ANNIVERSARY ADDRESS OF THE PRESIDENT. lix 



of the stream is slight as compared with what it was of old. On 

 the great change which must have occurred in the climate of the 

 country, M. Lartet proposes the following means of accounting for 

 the phenomena : — 



1st. That they are due to a general elevation of the temperature, 

 increasing the rate of evaporation. 



2nd. That the emergence of a large tract of land in the region 

 from which the prevalent winds had come would cause them to 

 arrive dry and hot instead of laden with moisture, and that the 

 rise of the Sahara above the sea probably took place about this period. 

 3rd. Or that a lofty chain of mountains might in early times 

 have been upraised in the path of the same winds, the cool summits 

 of which would tend to condense the watery vapour, and thus to 

 precipitate the rain which otherwise would have been conveyed to 

 a greater distance. 



A further question of high importance is answered in the affir- 

 mative by Dr. Fraas, viz. whether these climatal changes have not 

 been continued during the historical period, and whether we have not, 

 in the general absence of vegetable soil in Palestine, and in Egypt 

 outside the area of Nile irrigation, as also in the contrast afforded 

 between ancient records and present barrenness, sufficient evidence 

 of a secular deterioration altogether independent of human agency. 

 As regards the probability of a great revolution in meteorological 

 phenomena caused by the elevation of the Sahara, the Society is 

 aware of the theory put forward a few years ago by the Swiss geo- 

 logists, that the retreat of the vast ancient glaciers of the Alps was 

 caused by the hot winds which arrived direct from that newly 

 raised portion of JSTorthern Africa. The violent Fohn wind, or 

 " snow-eater", as it is sometimes locally termed, which, whether in 

 summer or winter, so often impinges from the southward with fury 

 upon the mountain barrier of Switzerland, is now pointed out, by 

 peasant and naturalist alike, as the chief agency by which the snows 

 and glaciers are kept in check. MM. Desor and Escher von der 

 Linth visited Africa in 1863, especially for the purpose of inquiring 

 into the subject, and, finding reasons for concluding that the 

 Sahara has been elevated from the condition of a sea-bottom darino: 

 the Quaternary epoch, were satisfied that they had obtained ample 

 confirmation of the theory. 



In his Address to the British Association at Bath, Sir Charles 

 Lyell drew up a lucid and connected account of the various facts 

 and their explanations which support the opinions first propounded 

 by M. Escher von der Linth, and, admitting the correctness of the 

 data, showed the undoubted marks of the change of climate which 

 resulted from the prevalence, for a few days more or less, of the 

 Fohn or Scirocco. At the same time he brought forward additional 

 evidence for the existence in Posttertiary times, of a sea occupying 

 the place of a great part of Northern Africa, and separating the 

 highlands of the south and south-east from the hill-district of Algeria 

 and the Atlas. 



Professor Dove, the eminent meteorologist, who has for many 



e2 



