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° 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