+ Q 
ECONOMIC GEOLOGY 
Section A. PRIMARY ORES 
I. Gorp-QuarTz Fissurg LopEs—CALIFORNIA, BALLARAT, 
MysorE—The character of these lodes varies so greatly 
with the nature of the rock fissures that they fall into four 
divisions, those in older sedimentary rocks, in pre-Palzozoic 
gneisses and schists, in the younger volcanic rocks, and 
pneumatolytic lodes. 
The standard gold-quartz lodes are those in the Sierra 
Nevada of California. ~The Sierra Nevada consists of pre- 
Palaeozoic schists and gneiss, covered by slates, sandstones, 
and igneous rocks, which range in age from early Palzozoic 
to Lower Cretaceous. At the end of the Jurassic or begin- 
ning of the Cretaceous the country was invaded by massifs 
of granodiorite with offshoots of augite-porphyrite, diabase, 
and serpentine, and was uplifted by faults, beside which 
quartz was deposited in fissures and fractures and by re- 
placement of slate and limestone. Gold and gold-bearing 
pyrites were deposited in the quartz. The date of their intro- 
duction is shown by the placer gold in some conglomerates, 
which are at latest early Cretaceous. The quartz-veins 
form a series of belts, which extend 700 miles in length by 
from 20 to 60 miles in width. The greatest of these belts 
is * the Mother Lode of California,” which trends N.W. for 
[12 miles through Mariposa, Calaveras, and Eldorado, 
localities famous from the writings of Bret Harte. The 
“ Mother Lode” is a mineralized belt with innumerable 
irregular discontinuous veins, generally in slates near grano- 
diorite, and sometimes along the contact; but they also 
beeur in the granodiorite or at considerable distances from 
it, or in serpentine. The distribution of the gold is irregular; 
of two parallel adjacent veins one may be barren and the 
other rich. Pure white or * buck-quartz ” is generally 
barren. Quartz banded with thin lines of micas, such as 
mariposite (a potash-mica coloured by chromite), and ros- 
coelite (a mica with 25 per cent. of vanadium oxide) is usually 
richer. The other common veinstones are calcite and dolo- 
mite, and, in Places, barite. The metallic minerals are 
sulphides, chiefly pyrites, with a little galena and blende. 
The gold usually occurs free in pyrites, 
The mines have been worked to depths of about 4000 feet ;