EXPEDITION OF THE CHALLENGER. 41 



some means or other, over these areas, of the carbonate of lime, which forms 

 probably about 98 per cent, of the material of the GloMgerina ooze. "We can 

 trace, indeed, every successive stage in the removal of the carbonate of lime in 

 descending the slope of tiie ridge or plateau where the Glohigerina ooze is form- 

 ing, to the region of the clay. We find, first, that the shells of pteropods and 

 otlier surface Jlollusca, which are constantly tVJling on the bottom, are absent, 

 or, if a fovr 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 Fora- 

 oninifera 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 Fora- 

 minifera are attacked, and instead of being vividly white and delicately sculp- 

 tured, they become brown and worn, and finally they break up, each according 

 to its fashion; the chamber-walls of Glohigerina fall Into wedge-shaped pieces, 

 Avhich quickly disappear, and a thick rough crust breaks away from the surface 

 of OrMdina, leaving a thin inner sphere, at first beautifully transparent, but 

 soon becoming opaque and crumbling away. 



"In the mean time the proportion of the amorphous 'red clay' to the calca- 

 reous elements of all kinds increases, until the latter disappear, with the excep- 

 tion of a few scattered shells of the larger Foi'aminifera, 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 Glohigerina 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 Glohigerina 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 one per cent, of a reddish mud, consisting of silica, alumina,, and the red 

 oxide of iron. This experiment has been frequently repeated with different sam- 

 ples of Glohigerina ooze, and always with the result that a small proportion of 

 a red sediment remains, which possesses all the characters of the red clay .... 



"It seems evident from the observations here recorded, that clay^ which we 

 have hitherto looked upon as essentially the product of the disintegration of 

 older rocks, may be, under certain circumstances, an organic formation like 

 chalk ; that, as a matter of fact, an area on the surface of the globe, which we 

 have shown to be of vast extent, although we are still far from having ascer- 

 tained its limits, is being covered by such a deposit at the present day, 



"It is impossible to avoid associating such a formation with the fine, smooth, 

 homogeneous clays and schists, poor in fossils, but showing worm-tubes and 

 tracks, and bunches of branching things, such as Oldhamia, silicious sponges, 

 and thin-shelled peculiar shrimps. Such formations, more or less metamor- 

 phosed, are very familiar, especially to the student of palajozoic geology, and they 

 often attain a vast thickness. One is inclined, from the great resemblance 

 between them in composition and in the general character of the included fauna, 

 to suspect that these may be organic formations, like the modern red clay of 

 tlie Atlantic and Southern Sea, accumulations of the insoluble ashes of shelled 

 creatures. 



" The dredging in the red clay on the 13th of March was unusually rich. 



