Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/16

Rh ditions of temperature, depth, and kind of bottom, as regards mud, sand, and gravel, affording a similar fauna.

In deep seas the ashes would remain undisturbed at the bottom, mingled only with the marine animals living there, or with products chemically formed. Where finely comminuted detritus could be deposited from mechanical suspension, the discharge of volcanic ashes being sufficiently repeated, there would be alternations of such substances; and when from the action of waves in less depths the bottom could be occasionally stirred up, there might be an intimate mingling of volcanic ashes with sands and mud.

Some singular mixtures would be effected upon sea bottoms where, either from an accumulation of broken or whole shells and corals, or from calcareous matter chemically produced, limestones were forming, and ashes and lapilli were showered upon the sea above them. In cases where the quantity of the volcanic matter thus scattered over the surface was small, there might be a considerable intermixture of ashes and cinders with the calcareous matter, without materially injuring the marine creatures inhabiting the bottom; but when the volcanic dis- charges were so violent as to cause a considerable accumulation of ashes and cinders, there might be great destruction of the animal life existing on the bottom, and their harder parts would become permanently entombed, the whole eventually forming a solid bed.

The same volcanic causes would be greatly modified in their effects when in action beneath the level of the sea. Ashes being little else, commonly, than finely comminuted portions of the vitreous state of lava, requiring a certain freedom from mechanical pressure, could, independently of the absorption of several propelling gases and vapours by the water, scarcely be formed at considerable depths. Very near the surface they might be produced, and be mingled with the water; but until the crater was sufficiently raised that the discharges could be considered as sub-aerial, ashes could scarcely be dispersed so as to cover large areas.

At great depths in the sea we should expect that the molten rocks ejected from a volcanic vent would have a tendency to be flattened out by spreading, if the volcanic action were considerable, into somewhat broad sheets, thus differing materially from sub-aerial lava currents. In proportion as the pressure became less from the melted rock accumulating round the vents, and the process of cooling became much modified under less depth, so should we expect, under equal action, that the lava discharges would become thicker, extending to less distances around, thus gradually forming a sort of obtuse cone.

It should be borne in mind, that in proportion as the active vent rose in the sea, so would the mechanical pressure of the water be removed; and thus, the accumulations continuing, the volcano would gradually rise