Now, the sandstones, clays, gravels, and pebble-beds all represent, as will presently be explained, so much material worn away from the surface of the land and swept into the ocean (or in some cases into inland seas and lakes) by streams and rivers, which are the great transporting agents of the world. Hence such deposits of débris, supplied by the constant wear and tear of all rocks exposed to the atmosphere, are truly sedimentary and have a purely mechanical origin. But it is not so with the limestones. The latter were never transported, but grew at the bottom of the sea in very wonderful ways. They have nothing to do with the wear and tear of the land to which the others owe their existence, but represent vast quantities of carbonate of lime extracted from sea water. Sea water contains a certain amount of this substance in a dissolved state, or "in solution," as a chemist would say; and the way in which this is extracted by the agency of various creatures, such as coral polypes and little microscopic creatures that build their shells of carbonate of lime, of great beauty, forms one of the most interesting subjects presented to the student of physical geography. Hence, since limestone can only be accounted for by the agency of living organisms,[21] it is rightly termed an _organic deposit_, and the others are said to be _mechanical deposits_. But both are called "aqueous rocks," because they are formed under water. It is important to distinguish clearly between these two very different methods of rock-formation. [21] The flints usually found in limestone are also of organic origin.