352 
Tin American Geologist. 
December, 189* 
It is the general opinion of geologists that limestone rocks are the 
result almost entirely of the consolidation of lime removed from the 
sea water through the agency of life, and that they consist of the re- 
mains of foraminifera, crinoids, corals, etc., or their fragments, embed 
cd in a more or less crystalline matrix resulting from subsequent altera 
tion of the original deposits. This, however, has been seriously ques 
tioned. Sorby, in giving his general conclusions of an extensive mi- 
croscopic examination of limestones states that: 
Even if it were possible to study in a detached state the finer granu- 
lar particles which constitute so large a part of many limestone forma- 
tions, it would usually be impossible to say whether they had been de- 
rived 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 char- 
acter of the identifiable fragments do, indeed, prove that much of this 
must have been derived from the decayed and worn-down calcareous 
organisms; 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 of earlier date, or was deposited chemi- 
cally, 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 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 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 
the deposit covering the floor of the ocean is 36.83; of this it is esti- 
mated 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 lay on, or were attached 
to, the bottom. The estimated areaof 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 
CaCOqT of the 
Different Deposits.} 
Deposit. 
Area, scjuare 
miles. 
Mean Depth 
in 
fathoms. 
Mean per 
cent, of 
CaCOg. 
1 Red clay 
50,289,600 
2,790. 400 
10,420,01)0 
47,752,5(1(1 
887,100 
3,219,800 
27,899,800 
2.727 
2. >9I 
1,477 
1,990 
1,118 
710 
1,016 
6.70 
Oceanic Oozes ! Radiolarian ooze... 
Globigerina ooze. . . 
f Coral sands and 
Terrigenous 1 muds 
Deposits ; Other terrigenous 
deposits, blue 
4.01 
22 90 
64.53 
79.20 
86.41 
19.20 
*Quart. Jour. Geol. Soc. London, vol. 35. 1879, pp. 91-92, 
tProc. Royal K,,c. Edinburgh, vol. 17. 1890, p. 81. 
iLoc. cit.. p. 82. 
