[4 s ¥: —_ -— —— “4 - a = a ea a2 | Bayes =” UE 
sae! * - Fs i 
5 - e eM : 
& ret = 
ParrIL $i] | ROCK-FORMING MINERALS. 77 
Hypersthene, a massive and eranular mineral, isomorphous with 
enstatite, having a perfect brachydiagonal cleavage, black to dark 
green or brown colour, and metalloidal coppery lustre on the leading 
_ 
cleavage planes. H. 6. Gr. 3:3—3-4. Chemically like bronzite, but 
with rather less magnesia (11—26 per cent.) and more iron (10 to 
34 per cent.). Under the microscope distinctly pleochroic, with 
crowded lamelle of dark microliths, partly of magnetite. Occurs in 
hypersthenite and associated with other magnesian minerals among 
the crystalline schists. ; 
Omphacite, a granular variety of pyroxene, grass green in 
colour, and commonly associated with red garnet in the rock known 
as eclogite. 
Smaragdite, a grass green lamellar aggregate of pyroxene and 
hornblende, or sometimes rather of hornblende only. Occurs in gabbro 
and eclogite, always in crystalline indefinite pieces, never in regularly 
formed crystals. 3 
Uralite, a mineral having the crystalline form of augite (pyroxene) 
and the internal cleavage and structure of hornblende (amphibole). 
It is regarded as a product of the gradual alteration of augite into 
hornblende. A marked finely fibrous texture and silky lustre dis- 
tinguishes the cleavage planes. Under the microscope a still un- 
changed kernel of augite may in some specimens be observed in the 
centre of a crystal surrounded by strongly pleochroic hornblende, 
with its characteristic cleavage. 
Olivine (Peridot). Orthorhombic, in dispersed crystals or granules 
of a pale yellowish, olive-green, or bottle-glass green tint, transparent 
when fresh, but apt to become dull, dark, and opaque by weathering. 

A B Cc 
Fic. 6.—Sraces IN THE ALTERATION, OF OLIVINE. A, THE NEARLY FRESH CRYSTAL; 
B, THE ALTERATION HALF COMPLETED; C0, THE CRYSTAL WHOLLY SERPENTINIZED. 
H. 65—7. Gr. 3-°2—3:5. Composed of an isomorphous mixture of 
the normal magnesium silicate, Mg, Si O,, with the ferrous silicate, 
Fe, Si O, = silica 40°98, magnesia 49°18; protoxide of iron 9°84, 
Under the microscope with polarized light, olivine gives, when 
fresh, bright colours, specially red and green, but is not per- 
ceptibly pleochroic. Its orthorhombic outlines can sometimes be 
readily observed, but it often occurs in irregularly shaped granules or 
