[02 
ECONOMIC GEOLOGY 
from them, and depositing them in the fault fissure in itregu- 
larly distributed rich bodies known as Bonanzas. The deep- 
seated origin of the ore is however probable from the abnor- 
mally high temperature, which rises in the mine 1° F. for 
every 33 feet of descent. The lode has been worked to a 
depth of 3000 feet despite the difficulties due to the heat and 
disastrous irruptions of boiling alkaline waters. 
The lead lodes at Aspen in Colorado were formed along 
fractures in shale, and that at Ceeur d’Alene in Idaho in a 
band of quartzite crushed between faults. 
II. ReprLAcEMENT ORrEe-BODIES 
(a) Massive Primary OrE-BODIES—BURMA—Primary fis- 
sure lodes and replacement bodies of lead ore often occur 
together. The Bawdwin Mines in Burma work extensive 
lead, zinc, and silver deposits (Coggin Brown, Rec. G.S. India, 
xlvii, 1917), of which the outcrops were mined by the Chinese. 
The ores are in a dome-shaped sheet of pre-Palzozoic rhyo- 
litic tuffs underlain by granite. This dome of tuffs has been 
broken through by compound faults, which form shear- 
zones up to 500 feet in width. Some thin persistent lodes 
have been formed on the faults ; the Burman Lode is usually 
2 feet in thickness, and its ores, according to Coggin Brown, 
yield from 15 to 37 per cent. of lead, 13 to 27 per cent. of 
zinc, and from 9 to 4I oz. to the ton of silver. The Shan 
Lode is a similar parallel lode in which the appearance of 
chalcopyrite on the 300 feet level indicates the probability 
of more copper at a greater depth. In the 500 feet shear- 
zone the rocks have been faulted and so altered that their 
volcanic origin is only recognizable in microscopic sections ; 
in this zone an enormous replacement deposit, the Chinaman 
ore-body, reaches 100 feet in width. It was formed, like 
the pyritic masses (p. 86), by the replacement of a com- 
pletely shattered block of country rock. 
(6) Ores write IeNEOUS Rocks—CoNTacT ORrEs—Lead 
ores formed by direct contact are uncommon as they are 
driven from positions of high temperature; but some 
deep-seated contact ores beside granite masses are known as 
at South Hill, Idaho. 
Ores WITH QUARTZ-PORPHYRY SHEETS—LEADVILLE— 
Important deposits of lead ores occur with sheets of quartz-