REPORT ON THE DEEP-SEA DEPOSITS. 
25 
light, limonite is reddish or deep brown ; after heating these particles become black, 
opaque, and magnetic. Although we can rarely distinguish limonite from red hematite, 
it seems probable that in some cases the latter is represented in the deposits. The 
distinctive characters are that hematite is not at all so soluble in acids ; it is red in 
reflected light and in transmitted light slightly transparent with a reddish tinge. These 
argillaceous matters are mixed up with : — 
h. Organic substances, heated on a platinum foil they disappear or leave cinders, 
not decomposed by hydrochloric acid ; soluble in caustic potash. Generally no organic 
structure is to be seen ; amorphous, colour greenish, yellowish, brown, or grey, between 
crossed nicols behaving as isotropic bodies ; in some cases when exhibiting an organic 
structure they are birefrangent. 
c. Siliceous organisms, owing to their low specific weight (1'9 to 2 ’3), and their small 
size, these are carried away with the fine washings, and are fragments of the siliceous 
organisms found in the deposit. 
d. Mineral particles, fragments of the same species as mentioned under “minerals.” 
We see thus that what goes by the name of deep-sea clay has no complete analogy 
with what should be included under the name of pure clay. It is not chemically or 
physically similar to kaolin, but is more nearly allied to bole-clay, rich in iron and 
manganese, and the clayey matter in the fine washings plays a much less important 
part than might be suspected before microscopical examination. Further on it will be 
shown that the proportion of silica in these clays, and therefore the proportion of silica 
in the fine washings, indicates generally free silica. This is to be attributed to the 
presence of remains of siliceous organisms, or small quartz particles. The fragments of 
minerals which pass away in the first decantations are always less than 0'02 mm. in 
diameter; their diminutiveness thus makes them float suspended in the water for some 
time, when the latter is agitated by shaking. Particles of this size might perhaps be 
determined in a rock section under the mineralogical microscope, but this is not possible 
with minute, isolated, irregular, chemically and physically altered, fragments, generally 
without any crystallographic outlines. There is, however, one exception in the case of 
splinters from pumice stone and vitreous volcanic rocks. The structure and form of 
these glassy particles makes them much more readily distinguishable than other mineral 
particles, so that particles even 0'002 mm. in diameter can be recognised. This can 
easily be tested by grinding a piece of pumice to powder in an agate mortar, when it 
will be found that the abundance of gaseous bubbles, the filamentous structure, curvi- 
linear outline and jagged appearance due to the presence of the bubbles, enable the 
minutest fragments to be detected. The fragments from basic and acid pumice can 
even be detected in some cases ; the latter yields elongated and nearly colourless 
fragments, while the former shows darker particles, and some of the bubbles are of a 
circular form rather than elongated. 
(deep-sea deposits chall. exp. — 1890 .) 
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