However this may be, the explanation thus offered of the presence of the
fine mud, and of the absence of organisms which ordinarily live at the
bottom, does not account for the absence of the skeletons of the
organisms which undoubtedly abound at the surface of the Mediterranean;
and it would seem to have no application to the remarkable fact
discovered by the _Challenger_, that in the open Atlantic and Pacific
Oceans, in the midst of the great intermediate zone, and thousands of
miles away from the embouchure of any river, the sea-bottom, at depths
approaching to and beyond 3,000 fathoms, no longer consists of
_Globigerina_ ooze, but of an excessively fine red clay.

Professor Thomson gives the following account of this capital
discovery:--

"According to our present experience, the deposit of _Globigerina_ ooze
is limited to water of a certain depth, the extreme limit of the pure
characteristic formation being placed at a depth of somewhere about 2,250
fathoms. Crossing from these shallower regions occupied by the ooze into
deeper soundings, we find, universally, that the calcareous formation
gradually passes into, and is finally replaced by, an extremely fine pure
clay, which occupies, speaking generally, all depths below 2,500 fathoms,
and consists almost entirely of a silicate of the red oxide of iron and
alumina. The transition is very slow, and extends over several hundred
fathoms of increasing depth; the shells gradually lose their sharpness of
outline, and assume a kind of 'rotten' look and a brownish colour, and
become more and more mixed with a fine amorphous red-brown powder, which
increases steadily in proportion until the lime has almost entirely
disappeared. This brown matter is in the finest possible state of
subdivision, so fine that when, after sifting it to separate any