IXx REPORT 1864. 



ronce of erratic blocks smoothed and scratched, at the bottom of the same 

 drift. 



The evidence of a period of great cold in England and Iforth America, in 

 the times referred to, is now so universally admitted by geologists, that I 

 shall take it for granted in this Address, and briefly consider what may have 

 been the probable causes of the refrigeration of central Europe at the era in 

 question. One of these causes, first suggested eleven years ago by a celebrated 

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

 When I proposed, in 183.3, 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 monu- 

 ments of the ages which preceded the historical, the more decided become the 

 proofs of a general alteration in the position, depth, and height of seas, con- 

 tinents, 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 height of mountain- 

 chains, but also on 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 groat African desert, or Sahara, was 

 submerged within the modern or jiost-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 fh'st threw out this hint, the fact that the 

 Sahara was reaUy 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, of Neuchatel, 

 and Professor Martins, of Mont]^)eUier. These three experienced observers 

 satisfied themselves, during the last winter, that the Sahara was under water 

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

 1856, from a memoir by M. Charles Laurent, that sands identical with those 

 of the nearest shores of the Mediterranean, and containing, among other 

 recent shells, the common cockle {Cardium 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. 

 These shells have been met with at heights 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 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 

 \\]) 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 forming successive 

 terraces, in which recent shells and the casts of them were agglutinated 

 together with sand and pebbles, the whole having the form of a conglomerate. 

 The ancient sea appears once to have stretched frorCi the GuK of Cabes, in 

 Tunis, to the west coast of Africa north of Senegambia, having a vsidth of 



