280 
THE VOYAGE OF H.M.S. CHALLENGER. 
So far as we can judge, these shells appear to he formed of calcite. But whether the 
shells be calcite or aragonite they all disappear in the greatest depths of the ocean, 
while only those with very thin or very porous shells are removed from the shallower 
deposits. Aliy shells may be preserved in marine deposits if they be rapidly covered up 
by other shells, or may be removed if long enough exposed to the solvent action of 
normal sea-w’ater. So far as we have been able to observe, the crystalline form of the 
carbonate of lime in these shells does not enter into the problem as to the causes of 
their gradual removal from marine deposits with increasing depth. 
If we take the Challenger deposits as representative of those covering the whole 
floor of the ocean, then the average proportion of carbonate of lime in deep-sea 
deposits as a whole is about 37 per cent., and of this carbonate of lime it is estimated 
that fully 90 per cent, is derived from the remains of pelagic organisms that lived in the 
surface waters, and therefore belonging to the pelagic Plankton. 
Coral LIuds, Coral Sands, Pteropod and Globigerina Oozes are estimated to cover 
over 52,000,000 square miles of the sea bottom, and the average percentage of carbonate 
of lime in these deposits, taking the Challenger samples as a basis, is 76 ‘44. 
Beyond the fact that the sounding tube and dredge have occasionally penetrated 
about 18 inches or two feet into these deposits, there is little, if any, information as to 
the depth or thickness of these beds, but judging from what has taken place in past 
geological periods they may undoubtedly have a very great thickness.^ 
The following table exhibits the percentage of carbonate of lime in each of the types 
of deep-sea deposits according to the analyses of the Challenger samples, together 
with the average depth of each type of deposit, and the estimated area which each type 
covers on the sea-floor, the extent of the areas being founded on a consideration of all 
available information on the subject. 
Table showing the Mean Depth, Mean Percentage of Carbonate of Lime, and the Estimated Area of the 
various Deep-Sea Deposits. 
1 
Mean Depth in 
Fathoms. 
Mean Percentage of 
CaCOg. 
Area, Square Miles. 
i Piod Clay, ..... 
2730 
6-70 
61,500,000 
Iladiolarian Ooze, 
2894 
4-01 
2,290,400 
10,880,000 
Diatom Ooztj, .... 
1477 
22-96 
fJlobigerinii Ooze, 
1996 
64-53 
49,520,000 
I’teropo<l Ooze, .... 
1044 
79-26 
400,000 
Coral .Mud, ..... 
Coral Sand, . ... 
Other terrigenous deposits. Blue 
740 1 
176 / 
86-41 
2,556,800 
Muil, Arc. ..... 
1016 
19-20 
16,050,000 
1 
* ilurrav, Scot. Oeoyr. Mag., vol. vi. pp. 468-473, 1890. 
