Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/17

4 to and above the surface of the waters, as is well known to have been the case in our own times. Ashes and cinders would be ejected to be washed away by the breakers, if the volcanic action were not sufficiently powerful to throw out barriers of lava, and thus keep the crater permanently above the sea, as has been the case with many a volcanic island.

Once above the sea, the ashes, cinders, and detritus from the volcanic rocks, produced in the ordinary manner from atmospheric influences, running water, and the action of breakers, would form mud, sand, and gravel around, on and in which molluscs and other animals would live; so that we should have nearly similar rocks, as regards mineral composition, accumulated from combined igneous and aqueous action, in the latter case intermingled probably with the remains of animal and vegetable life.

In some situations, such volcanic accumulations may rise boldly up from those depths usually termed considerable, as is the case with the Sandwich Islands; at others they may be distributed in shallower seas, the detritus and ashes from them becoming intermingled with the mechanically transported detritus of other rocks. We can readily imagine some singular mixtures of solid lava, cinders, and ashes, with ordinary mud, sand, and gravel, among which are dispersed the hard parts of molluscs and other animals. We may have masses of coral reefs entangled, and the calcareous matter incased amid lava, or beds of volcanic ashes, cinders, and conglomerate.

Let us now turn to those deposits or accumulations of matter which are effected by the action or through the agency of water. The de- composition of rocks from atmospheric influences, the transport of the decomposed parts by brooks and rivers, the wearing away of the various substances geologically termed rocks, by the mechanical action of these running waters, and the destructive effects of breakers on coasts, afford a mass of detrital matter to the sea, to be dealt with by it, and distributed according to a variety of conditions.

Independently of the matter thus mechanically suspended in, or moved by, waters, we have substances chemically mingled with them. The rains falling upon the land and infiltrating into it, come out as springs carrying with the water the soluble parts of the rocks they have passed through, the substances in solution varying according to the rocks thus traversed by the water. The rain falling on limestone countries and containing carbonic acid, obtained either in the atmosphere or from infiltration amid the decaying vegetation of the surface, dissolves the rock, and carries it away in the soluble form of a bicarbonate of lime, affording the clearest spring water, yet containing the needful material for the shells of molluscs and other creatures requiring carbonate of lime, and the elements of new limestone beds; in fact, in the latter case, invisibly removing limestone from one place to another.