280 A. EARLAXD ON EADIOLARIA. 



(5) The abyssal zone, in which the accumulation of oceanic- 

 deposits and the influence of bottom currents create new 

 conditions of existence. 



Speaking generally, the first and second zones consist mainly 

 of the Porulosa, while as the depth increases these disappear and 

 are replaced by the Osculosa. The obscure zone is the poorest 

 in species. The morphological characters of the zonarial fauna 

 appear to change gradually upwards into the delicate pelagic 

 forms, and downwards into the robust abyssal, while the average 

 size (within the limits of the same family) increases as we go 

 upwards, and decreases downwards. 



We have already seen that the skeletons of the second legion,, 

 the Acantharia, owing to their solubility, do not occur in bottom 

 deposits, while the carbon silicate skeletons of the Phseodaria, 

 which are more refractory, are sparingly represented. The 

 purely siliceous skeletons of the Spumellaria and Nassellaria are, 

 however, almost indestructible, and sink to the bottom after the 

 death of the animal. They must be constantly falling in a 

 gentle rain on the sea bottom at all depths beyond the influence 

 of terrigenous deposits, say 100 to 200 miles from the coast line ; 

 but owing to their minute size as compared with the calcareous 

 shells of the Foraminifera and other pelagic animals, they are 

 more or less masked in all oceanic deposits of depths under 2000 

 fathoms, although their presence may nearly always be detected 

 in varying quantities by the removal of the calcareous organisms. 

 The Globigerina oozes, which cover the greater portion of the 

 ocean bottom between 250 and 2500 fathoms, contain a large 

 though varying number of Radiolarian skeletons. 



Below 2000 fathoms the solvent action of carbonic acid in 

 solution quickly dissolves the calcareous organisms, and Globi- 

 gerina oozes gradually disappear, to be replaced by the Red Clay, 

 which covers an enormous area of the ocean bed. The Red Clay 

 is principally composed of silicate of alumina, derived from the 

 decomposition of volcanic ash, pumice, etc. It contains very few 

 calcareous organisms, but Radiolarian remains are frequent, and 

 in some cases so abundant that the Red Clay passes gradually 

 into a true Radiolarian ooze (e.g., Challenger stations 238 to 253 

 in the North Pacific). 



In certain limited areas of the Pacific and Indian Oceans, at 

 depths of from 2000 to 4500 fathoms, the bottom ooze contains 



