The operation of water in its various forms, of wind, changes of temperature, and other agents of destruction cannot be fully dealt with in this short summary. A river flowing to the sea or emptying itself into an inland lake, carries its burden of gravel, sand, and mud, and sooner or later, as the rate of flow slackens, it deposits the materials in the river-bed or on the floor of the sea or lake. Fragments of rock, chipped off by wedges of ice, or detached in other ways from the parent mass, find their way to the mountain streams, and if not too heavy are conveyed to the main river, where the larger pieces come to rest as more or less rounded pebbles. Such water-worn rocks accumulate in the quieter reaches of a swiftly flowing river, or are thrown down at the head of the river’s delta. If such a deposit of loose water-worn material became cemented together either by the consolidating action of some solution percolating through the general mass, or by the pressure of overlying deposits, there would be formed a hard rock made up of rounded fragments of various kinds of strata derived from different sources. Such a rock is known as a CONGLOMERATE. The same kind of rock may be formed equally well by the action of the sea; an old sea-beach with the pebbles embedded in a cementing matrix affords a typical example of a coarse conglomerate. Plant remains are occasionally met with in conglomerates, but usually in a fragmentary condition. From a conglomerate composed of large water-worn pebbles, to a fine homogeneous sandstone there are numerous intermediate stages. A body of water, with a velocity too small to carry along pebbles of rock in suspension or to roll them along the bed of the channel, is still able to transport the finer fragments or grains of sand, but as a further decrease in the velocity occurs, these are eventually deposited as beds of coarse or fine sand. The stretches of sand on a gradually shelving sea shore, or the deposits of the same material in a river’s delta, have been formed by the gradual wearing away and disintegration of various rocks, the detritus of which has been spread out in more or less regular beds on the floor of a lake or sea. Such accumulations of fine detrital material, if compacted or cemented together, become typical SANDSTONES.