98 EXPEDITION OF THE " CHALLENGER " m 



depressed regions on account of some law regulating its deposi- 

 tion, 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. 



