216 THE ATLANTIC. [CHAP. II. 
with pelagic mollusca; and in moderate depths, the shells of 
these are constantly mixed with the globigerina ooze, some- 
times in number sufficient to make up a considerable portion 
of its bulk. It is clear that these shells must fall in equal 
numbers upon the red clay; but scarcely a trace of one of 
them is ever brought up by the dredge on the red-clay area. 
It might be possible to explain the absence of shell-secreting 
animals living on the bottom by the supposition that the nature 
of the deposit was injurious to them; but the idea of a current 
sufficiently strong to sweep them away, if falling from the sur- 
face, 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 by chemical action. 
We conclude, therefore, that the red clay is not an addi- 
tional substance introduced from without, and occupying cer- 
tain 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 glo- 
bigerina 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 mollusea, which are con- 
stantly falling on the bottom, are absent; or, if a few remain, 
they are brittle and yellow, and evidently decaying rapidly. 
These shells of mollusea 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, un- 
der a high power, as minute cylinders scattered over the field. 
