But not only the loose materials on the shore suffer in this way an incessant wear and tear, the solid rocks underneath, wherever they come to the surface, are ground down in the same process. When the waves dash against a cliff they hurl the loose stones forward, and batter the rocks with them. Here and there in some softer part, as in some crevice of the cliff, these stones gather together, and when the sea runs high they are kept whirling and grinding at the base of the cliff till, in the end, a cave is actually bored by the sea in the solid rock, very much in the same way as holes are bored by a river in the bed of its channel. The stones of course are ground to sand in the process, but their place is supplied by others swept up by the waves. If you enter one of these sea-caves when the water is low, you will see how smoothed and polished its sides and roof are, and how well rounded and worn are the stones lying on its floor. So far as we know, the bottom of the sea is very much like the surface of the land. It has heights and hollows, lines of valleys and ranges of hills. We can not see down to the bottom where the water is very deep, but we can let down a long line with a weight tied to the end of it, and find out both how deep the water is, and what is the nature of the bottom, whether rock or gravel, sand, mud, or shells. This measuring of the depths of the water is called sounding, and the weight at the end of the line goes by the name of the sounding-lead. Soundings have been made over many parts of the sea, and something is now known about its bottom, though much still remains to be discovered. The Atlantic Ocean is the best known. In sounding it, before laying down the telegraphic cable which stretches across under the sea from this country to America, a depth of 14,500 feet, or two miles and three-quarters, was reached. But between the Azores and the Bermudas a sounding has been obtained of seven miles and a half. If you could lift up the Himalaya mountains, which are the highest on the globe, reaching a height of 29,000 feet above the sea, and set them down in the deepest part of the Atlantic, they would not only sink out of sight, but their tops would actually be about two miles below the surface. A great part of the wide sea must be one or two miles deep. But it is not all so deep as that, for even in mid-ocean some parts of its bottom rise up to the surface and form islands. As a rule it deepens in tracts furthest from land, and shallows toward the land. Hence those parts of the sea which run in among islands and promontories are, for the most part, comparatively shallow.