Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/15

2 viscous or liquid body would, until a balance to the pressure is effected. Many a volcano, if it could be so dissected that all the ashes and lapilli were removed, and the solid matter of the lava currents and dykes left, would, no doubt, present a singular skeleton,—one necessary to be borne in mind when we compare the products of sub-aerial volcanos with the rocks referable to igneous origin at previous geological epochs.

From changes of vent, cones of ashes and cinders, more or less braced together by lava currents and by dykes of lava, become intermingled with each other. Sometimes a large cone becomes studded around by others of minor dimensions; sometimes the most active vent changes its place, so as to leave part of the old crater and cone adjoining it. We can conceive that in a great volcanic region, such as Iceland, the intermixtures of lines and dykes of melted rock with ashes and cinders must be most complex.

Ashes are at times, and during great eruptions, poured forth in vast abundance, covering the adjacent country, and being occasionally transported by the winds even to great distances. These ashes, under many conditions of the relative position of the volcanos and the sea, fall abundantly into the latter; and the brooks and rivers often carry down such ashes washed into them by the rains; so that the sea bottom ad- joining volcanic districts, and especially near volcanic islands, receives a coating of volcanic ashes and lapilli during eruptions in which these products were distributed around, the lapilli disappearing as the distance from the volcanic vent is increased.

When thus thrown into the sea, the ashes and cinders are subjected to the usual action of breakers, waves, tides, and currents. On the coast there is an intermingling of the various triturated portions of the shore rocks, in some places forming shingle beaches, at others sands. If there be much volcanic matter of various kinds, the shingle beaches possess that character; and where sandstones, limestones, and other rocks also occur, there is a mixture of these also.

Marine, lacustrine, and fluviatile creatures live among the ashes and lapilli, especially the former, as amid the mud, sands, and gravels resulting from the destruction of ordinary rocks; more especially after the ashes and cinders have become well washed, and any salts, arising from chemical compounds formed while within the influence of volcanic vapours, are removed. We can readily conceive much mixture of the calcareous matter of molluscs and other marine creatures with the ordinary ashes, and a continuous deposit containing little else than the materials vomited forth from volcanos in one place losing that character gradually in distance, so that it should be volcanic in one part of the area, of a mixed kind in another, and composed of ordinary mud, sand, and gravel, in a third portion; yet the whole, according to equal con-