2S2 Professor Agassiz on the Glacial Theory. 



countries. Hence it appears natural, that during the retreat 

 of this covering of ice, there must have been a period when 

 the mountains of Scotland were the focus of numerous gla- 

 ciers, which at first descended from their summits into the 

 jilains, but afterwards occupied only the interior valleys, be- 

 fore disappeaxing completely. 



There would thus be two very distinct periods to be parti- 

 cularized in the epoch of the existence of ice in the north of 

 Europe, — that during which the general covering enveloped 

 the region, and that when glaciers existed only in the high 

 valleys. The dispersion of erratic blocks over great spaces, 

 across considerable depressions of surface, the formation of 

 the till, the furrowing and uniform slrlation of the polished 

 rochs of Sweden and of Finland, seem to me the chief phe- 

 nomena which have been produced by the northern covering 

 of the epoch of ice. The differences which exist as to the er- 

 ratic phenomenon between the north and the centre of Europe, 

 appear to me to be susceptible of easy explanation by the 

 differences of latitude and of the configuration of the surface. 

 In Britain, the ice, at the time of its greatest extension, 

 seems to have covered completely great tracts of country, 

 and consequently rendered the fall of blocks on its surface, 

 if not impossible, at least extremely rare ; so that the great 

 mass of the blocks was necessarily buried under the ice, and 

 was therefore subjected to all the effects of a gradual and 

 long- continued trituration, just as is observed beneath the gla- 

 ciers of the present day. Mountains of considerable eleva- 

 tion in Scotland — Schihallien, for example — have their sum- 

 mits as polished as their flanks ; whereas in Switzerland there 

 exists a limit, at about 9000 feet,* in the centre of the Alps, 

 above which the summits are no longer polished, but where 

 the rugged peaks present a very striking contrast to the 

 lower surfaces, which are polished, or, at least, moutonnts.f 



• All the measurements given in tliis paper are in pieds de Roi, orFrenci» 

 feet ; and the temperatures are all indicated in centigrade degrees, unless 

 where other measurements or other degrees are specially mentioned. 



t Vide the Comptes Eendus de I'Academie des Sciences, 1842 ; tortie 14, 

 p. 412. 



