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of volcanic eruptions at least is the expansive force of steam. The crust of the earth, though upon the whole solid, is believed to contain cavities or lakes of molten rock, and volcanoes are orifices of communication between them and the surface. Into these cavities, water, sinking down through crevices from the ocean or the land, must be constantly finding its way; and the steam thus generated exerts such enormous pressure as to force the molten rock to the surface, itself mingling and escaping along with it. While under pressure, the molten matter becomes highly charged with steam condensed in its substance; and as it approaches the orifice, and has the pressure removed, what happens is comparable to the evolution of carbonic acid in sodawater when the cork is removed, which is often violent enough to eject the whole contents of the bottle into the air-a veritable eruption. Owing to the constant shrinking and cracking of the crust, masses of water must frequently be precipitated suddenly into these hot caverns, and the explosion of steam will produce a concussion, and, where there is no vent, may be suffi cient to convulse and rend the superincumbent strata.

Another cause of earthquakes has been suggested in the falling of large rock masses from the roofs of subterranean cavities, or in the cracking of the rocky strata when bent or crushed in the process of shrinking.

163. Extinct Volcanoes.-Every considerable region on the globe bears marks of former volcanic action now extinct. During the geological history of our planet, the seat of activity seems to have often shifted, the eruptions dying out at one place only to break out fresh at another. In some places, as in Auvergne in France, there are yet to be seen cones and craters and descending currents of lava as perfect and fresh as if they had been

in activity a few years ago. In other places, as in the British Islands, the volcanic rocks have been so long exposed to the waste of the elements, that no actual crater is now to be seen. There is evidence, however, that a chain of volcanic islands extended from the north of Ireland to Iceland, and formed the hills of Antrim, Mull, Eigg, Skye, Faroe, and the older parts of Iceland. The present active volcanoes of Iceland are probably a remnant of this band (see par. 127).

UPHEAVAL AND SUBSIDENCE.

164. It sometimes happens that after an earthquake the level of the ground remains altered over a considerable area. Thus, in 1819, the Great Runn of Kutch in Hindustan was depressed over an area of several square miles, so as during the monsoons to become a salt lake. In New Zealand, on the other hand, the ground in the neighbourhood of Wellington, during an earthquake in 1855, was raised several feet. But these sudden changes are not always permanent. In 1835, a considerable extent on the western shore of South America rose a yard or more; but after a few months the ground sank again to its former level.

But these sudden alterations are of little moment compared with the tranquil movements of elevation and depression that are always going on. Observations carried on for more than a century show that the south of the Scandinavian peninsula is sinking; in the towns of Ystadt and Malmö whole streets have been abandoned

to the sea. The northern part of the peninsula, on the contrary, is rising from two to three feet in a century. A similar movement of elevation is going on in Spitzbergen, Northern Siberia, Greenland, the whole western coast of South America, Japan, Asia Minor, &c.

165. Raised Beaches.-We see in many places on our own coasts flat terraces at a little distance above the level of the sea; and such a terrace has often at its landward margin a range of cliffs which bear evidence that the waves once dashed against them, as they now do against their present rocky border. We have here a visible proof that the ground has been pushed up several feet from below.

166. Submerged Forests afford a striking proof of subsidence. In the south of Devonshire, remains of a submerged forest are visible under five fathoms of water; and on the coast near Lincoln, low-water exposes stumps of trees protruding through a deposit of peat. From these and other facts, it is believed that the British Islands and the west of Europe generally must have subsided at least seventy feet since these trees were alive. But nowhere are there marks of such extensive subsidence as in the southern oceans, where many coral reefs and islands speak of the submergence of the foundations on which they were built (see par. 139).

167. Repeated Alternations of Level.-The sedimentary strata of the earth's crust everywhere bear evidence that these sinkings and risings have been going on since the beginning of geologic time. Nor has the movement been always in the same direction. The same portions have been repeatedly submerged, and as often upheaved. This is most strikingly seen in the 'Coal Measures,' as they are called. In the same 'field' there are often several seams of coal one above another, with sedimentary strata between. Now, from what was said above as to the origin of coal, it is clear that each seam bespeaks an oscillation of the level—a submergence and an upheaval.

168. Causes of Upheaval and Subsidence.-Some have maintained that these changes of level are due to the water, and not to the land-it is not the land that has risen, but the sea that has sunk. To refute this theory, we have only to look at Scandinavia. A rise of the sealevel would submerge the streets of Malmö; but from the very nature of water, if it rose at Malmö, it must rise at Tornea, and yet it there appears to sink. The most probable theory is, that the upheavals and subsidences of the ground arise from the gradual shrinking of the earth as it cools. The warm interior is losing heat faster than the comparatively cold exterior; and this causes the core to shrink away from the crust. But owing to its curved form, the crust cannot follow the core except by bending downwards at one place and upwards at another. In short, it is thrown into puckers like the rind of a shrivelled apple. In this way a continent may sink below the level of the waters, and an adjoining part of the sea-bottom be thrust up into the air. In this way also it has been brought about that beds of rock are seldom horizontal, but incline or dip in various directions; and where the bendings have been abrupt, they bear marks of having been crushed, twisted, and dislocated. Mountain-ranges may thus be regarded as simply corrugations produced by the shrinking of the earth's crust.

THE SEA.

169. Composition of Sea-water.-In every hundred parts of sea-water there are three and a half parts of solid matter in a state of solution. This solid matter is made up of a variety of salts; but four-fifths of the whole consists of common salt (chloride of sodium). The other

ingredients are chloride of magnesium, chloride of potassium, bromide of magnesium, sulphate of magnesia, sulphate of lime, carbonate of lime, with traces of silica, iodine, and ammonia. It is chiefly to the chloride and bromide of magnesium that the peculiar bitterness of sea-water is owing.

It has been made a question, whence the sea got its saltness. But in this there is no mystery. Salt is one of the substances that make up the globe, just as quartz is; and when the water began to condense from its original steam, and to gather together into the hollows, it would of course carry a large share of such a soluble substance as salt along with it. This original saltness must have been on the increase all along; for the water that falls from the clouds is continually dissolving the soluble parts of the solid earth and carrying them to the sea; and as evaporation brings none of this mineral matter back again, it must accumulate there.

170. Why there is little Lime or Silica in Sea-water. -There is little chloride of sodium now remaining in the superficial strata of the earth, and the chief substance now carried down in solution by rivers is carbonate of lime. Yet there is exceedingly little of this salt to be found in sea-water. The mystery ceases when we think of the countless marine organisms that are constantly using it up to form their shells and skeletons (see par. 137). From the same cause, the quantity of silica is comparatively small.

171. Depth and Form of Sea-bottom.-Much has been done in recent years to make us acquainted with the depths of the ocean. It is only of late that deep-sea sounding has been brought to perfection. A heavy weight attached to a fine line (or still better, to a piano wire) is let rapidly down; the weight, on touching the

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