Geologic Time. — Walcott. 353 
"We have little knowledge as to the thickness of these deposits, still 
such as we have goes to show that in these organic calcareous oozes and 
muds, we have a vast formation greatly exceeding in bulk and extent 
the coral reefs of tropical seas; they are most widely distributed in equa- 
torial regions, but some patches of Globigerina ooze are to be found 
even within the Arctic circle, in the course of the Gulf Stream.'"* 
The percentage of carbonate of lime contained in deposits accumu- 
lating at different depths, obtained from 231 samples collected by the 
Challenger, is shown in the following tabulation: 
"14 cafes under 500 fathoms, m. p. c 86.04 
66.86 
" 70.81 
69.55 
4(1.73 
" 17.36 
" 0.88 
" o.OO 
" trace." 
7 
500 to 1,000 
24 
" 1,000 to 1,500 
\i 
" 1,500 to 2.000 
68 
•' 2.0(10 to 2.500 
• ir, 
" 2,500 to 3,000 
8 
" 8,000 to 3,500 
•> 
" 3,500 to4,000 
i 
" 4.000 
The fourteen samples under .">00 fathoms are chiefly coral muds and 
sands, and the seven samples from 500 to 1,000 fathoms contain a con- 
siderable quantity of mineral particles from continents or volcanic 
islands. In all the depths greater than 1,000 fathoms the carbonate of 
lime is mostly derived from the shells of pelagic organisms that have 
fallen from the surface waters, and it will be noticed that these wholly 
disappear from the greater depths.+ 
By a series of experiments Messrs. Murray and Irvine found: "That 
although sea water under certain conditions may take up a consider- 
able quantity of carbonate of lime in solution, yet it is unable perma- 
nently to retain in solution more than is usually found to be present in 
sea water, and it is owing to this that the amount of carbonate of lime 
is so constantly low. The reaction between organic matter and the sul- 
phates present in sea water (to which we have referred) tends also to 
keep the amount of carbonate of lime in solution at about one-half 
(0.12 grms.) of what it might contain (0.28 grms. per litre). This pecul- 
iarity of sea water in taking up a large amount of amorphous carbon- 
ate of lime and throwing it out in crystalline form, accounts for the 
rilling up of the interstices of massive coral with crystalline carbonate 
in coral islands and other calcareous formations, so that all traces may 
ultimately be lost of the original organic structures.^ 
The authors explain the disappearance of shells and lime deposits in 
the greater depths of the ocean by their being dissolved by the carbonic 
acid in the water which is present in larger quantity at great depths 
and also is produced by the decomposition of the animal matter of the 
shell and of the various organisms living in the water and on the bot- 
tom. They conclude that: 
On the whole, however, the quantity of carbonate of lime that is 
secreted by animals must exceed what is re-dissolved by the action of 
sea water, and at the present time there is a vast accumulation of the 
carbonate of lime going on in the ocean. It has been the same in the 
*Loc. cit., pp. 82-83. 
fLoc.cit., p. 84. 
JLoc. cit., pp. 94-95. 
