The moment it had shores to beat upon, that moment the ocean began to knock them to pieces under its pounding surf, and to grind the fragments so small that they could be drifted away, reassorted, and deposited wherever the water was sufficiently quiet to let them fall. The original rocks—chiefly granite—held the different forms of lime, magnesia, etc., to make the limestones; the silica to make the gritty sandstones; the alumina to make the clays; and so on. The sea not only was the agent to eat this old, rich crust to pieces and respread it into strata, but to sort out for us the materials to a considerable extent, laying down beds of limestone by themselves, and sandstone, shales, marl, etc., by themselves. It is probable, says Professor Shaler, that layers of rock twenty miles in thickness have thus been laid down on the gradually settling ocean floor, much of which has been raised again to form continental lands. Hitherto we have spoken of the waters that surround the continents as if they formed one mass, as, practically, they do; but for convenience sake we may designate certain areas by separate names, which ought now to be defined. Thus the larger, more open spaces are known as _oceans_, and of these five are recognized, namely, Pacific, Atlantic, Indian, Arctic, and Antarctic. Parts or branches of these, more or less inclosed by land and usually comparatively shallow, are termed _seas_. _The Pacific Ocean_ is the largest, it alone covering more space than all the continents combined, having a breadth, east and west, of ten thousand miles (about the length of the Atlantic), and an area of seventy million square miles. The equator divides it into the North and South Pacific. The former is comparatively free from islands, and is inclosed northward by the approaching extremities of Alaska and Siberia; while the latter widens at the south into the boundless Antarctic Ocean. Its basin is a vast depression of fairly uniform depth, studded in the western part by island peaks,—the summits of submerged volcanic mountain-ranges. The name “Pacific,” or “Peaceful,” was given to it by Magalhaens (Magellan), its first navigator, in 1540 (see Chapter IV), in his joy at having escaped from the tempestuous experience he had long endured in the South Atlantic. On the whole the Pacific deserves its name as compared with the Atlantic—a fact chiefly due to its great size. The term “South Sea” was formerly much used for it, but English-speaking persons now usually mean by that phrase the island-studded district between Hawaii and Australia. [Illustration: PERCÉ ROCK, IN THE GULF OF ST. LAWRENCE, SHOWING DESTRUCTION OF SHORE-ROCKS BY WATER.]