Of course they are spread out far and wide by the numerous ocean currents, some of which flow for hundreds of miles; and so the bed of the sea can only be very slowly raised by their accumulation. Still the geologist can allow plenty of time, for there is no doubt that the world is immensely old; and if we allow thousands of years, we may easily comprehend that deposits of very considerable thickness may in this way accumulate on the floors of the oceans. Also the coasts of continents and islands suffer continual wear and tear at the hands of sea waves; and thus the supply of sediment is increased. When the geologist comes to study the great rock-masses--hundreds, and even thousands, of feet in thickness--of which mountain-ranges are composed, he finds all those kinds of rock which we have just been considering,--sandstones, shales (or hardened clays), pebble-beds, and limestones,--and endeavours to picture to himself their gradual growth in the ways we have described. In so doing, he is driven to the conclusion that many thousands of years must have been occupied in their construction. We must now say a few words about those other aqueous rocks which have an organic origin, of which limestone is the chief. It is indeed a startling conclusion that deposits of great thickness, and ranging for very many miles over the earth's surface, have been slowly built up through the agency of marine animals extracting carbonate of lime from the sea. Yet such is undoubtedly the case. Of this important process of rock-building coral reefs are the most familiar example. The great barrier reef along the northeast coast of Australia is about 1,250 miles long, from ten to ninety miles in width, and rises at its seaward edge from depths which in some places certainly exceed eighteen hundred feet. It may be likened to a great submarine wall. Now, all this solid masonry is the work of humble coral polypes (not "insects"), building up their own internal framework or skeleton by extracting carbonate of lime from sea water. Then the breakers dashing against coral reefs produce, by their grinding action, a great deal of fine "coral-sand" and calcareous mud, which covers the surrounding bed of the sea for many miles.