Pakeontology of the Libyan Desert, etc. 173 



description merely to the territory wliich came under his own notice; 

 but, by bringing together the scientific investigations made by various 

 travellers in the desert regions of North Africa, he has given, in the 

 first portion of the work, a clear and instructive account of the 

 geological history of the great African Sahara, of which the Libyan 

 desert forms the north-easterly portion. 



Over nearly the entire breadth of North Africa, between 17° and 

 30° north latitude, embracing an area of about 160,000 square miles, 

 the surface layer is formed either of sand or naked rock, and the 

 rainfall is either altogether wanting or so slight that there is a con- 

 sequent poverty or complete absence of plant and animal life. 

 Throughout this territory the geological structure is of an extremely 

 regular and simple character. The surface of the Sahara is anything 

 but a dej)ressed basin ; for, without reckoning the mountain ranges of 

 Tuareg, Tripoli, and Tibesti, of which the highest summits are from 

 1500 to 2000 metres S.L., the average elevation of the whole country 

 is from 300 to 400 m. above the sea. It presents an elevated central 

 portion which graduallj'^ slopes to the west, east, and south, but more 

 towards the north, so that near the Mediterranean the surface is even 

 below the sea-level. 



The author remarks that although the causes of desert formation 

 are of themselves independent of the surface charactei-s, and wholly 

 controlled by meteorological laws, yet there is a connection between 

 the characters of a desert country and certain geological conditions. 

 Countries with a strongly developed relief, with frequent diversities 

 of mountain and plain, are vei'y seldom destitute of rainfall ; the 

 moisture-laden clouds collect round the mountains, and their contents 

 are deposited on the adjoining plains. Only where the wind blows 

 unhindered over wide, level land-surfaces, does it become dry, and 

 the land is changed into desert. Level plains or widely extended 

 table-lands are almost without exception formed of horizontal strata, 

 and this is the case in the Sahara, where, as far as known, all the 

 sedimentary rocks are either horizontal or very slightly inclined. 

 .Mountains composed of arched or vertical rocks like those of Europe 

 and Asia are unknown in the Sahara ; the mountain-ranges of this 

 region are merely so many platforms one rising over the other. 



Three typical conditions of surface structure prevail throughout 

 the Sahara ; (1.) The plateau-desert or Hammada. (2.) The erosion- 

 desert (Sebcha, Djtif, Schott) and (3.) The sand-desert proper (Erg 

 or Areg). To these the mountain-desert may be added, as constituting 

 another type peculiar to the mountain ranges. 



The commonest type is that of the Hammada, which is a level 

 stony surface destitute of elevations or depressions, without springs 

 or water-courses. The surface is a hard rock or harder clay, covered 

 with stony blocks and fragments of various sizes, which under the 

 influence of the strong changes of temperature, have been formed by 

 the splitting up of the surface strata. In many instances labyrinthic 

 passages have been worn in the horizontal strata and isolated columns 

 and masses of rock bear witness to the amount of denudation which 

 they have undergone. Some of these masses are from 30 to 50 m. 



