Slike strani
PDF
ePub

ENCYCLOPEDIA AMERICANA

D

D

ESERT, a term designating in its broadest application any uninhabited region. In its more ordinary use the word means a portion of the earth's surface that on acount of its barrenness cannot support inhabitants. Four classes of desert may be distinguished: (1) Ice-wastes occupy the central plateau of Greenland, the islands of the Arctic Sea, and probably the entire Antarctic c continent. (2) Tundras (q.v.) are flat plains, little elevated above sea-level, fringing the Arctic shores of the northern continents and especially characteristic of Siberia. (3) Temporary deserts, or steppe-lands, border the Asiatic deserts to the north and west. The saline steppes of the Caspian are true arid wastes; but the typical steppes in South Russia are luxuriantly clothed with verdure and flowers in spring. In the dry season they form a dusty plain of withered herbage. The llanos (q.v.) of the Orinoco have similar characteristics, but the pampas of South America include portions perennially green and suitable for agriculture. (4) Arid wastes, or deserts in the popular sense of the word, occur mainly in two zones encircling the world, and corresponding to regions of minimum rainfall. The greater zone extends from near the equator in an east-northeast direction across the whole breadth of North Africa, as the Great Sahara, Libyan and Nubian deserts, over the peninsula of Arabia, through Persia, Turkestan and the vast tracts of Gobi or Shamo to the confines of China. The zone, thus traced throughout the breadth of the ancient continents from western Africa to long. 120° E., has been computed to cover an area of 6,500,000 square miles; but the Asiatic portion of this tract includes many chains of mountains and fertile valleys, The great Indian Desert in the Punjab is the only extension of this belt south of the Himalayas. The ring is completed by the Great Basin of North America in lat. 40° N. The southern zone, less complete, comprises the Kalahari Desert in southwest Africa, the interior of Australia and districts in Chile and in the Argentine Republic.

Deserts occur at all elevations, from considerable depth beneath sea-level to many thousand feet above it, and with all varieties of surface, from a flat expanse of sand, where the view for days of travel is bounded by a sharp circle as at sea, to rocky mountain slopes rent by rough defiles bare and chiseled by the driving sand. The essential character of an arid waste is its rainlessness, and the scarcity of water on the

[ocr errors]
[ocr errors]

surface and of water vapor in the atmosphere. Radiation in the clear air is rapid and desert climate is consequently of an exaggerated continental type. The sand in the Sahara becomes heated to over 150° F. during the day, and chilled below the freezing point at night, while the diurnal and seasonal extremes in the lofty deserts of central Asia are much greater. Thus desert-regions are most effective in producing land and sea breezes and monsoon winds in consequence of the marked periodical changes in atmospheric density. Another effect is the mirage (q.v.), a phenomenon which, combined with the great loneliness that oppresses the occasional traveler, probably accounts for the widespread superstitions peopling deserts, above all other places, with evil and malicious spirits. The dreaded sand storm or simoom is a kind of tornado or whirlwind which raises the sand in tall rotating columns sweeping over the surface with tremendous velocity. Sand-dunes sometimes several hundred feet in height are raised by steady winds, and gradually shift their position. The rocks of desert regions are usually worn into fantastic shapes by wind-drifted sand, and many plains are strewn with "desert pavements" of pebbles, the harder remnants of the rocks etched away by natural sand blast erosion. Desert vegetation is extremely scanty, consisting mainly of hard prickly plants of the cactus, euphorbia and spinose kinds, whose surface exhales little moisture. Animal life is correspondingly restricted both in variety and number of individuals. The camel is par excellence the beast of burden for conducting traffic across arid wastes. When an overflowing river, such as the Nile, traverses a desert, the land becomes richly fertile in its immediate neighborhood, and wherever springs bubble up through the sand there are oases, bearing palm trees and grass. Artificial irrigation, especially the sinking of artesian wells, has done much to reclaim tracts of desert for agriculture in the Sahara and to a less extent in Australia, while the area of arid land in the western part of the United States, once considered irreclaimable, is being rendered by, irrigation increasingly productive.

Geological considerations show that arid deserts are not permanent features of the earth's surface. The most level expanses are believed by many to have once formed part of the ocean bed, or at least great inland seas. The orographical changes which cut off these seas and created inland drainage areas probably at the same time modified the rainfall of the locality.

[blocks in formation]

Excessive evaporation dried up the great lakes, leaving at present a series of diminishing salt lakes without outlet, receiving rivers which dwindle down by evaporation as they flow. The only commercial commodities yielded by deserts are the salts (common salt, borax, sodium carbonate and sometimes sodium nitrate) left in the dried-up lake beds. These salt lakes are subject to alternate long periods of desiccation and flooding; during the former the area of the desert extends, during the latter it contracts. These periods have been traced out in the case of the Great American Basin by a series of most interesting researches on the part of the United States Geological Survey. See GOBI; SAHARA.

DESERT, The Great American. The North American deserts possess all the physiographic, geologic and climatic elements which distinguish the African desert of Sahara. The chief difference between the two regions is the relatively larger area of the Sahara, the arrangement of the topographic units and the occurrence in the Great American Desert of a wealth of mineral resources which the Sahara does not possess. Through the application of modern mechanical agencies by American energies and brain, its wastes have become inhabited by an intelligent and progressive people, and its arid hills and plains made to yield a wealth twice as much per capita as that of any other portion of the United States.

The vast stretches lying between the Sierra Nevada and California and the eastern Cordilleran ranges (Rocky Mountains) in the United States, and between the Pacific Ocean and the eastern Sierra Madre of Mexico, constitutes the Great American Desert.

Of the total area of the Cordilleran province, three-eighths are forested mountains, oneeighth plateau and one-half waterless, treeless, turfless mountain and valley desert. The deserts occur in Nevada, Utah, eastern and southern California, Arizona, New Mexico and all of Texas west of the Pecos, 550,000 square miles. The American Desert is international, however, for in addition to the above area within the United States, it continues southward into Mexico, where it includes most of the states of Sonora, Chihuahua, Coahuila, San Luis Potosí and Sinaloa-another 500,000 square miles-making a total of 1,050,000 square miles, which although one-third the area is as truly a desert in every natural sense as is the Sahara, which embraces an area of 3,500,000 square miles, and has a population of 2,500,000 people.

In its entirety (with a few exceptional forested summits) this desert province is one of barren, stony mountain ranges, separated by equally barren stretches of desert plain, an aggregation of elongated arid plains and lower mountain ranges, which mostly follow the avial line of the Cordilleras. The individual deserts have many names, and each differs from the other in some minor aspects.

From a technical point of view an arid desert in its ultimate analysis is a region in which the rainfall is insufficient to produce run-off. The light rainfall, striking the heated rock surfaces and sandy soils, is soon evaporated or drunk in; even the large bodies of water which may start down the mountain sides as roaring torrents usually die out at the margins of the

[ocr errors]

plains. These waters are highly charged with mineral salts derived from the heated rock surfaces, and these salts are readily redeposited upon the surface or in the interstices of the permeable sands. The torrents locally transport the rock débris-boulders, pebbles and powder - from one locality to another, but only for short distances; and hence the desert plains are usually composed of the débris of the adjacent mountains, which in more humid regions of ample run-off would have been carried to the sea. The expansion and contraction from the daily temperature causes the desert rocks to fracture in situ into the desert waste. This is distributed by wind and torrent, and hence the features of the desert are largely air-made as well as water-wrought.

The scarcity of moisture results in the absence of vegetation of the root-twining, soilgathering and soil-making type that distinguishes the humid region. Every plant and species attests the aridity of the country. Exactly as in the Sahara, these plants are thorny, coriaceous bushes and shrubs of the cactus, aloe and acacia families, adapted to withstand their droughty environment and to defend themselves from attack by man or beast.

Physiographically there are two sub-provinces of the Great American Desert, lying to the east and to the west of the western Sierra Madre and Colorado Plateau, respectively. The westernmost of these may be termed the Nevadan and the eastern the Chihuahuan. The western, or Nevadan, Desert occupies much of the area of Utah, Nevada, Arizona, southern and eastern California in the United States, and the states of Sonora and Sinaloa in Mexico. The Chihuahuan Desert occupies the vast area of country lying between the eastern and western Sierra Madre of Mexico and their northern continuation into southern New Mexico and Texas west of the Pecos, and is the so-called Mexican Plateau.

The Great Basin Desert is marked by wide flatness and is largely a region of ancient lake beds. Its surfaces are in many instances what the geologists term constructional, built up largely of great alluvial fans or piedmont alluvial plains, constituting the so-called filled valleys of the inter-montane belts. Its flora is mostly sage brush and grease wood; its agricultural products are cereals and tubers, and minerals gold, silver and copper. The Sonoran Desert is of a more complicated geological type, and instead of being land-locked is bordered on one side by the Pacific Ocean. Some of its surfaces are also the result of what geologists term destructional processes. Its floral types are the saguara, the palo verde and the catsclaw. Its sparse agricultural products are fruit and wheat, its mineral resources gold and copper.

The Chihuahuan Desert, marked by parallel plains and ranges, is a relatively higher region; its features are a combination of destructional and constructional processes. Its floral types are the maguey cactus and yucca; its chief agricultural product maize (corn), and its principal mineral product silver.

While the desert plains may be extensive, they also have many phases of variation. There are the alkali plains, while crystal patches of saline efflorescence which vegetation abhors, and vast plains of "doby» (adobe) - brownish choc

[ocr errors]
[graphic][subsumed]
[graphic][ocr errors][subsumed][subsumed][subsumed][subsumed][ocr errors][subsumed][subsumed][ocr errors][subsumed][ocr errors][subsumed][ocr errors]
[graphic][subsumed]
« PrejšnjaNaprej »