PERSIA AS AN EXAMPLE OF AN ARID COUNTRY. 249
order of superposition, namely, silts or other fine materials at the bottom, then gravel, and lastly wind-blown sand on top. It is probable that this order of superposition represents the ordinary sequence of events in a country where basin-making and desiccation are both in progress.
The aqueous series of basin deposits.—The general appearance of the basin deposits is well described by Blanford (a, pp. 495-496):
In the smaller plains, and in the larger deserts at a short distance from their 'margins, the surface usually consists of very fine, .pale-colored rather sandy earth, which, although barren in general, is fertile wherever irrigation is practised, unless, as is not unfrequently the case, it is strongly impregnated with salts. . . . The margins of the desert plains . . . usually consist of a long slope covered with gravel and bowlders, and with a surface inclination of from one to three degrees. Such slopes often extend for a distance of from 5 to io miles from the base of the shills bounding the plains, the difference in level between the top and the bottom of the incline being frequently from 1,000 to 2,000 .feet or even more. What proportion of this depth consists of detritus it is impossible to say, but depth of The deposit must be great, because hills of solid rock but rarely emerge from it. The greater part of such slopes consists of sand and pebbles, the latter more or less angular and mixed with large blocks, all derived from the adjacent hills. . . . Fragments 2 or 3 feet in diameter are not uncommon, even at a distance of a mile or two from the base of the hills; but I only observed them near places where small streams issue from the higher ranges. At such spots the gravel deposits are naturally very often raised into a fan-shaped slope. Such a phenomenon is common enough in all countries, and so are strong slopes at the base of steep hills; but the peculiarity of these slopes in Persia consists in their great breadth, and in the enormous mass of detrital deposits .which they contain.
From many of the desert plains of Persia valleys of greatwidth extend afar into the more hilly regions. These valleys have, along their sides, precisely such long slopes of gravel as I have just described. The presence of a stream in the midst of the valley is by no aneans constant; but occasionally small rivulets coming from the sides run for miles along the slopes without descending to the bottom of the valley, and are finally absorbed by the soil, if not exhausted ,by being diverted for irrigation.
Even at great elevations (p. 497) up to 9,000 feet, similar immense accumulations of loose material occur in many places, and the higher peaks and ranges rise out of them. Among certain of the higher mountains, for example, near Shiraz (p. 498), where the rainfall is greater than usual and the streams flow perennially, the valleys present quite a different appearance, being flat-floored and having no gravel slopes along the sides. The reason for this peculiarity, as Blanford infers, is that the perennial streams are able to carry away the waste that is brought into the valley, whereas, if the streanis come to an end at the base of the hills, it is inevitable that the detritus which they carry should be deposited at once and fans should be built up indefinitely.
Blanford describes many fine examples of superficial deposits, especially of gravel slopes, and there are numerous others which might be mentioned. A small but typical example is the basin of Tabas, 6o miles east of Birjand, among the mountains, at an elevation of about 4,000 feet. The center of the plain, which is about 15 miles in diameter, is composed of the finest silt, and as water is relatively abundant, most of it is cultivated. Outside the cultivated area is a broad rim of fine gravel, difficult of cultivation, but very useful to the traveler, as we found during the melting of one of the occasional winter snows. The center of the plain was so muddy and slippery that it was utterly impassable for camels, which are the most helpless of beasts of burden when removed from their proper environ-