316 GEOLOGIC TIME. 



ited: (1) agency of organisms; (2) cliemical i)recipitation ; (3) mechan- 

 ical methods. 



It is tlie general opionion of geologists that limestone rocks are the 

 result ahnost entirely of the consolidation of lime removed from the 

 sea water through the agency of life, and that they consist of the 

 remains of foraminifera, crinoids, corals, etc., or. their fragments, em- 

 bedded in a more or less crystalline matrix resulting from subsequent 

 alteration of the original deposits. Tliis, however, has been seriously 

 questioned. Sorby, in giving his general conclusions of an extensive 

 microscopic examination of limestones, states that — 



"Even if it were possible to study in adetaclied state the finer granu- 

 lar particles which constitute so large a i>art of many limestone forma- 

 tions, it would usually be impossible to say whether they had been 

 derived from organisms which can decay down into granules, or from 

 other organisms which can only be worn down into granules, or from 

 ground-down older limestone, or, in some cases, from carbonate of lime 

 deposited chemically as granules. - - - The shape and character 

 of the identifiable fragments do, indeed, prove that much of this must 

 have been derived from the decayed and worn-down calcareous organ- 

 isms; and very often we may reasonably infer that the greater part, if 

 not tlie whole, was so derived; but at the same time it is impossible to 

 prove, from the structure of the rock, whether some or liow much was 

 derived from limestones of earlier date, or was deposited chemically, 

 as some certainly must have been."* 



In their memoir on coral reefs and other carbonate-of-lime forma- 

 tions in modern seas, Messrs. Murray and Irvine show that tempera- 

 ture 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 secretion of carbonate of lime than high 

 salinity. + 



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 tlie deposit covering the floor of the ocean is 36-83; of this it is esti- 

 mated that fully 90 j^er cent is derived from j)elagic organisms that 

 have fallen from the surface water, the remainder of the carbonate of 

 lime having been secreted by organisms that lay on, or were attached 

 to, the bottom. The estimated area of the various kinds of deiiosits, 

 the average depth, and the average percentage of carbonate of lime to 

 each are shown in the following table: 



" Quart. Jour. Geol. Soc, London, 1879, vol. xxxv, pp. 91-92. 

 t Proc. Boyal Soc, Etliuburgh, 1890, vol. xvii, p. 81. 



