i66 DEPTHS OF THE OCEAN 



many radiolarian skeletons. The frustules of diatoms and 

 skeletons of radiolarians may occur in all deposits, but gener- 

 ally they do not become characteristic or predominant when 

 calcareous shells are present in large numbers. 



Red Clay is characteristic of great depths, say beyond 2700 

 fathoms (as Globigerina ooze is characteristic of moderate 

 depths, between 1000 and 2500 fathoms), and is the most widely 

 distributed of all the deep-sea deposits, covering a larger area 

 of the sea-floor than any other deposit type. The basis of the 

 deposit is the hydrated silicate of alumina, or clay, derived 

 principally from the decomposition and disintegration of pumice 

 and other volcanic products long exposed to the action of sea- 

 water, often associated with secondary products derived from 

 the same source, such as manganese-iron nodules and phillipsite 

 crystals. Calcareous remains may be totally absent in the 

 greatest depths, while in lesser depths the percentage of calcium 

 carbonate may approach 30, and the deposit then passes gradu- 

 ally into Globigerina ooze. If the calcium carbonate in a 

 Globigerina ooze or a Pteropod ooze be removed by weak acid, 

 the residue resembles closely a Red clay. In other places the 

 siliceous remains of radiolaria may increase to such an extent 

 that the Red clay merges gradually into Radiolarian ooze. The 

 rate of accumulation is evidently at a minimum in the Red clay 

 areas, for the calcareous shells falling from the surface waters 

 have been gradually removed in solution either before, or 

 immediately after, reaching the bottom ; the ear-bones of whales 

 and teeth of sharks (some of them belonging to extinct species) 

 are there found in the greatest profusion, impregnated with and 

 coated by the peroxides of manganese and iron ; and there also 

 occur in greatest abundance (though always rare) minute 

 metallic and chondritic spherules supposed to have fallen from 

 interstellar space, and found there more abundantly simply 

 because of the sparse deposition of other materials. Radio- 

 active substances are also found more abundantly in Red clay 

 than in any other marine deposit, or in any continental rocks. 



A few facts relating to the horizontal distribution of marine 

 deposits may now be Indicated. The terrigenous deposits 

 include a number of varieties, but as a whole they surround all 

 continents and islands in all latitudes, and extend to varying 

 distances from the shore. The Coral muds and sands included 

 in this class are limited to the coral-reef regions of tropical and 

 subtropical latitudes, and the presence of the calcareous shells 



