must
become exceedingly slow.

Another consideration appears to me to be in favour of the view that the
_Globigerinoe_ and their allies are essentially surface animals. This is
the fact brought out by the _Challenger's_ work, that they have a
southern limit of distribution, which can hardly depend upon anything but
the temperature of the surface water. And it is to be remarked that this
southern limit occurs at a lower latitude in the Antarctic seas than it
does in the North Atlantic. According to Dr. Wallich ("The North Atlantic
Sea Bed," p. 157) _Globigerina_ is the prevailing form in the deposits
between the Faroe Islands and Iceland, and between Iceland and East
Greenland--or, in other words, in a region of the sea-bottom which lies
altogether north of the parallel of 60 deg. N.; while in the southern seas,
the _Globigerinoe_ become dwarfed and almost disappear between 50 deg. and
55 deg. S. On the other hand, in the sea of Kamschatka, the _Globigerinoe_
have vanished in 56 deg. N., so that the persistence of the _Globigerina_
ooze in high latitudes, in the North Atlantic, would seem to depend on
the northward curve of the isothermals peculiar to this region; and it is
difficult to understand how the formation of _Globigerina_ ooze can be
affected by this climatal peculiarity unless it be effected by surface
animals.

Whatever may be the mode of life of the _Foraminifera_, to which the
calcareous element of the deep-sea "chalk" owes its existence, the fact
that it is the chief and most widely spread material of the sea-bottom in
the intermediate zone, throughout both the Atlantic and Pacific Oceans,
and the Indian Ocean, at depths from a few hundred to over two thousand
fathoms, is established. But it is not the only extensive deposit which
is now taking place. In 1853, Count Pourtales, an officer of the United
States Coast Survey, which has done so much for scientific hydrography,
observed, that the mud forming the sea-bottom at depths of one hundred
and fifty fathoms, in 31 deg. 32' N., 79 deg. 35' W., off the Coast of Florida,
was "a mixture, in about equal proportions, of _Globigerinoe_ and black
sand, probably greensand, as it makes a green mark when crushed on
paper." Professor Bailey, examining these grains microscopically, found
that they were casts of the interior cavities of _Foraminifera_,
consisting of a mineral known as _Glauconite_, which is a silicate of
iron and alumina. In these casts the minutest