REPORT ON THE DEEP-SEA DEPOSITS. 
233 
the average found in a Red Clay. The excess of silica must, as in the other deposits, 
be attributed to the presence of siliceous organisms. The carbonate of lime is due, as in 
pelagic deposits, to the shells of calcareous organisms ; the presence of carbonate of mag- 
nesia and sulphate and phosphate of lime is to be interpreted in the same manner as in the 
case of these substances in the pelagic deposits.^ In the insoluble portion the excess of 
silica must be attributed to quartz, but besides there are present anhydrous silicates, which 
microscopic examination showed to be monoclinic and triclinic felspars, mica, augite, 
magnetite, hornblende, and the debris of pumice. 
An additional analysis of a sample of the Blue Mud from Station 323, in 1900 
fathoms, gave the following results : — 
station. 
Depth in 
Fathoms. 
No. 
SiOa 
AloOs 
FeaOg 
Phosphoric and 
Sulphuric Acids. 
CaO 
MgO 
K 2 O 
NajO 
Loss on 
Ignition. 
Total. 
323 
1900 
65 
59-54 
19-42 
7-15 
traces. 
1-68 
1-93 
1-35 
2-68 
6-24 
99-99 
If this analysis be compared with the preceding one from the same station, some 
differences will be perceived, the total amount of silica in analysis No. 64 being 64'20 
per cent., and in No. 65, 59'54 per cent. ; alumina, 13'55 per cent, and 19*42 per cent. 
Although the general result may lead in the two cases to the same interpretation as 
to the essential mineralogical composition of the deposit, these divergences show how 
much different samples of the deposit may vary even from the same dredging, and how 
difficult it is to pronounce upon the mineral nature of a deposit solely by chemical 
analysis. The variations in the composition of the deposit from the same trawling or 
dredging may arise from some portions being from deeper layers than others, or from 
differences of depth and position when the instrument was being dragged over several 
miles. The mode of collection and preservation, by separating the finer and coarser parts 
of the same sample, may also give rise to differences in the analyses. This remark may 
be applied to all the deposits, as already stated, but particularly to the terrigenous 
deposits which, according to the conditions of formation, are seldom so homogeneous as 
the pelagic deposits. 
The Blue Muds surround nearly all coasts and fill nearly all enclosed seas, like the 
Mediterranean, and even the Arctic Ocean ; of all the terrigenous deposits they occupy 
by far the largest area of the earth’s surface, being estimated to cover about 14,500,000 
square miles, of which the Arctic Ocean would contain about four millions of square miles, 
the Pacific three millions, the Atlantic two millions, the Indian one and a half millions, 
the Southern one and a half millions, and the Antarctic about two and a half millions of 
square miles. The geographical position of these muds is represented on Chart 1. 
^ See pp. 200, 201. 
(dSJP-SEA deposits CHAI.L, EXP. 1890 .) 
30 
