102 



KNOWLEDGE. 



[May 2, 1898. 



they Lave been raised at a comparatively recent epoch. 

 At any rate, there are several valuable books, published 

 not very many years ago, in which it is stated in so many 

 words that the Sahara represents the bed of an ancient 

 sea, which formerly separated Northern Africa from the 

 regions to the southward of the tropic. 



As a matter of fact, these opinions with regard to the 

 origin and nature of deserts are scarcely, if at all, less 

 erroneous than the deeply ingrained popular superstition 

 as to the growth of flints and pudding-stones. And a little 

 reflection will show that the idea of the loose sands of the 

 desert being a marine deposit must necessarily be erroneous. 

 Apart from the difficulty of accounting for the accumulation 

 of such vast tracts of sand on the marine hypothesis, it 

 will be noticed, in the first place, that desert sands are not 

 stratified in the manner characteristic of aqueous formations ; 

 and, secondly, even supposing they had been so deposited, 

 they would almost certainly have been washed away as 

 the land rose from beneath the sea. Then, again, we do 

 not meet with marine shells in the desert sands, of which 

 at least some traces ought to have been left had they been 

 marine deposits of comparatively modern age. 



Whether or no the subjacent strata have ever been 

 beneath the ocean, it is absolutely certain that the sands 

 of all the great deserts of the world have been formed (?i 

 situ by the disintegration of the solid rocks on which they 

 rest, and have been blown about and rearranged by the action 

 of wind alone. All deserts are situated in districts where 

 the winds blowing from the ocean's surface have to pass 

 over mountains or extensive tracts of land, which drain them 

 more or less completely of their load of moisture. Hence, 

 in the desert itself, when of the typical kind, little or no 

 rain falls, and there is consequently no flow of water to 

 wash away the de'luis resulting from the action of the 

 atmosphere on the rocks below. 



In other words, as has been well said, desert sands 

 correspond in all respects, so far as their mode of origin is 

 concerned, to the dust and sand which accumulate on our 

 high roads during a dry summer. On our highways, 

 indeed, the summer's dust and sand are removed by the 

 rains of autumn and winter, only to be renewed the 

 following season ; but in a desert no such removal takes 

 place, and the amount of sand increases year by year, 

 owing to the disintegration of the solid rock exposed here 

 and there. 



Only one degree less incorrect than the idea of their 

 submarine origin is the notion that deserts consist of 

 unbroken tracts of sand. It is true that such tracts in 

 certain districts may extend on every side as far as the eye 

 can reach, and even much farther; but, sooner or later, 

 ridges and bands of pebbles, or of solid rock, will be met 

 with cropping up among the sand, while frequently, as in 

 the Lybian desert, there are mountain ranges rising to a 

 height of several thousand feet above the level of the 

 plain. And it is these exposed rocks which form the source 

 whence the sand was, and still is, derived. Such moun- 

 tains naturally attract wh at moisture may remain in the ' 

 air, and in their valleys are found a more or less luxuriant I 

 vegetation. Oases, too, where the soil is more or less I 

 clayey, occur in most deserts ; and it is in such spots that [ 

 animal and vegetable life attains the maximum develop- 

 ment possible in the heart of the desert. 



In the most arid and typical part of the Lybian desert 

 the sand is blown into large dunes, which are frequently 

 flat-topped, and show horizontal bands of partly con- 

 solidated rock ; and between these are open valleys, partly 

 covered with sand, and partly strewn with blocks of rock 

 polished and scored by the sand-blast. In such sand 

 wastes the traveller may journey for days without seeing 



signs of vegetation, or hearing the call of a bird or the hum 

 of an insect's wing. But even in many of such districts 

 it is a mistake to suppose that vegetable and animal life 

 is entirely absent throughout the year ; in the western 

 Sahara, for instance, showers generally moisten the ground 

 two or three times a year, and after each of these a 

 short-lived vegetation springs suddenly up, and if no other 

 form of animal life is observable, at least a few passing 

 birds may be noticed. 



Among the most important and extensive deserts of the 

 world we have first the great Sahara, with an approximate 

 area of sixteen thousand square miles, nearly connected with 

 which is the great desert tract extending through Arabia, 

 Syria, Mesopotamia, and Persia. By means of the more or 

 less desert tracts of Baluchistan, Sind, and Kuch, this area 

 leads on to the great Rajputana desert of India. More 

 important is the vast Gobi desert of Mongolia, and other 

 parts of Central Asia. In Southern Africa there is the 

 great Kalahari desert, of which more anon. In North 

 America there is a large desert tract lying east of the 

 Rocky Mountains, and including a great part of Sonora ; 

 while in the southern half of the New World there is the 

 desert of Atacama, on the borders of Peru and Chili. 

 Lastly, the whole of the interior of Australia is desert of 

 the most arid and typical description. 



But among these, there are deserts and deserts. Tracts 

 of the typical barren sandy type are, as already said, 

 extensively developed in the Sahara, as they are in the 

 Gobi and the Australian deserts. Between such and the 

 plains of the African veld there is an almost complete 

 transition, so that it is sometimes hard to say whether a 

 given tract rightly comes under the designation of a desert 

 at all. A case in point is afiforded by the South African 

 Kalahari. Although there are endless rolling dunes of 

 trackless sand, and rivers are unknown, yet in many places 

 there is extensive forest, and alter a rain large tracts 

 could scarcely be called a desert at all. Mr. H. A. Bryden, 

 for instance, when describing the Kalahari, writes as 

 follows : — " And yet, during the brief weeks of rainfall, no 

 land can assume a fairer or more tempting aspect. The 

 long grasses shoot up green, succulent, and elbow-deep ; 

 flowers spangle the veld Ln every direction ; the giraffe 

 acacia forests, robed in a fresh dark green, remind one of 

 nothing so much as an English deer park ; the bushes 

 blossom and flourish ; the air is full of fragrance, and pans 

 of water lie upon every side. Another month, and all is 

 drought ; the pans are dry again, and travel is full of 

 difficulty." During the grassy season herds of springbok 

 used to migrate in the old days to the Kalahari, in the 

 northern part of which giraffes live the whole year, although 

 they must exist without tasting water for months. 



While such a district can scarcely be termed a desert 

 in the proper sense of the word, yet its sands have pre- 

 cisely the same origin as those of deserts of the typical 

 description. 



For sand to accumulate to the depths in which it occurs 

 in many parts of the Sahara and the Gobi by the slow 

 disintegration of the solid rocks under the action of 

 atmospheric agencies, must require an enormous amount of 

 time, to be reckoned certainly by thousands, and, for all we 

 know, possibly by millions of years. And we accordingly 

 arrive at the conclusion that the larger desert tracts must 

 not only have existed as land for an incalculable period, 

 but also as desert. Hence we can readily understand why 

 the animals of Algeria and the rest of Northern Africa 

 diSer for the most part from those of that portion of the 

 continent lying to the south of the northern tropic, the 

 Sahara having for ages acted as an impassable barrier-to 

 most if not all. 



