REPOKT ON THE DEEP-SEA DEPOSITS. 
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hematite, chlorite, &c. The minerals associated with the quartz grains give a clue as to the matrix rock : 
quartz fragments derived from granite are generally characterised by numerous liquid inclusions, some with 
cubic crystals, the grains have not crystallographic outlines. The same may be said of quartz derived 
from crystalline schists. Quartz of porphyritic rocks, which is very rare, is distinguished in some rare cases 
by its more or less regular outlines, corrosion, and by glassy geometrical inclusions ; moreover, liquid inclusions 
are not so often met with as in granitic quartz. The quartz of clastic rocks is angular or rounded, the micro- 
structure of the grains being the same as that of the same mineral in the rocks in which it was originally formed. 
Vein quartz presents irregular grains or aggregations of grains containing numerous liquid inclusions, sometimes 
fibrous or milky. Fragments of chalcedony are fibrous with a radiated structure, the fibres being very thin 
double refraction negative, aggregate polarisation or spherulitic interference cross. 
The silica derived from organic remains, which, like quartz, plays a very important part in marine deposits, 
behaves, between crossed nicols, like an isotropic body ; to this character must be added the property which 
this variety of silica possesses of preserving, notwithstanding its tenuity or disaggregation, a special organic 
structure (Radiolaria, Diatoms, Sponge spicules). This structure enables us in numerous cases to determine 
the organisms from which the siliceous particles are derived. This variety of silica is soluble in caustic 
potash. 
Rutile. — This mineral is very rarely found in isolated prismatic crystals or in small grains, it is generally 
imbedded in fragments of rocks. In some shore deposits rutile is observnd in the form of fine needles, or 
fragments of microscopic prismatic crystals often twinned as in the slates. Sometimes these small crystals are 
arranged in groups {sagenite), brown or reddish in colour. Double refraction very strong, high relief and brilliant 
polarisation colours for the smallest particles, not acted upon by acids. Associated with debris of older schisto- 
crystalline rocks, slates, &c. 
Serpentine. — Compact or fibrous grains, in some cases mesh- or lattice-structure, yellowish, greenish, or 
brownish ; in the veins, secondary deposition of metallic oxides ; remains of the primitive mineral imbedded in 
the serpentine, birefrangent, colours of chromatic polarisation generally weak ; in polarised light the fibrous or 
special structure appears most distinctly. Attacked by hot hydrochloric acid, and by sulphuric acid, with separa- 
tion of gelatinous silica. 
Tourmaline. — Often found in small prismatic fragments or hemimorphic crystals. Transparent, brownish, 
bluish grey, reddish or greenish. Strong pleochroism, the ordinary ray very much absorbed, extinction 
parallel to the length, no cleavage but cracks somewhat parallel to the base, tolerably numerous inclusions 
often grouped together. Unattacked by ordinary acids. Almost always associated with debris of crystalline 
schists, granitic rocks, slates, &c. 
Zeolites. — Sometimes granular aggregations, isotropic {analcim), or with rhombohedric cleavage {chahasite), 
more frequently divergent or radiated aggregations of small prismatic crystals, exceptionally isolated crystals or 
fragments of crystals bounded by prismatic and pyramidal faces, generally attached to fragments of rocks or 
of volcanic glass; colourless, transparent, low polarisation colours. Easily acted upon by hydrochloric acid, with 
formation of gelatinous silica. It was only possible in some exceptional cases to give a specification of the 
various zeolites, by microscopical examination or by. microchemical reactions (see chapter on formation of 
zeolites in the deep-sea deposits). 
Zircon. — Small quadratic crystals more or less rounded, prismatic but generally short, with indications of 
pyramidal faces. Colourless, yellowish, reddish, strong double refraction, relief very marked, colours of 
polarisation bright red and green, no pleochroism. Sometimes zonary structure, liquid and other inclusions. 
Reaction for zirconia by fusing with bicarbonate of soda. Found with debris of crystalline schists and of older 
and recent eruptive rocks, associated also with quartz grains, and other minerals derived from the disintegration 
of sedimentary rocks. 
The above are the minerals most frequently met with in the deposits, and the micro- 
scopical characters on which we rely for their determination. It is true that all the 
characters used in lithological investigations are not indicated in the foregoing short 
