Falceontology of the Libyan Desert, etc. 175 



9. The Soutliern, and part of the Central Sahara, have been land 

 since the close of the Devonian period ; the greater part of the 

 remainder of the Sahara became dry after the Cretaceous period, 

 and only in the Libyan desert did the sea extend during the Eocene, 

 and, in the northern portion, until the middle of the Miocene period. 



10. The eruption of basaltic, phonolitic and trachytic rocks in 

 Tripoli, the Libyan and Arabian deserts, and also probably those in 

 the mountain territory of Ahaggar and Tubu, caused but slight dis- 

 turbance and metamorphisin in the surrounding rocks, and may 

 probably have taken place in the newer Tertiary period. 



11. During the Quaternary period (Diluvialzeit) the Sahara as 

 well as a portion of the southern and eastern area of the Mediter- 

 ranean was dry land. 



VZ. The hypothesis of a Quaternary sea over the Sahara is not 

 supported either by the geological structure or by the surface 

 characters of the desert. At the utmost only the region of the 

 Tunisian Schotts was connected with the Mediterranean, and the 

 slight depression between Alexandria and the Ammon oasis may 

 have been united with the Red Sea. 



13. During the Quaternary period a moist climate prevailed in 

 North Africa, which probably continued until the commencement 

 of the present period. 



14. The characteristic surface features of the Sahara, such as the 

 excavation of the nuuierous dry water-courses and basin-like depres- 

 sions, the formation of the steep precipices and isolated I'ock masses, 

 may be attributed to the eroding action of fresh- water. 



15. The desert-sand results from the decomposition of the sand- 

 stone strata, which is the prevailing rock throughout the Central and 

 Southern Sahara. Its distribution and its heaping up into dunes 

 have been effected by the wind. 



16. The salt bogs and also the surface incrustations of salt and 

 gypsum have been derived from the solution by water of these 

 materials in the older rocks and the subsequent evaporation of the 

 water in depressed areas. 



17. There is no proof of any important change in the climatic 

 conditions of the Sahara during the historic period. 



The second part of the work, relating more particularly to the 

 Libyan desert, commences with a review of the literature which has 

 already appeared on the geology of this district and Egypt proper. 

 It is followed by a detailed description of the character, the 

 boundaries, and the fossil contents of the various strata. The 

 geological chai'acter of the entire territory west from the Nile is 

 astonishingly simple. With the exception of some inconsiderable 

 landslips bordering the escarpments, there is no alteration of the 

 strata worth mentioning, not even a single well-marked fault. All 

 the sedimentary rocks appear to the observer to be horizontal, and it 

 is only by extended investigations that a slight incline towards the 

 north and east can be distinguished. From this it results that the 

 oldest strata are in the south and the newest in the north. The 

 former belong to the Cretaceous, and the latter to the Tertiary period. 



