Here is a fact as to the strength of ocean-waves. Some of us may have watched the majestic ground-swell which beats upon the western coast of Scotland. It is said that, upon a rough average, taking smooth and tempestuous weather together, each summer wave that breaks upon that shore from the Atlantic does so with a force of over six hundred pounds upon the square foot. For winter months alone, when gales have sway, the average blow rises to about one ton; and some mighty billows are known to batter with a weight of three tons.
Such a fierce assault as this, continued through thousands of years, might well in time wear away the hardest rocks, demolish the loftiest cliffs.
A counteracting force, however, exists, and has already been named. Our earth-crust is in motion. Parts of it are sinking, and in those regions the ocean has the best of the contest; for fight as man may, raise walls and bulwarks and breakwaters as he will, he can but retard the inevitable. If the sinking continue, no matter how sluggishly, he must in the end be beaten, and the persistent sea will encroach upon the land’s domain. But many coasts are slowly rising, slowly lifting themselves out of the sea, slowly shaking themselves free from Ocean’s dominion; and in those regions, no less inevitably, the action of the sea is thwarted.
As to the why and the wherefore of such Crust-movements, science suggests explanations.
Once upon a time, long ago, our Earth was a glowing molten mass. Since that period it has been cooling, and it is cooling still. Many facts point to the great probability of a still molten centre.
Not only do we find volcanoes scattered over the Earth, on land and under the ocean; but numberless hot-water springs exist, some of them a thousand miles away from any known volcano. Then again, when deep mining or boring operations take place, it is noted that the degree of warmth for a while corresponds to the state of the weather above, but that beyond the limits of seasonal change the temperature rises with increase of depth. This looks like greater heat below.
Many scientists have held strongly that, as above suggested, the whole inside of our Earth is so far heated as to be in a molten condition, contained within a cool hard crust, which may be from twenty to fifty miles in thickness.
Others have maintained that the said crust cannot be less than two thousand miles thick, with a small molten core.
Others believe that the entire Earth is actually solid throughout; the outer parts from coolness; the inner parts from great pressure.
Again, a theory has been started of a solid centre and a solid crust, with an intervening “fire-sea” of molten rocks; and another of a gaseous inner globe, surrounded first by molten layers, then by a firm inclosing crust.