360 KEPOBT— 1882. 



The last but not the least important geological element of the Sahara- 

 Lybian desert, is the sand, which contains no organic remains, except at 

 places where it is intermingled with underlying older diluvial deposits. 

 Those enormous sandy accumulations doubtless are not of marine but of 

 Bubaerial origin. Still their vast proportions render it difficult to decide 

 the question whence they came and how they have been formed. Dr. 

 Zittel, to whom we owe valuable observations on the Lybian desert, 

 thinks that these accumulations of sand are on too large a scale to be 

 explained by Baron Richthofen's theory of the formation of the Loss in 

 China, though he concedes to the action of winds an important part in 

 the Afi'ican deserts also. In consequence Dr. Zittel admits that those 

 sands have been transported not only by atmospheric movements but also 

 by water-floods. And as the desert sands consist of quartz which could 

 not have been furnished by the limestone and marly rocks prevailing in 

 the desert, he supposes that it may have been derived from the so-called 

 Nubian sandstone which composes the mountainous range situated in the 

 southern part of the Lybian desert, and which, after long discussions 

 among geologists, has definitely been placed in the cretaceous forma- 

 tion. Dr. Zittel thinks that the disintegration of this sandstone has been 

 produced by water, the erosive power of which has left in the desert sa 

 many conspicuous traces, as, for instance, the high and steep sides of the 

 oasis, the deep depressions, and particularly the isolated rocks he calls 

 ' insular mountains,' considering them as the scattered remnants of once 

 connected masses, so that, according to him, such gigantic denudations 

 and erosions can only be the work of violent fresh-water floods coming- 

 from the south and carrying the large quantity of petrified tree-trunks 

 so frequent in the Lybian desert. Several objections arise against this 

 theory, at least as far as the Sahara is concerned, the southern boundaries 

 of which are not composed, as in the Lybian desert, of sandstone, but 

 chiefly of limestone, marls, and clay, and consequently could not furnish 

 the quartz grains yielded by the Nubian sandstone. Moreover, the 

 violent fresh-water floods proceeding from the south, which, according- 

 to Dr. Zittel, caused in the Lybian desert the above-mentioned enormous 

 denudations and erosions, suppose an extraordinary change in the climate 

 of that country ; for, though Dr. Zittel admits that in Egypt, and con- 

 sequently in the neighbouring countries, the atmospheric humidity was, 

 at a comparatively i-ecent epoch, ranch greater than now, such power- 

 ful floods could only be pi'oduced by rains unknown in the most rainy 

 regions of our globe. At all events M. Rollaud thinks that as far as the 

 Sahara is concerned, the quaternary and alluvial sandstones which the 

 desert contains are sufficient for the production of sand-accumulations, 

 which, according to him, are derived from actual atmospheric agencies, 

 namely, first, by disintegration, and then by the winds scattering the 

 grains over the surface of the desert. 



The hypothesis of the subaerial origin of those sands receives a strong 

 support, as well from the facts ascertained in reference to the large 

 distances to which such sands may be carried as from the comparative 

 study of atmospheric sands fallen in different countries. Now, even in 

 the twelfth century the celebrated Ai-ab geographer Edrisi spoke with 

 astonishment of the clouds of red dust and dry fog which frequently 

 obscured the sky of the Atlantic between the Green Cape and South 

 America, a tract which Edrisi qualified for that reason as the Dark Sea 

 (Bar-el-Mecdolin), and after him the authors of the middle age as Mare 



