DEEP-SEA DEPOSITS. 561 



extreme fineness renders them all tlie more susceptible of influence. 

 The mineral substances which sea water holds in solution contribute 

 no doubt actively in these modifications. 



Before the expedition of the ChaVicnger the results of these reactions 

 and the mineral species i)roduced therefrom were, for the most part, 

 unknown, although such species occupy a large portion of the ocean 

 bed. The exact study which has been made of them by the Challenger 

 constitutes for geologists and mineralogists perhaps the most interest- 

 ing part of the exploration. We will review in succession the species 

 which have been ascertained. 



Red da I/. — Of all marine sediments the tyi»e most widely spread over 

 the deep seas has received the name of red clay. It is essentially a 

 hydrated silicate of alumina, the color of which is due to an intimate 

 mixture of [)eroxide of iron ; sometimes also it takes a brown (;oIor from 

 the oxide of manganese. Plastic, like most of the clays, greasy to the 

 touch, it can be molded in the fingers. When dry it adheres in a cohe- 

 rent mass, and subjected to the blow-pipe, it is fused into a black, mag- 

 netic globule. 



In spite of its homogeneous appearance it is rare that red clay is not 

 mixed with very small fragments of pumice and other volcanic produc- 

 tions. When they are not recognizable by the naked eye this debris 

 reveals its granular nature to the touch. Accidentally red clay may 

 also contain detritus of continental origin, drifted by floating ice or 

 carried far by winds. All this debris is very fine, and it rarely exceeds 

 one-twentieth of a millimeter. 



Usually led clay is associated with calcareous and siliceous debris, 

 coming from organisms of a microscopic size, which have been men- 

 tioned above. These organisms are mixed in variable proportions, and 

 sometimes predominate so as to greatly modify their aspect. Hence the 

 names (jlobigcrlna ooze and radiolarlan ooze, according as one or the 

 other of those beings characterize it. Each of these categories of 

 deposits in the great bottoms occupies vast extents. (The area of the 

 radiolarian ooze extends si)ecially between latitudes 20 degrees north 

 and 10 degrees south; the globigerina oozes occupy nearly 110 degrees 

 of latitude, aiul attain sometimes 5,000 meters in depth. Both disap- 

 pear near the polar regions.) The terrigenous deposits represent only 

 14 per (^ent of the superficies of sea bottoms, the red clay occupies 38 

 per cent, and the globigerina mud 30 per cent. The diatomacea, a sort 

 of alg;e with a siliceous skeleton, specially abound toward the polar 

 regions. Thus, as we have said, these various organisms have lived 

 for the most part in the waters of the surface, whence their solid debris 

 have fallen after death into the depths. Vast regions of the Pacific, of 

 the Atlantic, and of the Indian Ocean are occupied with red clay, 

 associated with microscopic organisms. According to a numerous 

 series of soundings, as the depth is greater, the calcareous shell of 

 various organisms disappears gradually from the slimy sediment, so 

 SM 93 30 



