Mesozotc and Ccenozoic Geology and Palceontology. 239 



in the same proportion. The Mid-Atlautic swarms with pelagic mol- 

 lusca; and in moderate depths, the shells of tliese are constantly mixed 

 with the globigerina ooze, sometimes in nnmber 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 b}' the supposition that the nature of the deposit was in- 

 jurious to them; but the idea of a current sufficiently strong to sweep 

 them awa}'^, if falling from the surface, is negatived hy the extreme 

 fineness of the sediment which is being laid down. The absence of 

 surface shells appears to be intelligible onl}^ on the uupposition that 

 the^^ are in some waj^ removed by chemical action. 



"We conclude, therefore, that the red clay is not an additional sub- 

 stance introduced from without, and occup^'ing certain depressed re- 

 gions 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 succes- 

 sive 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 raollusca, 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 easil}', and disap- 

 pear sooner than the smaller, and apparently more delicate shells of 

 rhizopods. The smaller foraminifera now give wa}', 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 minute 

 cylinders scattered over the field. The larger foraminifera are at- 

 tacked, and instead of being vividly white and delicately sculptured, 

 they become brown and worn, and finallj^ they break up, each accord- 

 ing to its fashion: the chamber-walls of G^Zo6/(/erM2a fall into wedge- 

 shaped pieces, which quickly- disappear; and a thick rough crust 

 breaks away from the suiface of Orbidina, leaving a thin inner sphere, 

 at first beautifully transparent, but soon becoming opaque and crumb- 

 ling awa3^ 



" In the mean time, 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. 



