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The beds of many rivers, again, become elevated by alluvial deposits in the course of years, as, for example, that of the Po in Italy: while the same cause often produces triangular plains at the mouths of rivers, called deltas; the most remarkable of which is the delta of the Nile.

50. Most of the large rivers discharge their waters into the sea by several mouths; as the Nile, the Ganges, the Volga, the Niger, the Orinoco, and the Rhine.

51. Some rivers are subject to periodical floods; as the Nile, the Ganges, the Indus, and the Missisippi. These are produced by the heavy rains which fall during the wet season in the torrid zone, and by the annual melting of the snow on the mountains from which the rivers rise. These floods, though sometimes destructive, irrigate soils otherwise barren; and in some cases create extensive plains of remarkable fertility.

52. Rivers are occasionally engulphed, especially in limestone districts; they sink under ground, flow on for several miles, and then reappear. This is the case with the Rhine, and the Guadiana in Spain. Subterranean streams likewise abound in Greece.

53. Changes of Land and Water.—To trace the history, to note the phenomena, and to investigate the causes, of the remarkable interchanges which land and water undergo, together with their mutual action, belongs to the province of geology. We shall here briefly notice them only as they affect geography.

54. Portions of the land, wasted away into detritus, are being continually carried down into the sea, and so change the relative level of the land and the The Gulph of Mexico is rapidly filling up by

ocean.

the sedimentary deposits from the waters of the Missisippi. The Lake of Geneva is filling up in like manner by Alpine debris carried down into it by the Rhone.

55. The encroachments of the ocean on the land are strikingly shown by the changes on our own coasts, and on those of Holland and Friesland. The chalky heights of Dover and the softer strata along the east coast are annually giving way. Sometimes the invasions of the sea are more rapid and devastating; as in the destruction, in the beginning of the thirteenth century, of the isthmus which united Holland with Friesland, by which the Zuyder-Zee was formed.

56. Earthquakes.- Earthquakes are powerful agents in changing the aspect of a country. Thus in 1755, Lisbon was in a great measure destroyed by one of the most awful earthquakes that ever visited Europe: the new mole suddenly sank into a hideous abyss, and on its former site there are now 100 fathoms of water. The earthquakes that desolated Calabria in 1783, depressed the land in many places, in others opened extensive fissures, in others effected partial elevations. In 1797, Upper Peru was terribly convulsed, and the face of the country totally changed. In 1811, violent earthquakes shook the valley of the Missisippi, by which lakes of considerable extent disappeared, and new ones were formed. Earthquakes in South America, during 1822-3, permanently raised the level of the land to heights varying from two to seven feet, over an area, extending from the base of the Andes to the sea, of not less than 100,000 square miles.

57. Volcanoes.-Most of the phenomena of earthquakes are the consequences of volcanic fire. At present there are nearly two hundred active volcanoes in different parts of the world. Tierra del Fuego, Peru, Chili, and New Granada are filled with them. Numerous islands in the Pacific are volcanic. Java alone contains thirty-eight. In Europe, a great volcanic zone traverses Greece, Italy, Germany, and France; but it is no longer active in the two latter countries. Iceland abounds in volcanoes; of which Hekla, though not the most considerable, is the best known. Several of the most lofty mountains of South America are active volcanoes. Volcanic rocks prevail over the whole table-land of Mexico; and there, amid other cones, rises the lofty Popocateptl, whose elevation exceeds 17,000 feet. But the most remarkable, perhaps, is the volcano of Jorullo, formed as recently as 1759. In one night there issued from the earth, in the midst of more than two thousand burning apertures, called hornitos or ovens, six vast mountain masses rising to heights varying from 300 to nearly 1700 feet above the original level of the plain; constituting, says Humboldt, "one of the most remarkable physical revolutions in the annals of our globe."

SECTION III.

THE ATMOSPHERE.

58. The extent, the weight, the composition, and the mechanical properties of the atmosphere; the

formation of clouds, fogs, rain, snow, and dew, and indeed the whole range of meteorological phenomena, are points of interest in physical geography: but we must here be content to consider only one or two of them.

59. A portion of the atmosphere exposed to the sun's rays becomes heated, expands, and, being thus made lighter than before, ascends; while the surrounding air, being colder and heavier, rushes in to supply its place. The current of air thus produced is called wind. Winds are generally divided into three classes-permanent, periodical, and variable.

60. The permanent winds extend nearly thirty degrees on each side of the equator, and blow almost always in the same direction, namely, from the east. They are usually called trade winds, because of the valuable assistance they render to navigation. They are caused by the higher temperature, and the greater rotatory motion of the equatorial regions of the earth. To supply the place of the air which is continually ascending, as already explained, from the torrid zone, continual currents move from the colder regions towards the equator. These regions having a slower rotatory motion than the equatorial, the currents which proceed thence are, in fact, left behind when they reach the equator; and since the earth rotates towards the east, they appear to move, as already stated, from the east. Since, however, their original direction was towards the equator, that is, from the north or from the south,-they do not blow due westward except at the equator itself: at the northern tropic, they blow from the north-east, and at the

southern, from the south-east, varying a point or two of the compass either way. If the great equatorial band of the earth were covered with water, the trade winds would constantly and regularly blow in the direction here stated. But the unequal and varying temperature produced by the interposition of large tracts of land, of snow-clad mountains, and of heated plains of sand, diverts them from their course, and often subjects them to sudden and great irregularities. Hence it is that the trade winds are more constant and regular in the Pacific than in the Atlantic, and in the Atlantic than in the Indian Ocean.

61. These irregularities give rise to the periodical winds, which change with the changing seasons. The most important of these are the Monsoons, which chiefly prevail in the Indian Ocean.

From

April to October they blow from the S.W.; during the other six months, the regular trade wind from the N. E. resumes its course. When these changes take place, violent tempests and thunder storms ensue. The N. E. monsoon is produced by the same causes as the general trade wind. The S. W. monsoon appears to be occasioned by the great rarefaction of the air over the extensive regions of Eastern Asia when the sun is north of the equator, and the rushing in of the dense air of the Indian Ocean to occupy the place of the ascending current. The land and sea breezes, which are common on tropical sea-coasts, especially the latter, are produced by the various differences of temperature between the land and the sea; which give rise to several currents, in the manner already explained.

62. In temperate climates the winds are variable,

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