876 REPORT—1890. 
separation between Europe and Africa. We find the trout in the Atlantic region 
and in all the snow-fed rivers falling into the Mediterranean; in Spain, Italy, 
Dalmatia; it occurs in Mount Olympus, in rivers of Asia Minor, and even in the 
Lebanon, but nowhere in Palestine south of that range, in Egypt, or in the Sahara, 
This fresh-water salmonoid is not exactly the same in all these localities, but is 
subject to considerable variation, sometimes amounting to specific distinction. 
Nevertheless it is a European type found in the Atlas, and it is not till we advance 
into the Sahara, at Tuecurt, that we come to a purely African form in the 
Chromide, which have a wide geographical distribution, being found everywhere 
between that place, the Nile, and Mozambique. 
The presence of newts, tailed batrachians, in every country round the Mediter- 
ranean, except again in Palestine, Egypt, and the Sahara, is another example of 
the continuity of the Mediterranean fauna, even though the species are not the 
same throughout. 
The Sahara is an immense zone of desert which commences on the shores of 
the Atlantic Ocean, between the Canaries and Cape de Verde, and traverses the 
whole of North Africa, Arabia, and Persia, as far as Central Asia. The Mediter- 
ranean portion of it may be said roughly to extend between the 15th and 30th 
degrees of north latitude. 
This was popularly supposed to have been a vast inland sea in very recent 
times, but the theory was supported by geological facts wrongly interpreted. It 
has been abundantly proved by the researches of travellers and geologists that such 
a sea was neither the cause nor the origin of the Libyan Desert. 
Rainless and sterile regions of this nature are not peculiar to North Africa, but 
occur in two belts which go round the world in either hemisphere, at about similar 
distances north and south of the equator. These correspond in locality to the 
great inland drainage areas from which no water can be discharged into the ocean, 
and which occupy about one-fifth of the total land surface of the globe. 
The African Sahara is by no means a uniform plain, but forms several distinct 
basins containing a considerable extent of what may almost be called mountain 
land. The Hoggar Mountains in the centre of the Sahara are 7,000 feet high, and 
are covered during three months with snow. The general average may be taken at 
1,500. The physical character of the region is very varied ; in some places, such 
as at Tiout, Moghrar, Touat, and other oases in or bordering on Morocco, there are 
well-watered valleys, with fine scenery and almost European vegetation, where the 
fruits of the North flourish side by side with the palm tree. In others there are 
rivers like the Oued Guir, an affluent of the Niger, which the French soldiers, who 
saw it in 1870, compared to the Loire. Again, as in the bed of the Oued Rir, 
there is a subterranean river, which gives a sufficient supply of water to make a 
chain of rich and well-peopled oases equal in fertility to some of the finest portions 
of Algeria. The greater part of the Sahara, however, is hard and undulating, cut 
up by dry watercourses, such as the Igharghar which descends to the Chott 
Melghigh, and almost entirely without animal or vegetable life. 
About one-sixth of its extent consists of dunes of moving sand, a vast accumu- 
lation of detritus washed down from more northern and southern regions—perhaps 
during the glacial epoch—but with no indication of marine formation. These are 
difficult and even dangerous to traverse, but they are not entirely destitute of 
vegetation. Water is found at rare but well-known intervals, and there is an 
abundance of salsolaceous plants which serve as food for the camel. This sand is 
largely produced by wind action on the underlying rocks, and is not sterile in - 
itself, it is only the want of water which makes it so. Wherever water does — 
exist, or artesian wells are sunk, oases of great fertility never fail to follow. ‘ 
Some parts of the Sahara are below the level of the sea, and here are formed 
what are called chotts or sebkhas, open depressions without any outlets, inundated 
by torrents from the southern slopes of the Atlas in winter and covered witha 
saline efflorescence in summer, This salt by no means proves the former existence _ 
of an inland sea; it is produced by the concentration of the natural salts, which _ 
exist in every variety of soil, washed down by winter rains, with which the salt 
evaporated residue of water becomes saturated. 
