ORES OF LEAD, ZINC, AND SILVER ror 
Devon generally occur in the Devonian slate or killas, 
aS cross-courses through the copper lodes; they have been 
worked to the depth of 1800 feet; the ores were deposited 
after the tin and copper. 
The Spanish lead mines were opened by the Pheenicians and 
were worked by the Romans to the depth of 600 feet. The 
Linares field, 160 miles S. of Madrid, was from 1870 to 1910 
the most prolific lead-field in the world. Its mines are of 
great variety; they are mainly vertical lodes trending 
W.SW parallel to the great Guadalquivir fault—a con- 
SPicuous feature in the geology of Southern Spain. The 
Arroyanes Lode consists of several parallel veins along faults 
In granite; it is 5 miles long, usually 3 feet thick, and has 
been worked to the depth of 1300 feet. Further N. is the 
Guindos line along fractures in Silurian and Ordovician 
rocks; the lodes are productive in quartzite, but are closed 
When the lode track crosses slate. The lodes were formed 
slowly, for one generation of veins was fractured and dis- 
Placed, before a second set was deposited. 
Further N. in the Sierra Morena is the Centenillo Mine, 
Which was also worked by the Romans; one of its lodes, 
the Mirador, is a sheet of pug, which has to be valued by 
Sampling as the galena is invisible; it is a Hercynian lode 
that was ground to powder by Alpine faulting. The leaching 
of the ore is indicated by the absence of zinc. 
‘The age of the Linares system is probably Hercynian, as the 
local Triassic conglomerates contain fragments of the ore 
and lode-quartz. A little ore extends into the Trias by 
Secondary migration. 
The Comstock Lode in Nevada is one of the most famous 
And richest in mining history (Becker, U.S.G.S., Mon. 3, 
1882). Alluvial gold was worked in the field for years before 
the discovery in 1859 of its silver-lead ores. The German 
Physiographer, von Richthofen, described the lode as a con- 
tact formation in a long series of igneous rocks, which he 
regarded as lavas, each characteristic of a separate age. They 
ar NOW regarded as variously altered andesites (U.S.G.S., 
ull. 17, 1885), and the lode as formed along a fault up which 
thse Superheated water that altered the andesite into propylite. 
At one time the lode was attributed to lateral secretion by 
Water percolating through theadjacent rocks, dissolving metals