330 GEOLOGY 



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 considerable that the product is referred to this class. 

 Where sedimentation is rapid and the sediment coarse, the chemical 

 change is relatively slight; but where the sedimentation is slow 

 and the sediment fine, the chemical change is relatively great; for 

 both the longer exposure to the sea-water and the greater propor- 

 tion of surface exposed to attack, favor change. 



The red day 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, 

 zeolitic crystals, and extra-terrestrial spherules, and doubtless the 

 insoluble parts of the shells of pelagic life; 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 life does not seem to be less abundant at the 

 surface where the water is deep than where it is shallow, and it 

 would appear that the shells must sink in such situations as else- 

 where. If the lime carbonate of globigerina ooze is removed by 

 dilute acid, the inorganic residue is similar to the red clay in the 

 ocean-bottom. This suggests that owing to the more complete 

 solution in the very deep water, the lime carbonate of the shells has 

 been dissolved, leaving as a residuum material which makes up a 

 part of the red clay. The more complete solution at the bottom 

 might be the result either of the greater pressure, or of a greater 

 percentage of CO 2 in the water due to emanations from the sea- 

 floor, or to both; but the rather sudden 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 inclined 

 the students of these materials to the conclusion that volcanic 

 materials, rather than shells, are the principal sou roc of the red 

 clay. 1 The volcanic materials are thought to have accumulated 

 slowly, and to have been lon; exposed to the action of sea-water. 



1 Murray, Challenger Report on iwp Sen Drpoxiu. p. :;.!7 ct scq., and 

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



