; that this gravel 
was in general produced by the trituration of the sheets of ice upon 
the subjacent surface ; that moraines, as before stated, are the effects 
of the retreat of glaciers; that the angular blocks found on the sur- 
face of the rounded materials were left in their present position at 
the melting of the ice, and that, as the advance and disappearance 
of great bodies of ice produce debacles and considerable currents, so 
it may be inferred, that by such operations in times past masses of ice 
were set afloat, and conveyed in diverging directions the blocks with 
which they were charged. He believes that the Norwegian blocks 
found on the coast of England have been correctly assigned by Mr. 
Lyell to a similar origin. 
Another class of phenomena connected with glaciers, is the form- 
ing of lakes by the extension of lateral moraines into a main valley ; 
and M. Agassiz is of opinion, that the parallel roads of Glen Roy are 
the effects of a lake which was produced in consequence of a glacier 
projecting across the glen near Bridge Roy, and another across the 
valley of Glen Speane. Lakes thus formed naturally give rise to 
stratified deposits and parallel roads, or beds of detritus at different 
levels. 
The connexion of stratified very recent deposits with glacier- 
detritus, M. Agassiz observes, is difficult to explain ; but he conceives 
that the same causes which can bar up valleys and form lakes, like 
those of Brientz, Thun and Zurich, may have formed analogous bars 
at the point of contact of glaciers with the sea sufficiently extensive 
to have produced large salt-marshes to be inhabited by animals, the 
remains of which are found in the clays superimposed on the till of 
Scotland ; and he adds, that the known arctic character of these fos- 
sils ought to have great weight with those who study the vast subject 
of glaciers. 
In conclusion, the author remarks, that the question of glaciers 
forms part of many of the great problems of geology ; that it accounts 
for the disappearance of the great mammifers inclosed in the polar 
ice, as well as for the disappearance of the organic beings of the so- 
called diluvian epoch; that in Switzerland it is associated with the 
