:tt>'l Tin American Geologist. December, 1893 
an allowance being made for the presence of 6ilica, alumina and mag- 
nesium in the limestones. 
Rate of deposition in recent deposits. Of the rate of deposition 
in recent deposits Messrs. Murray and Renard state, in their report 
on the deep-sea deposits, that: "It must be admitted that at the 
present time we have no definite knowledge as to the absolute rate of 
accumulation of any deep sea deposit, although we have some infor- 
mation and some indications as to the relative rate of accumulation 
of the different types of deposits among themselves. The most rapid 
accumulation appears to take place in the Terrigenous Deposits, and 
especially in the Blue Muds, not far removed from the embouchures 
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, al- 
though 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 constituents, evidently ac- 
cumulate at a much lower rate than the terrigenous deposits, in which 
the materials washed down from the land play so large a part. The Pter- 
opod and Globigerina oozes of the tropical regions, being chiefly made 
up of the calcareous shells of a much larger number of tropical species, 
must necessarily accumulate at a greater rate than the Globigerina 
oozes in extra-tropical areas or other organic oozes. Diatom ooze, be- 
ing 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."* 
Prof. James D. Dana estimates that the rate of increase of coral reef 
limestone formations, where all is most favorable, does not exceed per- 
haps a sixteenth of an inch a year, or five feet in a thousand years. Of 
this he says: "And yet such limestones probably form at a more rapid 
rate than those made of shells."! 
Messrs. Murray and Irvine, in their valuable paper on coral reefs and 
other carbonate of lime formations in modern seas, calculate the total 
amount of calcium in the whole ocean to be 628,340,000 million tons; 
also they estimate that 925,866,500 tons of calcium are carried into the 
ocean from all the rivers of the globe annually. At this rate it would 
take 680,000 years for the river drainage from the land to carry down an 
amount of calcium equal to that at present existing in solution in the 
whole ocean. They say further: "Again, taking the 'Challenger' de- 
posits as a guide, the amount of calcium in these deposits, if they be 22 
feet thick, is equal to the total amount of calcium in solution in the 
whole ocean at the present time. It follows from this that if the salin- 
*Report on the scientific results of the voyage of H. M. S. Challenger; Deep-8ea 
Deposits. 1891, pp. 411-412. 
tCorals and Coral Islands, 3d Ed., 1890, pp. 396-397. 
