20 
THE VOYAGE OF H.M.S. CHALLENGER. 
and microstructure ; sometimes, as in tlie case of otoliths of fish, Foraminifera, and Mollusc shells, one observes 
between cnissed nicols the black cross of sphcrulitic aggregation. 
CoLoiuTE. — Under this name are included all the minerals of the chlorite group. Lamellae or scales more 
or less curved, with irregular outlines, often with parallel structure, sometimes forming fibro-radiated aggregates. 
.Vlso observed in the form of minute amorphous particles attached to various minerals or coating rock frag- 
ments ; perfect cleavage parallel to the basal plane ; colour green, more or less dark, index of refraction low. 
The lamelhe parallel to the cleavage are isotropic, extinction parallel to the cleavage, interference colours 
bluish, pleochroic, fibro-radiated aggregates give the interference cross of spherulites, easily acted upon by 
acids, microchemical reaction of magnesia, iron, and alumina; becomes opaque, brown, or blackish when heated 
on a platinum foil. Generally found with debris of schistose and of older crystalline rocks, principally 
amphibolic or gneissic, also as coating of some specimens of continental rocks and minerals. 
Chromite. — Often in small octahedral crystals, more usually as irregular grains; fine splinters are trans- 
j)arcnt, reddish brown, or bro^vn. In reflected light black metallic lustre, not magnetic, not attacked by acids, 
reaction of chrome before blowpipe. Found with debris of crystalline rocks, especially of olivine rocks. 
Deles.site. — Fibrous or fibro-radiated aggregates, often zonary structure, green, yellowish green, brownish, 
double refraction weak, slightly pleochroic. Often observed on fragments of basaltic rocks, of tachylite, or of 
jmlagonite. Easily acted upon by acids, when heated becomes black or brownish. 
Dolomite. — Sometimes found as aggregations of crystals, which have almost always the outlines of the 
fundamental rhombohedron I\, forming small rock fragments, with saccharoidal structure ; colourless or 
brownish, not acted upon by acetic acid or cold dilute hydrochloric acid, microchemical reaction of magnesia. 
Occurs in the deposits as fragments of dolomitic rocks associated with blocks or gravel of older crystalline and 
sedimentary rocks transported by icebergs. 
Epidote. — Generally occurs as fragments of crystals, mostly prismatic, rarely with sharp crystallographic 
faces, elongated parallel to the axis of symmetry, cleavage cracks following M. Several crystals often found 
attached parallelly, sometimes more or less radiated aggregates or grains. Yellowish green colour, sometimes 
almost colourless, uneven surface and strong relief, high interference colours, strong pleochroism : yellow, 
brown, green ; extinction parallel to the cleavage, unattacked by cold acids. Occurs with debris of eruptive 
or schistose rocks. 
Felspar-s. — (a) Monorlinic . — Generally fragments bounded by the cleavage planes, following P and M, 
intersecting at right angles, colourless or coloured by interpositions, dull or sometimes opaque, in other cases 
glassy. Weak double refraction, low interference colours, no pleochroism nor differences in absorption, 
extinctions of the monoclinic system, twins following the Carlsbad, Baveno, or Manebach laws. Sanidine, 
often in crystals more or less fragmentary, with glassy habit, colourless and transparent, presenting the ordinary 
crystallographic form, tabular parallel to M or prismatic parallel to the edge P/M, separation-planes parallel to 
the orthopinakoid, glass inclusions often regularly disposed. Associated with recent volcanic products, often 
covcreil entirely or partly with a glassy coating. Orthoclase, fragments bounded by cleavage planes, often 
irregular grains, no glassy habit, dull and milky, no glass inclusions, intergrowth with quartz or with triclinic 
felspar, decomposition into kaolin or muscovite, microchemical reaction of potash. Associated in the deposits 
with debris of crystalline schists and of older eruptive rocks. 
(h) Trirlinir, — Microclinp, colourless or dull grains or fragments often bounded by the cleavage planes 
parallel to P ami .M, j»olysynthetic lamella; following the albite and pericline laws. In parallel polarised light 
(’bamcteristic cross-hatched appcaninco produced by twin lamellae parallel and perpendicular to P/M. 
Extinction of 15* .30' on P, alterations as for orthoclase. Associated with debris of older eruptive and schisto- 
cr}'iitallinc rocks. 1‘laijiodane^ under this designation are included all the triclinic felspars with the exception 
of microcline. As a rule, in the deposits, the s|)ccification of pdagioclase by optical determination is difficult 
or uncertain ; when a specification of the plagiocluscs has been made it is stated in the descriptions. They 
occur in ileep-sea tkqiosits in most coses ns fragments of crystals often bounded by the cleavage jilanes ; when 
they arc iml>c<lded in glassy or pnlagonilic matter, ns is frequently the case in the true pelagic deposits, they 
pre. nt the onlinary crystallographic forms or are crystallised in very thin tables parallel to M. Polysynthetic 
twinning following the albite law with frcriuent repetition, and following the pericline law, sometimes Carlsbad 
