(JKOLOOY. 277 



so universally admitted by geologists, that I shall take it for granted, 

 and bricllv consider what niav have lie-on the probable causes of the 



*. ** 



refrigeration of central Europe at the era in question. One of these 



causes, first suggested 11 years ago by a celebrated Swiss geologist, 

 has not, I think, received the attention which it well deserved. When 

 I proposed, in 1883, 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 depression were too local to account 

 for such general changes of temperature. 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 1 , monuments of the ages 

 which preceded the historical, the more decided become the proofs of 

 a general alteration in the position, depth, and hight of seas, conti- 

 nents, and mountain-chains since the commencement of the glacial 

 period. The meteorologist also has been learning of late years that 

 the quantity of ice and snow in certain latitudes depends not merely 

 on the hight of mountain-chains, but also on the distribution of the 

 surrounding sea and land even to considerable distances. 



M. Eschcr von Li nth gave it as his opinion in 18-32, that if it were 

 true, as Hitter had suggested, that the gieat African desert, or Sa- 

 hara, was submerged within the modern or post-tertiary period, that 

 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 Escher 

 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 eastern part of the Algerian 

 Desert, in which he was accompanied by M. Desor, and Prof. Mar- 

 tins. These three experienced observers satisfied themselves, during 

 the last winter, that the Sahara was under water during the period of 

 the living species of Testacea. We had already learnt, in 1856, from 

 a Memoir by M. Laurent, that sands identical with those of the near- 

 est shores of the Mediterranean, and containing, among other recent 

 shells, the common cockle (Cardium eclule), 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. These shells have been met with at hights 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 beloAV 

 the level of the sea. The same cockle has been observed still living 

 in several salt-lakes in the Sahara ; and superficial incrustations of 

 salt in many places seem to point to the drying up by evaporation of 

 several inland seas in certain districts. 



Mr. Tristram, in his travels in 1859, traced for many miles along 

 the southern borders of the French possessions in Africa, lines of 

 inland sea-cliffs, with caves at their bases, and old sea-beaches form- 

 in^ successive terraces, in which recent shells and the casts of them 



j^ 



were agglutinated together with sand and pebbles, the whole having 

 the form of a conglomerate. The ancient sea appears once to have 

 24 



