CHAPTER IV. 
GENERAL GEOLOGY OF LEAD AND ZINC ORES. 
General Considerations. 
LEAD and zinc ores occur in rocks of all kinds, the massive 
or crystalline rocks, and the fragmental or sedimentary 
ones. Of the former, granites and porphyries in places are 
comparatively productive, and even the more basic crys- 
tallines produce ore. Of the latter, limestones, sandstones 
and shales alike yield both lead and zinc; but the more im- 
portant deposits commercially yet known are found in imme- 
diate relation with limestones. In some way this rock seems 
to be more productive of lead ore and zinc ore. The prin- 
cipal deposits of lead ore are found in limestone throughout 
Colorado, Nevada, Montana and Idaho. The entire group 
of lead ores and zinc ores in the Mississippi valley are found 
in the same and associated flint, as are also those of Virginia, 
Pennsylvania and New Jersey. In Europe the principal 
mines of Upper Silesia, Laurium, Carthagena, Santander, 
Bleiberg and Vieille Montagne as well as those of northern 
England are found either in limestone or in immediate con- 
nection therewith. It is probable that more than half of all 
the lead ores and zinc ores of the world come from limestone 
or associated impurities such as flint or chert of the Missouri- 
Arkansas area. 
Lead and zinc ores also occur in rocks of all geologic ages 
from the Archean to the Tertiary. Here also we find that 
in certain places they are more abundant than elsewhere. 
The rocks of the Archean age usually are cut with fissures, 
and the ores occur in the fissures, probably being of much 
more recent age than the enclosing rocks. The Silurian and 
Carboniferous rocks contain much greater quantities of ore 
than all the others combined. 
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