ous to them; but then the idea of a
current sufficiently strong to sweep them away is negatived by the
extreme fineness of the sediment which is being laid down; the absence of
surface shells appears to be intelligible only on the supposition that
they are in some way removed.

"We conclude, therefore, that the 'red clay' is not an additional
substance introduced from without, and occupying certain depressed
regions on account of some law regulating its deposition, but that it is
produced by the removal, by some means or other, over these areas, of the
carbonate of lime, which forms probably about 98 per cent. of the
material of the _Globigerina_ ooze. We can trace, indeed, every
successive stage in the removal of the carbonate of lime in descending
the slope of the ridge or plateau where the _Globigerina_ ooze is
forming, to the region of the clay. We find, first, that the shells of
pteropods and other surface _Mollusca_ which are constantly falling on
the bottom, are absent, or, if a few remain, they are brittle and yellow,
and evidently decaying rapidly. These shells of _Mollusca_ decompose more
easily and disappear sooner than the smaller, and apparently more
delicate, shells of rhizopods. The smaller _Foraminifera_ now give way,
and are found in lessening proportion to the larger; the coccoliths first
lose their thin outer border and then disappear; and the clubs of the
rhabdoliths get worn out of shape, and are last seen, under a high power,
as infinitely minute cylinders scattered over the field. The larger
_Foraminifera_ are attacked, and instead of being vividly white and
delicately sculptured, they become brown and worn, and finally they break
up, each according to its fashion; the chamber-walls of _Globigerina_
fall into wedge-shaped pieces, which quickly disappear, and a thick rough
crust breaks away from the surface of _Orbulina_, leaving a thin inner
sphere, at first beautifully transparent, but soon becoming opaque and
crumbling away.

"In the meantime the proportion of the amorphous 'red clay' to the
calcareous elements of all kinds increases, until the latter disappear,
with the exception of a few scattered shells of the larger
_Foraminifera_, which are still found even in the most characteristic
samples of the 'red clay.'

"There seems to be no room left for doubt that the red clay is
essentially the insoluble residue, the _ash_, as it were, of the
calcareous organisms which form the _Globigerina_ o