118 Lieui-Colonel Portlock on the Scratching of Bocks 



not only with the drift of mountain valleys, but also with the 

 immense detritic coating of the level plains below them. 

 The effect of his first exposition of the phenomena to British 

 geologists, who were not, like those of the Continent, familiar 

 with the writings of Hugi, Charpentier, and Venetz, was 

 truly surprising ; and many of the most distinguished, headed 

 by DrBuckland, entered upon the search for glaciers with the 

 utmost enthusiasm. Nor has the subject lost its interest even 

 now, as almost every season produces new examples of former 

 glacial action on the mountains, humble as they compara- 

 tively are, of the United Kingdom. Agassiz was not origin- 

 ally an advocate of the glacier theory, but, as he informs us, 

 considered the explanation of drift and its effects by marine 

 currents more simple and rational. Personal intercourse, 

 however, with Charpentier soon brought him to concur in the 

 opinions of that author, and he became their most zealous 

 and successful expounder. In one respect, the advocate 

 for glacial action has this great advantage, that he can 

 appeal to nature for proofs of the transporting power of ice — 

 whether on land, as exhibited in the glacier, or in water, as 

 manifested in the iceberg, or in the icefloe ; and the real 

 question, therefore is, whether that transporting power is 

 equivalent to the effects which have been produced. On 

 this point we may freely admit, as had been done long before, 

 that the transport of large erratics is best explained by re- 

 ferring them to icebergs or icefloes. In like manner, the 

 grooving and polishing of rocks within mountain chains, and 

 even the arrangement of mountain drift, may be fairly as- 

 cribed to glaciers ; but a doubt may be reasonably expressed 

 whether either or both of these actions can explain the ar- 

 rangement of the more widely spread drift of plains ; and we 

 are forced to call into our aid marine currents, more especi- 

 ally tidal currents, of which we shall again speak in the 

 sequel of these remarks. It is right, however, to inquire 

 first into the peculiar relations of the glacier to the effects 

 we are now studying. M. Agassiz states (Etudes sur les 

 Glaciers, 1840, p. 184), " The bottom of the glacier does not 

 always rest immediately on the rock or ground, but is usually 

 separated by a bed of sand or mud, which, according to its 



