76 - GEOGNOSY. [Boos TI, 
It is often remarkable for the amount -of extraneous materials 
enclosed within its crystals. Like some felspars, augite may be 
found in basalt with merely an outer casing of its own substance, 
the core being composed of magnetite, of the ground-mass of the 
surrounding rock, or of some other mineral (Fig. 5). - 
The distribution of augite resembles that of hornblende ; the pale, 
non-aluminous varieties are more specially found among gneisses, 
marbles, and other crystalline, foliated, or metamorphic rocks; the 
dark green or black varieties enter as essential constituents into 
many igneous rocks of all ages, from paleeozoic up to recent times 
(diabase, basalt, andesite, &c.) 
Its weathering also agrees with that of hornblende. The aluminous 
varieties containing usually some lime give rise to calcareous and 
ferruginous carbonates, from which the fine interstices and cavities of 
the surrounding rock are eventually filled with ‘threads and kernels 
of calcite and strings of hydrous ferric oxide. In basalt and dolerite, 
for example, the weathered surface acquires often a rich yellow 
colour from the oxidation and hydration of the ferrous oxide. 
Diallage. Monoclinic, but usually with undefined contours, 
distinguished by a very perfect cleavage in the direction of the 
orthopinakoid, feeble pleochroism, a finely fibrous structure, and a — 
pearly to metalloidal lustre on cleavage faces. H.4. Gr. 3-23—3-34, 
Essentially similar in chemical composition to augite, of which it 
may be only a variety, containing silica 50—53, magnesia 15—17, 
alumina 1—4, lime 15—22, ferrous oxide (and usually also man- 
ganous oxide) 5—138 per cent. A constituent of gabbro. | 
Enstatite. Orthorhombic, with cleavage parallel to faces of prism ; 
colourless, light grey, yellowish, greenish, or brown, with pearly lustre 
on the cleaved surfaces. H. 5:5. Gr. 8:°10—3:°29,. Under the micro- 
scope it presents irregularly defined forms with usually a finely 
fibrous structure ; pleochroism feeble or absent. A meta-silicate of 
magnesium (with silica 60, magnesia 40 per cent., but commonly with 
a little ferrous oxide and alumina). Occurs in lherzolite, serpentine, 
and other olivine rocks. LBastite is probably a hydrated enstatite. 
Bronzite. Orthorhombic, with very perfect brachydiagonal 
cleavage; brown, green, and yellow with a characteristic pearly 
metalloidal lustre and a finely fibrous surface on cleavage planes. 
H, 4—5. Gr. 3—38'5, Under the microscope shows weak pleochroism. 
Like enstatite, is found to occur as a constituent of rocks only in 
irregularly defined crystalline grains, and not in definite crystals; both 
minerals usually present the finely fibrous texture above referred to, 
the structure being on the whole straighter in enstatite and more 
undulating in bronzite.—An isomorphous mixture of silicates of mag- 
nesium and of iron, with silica 55—57 per cent., magnesia 25—86, 
protoxide of iron 7 to 10, and frequently a little alumina and 
manganese. It occurs under similar conditions to enstatite and is 
found also in some basalts and even in meteorites. Bronzite and 
enstatite weather into dull green serpentinous products, 
a 
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in aa 
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pi 
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