The amount of land débris thus transported depends partly on the carrying power of rivers, which varies with the seasons and the annual rainfall; partly on the size of the area drained by a river; and again, partly on the nature of the rocks of which that area is composed. A stream, moving along at the rate of about half a mile (880 yards) an hour, which is a slow, rate, can carry along ordinary sandy soil suspended in a cloud-like fashion in the water; when moving at the rate of two thirds of a mile (about 1,173 yards) an hour, it can roll fine gravel along its bed; but when the rate increases to a yard in a second, or a little more than two miles an hour, it can sweep along angular stones as large as an egg. But streams often flow much faster than this, and so do rivers when swollen by heavy rain. A rapid torrent often flows at the rate of eighteen or twenty miles an hour, and then we may hear the stones rattling against each other as they are irresistibly rolled onward; and during very heavy floods, huge masses of rock as large as a house have been known to be moved. These are the two principal ways in which streams and rivers act as transporting agents: they carry the finer materials in a suspended state (though partly drifting it along their beds); and they push the coarser materials, such as gravel, bodily along. But there is one other way in which they carry on the important work of transportation, which, being unseen, might easily escape our notice. Every spring is busily employed in bringing up to the surface mineral substances which the water has dissolved out of the underground rocks. This invisible material finds its way, as the springs do, to the rivers, and so finally is brought into that great reservoir, the sea. Rain and river water also dissolve a certain amount of mineral matter from rocks lying on the surface of the earth. Now, the material which is most easily dissolved is carbonate of lime. Hence if you take a small quantity of spring or river water and boil it until the whole is evaporated, you will find that it leaves behind a certain amount of deposit. This, when analysed by the chemist, proves to be chiefly carbonate of lime; but it also contains minute quantities of other minerals, such as common salt, potash, soda, oxide of iron, and silica, or flint. All these and other minerals are found to be present in sea water.