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THE JOURNAL OF GEOLOGY.

and very often we may reasonably infer that the greater part, if not the whole, was so derived; but, at the same time, it is impossible to prove, from the structure of the rock, whether some or how much was derived from limestones or earlier date, or was deposited chemically, as some certainly must have been.[1]

In their memoir on coral reefs and other carbonate of lime formations in modern seas, Messrs. Murray and Irvine show that temperature of the water has a controlling influence upon the abundance of species and individuals of lime-secreting organisms; high temperature is more favorable to abundant secretions of carbonate of lime than high salinity.[2]

Taking the samples of deep sea deposits collected by the Challenger as a guide, the average percentage of carbonate of lime in the whole of the deposit covering the floor of the ocean is 36.83; of this it is estimated that fully 90 per cent. is derived from pelagic organisms that have fallen from the surface water, the remainder of the carbonate of lime having been secreted by organisms that laid on, or were attached to, the bottom. The estimated area of the various kinds of deposits, the average depth, and the average percentage of carbonate of lime to each are shown in the following table:

Table showing the Estimated Area, Mean Depth, and Mean Percentage of CaCO3, of the different Deposits.[3]
Deposit. Area square miles. Mean depth in fathoms. Mean per ct. of CaCO3.
Oceanic Oozes and Clays Red clay, 50,289,600 2727 6.70
Radiolarian ooze, 2,790,400 2894 4.01
Diatom ooze, 10,420,600 1477 22.96
Globigerina ooze, 47,752,500 1996 64.53
Pteropod ooze 887,100 1118 79.26
Terrigenous Deposits Coral sands and muds, 3,219,800 710 86.41
Other terrigenous deposits, blue mud, etc. 27,899,300 1016 19.20

"We have little knowledge as to the thickness of these deposits, still such as we have goes to show that in these organic cal-

  1. ↑ Quart. Jour. Geol. Soc. London, Vol. 35, 1879, pp. 61-92.
  2. ↑ Proc. Roy. Soc. Edinburgh, Vol. 17, 1890, p. 81.
  3. ↑ Loc. cit., p. 82.
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