84

CONTRACTION AND GRAVITATION.

[CHAP.

out what power there is in nature competent to produce them; and having found such a power, we must examine whether the effects to be accounted for are such as might be expected from what we know of its action.

There is such a power in the gravitation of the earth: that is to say, in the force which draws every part of the earth towards its own centre; if we suppose that, by any means, the interior of the earth has contracted, and so ceased to afford a complete support to the solid layer that forms the external shell. To render this idea clear, let us suppose that the accompanying figure represents a section of

the earth, cut in half, and exposing a a, the external shell, and bb all that lies within it. Next, let us suppose that, from some cause, which has yet to be assigned, the interior portion bb shrinks so as only to occupy the shaded space, while the shell is affected by no such shrinking, or is, at all events, less shrunken. The shell will then be left without support; and, under the influence of gravitation, it must somehow adapt itself to the diminished size of the core. It can do so only by acquiring a diminished surface, which will be effected somewhat in the manner represented in

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CONTRACTION AND GRAVITATION.

85

Figs. 11 and 12, which represent a small portion of the shell and nucleus, the dotted lines a a showing the original, and the solid lines a' a' the final position of the former. It is evident from the figures, that the portion of the shell a a must somehow squeeze itself into the narrower space at a' a'; and to do so, either it must undergo fracture, and some portion of the fractured mass must protrude beyond the rest, as shown in Fig. 11; or else, if flexible, it may be crumpled up as represented in Fig. 12. Most probably the

adaptation will be effected partly by fracture and dislocation, and partly by contortion. Now let us turn back to Fig. 9. Here we have fractures and dislocations of the same kind as in Fig. 11-and in the intervals, the bedded rocks are contorted just as we should expect, from their having to adapt themselves to a diminished space. Such is the general character of mountain ranges; and we must, therefore, regard them as portions of the earth's surface, which have been broken and crumpled up, in a manner such as would follow from the shrinking or contraction of the interior. Thus we may have a portion of the surface, that has for ages been level, thrown up into a series of ridges, producing ranges of mountains as lofty as the Himálaya and the Andes. Another effect of this contortion and squeezing is a great change in the character of the rocks, viz., that which was described in the fourth chapter as metamorphism. The rocks have been so heated by the

86

MOUNTAIN FORMATION GRADUAL.

[CHAP. pressure that they have become softened and partially fused; and when cooled again, very hard, compact, and crystalline. In the Himalaya it is only on the flanks of the chain, viz., in the neighbourhood of the Gangetic plains on the south, and in the valleys of the Spiti and the Sutlej on the north, that the rocks retain much of their original characters distinctly as water-formed deposits. All the great intervening mass, including the loftiest snow-covered peaks, consists of metamorphosed crystalline rocks, and such is also the case in the Alps, the Andes, and other great mountain chains. Here and there, masses of granite and other rocks of the same class, breaking through the fractured and contorted gneiss, show that, in some cases, the heat aided by water has been sufficient to liquify the rock; and elsewhere, intruded rocks of the volcanic class bear witness to a similar action.

Paradoxical as it may seem, the structure of mountains shows us then, that they have been produced by crushing; by the necessity that some portion of the earth's surface shall accommodate itself within narrower limits; in consequence of which, that which is in excess, is squeezed up above the general level. This may form one or many parallel ridges, according to the magnitude of the crushing, and to the extent of the country affected. In the case of the Himalaya there are many parallel ridges; and the whole of them regarded as forming only the southern border of a much more extensive elevation, the table-land of Tibet. It must not be supposed that the great movements, by which such mountain masses are produced, take place all at once. In the case of the Himálaya, for instance, it can be proved that the same kind of movement has been repeated again and again, at long intervals; so that sedimentary formations which were formed of the waste of the primitive ranges, have been themselves lifted up and contorted during later movements. The same is true of the Alps, and probably of all the greater mountain ranges. In the next and following chapters, we shall have more to say about the wearing down to which they are and

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CAUSE OF EARTH'S SHRINKING.

87

have been subjected; and in consequence of which, they and all existing mountains present an appearance very different from that of the broken masses, which would result immediately from such a process as I have described.

We must now turn our attention to the cause of the earth's shrinking, which I have hitherto only assumed as the ulterior cause of the formation of mountains; and we must bear in mind, that it is not the mere fact of the shrinking that we have to account for; but that, to produce the effects described, this must take place unequally, and the interior must contract more than the surface. In the description, and Figs. 10, 11 and 12, on a previous page, I have supposed, for the sake of clearness, that the external shell is quite distinct from the inner mass, and that the latter contracts as a whole, so as to separate itself from the shell, which remains uncontracted; lastly, that the breaking up and falling in of the shell takes place subsequently as a distinct and independent movement. This of course is really not the case. The shell could not remain for an instant unsupported as it is represented in the figure, and must accommodate itself to the contracted nucleus, as fast as the latter contracts. But the final result will be the same.

The only cause we know of, competent to produce such an unequal shrinking, is that mentioned at the end of the last chapter, viz., the unequal loss of heat. A very familiar illustration of the shrinking of a cooling body may be seen in any blacksmith's shop, When a blacksmith makes an iron tire for the wheel of a bullock cart, he makes it a little smaller than the wooden wheel to which it is to be fitted. He then covers it with burning cow-dung, and when thoroughly hot, puts it on the wheel. The heat expands it, and it now fits easily. Finally he cools it by throwing cold water upon it, causing it at the same time to contract to its former size; and, in so doing, it closes tightly on the circumference of the wheel and squeezes all its parts firmly together. With very few exceptions, all substances expand in like manner when they are heated,

888

WHY THE INTERIOR COOLS.

[CHAP.

and contract when cooled. If, therefore, it can be shown that the interior of the earth is cooling, while the surface remains always of the same temperature, or nearly so, we shall have just the state of things required to produce those disturbances of the surface, that have been described in this and the previous chapters.

Now we know from experience, that whenever two parts of one and the same body are unequally hot, heat travels from the hotter to the colder part. In the case of metals (especially silver and copper) and some other substances, this flow of heat is rapid; in that of other substances, on the other hand, it proceeds very slowly. If we take a bar of iron, or still better, one of copper, and put one end in the fire, before very long the heat will have travelled along the bar, so that the end furthest from the fire will have become too hot to hold. If however a number of bricks be built up with mortar or clay in a furnace wall and a fire be made inside, it must burn a long time and very fiercely before the bricks become too hot on the outside to be touched; and this will be due to two causes : one is that heat travels through a brick much less quickly than through a metal; and the other, that the outside being exposed to the air, the heat is thrown off from it almost as fast as it reaches it; so that the outside remains but little warmer than the air.

But if one of the bricks be removed from the outside of such a wall, the interior will be found quite hot; showing that the heat has penetrated it. Now, for the furnace and its wall, let us substitute the highly-heated interior and cool shell of our earth. The heat of the former must travel towards the surface, and on reaching it must be thrown off into free space. The surface always remains therefore at much the same temperature. All the heat that it receives from below is at once lost; and were it not for that of the sun which it receives during the day time, it would be so intensely cold that no living thing could exist upon it. But it is maintained at a moderate degree of warmth by the sun, and as this varies but little from year to year, or, as far as we know, from century to century, it undergoes but little change