The observations which have been detailed leave no doubt that the Antarctic sea bottom, from a little to the south of the fiftieth parallel, as far as 80 deg.  S., is being covered by a fine deposit of silicious mud, more or less mixed, in some parts, with the ice-borne debris of polar lands and with the ejections of volcanoes.  The silicious particles which constitute this mud, are derived, in part, from the diatomaceous plants and radiolarian animals which throng the surface, and, in part, from the spicula of sponges which live at the bottom.  The evidence respecting the corresponding Arctic area is less complete, but it is sufficient to justify the conclusion that an essentially similar silicious cap is being formed around the northern pole.

There is no doubt that the constituent particles of this mud may agglomerate into a dense rock, such as that formed at Oran on the shores of the Mediterranean, which is made up of similar materials.  Moreover, in the case of freshwater deposits of this kind it is certain that the action of percolating water may convert the originally soft and friable, fine-grained sandstone into a dense, semi-transparent opaline stone, the silicious organized skeletons being dissolved, and the silex re-deposited in an amorphous state.  Whether such a metamorphosis as this occurs in submarine deposits, as well as in those formed in fresh water, does not appear; but there seems no reason to doubt that it may.  And hence it may not be hazardous to conclude that very ordinary metamorphic agencies may convert these polar caps into a form of quartzite.

In the great intermediate zone, occupying some 110 deg. of latitude, which separates the circumpolar Arctic and Antarctic areas of silicious deposit, the Diatoms and Radiolaria of the surface water and the sponges of the bottom do not die out, and, so far as some forms are concerned, do not even appear to diminish in total number; though, on a rough estimate, it would appear that the proportion of Radiolaria to Diatoms is much greater than in the colder seas.  Nevertheless the composition of the deep-sea mud of this intermediate zone is entirely different from that of the circumpolar regions.

The first exact information respecting the nature of this mud at depths greater than 1,000 fathoms was given by Ehrenberg, in the account which he published in the “Monatsberichte” of the Berlin Academy for the year 1853, of the soundings obtained by Lieut.  Berryman, of the United States Navy, in the North Atlantic, between Newfoundland and the Azores.

Observations which confirm those of Ehrenberg in all essential respects have been made by Professor Bailey, myself, Dr. Wallich, Dr. Carpenter, and Professor Wyville Thomson, in their earlier cruises; and the continuation of the Globigerina ooze over the South Pacific has been proved by the recent work of the Challenger, by which it is also shown, for the first time, that, in passing from the equator to high southern latitudes, the number and variety of the Foraminifera diminishes, and even the Globigerinoe become dwarfed.  And this result, it will be observed, is in entire accordance with the fact already mentioned that, in the sea of Kamschatka, the deep-sea mud was found by Bailey to contain no calcareous organisms.