SILICEOUS OOZES 457 



rods, radially arranged in the Rhabdosphsera. Both are members 

 of the family Coccolithophoridse, of the order Chrysomondacese, 

 minute organism5 referred by some to the algae, and by others to 

 the Flagellata among the Protozoa. (See Chapter XXIV.) They 

 make up an important part of the Globigerina ooze of the present 

 day, and their remains also abound in older deposits. In the North 

 Atlantic certain dredgings in 4,004 m. brought up ooze in which 

 coccoliths formed 68 per cent, of the sediment. Certain chalks 

 have also been found to consist almost wholly of coccoliths and 

 rhabdoliths. The size of the individual coccolith plates ranges 

 from 0.00 1 to 0.003 "^"1- i^"* diameter, and Huxley has divided them 

 into two groups : discoliths, disk-shaped, convex on one side and 

 concave on the other, and cyatholiths, of a form somewhat resem- 

 bling that of a shirt stud. Minute calcareous disks of this kind are 

 separated out of a solution of lime sulphate or lime chloride by the 

 action of ammonia generated by the decomposition of organic mat- 

 ter, and from this it has been inferred that coccoliths are of inor- 

 ganic origin. 



2. The Siliceous Oozes. 



These comprise the radiolarian and diatomaceous oozes, the first 

 animal and the second plant structures. 



Radiolarian Oozes. (Fig. 108.) These are typically abyssal de- 

 posits; the shells, owing to the resistance which they ofifer to solu- 

 tion, can sink into depths below those at which foraminiferal shells 

 dissolve, and so they are typically found associated with the deep- 

 sea red mud. Indeed, the deposit is seldom if ever free froni the 

 red mud, which forms a sort of medium in which the radiolarian 

 shells are embedded. When more than 20 per cent, of the sediment 

 is made up of Radiolaria, it is called a Radiolarian ooze. The most 

 abundantly represented and best preserved of these planktonic 

 Radiolaria are the Nessellaria and Spumellaria, while the Phaeo- 

 daria are more sparingly and the Acantharia not at all represented 

 in the deposits, though abundant in the plankton. This is due to 

 the fact that the skeletons of the last group are not siliceous, but 

 consist of acanthin, which, like chitin, is readily destroyed after the 

 death of the organism. 



The Challenger found this ooze in its deepest soundings in the 

 western Marian deep in 8,184 meters. It contained 54.4 per cent, 

 of Radiolaria and other siliceous organisms, 3.1 per cent, of pelagic 

 and 0.1 per cent, of benthonic Foraminifera, 0.8 per cent, of other 



