THE WORK OF TEE OCEAN. 383 



When the percentage of Hme carbonate becomes very low, the cal- 

 careous oozes grade off into the red clay with which the sea- floor below^ 

 2400 to 2600 fathoms is covered. 



Chemical deposits. — The chemical deposits of the deep sea are chiefly 

 the alteration products of sediments which reach the sea-bottom by 

 mechanical means. All sediment deposited in the sea undergoes more 

 or less chemical change, but it is only when the change is very con- 

 siderable that the product is referred to this class. AATiere sedimenta- 

 tion is rapid and the sediment coarse, the chemical change is relatively 

 shght ; but where the sedimentation is slow and the sediment fine, the 

 chemical change is relatively great ; for the longer exposure to the sea- 

 water and the greater proportion of surface exposed to attack, both 

 favor change. Both the area and the mass of sea-bottom sediment 

 radically changed in this way are large, but most of the deposit does 

 not correspond to any formation known on the land. 



The red clay already referred to belongs to this class of deposits. 

 Its origin has been the subject of much discussion. It contains much 

 volcanic debris, various concretions, bones of mammals, zeohtic crystals, 

 and extra-terrestrial spherules, and doubtless the insoluble products 

 of the shells of pelagic hfe ; but it is still a mooted question how far the 

 clay itself is the product of decomposed shells, and how far the altered 

 product of pulverized pumice, volcanic ash, dust, etc. Pelagic hfe does 

 not seem to be less abundant at the surface w^here the water is deep than 

 where it is shallow, and it would appear that the shells must sink in 

 such situations as elsewhere. If the hme carbonate of globigerina ooze 

 be removed by dilute acid, the inorganic residue is similar to the red 

 clay in the ocean-bottom. This suggests that owing to the more com- 

 plete solution in the very deep water, the hme carbonate of the shells 

 has been dissolved, leaving the red clay as a residuum. The more 

 complete solution at the bottom might be the result either of the greater 

 pressure, or of a greater percentage of CO2 in the water due to emanations 

 from the sea-floor, or to both; but the suddenness of the transition 

 from oozes to red clay, with increasing depth, does not seem to be fully 

 explained by these assumptions. The study of the dredgings has 

 inchned the students of these materials to the conclusion that volcanic 

 materials, rather than sheUs, are the principal source of the red clay.^ 



^ Murray, Challenger Report on Deep Sea Deposits, p. 337 et seq., and Buchanan 

 Proc. Roy.'Soc. Edin., Vol. XVIII, 1892, pp. 17-39. 



