vi.]

RECAPITULATION.

89

on the whole, while the interior of the earth is always losing heat, or cooling.

The train of observation and reasoning which we have followed in this discussion of the causes that produce mountains, is a good example of that followed generally in explaining the phenomena of nature. Let us briefly sum up its leading points. First of all, we learn by examining the structure of mountain chains, that they are composed of rocks, a great part of which have at one time formed. level layers at the bottom of some ocean; that in mountains these are contorted and broken in a manner that proves them to have been squeezed into a space less than that which they must once have occupied. And reason tells us that, as the surface of a small ball is less than that of a large ball, such an effect would follow, if the outer layer of a larger ball were · forced down upon the surface of one which is smaller. In the case of our earth, if the interior were to shrink, while the outer shell were not so affected, gravitation would constantly force it to adapt itself to the diminished size of the former. And finally, our knowledge of the manner in which heat is propagated from the heated end of a bar to the cool end, and therefore from the heated interior to the cool surface of our earth, whence it is radiated away into space-combined with the further fact, that, with rare exceptions, all cooling bodies shrink as they cool,-assures us that the interior of the earth must be contracting, and that the outermost part of the shell does not so contract, except under the influence of gravity.

From all this it follows that our earth must once have been larger than it now is, and that it will some day be smaller. Undoubtedly it is so but the change is a very slow one. The mountain ranges on its surface are the cracks and ridges that have been produced by its contraction; and since there are other causes at work which we have yet to study more in detail, which are always wearing down all irregularities, we might conclude that the most ancient mountains must long ago have been worn down to low hills, and that the loftiest mountains are some of the most recent. Geology

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CHANGES OF LEVEL EXPLAINED.

[CHAP.

assures us that this is so; as will be described more at length in Chapter X.

The causes that we have found to explain the formation of mountains will also explain those gradual upheavals and depressions of the surface described in the last chapter. Let the dotted lines a a in Fig. 13 represent a portion of the surface evenly covered with water; and, in consequence of the contraction of the earth's interior, let it be forced to accommodate itself within the smaller space á á. Since all rocks are more or less flexible, if the difference be not very great, this portion of the superficial crust, instead of being

contorted or broken up, may be simply bent into the double curve represented in the figure. Dry land, a portion of a new continent for example, will emerge at b; and at c a deeper depression will be formed, in which all the water will accumulate. Now it has long been observed that when any great tract of land is being upraised, there is, as a rule, at no great distance, another tract undergoing depression. While the north of Scandinavia is rising, the south is being depressed; and Mr. Darwin has shown reasons for inferring that, in the Pacific Ocean, the line of volcanic islands mentioned in the last chapter (and consisting of New Britain, the Solomon Islands, and the New Hebrides,) are undergoing occasional slow elevation; while, on either side, the existence of coral islands of the kind termed atols and barrier reefs gives evidence that gradual depression is in progress. On

VI.]

METAMORPHISM EXPLAINED.

91

the north are the Caroline Islands, the Marshall Islands, &c., and on the south the Louisiade, New Caledonia, and the great barrier reef off the north-east coast of Australia. Still nearer home, we have another example. The east coast of Arabia has been raised above the sea, at a period, geologically speaking, recent; while the existence of the Laccadives and Maldives and other groups of coral islands to the south of these, show that the North Indian Ocean has become of greater depth than formerly.

Before we quit the subjects treated of in the last two chapters, it remains to show how the contraction of the earth and the consequent distortion of its crust may give rise to volcanic eruptions, and produce that metamorphism of the central parts of great mountain chains which we have seen to be their general characteristic.

It has long been known to physicists, that if any substance be placed under great pressure, it becomes heated. Dr. Tyndall showed some years ago that ice, for instance, can be melted by squeezing it under a powerful press; and the fact that pieces of metal are heated under the like circumstances is well known to all persons who have to deal with powerful machinery. The crushing force by which mountain masses have been elevated, must therefore have produced heat proportional to the intensity of the pressure and the space through which the rocks have been compressed; and Mr. Mallet has shown, by experiments on different kinds of stones, that this has been sufficient to metamorphose the rocks, (already perhaps at a high temperature owing to their depth below the surface,) and even to melt them in those central parts of the chain, where, as a rule, the lowest rocks have been thrust up to the surface. In this way, probably, may be explained the fact noticed above, that the central parts of great mountain chains are generally formed of metamorphic rocks.

I cannot better sum up the teachings, and point the moral of these two chapters, than in the words of Sir John Herschel. "The land," he says, "is maintained in its elevated position by internal force, locally exerted, and

92

VOLCANIC ACTION CONSERVATIVE. [CHAP. VI.

varying its locality from age to age, Whatever be the nature and ultimate origin of that force, it is manifested to us from time to time in the volcano and the earthquake, which thus we learn to regard as very far from purely destructive arrangements in the great scheme of nature; since without the agency of which they are part and parcel, there would by this time have been no dry land whatever. The fact that all our present continents consist of beds or strata, which have resulted from the destruction of former ones, and the distribution of their materials at the bottom of the sea, and of granitic masses, forcibly thrust up through those strata, disturbing and dislocating them, leads direct to the conclusion that, had the primeval world been constructed as it now exists, time enough has elapsed and force enough, directed to that end, [has] been in activity, to have long ago destroyed every vestige of land, but for the reproductive efficiency of those internal forces, bringing up continually new lands to replace the old."

CHAPTER VII.

ICE AND SNOW.

No single mountain range now presents to our eyes the aspect it must have had when newly upheaved, a contorted and fractured mass, under the enormous pressure of the contracting crust of the earth. Lofty as are still the snow-capped peaks-Mount Everest, Kanchinjinga, and Doulagiri-they have all been wasted under the eroding tooth of frost, and began to contribute their quota to the soil of the Bengal plains and the sediment of the ocean from the very first moment of their upheaval. Destruction and renovation are the law of the insentient rocks, not less than of our own bodies and other living things. I have briefly sketched out the latter; let us now turn our attention to the former process.

The agents by which mountains and continents are planed down and destroyed are water and a gas; water in its several forms of ice or frost, springs, rivers, and the ocean; and a gas which exists in the atmosphere and is called in chemical language Carbon dioxide, and when dissolved in water Carbonic acid. Of these, water is by far the most important and the most varied in its action, and it is only in the presence of water, and when dissolved in it as carbonic acid, that the latter takes part in the work of destruction. The vehicle of both is the atmosphere.

We have seen in the second chapter how water vapour is taken up by the winds from the surface of seas, lakes, and