99 
(hornblende) ; crystals not infrequently twinned, decompos- 
ing with a large deposition of red oxide of iron. Mica scarce 
as well-defined flakes of strongly pleochroic brown biotite. 
Magnetite jDlentiful, arranged in groups, the individual 
constituents of which have a strong tendency to parallel ar- 
rangement, as though conforming with some pre-existent 
crystal constant. Hornblende has, no doubt, yielded to its 
formation. 
The absence of quartz, feeble development of orthoclase, 
and the brown tint of the hornblende indicate a basic type of 
diorite. 
DOLERITE. 
Localiti/. — Mount Olga. 
Macroscopically. — Slate-coloured, uniformly crystalline 
rock of fine grain. The minute needles of felspar are dimly 
recognisable, and here and there larger secretions of a green 
mineral are apparent (olivine). The rock decomposes to a 
richly coloured ochreous powder. 
Mici'oscopicaJly. — Holocrystalline : of fine texture. The 
lath-shaped felspars, on an average about '2 mm. in length, 
are clouded : on that account they exhibit twin lamination 
and cleavage cracks very imperfectly, and are variously tinted 
in polarised light. The arrangement of the laths produces a 
poor ftuxion structure. 
The augite in the sections examined has been almost com- 
pletely altered to a scaly, green, chloritic mineral, possessing 
a very faint double refraction. 
Between the felspars a subsequent crystalline segregation 
has taken place radially to small granules of magnetite. These 
aggregates show the characteristic black cross under crossed 
nicols, having its arms parallel to the cross wires of the micro- 
scope. 
Olivine as greenish, irregularly bounded individuals. 
Magnetite is distributed generally through the mass as 
small granules and cubes ; or it darkens the constituent mine- 
rals in the form of a very fine dust. 
A secondary serpentinous, fibrous mineral present is pro- 
bably another product of the decomposition of the augite. 
Ordovician Quartzite. 
Locality. — Mount Chandler. 
MacroscopicaUy. — A highly compact, fine-grained, white, 
siliceous quartzite, breaking with a splintery fracture. 
Microscopically. — Consists of closely set, rolled grains of 
clear quartz, so compacted by pressure as to have left but 
little space for interstitial cement, which is also of silica. The 
interstices are slightly stained by iron salts. The average 
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