GEOLOGIC TIME. 667 



so largely formed of carbonate of lime that I have taken the 

 latter as a basis for estimates upon the rate of chemical sedi- 

 mentation, an allowance being made for the presence of silica, 

 alumina and magnesium in the limestones. 



Rate of Deposition of Recent Deposits. — Of the rate of depo- 

 sition in recent deposits Messrs. Murray and Renard state, 

 in their report on the deep-sea deposits, that : " It must be ad- 

 mitted that at the present time we have no definite knowl- 

 edge as to the absolute rate of accumulation of any deep- 

 sea deposit, although we have some information and some 

 indications as to the relative rate of accumulation of the different 

 types of deposits among themselves. The most rapid accumula- 

 tion appears to take place in the Terrigenous Deposits, and 

 especially in the Blue Muds, not far removed from the embouch- 

 ures of large rivers. Here no great time would seem to have 

 elapsed since the deposit was formed, so far at least as the 

 materials collected by the dredge, trawl, and sounding tube are 

 concerned. 



" Around some coral reefs the accumulation must be rapid, 

 for, although pelagic species with calcareous shells may be 

 numerous in the surface waters, it is often impossible to detect 

 more than an occasional pelagic shell among the other calcareous 

 debris of the deposits. 



"The Pelagic Deposits as a whole, having regard to the. 

 nature and condition of their organic and mineralogical constit- 

 uents, evidently accumulate at a much slower rate than the 

 terrigenous deposits, in which the materials washed down from 

 the land play so large a part. The Pteropod and Globigerina 

 oozes of the tropical regions, being chiefly made up of the cal- 

 careous shells of a much larger number of tropical species, must 

 necessarily accumulate at greater rate than the Globigerina oozes 

 in extra-tropical areas or other organic oozes. Diatom ooze, being 

 composed of both calcareous and siliceous organisms, has, again, 

 a more rapid rate of deposition than the Radiolarian ooze, while 

 in a Red Clay there is a minimum rate of growth." T 



1 Report on the scientific results of the voyage of H. M. S. Challenger ; Deep-Sea 

 Deposits. 1891, pp. 411-412. 



