silting up estuaries. Currents assort the materials transported by them, depositing the heavy fragments of rock very near where they receive them, conveying the sand to a greater distance, while the mud settling very slowly may be transported a very great distance. Since the direction. and velocity of the current are tolerably uniform, the deposits will be in a good degree homogeneous. The bed of the German ocean is traversed by sand banks, one of which "the Dogger Bank," is three hundred and fifty-four miles long and eighty feet high. The greatest deposits, especially of fine clay, are probably made in the quiet depths of the ocean, beyond our observation. By the agency of tidal waves, sand is forced upon beaches; and when aided by storms, the sand is carried beyond the reach of succeeding waves. The isthmus of Suez, between the Mediterranean and Red seas, has doubled its breadth since the time of Herodotus. The collection of loose round water-worn pebbles, accumulated on beaches by waves, is called shingle. Very many bays and estuaries, are shoaling rapidly with the sediment conveyed into them by the waves of the ocean. The amount of matter brought down by rivers and deposited as deltas, is quite insignificant when compared with ocean deposits. So great is the quantity of detritus held in suspension by sea-water, that extensive tracts of land which have been purposely flooded with it repeatedly, have been raised five or six feet. The ocean also disperses through its vast extent, the saline substances, as common salt and carbonate of lime, which it obtains either directly from the rocks, or from the rivers which empty into it.

45. The general result of atmospheric and aqueous agencies, is the reduction of elevated portions of the earth to lower levels. The loss of land through their influence

greatly exceeds all deposits in the form of dry land; consequently a large portion of the detritus is spread over the bottom of lakes and of the ocean.

IGNEOUS AGENCIES.

46. We have seen that the tendency of atmospheric and aqueous agencies is to destroy the inequalities of the Earth's surface, and deposit the materials thus separated at the mouths of rivers and on the bottoms of lakes and seas. An antagonist force, however the igneous agency-is as constantly operating to restore and produce these inequalities. This agency exhibits itself in the phenomena of volcanoes, earthquakes, hot-springs, and gradual elevations of extensive lines of coasts and continents.

47. Volcanoes are openings in the earth, through which melted rock, lava, smoke, ashes, gases or vapors are discharged. They are usually inverted cones or craters at the summits of conical hills or mountains, which vary in height from the smallest hill to nineteen thousand feet, (Cotopaxi.) When they exist upon land they are called subaerial; when under the sea, submarine. Those which exhibit no evidence of action since the commencement of the historic period, are deemed extinct. Of the active volcanoes, some are constant, others intermittent. The periods of inter- a pè exte mission vary from a few months to centuries. Monte Epoqa/a meo, in Ischia, after remaining dormant one thousand seven hundred years, again burst forth in the early part of the fourteenth century of the Christian era. Volcanic vents, which emit only sulphurous, watery, and acid vapors, are called solfataras.

48. The number of active volcanoes is about three

hundred; of which two-thirds are on islands of the ocean,

and the others are usually near the sea. Some, however, are remote from large bodies of water, as Peschan in Central Asia, which is twelve hundred miles, and some of the Mexican and South American volcanoes, one hundred miles distant from the sea. Volcanoes are grouped as central, in which the disturbing power manifests itself in radii from a central point, as the Peak of Teneriffe and the Isle of Palma (one of the Canary islands); or volcanic chains or bands, in which a number of vents occur in a line extending over many miles, oftentimes coinciding with mountain chains. Such a band is presented by the volcanoes of America, those of the Andes and Rocky Mountains being connected by the Cordilleras of Mexico. The same chain may also connect with the remarkable line of volcanoes passing through the Aleutian Islands, Kamschatka, Japan, and the Molucca islands; thence by the Antartic Land (on which Captain Ross observed active volcanoes) to Terra del Fuego, thus encircling the globe.

49. The phenomena of an eruption usually commence with rumbling sounds in the earth and emission of smoke, sulphurous and acid gases from the mountain; stones and ashes are thrown with violent explosions from the crater, the earthquake increasing, until the molten lava flows freely down the mountain's sides. Toward the close of the eruption, cinders, red hot stones, and smoke are again thrown out. Impetuous showers of rain, with vivid lightning, and if the mountain is snow-clad, the sudden melting of snow and ice, render the scene still more complicate and awful. The lava sometimes does not rise to the brim of the crater, but bursts through the sides of the mountain and flows over the surrounding country.

50. The greatest exhibition of eruptive violence on record occurred in the island of Sumbawa (one of the Sunda group), in the year 1815. It commenced on the fifth of April, increased in violence until the twelfth, and ceased in July. The explosions were heard in Sumatra, nine hundred and seventy miles west, and at Ternato, seven hundred and twenty miles east of the island. The ashes were carried three hundred miles in the direction of Java, and two hundred and seventeen miles northward toward Celebes in sufficient quantity to produce darkness equal to what is ever witnessed in the darkest night. floating cinders westward of Sumatra formed a mass two feet thick, several miles in extent, through which ships with difficulty forced their way. Several streams of lava, issuing from the crater of the Tomboro mountains, covered extensive tracts of land and ran into the sea. The area convulsed by this volcanic paroxysm was one thousand miles in circumference. Out of a population of twelve thousand on the island, twenty-six only survived.*

The

51. Of all eruptions of modern times, the most remarkable in respect to the quantity of lava ejected was that of Skaptaa Jokul, in Iceland, in the year 1783. On the eighth of June of that year, clouds of smoke began to collect in the mountain, obscuring the light of day, showering down great quantities of ashes and sand. On the tenth, slight shocks of earthquake and flames were perceived. On the eleventh, the large river Skaptaa, which had been much swollen, entirely disappeared, and the next day a current of lava rushed down the mountain and overflowed the channel of the river, which was in some places from four hundred to six hundred feet deep, and *Lyell's Principles.

two hundred feet broad. The lava continued to flow until the twentieth of July, pouring over a lofty cataract, and filling up in a few days an enormous cavity which the river had been for ages hollowing out. On the night of the ninth of August, another torrent overflowed the country to the extent of more than four miles. The eruptions continued, with intervals, till the end of August, and closed with a violent earthquake. One of the streams of lava was fifty miles long by twelve broad, and the other forty miles long by seven broad. Their thickness was, in the narrow channels, five or six hundred feet; but on the plains rarely more than one hundred feet, and in some places only ten feet. Taking the lowest average thickness, the mass of lava can not have been less than twenty thousand millions of cubic yards. Thirteen hundred human beings lost their lives, more than one hundred and fifty thousand domestic animals were destroyed, the fisheries on the coast were ruined, and it is affirmed that Iceland has not yet recovered from the ravages of this eruption of Skaptaa Jokul. The great mass of lava spread over the land by this eruption, distended by gases, cooling on the surface and becoming solid, while the central parts continued liquid and flowing onward, left caverns of great extent and singular appearance.

The cavern of Surtsheller, or the "black cavern," is a long, winding canal, with several branches, enclosed by a crust of lava six feet thick. It is twenty-five feet wide, and its sides and vaulted roof are studded with stalactites of lava and ice.

52. The history of Vesuvius and Etna is more complete than that of any other center of volcanic action, because their phenomena have been for a longer time intelli