"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_ ooze, after the calcareous matter has been by some means removed. An ordinary mixture of calcareous _Foraminifera_ with the shells of pteropods, forming a fair sample of _Globigerina_ ooze from near St. Thomas, was carefully washed, and subjected by Mr. Buchanan to the action of weak acid; and he found that there remained after the carbonate of lime had been removed, about 1 per cent. of a reddish mud, consisting of silica, alumina, and the red oxide of iron. This experiment has been frequently repeated with different samples of _Globigerina_ ooze, and always with the result that a small proportion of a red sediment remains, which possesses all the characters of the red clay."