104 
ECONOMIC GEOLOGY 
view has been supported by the discovery (G. M. Butler, 
Econ. Geol., vii, 1912, p. 318) of the Louemma Vein (Fig. 33), 
along a fault which passes down from the limestone and por- 
phyry through quartzite into the underlying granite; it 
is a primary ore containing 8 oz. per ton of gold, 27 oz. per 
ton of silver, 11-1 per cent. of zinc, and 29-5 per cent. of lead. 
The Leadville ores have been followed to depths of 1500 
feet and have been traced far from the outcrop. Emmons 
and Irving (U.S.G.S., Bull. No. 
320, 1907)! showed that the 
ores extend beneath the De- 
vonian quartzite, where they 
are away from the porphyry. 
Nevertheless the ore-bodies in 
general follow the distribution 
of the porphyry. Those iso- 
lated in the Ordovician lime- 
stone may be near some un- 
discovered intrusion or due to 
solutions which rose along the 
course of the porphyry. The 
Leadville ores do not contain 
the lime silicates found in con- 
tact deposits and the country 
hasbeen affected by the hydro- 
thermal changes that happen 
near dykes and sills. 
Deposits analogous to those 
of Leadville occur in the Mag- 
dalena field in New Mexico 
(US.G.S., Prof. Pap. 68, 1910, 
pp. 51-6), and are associated with quartz-porphyry and 
granite-porphyry. Lead ores beside quartz-porphyry intru- 
sions also occur in Kelantan in the Malay Peninsula. 
BroxeN HiLr—The Broken Hill mining field in the arid 
plains of Western New South Wales is notable for its special 
geological features, its effect on metal prices and on metallurgy 
and the many mineral species first found in its gossan. The 
main lode is 3 miles long, in places 200 feet in width, and has 
Gr 
1 For results from later mining, cf. G. F. Loughlin, U.S.G.S., Bull 
770, 1926.