Page:Popular Science Monthly Volume 7.djvu/52

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It must be admitted that it is very difficult at present to frame any satisfactory explanation of the mode of origin of this singular deposit of red clay.

I cannot say that the theory put forward tentatively, and with much reservation by Prof Thomson, that the calcareous matter is dissolved out by the relatively fresh water of the deep currents from the antarctic regions, appears satisfactory to me. Nor do I see my way to the acceptance of the suggestion of Dr. Carpenter, that the red clay is the result of the decomposition of previously-formed greensand. At present there is no evidence that greensand casts are ever formed at great depths; nor has it been proved that Glauconite is decomposable by the agency of water and carbonic acid.

I think it probable that we shall have to wait some time for a sufficient explanation of the origin of the abyssal red clay, no less than for that of the sublittoral greensand in the intermediate zone. But the importance of the establishment of the fact that these various deposits are being formed in the ocean, at the present day, remains the same, whether its rationale be understood or not.

For suppose the globe to be evenly covered with sea, to a depth say of a thousand fathoms—then, whatever might be the mineral matter composing the sea-bottom, little or no deposit would be formed upon it, the abrading and denuding action of water, at such a depth, being exceedingly slight. Next, imagine sponges, Radiolaria Foraminifera, and diatomaceous plants, such as those which now exist in the deep sea, to be introduced: they would be distributed according to the same laws as at present, the sponges (and possibly some of the Foraminifera) covering the bottom, while other Foraminifera, with the Radiolaria and Diatomaceæ would increase and multiply in the surface-waters. In accordance with the existing state of things, the Radiolaria and diatoms would have a universal distribution, the latter gathering most thickly in the polar regions, while the Foraminifera would be largely, if not exclusively, confined to the intermediate zone; and, as a consequence of this distribution, a bed of "chalk" would begin to form in the intermediate zone, while caps of silicious rock would accumulate on the circumpolar regions.

Suppose, further, that a part of the intermediate area were raised to within 200 or 300 fathoms of the surface—for any thing that we know to the contrary, the change of level might determine the substitution of greensand for the "chalk;" while, on the other hand, if part of the same area were depressed to 3,000 fathoms, that change might determine the substitution of a different silicate of alumina