491 



DESERTS. 



DESERTS. 



432 



and cover up everything which does not oppose such domination, either 

 by its elevation above the general level of the plain or by means of its 

 waters ; and this is the reason why a variety of rocks have been sup- 

 posed to be originally connected with deserts, over which they have in 

 reality merely extended their sands derived from the sources already 

 stated. Thus, on the two borders of the Red Sea, granitic rocks, as 

 well as crystalline schists and cretaceous or tertiary limestones, form a 

 constituent part of the soil or substratum of the deserts. This also 

 appears to be the case with some parts of the Asiatic desert of Kobi, 

 and with the few deserts proper of South America, such as that of 

 Atacaina, covering crystalline schists. At the eastern base of the Rocky 

 Mountains, in North America, deserts without water have also been 

 produced, it is said, by the disaggregation of sandstones of uncertain 

 age. Still, of course, the sands of deserts must always rest on some 

 other geological formation, like the sandy beaches of the present 

 day. 



We proceed from the geology to the physics and meteorology of the 

 subject. Professor Tyndall (' Phil. Trans.,' 1853) having found that 

 there are the strongest experimental grounds for the belief that silica 

 possesses a higher conductive power for heat than some of the metals, 

 and shown that the average conducting power of wood may be taken at 

 12, while that of silica is 90 (according to the deflections of the gal- 

 vanometer employed in his method of measuring that power), remarks 

 that the part played by silica in exercising an influence upon climate 

 has hitherto had no particular importance attached to it ; and, having 

 also cited the former conclusions of De la Rive and De Candolle relative 

 to the influence which the feeble conducting power of wood must exert 

 in preserving within a tree the warmth which it acquires from the soil, 

 proceeds to contrast, in exemplifying the functions in nature of silica, 

 two regions overspread with the former and with the latter respectively. 

 " Let us consider for a moment the progress which takes place from 

 sunrise to the hour of maximum temperature in a region overspread 

 with forests, and compare it with that which must take place in the 

 African desert. In the former case, the heat slowly and with difficulty 

 penetrates the masses of wood and leaves on which it falls ; and after 

 the point of maximum temperature is passed, the yielding up of the 

 heat acquired is proportionately slow. In the desert, however, the 

 mass of silica exposed to the sun becomes burning hot as the hour of 

 maximum temperature approaches ; but after this is passed the heat is 

 yielded up with proportionate facility. Hence a maximum and a mini- 

 mum thermometer must, in the latter case, mark a far wider range of 

 temperature during the twenty-four hours than the former. This 

 agrees with observation. In Sahara, to use the words of Mrs. Somer- 

 ville, during ' the glare of noon the air quivers with the heat reflected 

 from the red sand, and in the night it is chilled under a clear sky 

 sparkling with its host of stars.' Were gypsum, however [of which 

 gome deserts'eonsist], the prevailing mineral, it is a priori certain that 



this could not be the case to anything like its present extent A 



cube of the latter substance, examined in the usual manner, produces a 

 deflection of 19 only. It is scarcely superior to wood." 



Deserts of sand are much more abundant in the Old than in the 

 New World. Almost every plain, even the best cultivated in Europe, 

 contains patches of sand of greater or less extent ; but to confine our- 

 selves to deterit, properly so called, let us consider the immense surface 

 which they occupy on the ancient continent. 



Africa, from its western coast, between the Senegal river on the 

 south and Marocco on the north, to the border of the Red Sea, pre- 

 sents one vast sea of sand, occupying upon an average about 48 degrees 

 of longitude and 10 of latitude, which is but partially interrupted by a 

 projecting part of Fezzan and by the narrow valley of the NUe. This 

 sterile region is divided into two unequal parts ; the more considerable, 

 that to the west, being called the Sahara, or Qreat Desert, and the 

 other, to the eastward, sometimes bearing the name of the Libyan 

 Desert. Both the one and the other inclose a few small fertile spots 

 called Oatet, which rise in the midst of the sands like islands in the 

 ocean. They serve as a resting-place to the merchants, who, by means 

 of camels, poetically termed the ships of the desert, traverse these 

 dismal regions. 



The Detert of Sahara [SAHARA, in OEOO. Div.] is represented to us 

 as covered with moving sands formed into ridges like the waves of the 

 sea. In the midst of these sands, whose position and aspect are con- 

 tinually changing by the effect of the wind, are dispersed a few rocky 

 hills, and small hollows where the collected waters nourish a few 

 shrubs, ferns, and grasses. The mountains which bound the desert on 

 the west present insulated pinnacles, descending gradually into a plain 

 covered with white and sharp siliceous stones, and which is at last con- 

 fmm.led with the sands. At Tegazza, and some other places rock-salt, 

 whiter than the whitest marble, extends in vast beds beneath a stratum 

 of rock. 



" During the greater part of the year," says Malte Brun, " the dry 

 and heated air presents the aspect of a red vapour ; the horizon seems 

 beaming with the fires of innumerable volcanoes. The rains, which 

 fall from July till October, do not extend their uncertain and momen- 

 tary benefits to all parts alike. An aromatic herb resembling thyme, 

 the njant which bears the Sahara berry, acacias, and other thorny 

 shrubs, nettles, and briars, such is the only vegetation which is met 

 with on a few spota in the desert. Rarely is a grove of date-trees or of 

 other plants to be seen. A few monkeys, a few antelopes, content 



ART8 AKD gCI. DIV. VOL. lit 



themselves with what little food they can find. The ostrich also 

 inhabits this region in numerous flocks, feeding upon lizards and snails 

 and a few coarse herbs. Lions, panthers, and serpents, often of a 

 monstrous size, add to the horror of these dreadful solitudes. The 

 desert presents no traces of a beaten path, and the caravans that 

 traverse it, directing their way by the polar star, describe a tortuous 

 road in order to profit by the oases, described as brilliant with 

 vegetation, but which probably owe a great part of their reputa- 

 tion to the contrast they form with the absolute barrenness of the 

 desert." 



Such is the dryness of the terrible burning wind named Samoom, 

 Samum, or more properly Samiel, that it frequently absorbs the water 

 contained in the skins borne by the camels for the use of the merchants 

 and drivers. A last cruise of water has been sold for ten thousand 

 drachmas of gold. In 1805, an akkabah or caravan, composed of 2000 

 persons and 1800 camels, not having found water in the usual halting 

 places, they all, men and animals, perished of thirst. It is in the desert 

 that is seen most particularly that singular optical illusion termed the 

 Mirage. [MIRAGE.] 



Desert of the Nwtli.-Ei.tit and East of Africa. The great sandy region, 

 as we have said, is contracted in one place by the country of Fezzan. 

 To the east and south of this are deserts occupied by the Tibboos, a 

 nation of Berbers ; whilst on the north-east the desert of Barca, the 

 ancient Cyrenaica, extends as far as the Mediterranean ; both these are 

 continued on the eastern side by the great desert of Libya, which bor- 

 ders on Egypt, forming its western boundary. These deserts differ 

 little in character from the Sahara. To the south, the Libyan desert 

 joins the equally sterile region of northern Nubia ; leaving which, and 

 crossing the Nile, we again meet with sandy and rocky tracts, which, 

 from Abyssinia on the south as far as Suez on the north, occupy the 

 whole space between the river and the Red Sea. 



A comprehensive induction due we believe originally due to the 

 late Edward Forbes, has shown that the Mediterranean and the Red seas 

 are the remainder of a former great Indian ocean and south Mediter- 

 ranean, which extended to the mountains of northern central Africa, 

 prior to the elevation of the eastern portion of its bed, now forming 

 the land of Arabia, Syria, and Palestine ; and probably also, it may be 

 inferred, before the Mediterranean depression had been opened to the 

 Atlantic waters on the west. Agreeably to this view, the Sahara and 

 Libyan deserts, as Forbes further showed, occupy the drained though but 

 slightly elevated bed of the western part of this ancient ocean, extending 

 northward from the mountains to the southern shores of the present 

 Mediterranean. As Dr. Bolle has evinced (Hooker's ' Journal of Botany,' 

 1852, p. 122) that Fuertaventura, the greatest of the Canarian islands, 

 is a Sahara in miniature, we may add that it must also have extended 

 westward beyond the line of the existing African coast into a part of 

 the area now occupied by the waters of the Atlantic. 



Deserti of Arabia. Passing from Africa to Arabia, we first meet 

 with the sandy hills which form the isthmus of Suez, and separate the 

 Arabian gulf from the Mediterranean, whose coast-line they follow as 

 far as Palestine. Immediately to the south of these sands extends the 

 stony and barren tract known by the nanle of Arabia Petraea. The 

 reputed Mount Sinai, an imposing mass of granite, is there surrounded 

 by rocks of inferior height, composed in part of sandstone, and inclosing 

 a few fertile valleys producing grapes, pears, and other excellent fruits ; 

 but the country in general is of frightful sterility, presenting nothing 

 but a few shrubs of Egyptian thorn (acacia vera), which furnishes the 

 gum arabic, with capers, and a few other plants, intermixed with rocks 

 of a blackish granite, of jasper, and of sienite, and plains covered with 

 sand, flints, and rolled stones. There are, however, large herds of 

 gazelles and other game. 



On the south, as far as Hadramaut, and bounded on the east by the 

 Euphrates and Persian Gulf, and on the west by Hejaz and Yemen, 

 extend the vast deserts of Nejd and Ahkaff, which produce nothing 

 but a few saline plants, and their general appearance differs little from 

 the deserts of Africa, unless it be that they contain many hilly oases 

 adorned with palms and date trees. 



Continuing our route towards the Euphrates, we leave on our left 

 that part of the Arabian desert which bears particularly the name of 

 the desert of Syria, which extends northward as far as Haleb, and in 

 the middle of which the traveller discovers the extensive and solitary 

 ruins of Palmyra, the once magnificent residence of Zenobia. 



Deiert of Mesopotamia. Having crossed the Euphrates, we find our- 

 selves in the ancient Mesopotamia, so called from two Greek words 

 signifying between the rivers, that is, the Euphrates and the Tigris, and 

 which, in the most extended sense of the name, includes Aljezireh on 

 the north, and Irak-Arabia or Babylonia on the south. 



With the exception of narrow slips along the two rivers just men- 

 tioned, Mesopotamia is a desert still more horrible than those of Africa 

 and Arabia, of which it may be regarded as the continuation. It is 

 covered with burning sands and sterile gypsum. Wormwood and 

 certain aromatic shrubs are the only vegetation, which, covering 

 immense spaces, banish all other plants. The waters of this desert, 

 mostly all saline or sulphurous, give rise to pestilential miasmata, 

 which remain suspended over the desert till, the equilibrium of the 

 air being disturbed, there is formed that pestiferous wind so justly 

 dreaded in Syria and the neighbourhood, and which suffocates any one 

 who has temerity enough to expose himself to its influence. 



