914 
THE VOYAGE OF H.M.S. CHALLENGER. 
very indefinite it is difficult to estimate, even approximately, the quantity present 
in a deposit. However, it augments in proportion as the deposit becomes more clayey, 
but only a small quantity of this substance is necessary to give a clayey character 
to a deposit. That which predominates in this fundamental base, are irregular frag- 
ments of minerals, vitreous rocks, and remains of siliceous organisms. These particles 
probably make up about 50 per cent, of the whole mass, and this large percentage of 
foreign matters must necessarily mask the character of the clayey matter in which they 
are imbedded. 
The mineral particles are seldom larger than O'Ol mm. in diameter, but descend 
from this size to the merest specks. It is impossible on account of their size to say 
to what mineral species they belong, their thickness not being sufficient to be distin- 
guished by polarised light, their outlines too irregular, and all special coloration having 
likewise disappeared. All that can reasonably be said is that these minute mineral 
particles probably belong to the same species as the larger particles in the same deposit, 
such as felspar, hornblende, magnetite. In the case of the pumice and siliceous 
organisms the fragments can be recognised when of a much less size than in the case of 
the above minerals. Here the structure and fracture of the pumice, and form and 
markings of the organisms, enable very minute particles to be detected. However, 
some particles of pumice are so small as to have lost entirely their fibrous structure, the 
bulbs of air, and the peculiar fracture with frequent semicircular contours, and are 
therefore indistinguishable from the minute fragments of minerals which are likewise 
isotropic. In the same way extremely minute fragments of siliceous organisms cannot 
be distinguished from minerals and pumice fragments of about the same size. 
The colouring substances are the hydrated oxides of iron and manganese. The former 
is scattered through all the mass in a state of very fine division. In some points, 
however, it is more localised, the argillaceous matter appearing with a brown tinge, but 
these spots are noticed gradually to disappear in the surrounding mass. The coloration 
given by the manganese is much more distinct. There are small, rounded, brown- 
coloured spots, with a diameter of less than O'Ol mm., which disappear under the action 
of hydrochloric acid with disengagement of chlorine. Among the magnetic particles, 
which consisted of small fragments of magnetite and small spherules, there was one 
spherule of bronzite (see p. 812). 
The mineral particles collected after dissolving away a very large quantity of the ooze 
were the same as to size and species as those indicated in the description of the deposit. 
A quantity of the ooze was taken from the dredge, and divided into three portions by 
decantations. The finest portion gave 88 T 5 per cent, of carbonate of lime, the second 
gave 98'84 per cent., and the third 98'56 per cent. The finest portion consisted chiefly 
of Coccoliths, Ehabdoliths, primordial chambers of Foraminifera, and amorphous clayey 
and calcareous particles, thus showing that most of the clayey matter passed away with 
