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 



