174 Revieivs — Prof. Br. Karl Zittel''s Geology and 



in height, and they stand out, as so many islands, not infrequently 

 miles in advance of the main body of the rock with which they were 

 formerly continuous. 



In the central districts of the Sahara, where the terraces reach an 

 elevation of 800 to 1000 m. S.L., the Hamraada type of desert passes 

 into the mountain-desert type characterized by plateaux from 500 to 

 1000 m. higher than the surrounding country, bordered by vertical 

 walls which have been deeply furrowed into gullies by water and 

 wind. From these mountain plateaux, water-courses now dry extend, 

 often hundreds of miles into the desert. 



Independent of these water-courses, the Sahara has numerous 

 basin-like depressions of various dimensions, frequently inclosed by 

 steep rock-walls, which belong to the erosion-desert type. The 

 bottom of these depressions is nearly level and covered with a crust 

 of salt or clay containing gypsum ; when it rains, the surface is 

 changed into a swamp. 



The sand-desert proper, to which fortunately only about one-ninth 

 of the Sahara belongs, consists of a wave-like carpet of pure quartz 

 sand, from which groups or parallel chains of dunes, from 50 to 150 m. 

 in height, project. 



The most important facts relating to the geology of the Sahara as 

 a whole are epitomized by the author as follows : — 



1. The Sahara is distinguished by a remarkably simple geological 

 structure, by the horizontal position of the major part of the sedi- 

 mentary rocks, and the absence of important discordances, folds 

 and faults. 



2. At the south foot of the Morocco Atlas, rocks of Devonian and 

 Carboniferous age are exposed, and further southwards, sandstones 

 and PalEeozoic slates, occasionally broken through by granite and 

 porphyry, and also quartzitic and Azoic clay-slates, appear. 



3. In the depression between the range of the Atlas and the 

 Ahaggar Mountains, there is a surface layer of Quaternary fresh- 

 water sands and clays holding gypsum and rock salt, which overlies 

 rocks of the age of tbe Middle and Upper Chalk. 



4. Similar Cretaceous deposits compose the surface of Hammada 

 el Homra and of the Harudj Mountains in Tripoli. To the south, 

 Devonian sandstones appear, which, with subordinate beds of lime- 

 stone and shale, extend to the southern borders of the desert. 



5. Formations of Permian, Triassic, Jurassic, and Lower Cre- 

 taceous age do not appear to exist either in the Sahara or in the 

 Egyptian border ranges. " 



6. The great mountain plateau of the Ahaggar in Air and Tibesti 

 appears to be principally composed of Palaeozoic sandstone, clay- 

 slate, gneiss and granite, and of eruptive rocks of a later age. 



7. Marine Tertiary deposits are only known to the north of the 

 Schotts of Tunis, and also covering a considerable area in the Lybian 

 and Arabian deserts. 



8. Eocene Nummulitic rocks in the North-east Sahara and Egypt 

 reach southwards as far as the latitude of Esneh, whilst the southern 

 limit of the Miocene is found ia_ the oasis of Siuah and the hills 

 between Cairo and Suez. 



