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the river. These agitations of the Mississippi valley ceased, when the South American cities were destroyed. This coincidence, together with the direction of the chasms, indicates a subterranean communication between the localities, which are more than two thousand miles apart. The violent earthquake of Guadaloupe, which occurred in A. D. 1842, extended in a direction North-west to South-east, from Charleston, South Carolina, to the mouth of the Amazon river, destroying several towns in the West India islands.

66. Hot-springs are common in the immediate vicinity of volcanoes. They are found upon the slopes of Etna and Vesuvius, but in great numbers, and with more imposing features in that remarkable field of igneous agency, Iceland, where they are called Geysers-raging fountains. Within a circuit of two miles more than one hundred of them may be found. They are situated in sight of Mount Hecla, in a plain at the foot of a hill of gray trachyte (ancient lava.) The crater of the great Geyser is a flattened cone of silicious matter; the basin within is of an oval figure fifty-six feet by forty-six, terminating at the bottom in a perpendicular pipe, seventy-eight feet deep. Usually the basin is filled with clear water of the temperature of 180°.

At times a subterranean sound, resembling that made by a volcano during an eruption, is heard, and then a slight tremulous motion is perceived on the rim of the fountain; the surface of the water in the basin becomes convex, and large bubbles of steam rise and burst, throwing up the boiling water several feet high: a heavier noise is heard below, and suddenly there shoots up a column of water to the height of one hundred feet, dis

persing at the summit into dazzling white foam. After a brief period, a column of steam issues, with a loud, roaring noise se; this is followed by another column of water higher than the preceding ones, mingled with stones, accompanied by loud detonations. The phenomenon lasts a few minutes, and then the basin resumes its tranquil state. These waters encrust, the surrounding soil, and all substances upon which they fall, with silica.

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Boiling springs are found also in the Azores, Java, and the volcanic regions of Central and South America.

Thermal springs are not confined to the vicinity of active volcanoes, but are found in the Alps, and Pyrenees, in Virginia, Arkansas, and Oregon. In all cases, however, they are situated near mountains or rocks which have been subjected to igneous agency. The temperature of the

water varies in different springs; that of the seventy springs in Arkansas ranges from 118° to 148°. Thermal waters are charged with salts and gases; hence they are mineral waters, as those of Bath in England, Wiesbaden in Germany, Saratoga, and the Sulphur Springs of Virginia. S 67. Another apparent manifestation of igneous agency is the gradual elevation or subsidence of portions of the earth's surface. An interesting example of this is presented by the remains of the temple of Jupiter Serapis, at Puzzuoli, on the shore of the Bay of Baiæ, near Naples. The Fig. 30.

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Pillars at Serapis.

building was originally of a quadrangular form, twenty feet in diameter, the roof being supported by twenty-four granite columns, and twentytwo of marble: three of them are still standing. The tallest of them is forty-two feet in height; the surface is smooth and uninjured to an elevation of twelve feet, where commences a band of perforations

the

made by the Lithodomus (lithos, stone, and domos, a house,) a shell-fish inhabiting the Mediterranean sea. These per forations, many of which still contain shells, cover a space of nine feet, and are so numerous and deep as to prove that the pillars were for a long time immersed in sea water; lower portions were protected by rubbish and puzzolanavolcanic tufa; the upper portions projected above the water, beyond the reach of the lithodomi. The platform of the temple is now one foot below high-water mark, and the sea is one hundred and twenty feet distant. These columns

have been depressed and again elevated more than twenty feet, the relative level of the land and sea having changed at least twice since the Christian era, so gently that these columns have not been prostrated. Professor Babbage attributes their tranquil depression and elevation to the contraction and expansion of the rocks on which they stand, in consequence of variations of temperature. A small volcano-solfatara-and a hot spring exist in their vicinity. The columns are again gradually subsiding.

68. The gradual change of relative level of sea and land, on an extensive scale, in regions remote from active volcanoes and violent earthquakes, is exemplified on the coasts of the Baltic sea and Northern ocean. Beds of marine shells and sunken rocks have been raised above the water-line, and the shells of species now living on the shore are found fifty miles inland and at an elevation, as ascertained by Bravais' measurements, of six hundred feet above the ocean. The northern portion of the coast rises most rapidly; the average elevation is stated to be four feet in a century. Raised beaches found in England at a height of from twenty to two hundred feet above the existing sea-level, with shells and all the features of the beaches of the present sea-coast, show the same process on that island. Mr. Darwin has also shown that the southern part of South America, at least twelve hundred miles on the east coast, from Rio de la Plata to the straits of Magellan, and a greater distance on the west coast, has been raised from one to four hundred feet. The north-eastern coast of the United States is supposed to be in the same process of gradual elevation. On the other hand, a large portion of the coast of Greenland has been for four centu

ries gradually sinking. Ancient buildings on low islands and near the coasts, have been submerged. The depression of extensive areas in the Pacific and Indian oceans is proved by the existence of banks of dead coral several hundred feet deeper than the limit at which the animal can have lived. On the coasts of Europe and America are found submarine forests, consisting of trees and stumps, together with peat, in the position in which they originally grew, depressed several feet below the level of the sea.

ORGANIC AGENCIES.

69. Although organic agencies are less influential in modifying the crust of the globe, than aqueous and igneous agencies, still vegetable and animal bodies not unfrequently make up a large portion of important and extensive rock formations; and are regarded by the geologist with special interest because they furnish the clearest indications of the physical conditions of the globe at the time and place in which they lived.

70. Marine plants, which oftentimes cover the surface of the ocean so thickly that ships are impeded in their progress by them, are very perishable, and contribute little to the formation of rocks. But the remains of terrestrial plants enter into the composition of soils, and form extensive deposits in the great swamps. Peat consists principally of the fibrous roots of mosses, especially of the Sphagnum, which continually throw up new shoots from the decaying extremities below. When dry it consists of from sixty to ninety-nine per cent. of carbonaceous matter, forming a valuable fuel. Peat beds, of from four to twenty feet thickness, are common in Ireland and Scotland. The "moss" on the

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