Slike strani
PDF
ePub

THE

ENCYCLOPEDIA AMERICANA

D

[ocr errors]

ESERT, a term designating in its surface and of water vapor in the atmosphere.

broadest application any uninhabited Radiation in the clear air is rapid and desert region. In its more ordinary use the climate is consequently of an exaggerated con

word means a portion of the earth's tinental type. The sand in the Sahara becomes surface that on acount of its barrenness can heated to over 150° F. during the day, and not support inhabitants. Four classes of desert chilled below the freezing point at night, while may be distinguished: (1) Ice-wastes occupy the diurnal and seasonal extremes in the lofty the central plateau of Greenland, the is deserts of central Asia are much greater. Thus lands of the Arctic Sea, and probably the entire desert-regions are most effective in producing Antarctic continent. (2) Tundras (q.v.) are land and sea breezes and monsoon winds in conflat plains, little elevated above sea-level, fring sequence of the marked periodical changes ating the Arctic shores of the northern continents mospheric density. Another effect is the mirage and especially characteristic of Siberia. (3) (q.v.), a phenomenon which, combined with the Temporary deserts, or steppe-lands, border the great loneliness that oppresses the occasional Asiatic deserts to the north and west. The saline traveler, probably accounts for the widespread steppes of the Caspian are true arid wastes; but superstitions peopling deserts, above all other the typical steppes in South Russia are luxu places, with evil and malicious spirits. The riantly clothed with verdure and flowers in dreaded sand storm or simoom is a kind of torspring. In the dry season they form a dusty nado or whirlwind which raises the sand in tall plain of withered herbage. The llanos (q.v.) of rotating columns sweeping over the surface with the Orinoco have similar characteristics, but the tremendous velocity. Sand-dunes sometimes pampas of South America include portions per several hundred feet in height are raised by ennially green and suitable for agriculture. (4) steady winds, and gradually shift their position. Arid wastes, or deserts in the popular sense of The rocks of desert regions are usually worn the word, occur mainly in two. zones encircling into fantastic shapes by wind-drifted sand, and the world, and corresponding to regions of mini many plains are strewn with desert pavements) mum rainfall. The greater zone extends from of pebbles, the harder remnants of the rocks near the equator in an east-northeast direction etched away by natural sand blast erosion. across the whole breadth of North Africa, as the Desert vegetation is extremely scanty, consisting Great Sahara, Libyan and Nubian deserts, over mainly of hard prickly plants of the cactus, the peninsula of Arabia, through Persia, Turke euphorbia and spinose kinds, whose surface exstan and the vast tracts of Gobi or Shamo to hales little moisture. Animal life is correspondthe confines of China. The zone, thus traced ingly restricted both in variety and number of throughout the breadth of the ancient continents individuals. The camel is par excellence the from western Africa to long. 120° E., has been beast of burden for conducting traffic across arid computed to cover an area of 6,500,000 square wastes. When an overflowing river, such as the miles; but the Asiatic portion of this tract Nile, traverses a desert, the land becomes richly includes many chains of mountains and fertile fertile in its immediate neighborhood, and whervalleys, The great Indian Desert in the Punjab ever springs bubble up through the sand there is the only extension of this belt south of the are oases, bearing palm trees and grass. ArtiHimalayas. The ring is completed by the Great ficial irrigation, especially the sinking of arteBasin of North America in lat. 40° N. The sian wells, has done much to reclaim tracts of southern zone, less complete, comprises the Kal desert for agriculture in the Sahara and to a ahari Desert in southwest Africa, the interior less extent in Australia, while the area of arid of Australia and districts in Chile and in the land in the western part of the United States, Argentine Republic.

once considered irreclaimable, is being rendered Deserts occur at all elevations, from consid by irrigation increasingly productive. erable depth beneath sea-level to many thousand Geological considerations show that arid desfeet above it, and with all varieties of surface, erts are not permanent features of the earth's from a flat expanse of sand, where the view for surface. The most level expanses are believed days of travel is bounded by a sharp circle as at by many to have once formed part of the ocean sea, to rocky mountain slopes rent by rough bed, or at least great inland seas.

The orodefiles bare and chiseled by the driving sand. graphical changes which cut off these seas and The essential character of an arid waste is its created inland drainage areas probably at the rainlessness, and the scarcity of water on the same time modified the rainfall of the locality,

sea.

Excessive evaporation dried up the great lakes, plains. These waters are highly charged with leaving at present a series of diminishing salt mineral salts derived from the heated rock surlakes without outlet, receiving rivers which faces, and these salts are readily redeposited dwindle down by evaporation as they flow. The upon the surface or in the interstices of the only commercial commodities yielded by deserts permeable sands. The torrents locally transport are the salts (common salt, borax, sodium car the rock débris — boulders, pebbles and powder bonate and sometimes sodium nitrate) left in the - from one locality to another, but only for dried-up lake beds. These salt lakes are sub short distances; and hence the desert plains are ject to alternate long periods of desiccation usually composed of the debris of the adjacent and flooding; during the former the area of the mountains, which in more humid regions of desert extends, during the latter it contracts. ample_run-off would have been carried to the These periods have been traced out in the case The expansion and contraction from the of the Great American Basin by a series of most daily temperature causes the desert rocks to interesting researches on the part of the United

fracture in situ into the desert waste. This is States Geological Survey. See GOBI; SAHARA. distributed by wind and torrent, and hence the

features of the desert are largely air-made as DESERT, The Great American. The

well as water-wrought. North American deserts possess all the physi

The scarcity of moisture results in the abographic, geologic and climatic elements which

sence of vegetation of the root-twining, soildistinguish the African desert of Sahara. The

gathering and soil-making type that distinchief difference between the two regions is the guishes the humid region. Every plant and relatively larger area of the Sahara, the arrange species attests the aridity of the country. ment of the topographic units and the occur

Exactly as in the Sahara, these plants are rence in the Great American Desert of a wealth

thorny, coriaceous bushes and shrubs of the of mineral resources which the Sahara does not

cactus, aloe and acacia families, adapted to possess. Through the application of modern

withstand their droughty environment and to mechanical agencies by American energies and defend themselves from attack by man or beast. brain, its wastes have become inhabited by an

Physiographically there are two sub-provintelligent and progressive people, and its arid inces of the Great American Desert, lying to hills and plains made to yield a wealth twice the east and to the west of the western Sierra as much per capita as that of any other portion Madre and Colorado Plateau, respectively. The of the United States.

westernmost of these may be termed the Nevadan The vast stretches lying between the Sierra and the eastern the Chihuahuan. The western, Nevada and California and the eastern Cordil

or Nevadan, Desert occupies much of the area leran ranges (Rocky Mountains) in the United

of Utah, Nevada, Arizona, southern and eastStates, and between the Pacific Ocean and the

ern California in the United States, and the eastern Sierra Madre of Mexico, constitutes the states of Sonora and Sinaloa in Mexico. The Great American Desert.

Chihuahuan Desert occupies the vast area of Of the total area of the Cordilleran prov country lying between the eastern and western ince, three-cighths are forested mountains, one Sierra Madre of Mexico and their northern eighth plateau and one-half waterless, treeless, continuation into southern New Mexico and turfless mountain and valley desert. The des Texas west of the Pecos, and is the so-called erts occur in Nevada, Utah, eastern and south Mexican Plateau. ern California, Arizona, New Mexico and all The Great Basin Desert is marked by wide of Texas west of the Pecos, 550,000 square flatness and is largely a region of ancient lake miles. The American Desert is international, beds. Its surfaces are in many instances what however, for in addition to the above area

the geologists term constructional, built up within the United States, it continues south largely of great alluvial fans or piedmont alluward into Mexico, where it includes most of the vial plains, constituting the so-called filled valstates of Sonora, Chihuahua, Coahuila, San leys of the inter-montane belts. Its Aora is Luis Potosí and Sinaloa — another 500,000 mostly sage brush and grease wood; its agrisquare miles — making a total of 1,050,000 cultural products are cereals and_tubers, and square miles, which although one-third the area minerals gold, silver and copper. The Sonoran is as truly a desert in every natural sense as is Desert is of a more complicated geological type, the Sahara, which embraces an area of 3,500, and instead of being land-locked is bordered on 000 square miles, and has a population of one side by the Pacific Ocean. Some of its 2,500,000 people.

surfaces are also the result of what geologists In its entirety (with a few exceptional for term destructional processes.

Its floral types ested summits) this desert province is one of are the saguara, the palo verde and the catsbarren, stony mountain ranges, separated by claw. Its sparse agricultural products are fruit equally barren stretches of desert plain, an and wheat, its mineral resources gold and aggregation of elongated arid plains and lower copper. mountain ranges, which mostly follow the avial The Chihuahuan Desert, marked by parallel line of the Cordilleras. The individual deserts plains and ranges, is a relatively higher region; have many names, and each differs from the its features are a combination of destructional other in some minor aspects.

and constructional processes.

Its floral types From a technical point of view an arid des are the maguey cactus and yucca; its chief ert in its ultimate analysis is a region in which agricultural product maize (corn), and its printhe rainfall is insufficient to produce run-off. cipal mineral product silver. The light rainfall, striking the heated rock sur While the desert plains may be extensive, faces and sandy soils, is soon evaporated or they also have many phases of variation. There drunk in; even the large bodies of water which are the alkali plains, while crystal patches of may start down the mountain sides as roaring saline efflorescence which vegetation abhors, and torrents usually die out at the margins of the vast plains of doby» (adobe) — brownish choc

THE IBAN DESERT

[graphic][subsumed]
« PrejšnjaNaprej »