22 
THE VOYAGE OF H.M.S. CHALLENGEE. 
with debris of altered sedimentary or f^eissic rocks or with minerals derived from the disintegration of granite. 
It is possible that under the name of white mica not only muscovite and sericite, but other varieties, and also 
altered black mica, are included. Generally speaking, the micas are the less altered of the silicates of the 
deejvsea deposits. 
Olivine, — Almost always irregularly-bounded grains, except when coated with volcanic glass ; in this case 
olivine has the crystallographic forms of this mineral in recent eruptive rocks ; often skeleton crystals. Im- 
perfect cleavage following the brachypinakoid and cracks following the macropinakoid. Transparent, colourless, 
greenish or reddish by alteration, without pleochroism, and with feeble absorption, high relief; colours of 
polarisation very bright green and red, even for the smallest fragments. Numerous inclusions of fluid, of 
vitreous grains, of magnetite and picotite. Decomposes into serpentinous matter, but generally the alteration 
of olivine in dee{vsca deposits is accompanied with a deposition of ferric oxide or hydrous oxide of iron. 
Gelatinises with sulphuric acid. Olivine is found in some exceptional cases with debris of crystalline schists 
or older eruptive rocks, but generally with clastic volcanic minerals and fragments of basalts, limburgite, 
tachylite, and in volcanic ashes. 
PvRiTES. — Sometimes observed in cubic crystals or in the form of pentagonal dodecahedrons, sometimes 
with the characteristic striie of oscillatory combination, sometimes in irregular grains, opaque, yellowish metallic 
lustre or bluish black in reflected light. Occasionally transformed into limonite. 
Pyroxene. — (a) Rhoinlric, lamellar aggregates, generally large fragments, are found with older eruptive rock 
debris, short prismatic fragments with products of more recent eruptions. Cleavage following the prism of about 
90*, in some cases also another cleavage parallel to the brachypinakoid. Index of refraction high, low inter- 
ference colours. Extinction parallel to the pinakoidal cleavage. Enstatite, colourless to grey, yellowish, not 
pleocbroic. Bronzite, often fibrous, yellowish to greenish, pleochroic, greyish green, yellowish. These two 
pyroxenes often contain metallic inclusions, intergrowth with monoclinic pyroxene. Associated with older 
rock debris, principally with peridotic rocks, and with volcanic ashes of ande.sitic or trachytic nature. In this 
case the fragments or crystals have often glass inclusions, but do not show intergrowth with monoclinic pyroxene. 
Thi.s or the previous variety found also in cosmic spherules. Hij'persthene, greenish, red or brownish fragments 
bounded by cleavage planes, prismatic when associated with the debris of more recent eruptive rocks, inter- 
growth with monoclinic pyroxene. In the massive variety characteristic tabular inclusions, strong pleochroism. 
Found rarely with debris of older eruptive rocks, in some cases with lapilli or minerals of andesitic or trachytic 
eruptions. 
(6) Monoelinir. — Augite, fragments irregularly bounded or with cleavage planes, often crystals of the 
ordinary form, coated with volcanic glass, twinning parallel to the orthopinakoid, cleavage parallel to the prism 
of 87* 6', greenish, yellowish, brownish, purplish, high index of refraction, strong double refraction, oblique 
extinction of 36* 54'. Weak plmchroism ; green, yellow, brown; in some cases not pleochroic. Gaseous or 
glassy inclusions frequent, encloses also crystals and grains of magnetite. Found frequently in the deposits 
with debris of basalts, andesites, &c., in volcanic ashes. DiaJlage, grains bounded by cleavage faces, cleavage 
parallel to the ortbopinakoic], fibrous structure ; greenish or brownish, tabular inclusions, double refraction 
strong, maximum extinction angle of alxmt 40°, cleavage plates show one of the systems of polar rings. Associated 
with debris of older eruptive rocks. 
Quartz. — It is genenilly in the form of quartz that free inorganic silicic acid is found in the deposits; 
particles of jasper, cliulcedony, A’c., are rtslatively rare. Only in exceptional instances was quartz observed as 
■mall cry'stals hounde<l by the planes of the hexagonal prism and pyramid. As a general rule the grains of 
this mineral are without any crystal lograjihic outlines ; they are angular or rounded, massive, without cleavage 
planes, with a characteristic conchoidal fracture. After cleaning with acid, the grains are transparent and 
colourleMi ; some arc clouded with inclusions, no traces of alteration, not attacked by acids. The latter 
characters : absence of cleavage planes, conchoidal fracture, rounded or angular form, without crystallographic 
faces, absence of any decomposition, distinguish at first sight this mineral from felspar. Interference figure of 
monaxial crystals, positive double refraction, in parallel polarised light bright colour, when the fragments 
attain, as they do in aome deposits, a certain thickness. In many cases this mineral is characterised by its 
inclusiona, aoraetimes arranged in planes or irregularly disposed, by liquid inclusions, some with carbonic acid 
or with amali cubic crystals ; sometimes these (quartz grains contain needles of tourmaline, rutile, scales of 
