Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/280

Rh 268 houses and fields ,- even whole parishes and districts once populous have been overwhelmed by them.‘ Along the margins of large lakes and inland seas many of the phenomena of an exposed se-a~coast are repeated, and on no inferior scale. Among these must be included sand- dunes, such as occur at the south-eastern end of Lake Michigan and on the eastern borders of the Caspian Sea. The shifting of vast waves of sand by the win(l is exempli- fied on the gr-andest scale in the sandy deserts of Africa, Arabia, and Central Asia. Such arid wastes of loose sand, situated far inland and far distant from any sheet of fresh water, suggest curious problems in physical geography. Their sites may have been at a comparatively recent geo- logic-al period covered by the sea; or, lying in rainless climates and having their surfaces exposed to the disinte- grating effects of great extremes of temperature, the tracts may have become sandy and barren through atmospheric disintegration. The desert of the Sahara furnishes a good illustration of a dried-up sea—bed. In the rainless tract to the east of the Red Sea lie the great sandy deserts and hills of Arabia, of which Mr Palgrave has given so graphic a narrative. Captain Sturt found vast deserts of sand in the interior of Australia, witl1 long lines of dunes 200 feet high, 1mite:l at the base and stretching in straight lines as far as the eye could reach. In the south—east of Europe great tracts of sandy desert occur in Poland, and run‘ through the southern provinces of Russia. Dust—s/zozrcrs, Bloocl—raz'n.—I11 tropical countries, where great droughts are succeeded by violent hurricanes, the dust or sand of dried lakes or river-beds is some- times borne away into the upper regions of the atmo- sphere, where, meeting with strong aerial currents which transport it for hundreds and even thousands of miles, it may descend again to the surface, in the form of “red-fog,” “sea—dust,” or “sirocco-dust.” This trans- ported material, usually of a brick—dust or cinnamon colour, is occasionally so abundant as to darken the air and. obscure the sun, and to cover the decks, sails, and rigging of vessels which may even be l1u11dreds of miles from land. lain falling through such a dust—cloud mixes with it, and descends either on sea or land as what is popularly called “ blood—rain.” This is frequent on the north-west of Africa, about the Cape Verd Islands, in the Mediterranean, and over the bordering countries. A 1uicro.scopic examination of this dust by Ehrenberg led him to the belief that it contains numerous diatoms of South American species; and he inferred that a dust-cloud n1ust be swimming in the I atmosphere, carried forward by continuous currents of air in the region of the trade-winds and anti-trades, but suffering partial and periodical deviations. ' But much of the dust must come from the sandy plains and desiccated pools of the north of Africa. Daubrée recognized in 1865 some of the Sahara sand which fell in the Canary Islands. On the coast of Italy a film of sandy clay, identical with that in parts of the Libyan desert, is occasionally found on windows after rain. In the middle of last century an area of northern Italy, estimated at about 200 square leagues, was covered with a layer of dust which in some places reached a depth of one inch. Should the travelling dust encounter a cooler temperature, it may be brought to the ground by snow, as has happened in the north of Italy, and more notably in the east and south-east of Russia, where the snows are sometimes rendered dirty by the dust raised by the winds on the Caspian steppes. It is easy to see that a prolonged continuance of this action must give rise to widespread deposits of dust, mingled with the soil of the land, and with the silt and sand of lakes, rivers, 1 This destruction has been, during the last quarter of a century, averted to a great extent by the planting of pine forests, the turpen- tine of which has become the source of a large revenue. GEOLOGY [u1. m'.'..ic.L. ' and the sea; and that the minuter organisms of tropical regions may thus come to be preserved in the same forum- tions with the terrestrial or marine organisms of temperate latitudes? Trtmsporli(lion qf Sec:/.~‘. llcsides the transport of dust and minute organisms for distances of many tliousands of miles, the same agency may come into play also in the transport of living seeds, which, finally reaching a congenial climate and soil, may take root and spread. We are yet, however, very ignorant as to what extent this cause has actu- ally operatcd in the establishment of any given local flora. With regard to the minute forms of vegetable lifc, indeed, there can be no doubt as to the etlicacy of the wind to transport them across vast distances on the surface of the globe. Upwards of 300 species of diatoms have been found in the deposits left by dust-showers. Among the millions of organisms thus transported it is hardly concci- able that some should not fall into a fitting l:.»c-dity for their continued existence and the perpetuation of their I species. 53. I 21]] name of (fee Air on ll'«u‘cr. The action of the air upon water will be more ﬁtly noticed in the section devoted to water (p. 245). It will be enough to notice here- I. Ocean Cm'7'e12{3.—'l‘ he in-streaming of air from cooler latitudes towards the equator causes a drift of the sea-water in the same direction. Owing to the rotation of the earth, these aerial currents tend to takea more and n1orc westerly trend as they approach the equator. This they comn1m1i- ' eate to the marine ciirrents, which, likewise moving into regions having agreater velocity of rotation than lllLll‘l, are all the more impelled in the same westerly direction. Hence the westerly belt or equatorial current, which flows across the great ocean. Owing to the position of the con- tinents across its path, this great current cannot. move un- interruptedly round the earth. It is split into l-ranches which turn to right and left, aml, bathing the shores of the land, carry some of the warmth of the tropics into more temperate latitudes. 2. ll'aves.—Tlie impulse of the wind upon a surface of . water throws that s1n'faee into pulsations which range in size from mere ripples to huge billows. Long-continued gales from the seaward upon an exposed coast indirectly effect much destruction, by the formidable battery of billows which they bring to bear upon the land. Wave—action is likewise seen in a marked manner when wind blows strongly across a broad inland sheet of water, such as Lake Superior. (See p. 279.) 3. A1[i£)')‘.(l{’:0)I of Me llYticr-Iez'cI.—Vhen the wind blows freshly for a time down a lake or into a bay or arm of the sea, it drives the water before it, and keeps it temporarily at a higher level, at the further or windward side. In a tidal sea, such as that which surrounds Great ]}ritain, and which sends abundant long arms into the land, this action ea11 often be studied. It is no infrequent occurrence that a high tide and a gale should happen at the same time. Whenever that takes place, then at those bays or firths which look windward the high tide rises to a greater height than elsewhere. Vith this conjunction of wind and tide, con- siderable damage to property has sometimes been done ly the flooding of warehouses and stores, while even -a sensible destruction of cliffs and sweeping away of loose materials may be chronicled by the geologist. On the other hand, a wind from the opposite quarter will drive the water out of the inlet, and thus make the water-level lower than it should otherwise be. i 9 See llumboldt on «lust whirlwinds of the Orinoco, .l.s_m’Cl3 ff Nature; also IIaur_v, l’l«_r/s. Geog. of Sm, chap. vi.; and Elirenhergs I I’assat-Sta1I.b uml ljlut-l.‘e_r/cn, 1847.