GEOLOGIC TIME. 327 



Rate of dejmsition in recent ileposlts. — Of the rate of depositiou in 

 recent deposits Messrs. Murray and IJenard state, in tlieir report on 

 the deep-sea deposits, that — 



" It must be admitted that at the present time we Inive no definite 

 knowledge as to the absohite rate of accumulation of any deep-sea 

 deposit, although we have some itifornuitioii and some indications as 

 to the relative rate of accumulation of the different types of deposits 

 among themselves. The most rapid accumulation ap})ears 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, 

 although i^elagic species with calcareous shells may be numerous in 

 the surface waters, it is often impossible to detect more than an occa- 

 sional pelagic shell among tlie otlier 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 

 accumulate at a much slower rate than the terrigenous deposits, in 

 which the materials washed down from the land play so large a part. 

 Tlie Pteropod and Globigerina oozes of the tropical regions, being 

 chietiy made up of the calcareous shells of a much larger number of 

 tro])ical species, must necessarily accumulate at a 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 liadiolarian ooze, while 

 in a lied Clay there is a minimum rate of growth."* 



Prof. James 1). Dana estimates that the rate of increase of coral reef 

 limestone formation's, where all is most favorable, does not exceed per- 

 haps a sixteenth of an inch a year, or 5 feet in 1 000 years. Of this he 

 says: "And yet such limestones probably form at a more rapid rate 

 than those made of shells.''t 



Messrs. Murray and Irvine, in their valuable paper on coral reefs 

 and other carbonate of lime fornuitions in modern setis, calculate the 

 total amount of calcium in the whole ocean to be (528,340,000,000,000 [628 

 trillion] tons; also they estinnite that 92r>, 806,500 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 CArt/- 

 leiKjer deposits 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 

 salinity of the ocean has remained the same as at the present during 

 the whole of this period, then it has taken 680,000 years for the deposits 

 of the above thickness, or containing calcium in amount e(pial to that 



* Report on the scientific results of thevoi/af/e of H. M. S. ChaUenger; Deep-Sea Deposits. 

 1891, pp. 411-412. 



\Corals and Coral Islands, 3d ed., 1890, pp. 390, 397 



