1864.] ADDRESS BY SIR CHARLES LYELL. 397 



losist, has not, I think, received the attention ^Yhich it well de- 

 served. When I proposed, in 1833, the theory that alterations in 

 physical geography might have given rise to those revolutions in 

 climate which the earth's surface has experienced at successive 

 epochs, it was objected by many that the signs of upheaval and de- 

 pression were too local to account for such general changes of tem- 

 perature. This objection was thought to be of peculiar weight 

 when applied to the glacial period, because of the shortness of the 

 time, geologically speaking, which has since transpired. But the 

 more we examine the monuments of the ages which preceded the 

 historical, the more decided become the proofs of a general altera- 

 tion in the position, height, and depth of seas, continents, and 

 mountain-chains since the commencement of the glacial period. 

 The meteorologist also has been learning of late years that the quan- 

 tity of ice and sn#w in certain latitudes depends not merely on 

 the height of mountain-chains, but also in the distribution of the 

 surrounding sea and land even to considerable distances. 



M. Escher von der Linth gave it as his opinion in 1852, that if it 

 were true, as Ritter had suggested, that the great African desert, 

 or Sahara, was submerged within the modern or post-tertiary period, 

 the same submergence might explain why the Alpine glaciers had 

 attained so recently those colossal dimensions which, reasoning on 

 geological data, Venetz and Charpentier had assigned to them. 

 Since Kscher first threw out this hint, the fact that the Sahara was 

 really covered by the sea at no distant period has been confirmed 

 by many new proofs. The distinguished Swiss geologist himself 

 has just returned from an exploring expedition through the east- 

 ern part of the Algerian desert, in which he was accompanied by 

 M. Desor, of Neuchatel, and Professor Martins, of Montpellier. 

 These three experienced observers satisfied themselves, during 

 the last winter, that the Sahara was under water dui-ing the 

 period of the living species of Testacea. We had already learnt in 

 1856, from a memoir by M. Charles Ijaurent, that sands identical 

 with those on the nearest shores of the Mediterranean, and contain- 

 ning, among other recent shells, the common cockle {^Carillum 

 edule), extend over a vast space from west to east in the desert, 

 being not only found on the surface, but also brought up from 

 depths of more than 20 feet by the Artesian auger, 'i'hese shells 

 have been met with at lieiii'hts of more than 900 feet above the sea- 

 level, and on ground sunk 300 feet below it; for there are in 

 Africa, as in Western Asia, depressions of land below the level o 



