234 
THE VOYAGE OF H.M.S. CHALLENGER. 
Red Mud. 
Along the Brazilian coast of South America the terrigenous deposits oflf shore arc 
different from the deposits found in similar positions along other continents, in that they 
are all of a red-brown or brick-red colour, in place of blue or green coloured, as is 
usually the case. The red colour of the deposits appears to be produced by the large 
quantity of ochreous matter carried into the ocean by the Amazon, Orinoco, and 
other South American rivers, and distributed by oceanic currents along these coasts. 
Although organic matters are probably as abundant in these as in the deposits along 
other coasts, still they do not seem to be sufficient to reduce the whole of the peroxide 
of iron to the state of protoxide, nor does sulphide of iron accumulate here as in the 
case of the Blue Muds, in both of which respects these Red Muds resemble the Red Clays 
of the abysmal regions. It is a remarkable fact that we do not find in these red deposits 
a trace of the green coloured glauconitic casts of Foraminifera and other calcareous 
organisms, nor any of the glauconite grains which usually accompany these casts in 
other terrigenous deposits. There are a few spicules of siliceous Sponges, but frustules 
of Diatoms and the remains of Radiolarians are exceedingly rare, or wholly absent. As 
regards the calcareous organisms, and mineral particles other than glauconite, they do 
not appear to differ from those present in the Blue or Green Muds. 
Of these Red Muds 10 samples are described in the Tables of Chapter II. These are 
from depths varying from 120 to 1200 fathoms, the average depth being 623 fathoms. 
3 are from less than 500 fathoms. 
5 ,, between 500 and 1000 ,, 
2 „ over 1000 „ 
In colour they are all red-brown. The carbonate of lime in these samples ranges from 
5 75 to 6079 per cent., the average being 32'28 per cent. The amount of carbonate of 
lime in the different samples is more in relation to the nearness or distance from the 
mouths of rivers than to the depth from which the samples were taken. The carbonate 
of lime derived from the sliells of pelagic Foraminifera ranges from 3 to 30 per cent., the 
average being 13‘44 percent.; that derived from bottom-living Foraminifera ranges from 
1 to 8 per cent., and averages 3'33 per cent.; that from other organisms ranges from 175 
to 4075 per cent., the average being 15‘51 per cent. 
'riic residue is in all cases reddish brown or yellow; it ranges from 39‘21 to 94’25 
per cent, of the whole deposit, the average being 6772 per cent. Siliceous organisms 
are relatively very rare, and in no case are they estimated to make up more than 1 per 
cent, of the wliole deposit ; they consist almost exclusively of Sponge spicules and a few 
arenaceous Foraminifera. With some doubtful exceptions. Diatoms were not observed 
during the examination of these deposits. 
