652 



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 impos- 

 sible 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 CaC0 3 , of 



the different Deposits. 



Loc. cit, p. 82. 



"We have little knowledge as to the thickness of these depos- 

 its, 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. 



