The cooling of the inside goes on slowly, and not uniformly, since conditions differ in different places. As the heated materials become cooler they lessen in size. Thus the nucleus is constantly getting a little too small for the inclosing crust. If the said crust were a solid shell, compact and strong enough in all its parts to resist the strain of its enormous weight, it might be expected to keep its shape and position unchanged, holding loosely within itself the shrinking centre. But it is formed of a great number of materials, some hard, some soft and yielding. Therefore, as the central parts lessen in size, the crust sinks downwards, and in so doing it wrinkles into huge folds, like the rind of a shrivelling orange or the skin of a very old man’s face. A ridge is pushed up here; a furrow extends there. Here a long range of mountains is found; there we see a succession of valleys. By these movements the Ocean is both helped and hindered in its “building.” As new land is formed under water, the “crust-creep” perhaps raises it gently, and a strip of sea becomes dry land. But another portion of the work, that of pulling land to pieces, is checked; for let the waves strive as they may to wear away a coast-line, a very slight upheaval is enough to counterbalance their utmost exertions. Such changes are usually gradual--so quiet, so deliberate, as only to become apparent in the course of centuries. Not that crust-movements are always sluggish. The ceaseless sinking and crinkling of vast extents of country, the crowding together into a smaller space of immense masses of material, cannot but cause enormous pressure; and here or there a portion suddenly _gives_ under the strain. Then the ground shakes and heaves, or some terrible landslip takes place. Earthquakes are now looked upon generally as signs of abrupt yielding to intense pressure. Sometimes they indicate that a huge weight of material deep down has slid to a lower level, packing more closely or pushing aside other masses, and causing what miners call “a fault”--that is a break or dislocation in the regular lines of the stratified rocks. Or slip after slip may take place, one mass being disturbed by the movements of a neighbouring mass; and so the ground above may tremble again and again with recurring shocks. Such shocks are fearfully common in some parts. In Japan, for instance, it was noted that nine thousand had been felt in the course of only eight years; and in one great earthquake alone--that of October 26th, 1891--almost ten thousand human beings perished. Movements of the crust, whether slow or rapid, are not due always and only to the shrinkage of the central portions of our globe, and to the sinking and re-adjustment of the strata.