They were designated "blue
gravel," the color being due to the action of sulphuret of iron and other
salts, the cementing auxiliaries requisite to form the hard conglomerate,
and on exposure to the atmosphere changes color to yellow and violet,
losing also its firmness by oxidation.
The "great blue lead" is another important mining term and designates the
alluvium found reposing in a well-defined channel on the bed rock, being
the well-worn path of an ancient river; and it is obvious that the
material in these channels should be richer than the general mass beyond
their limits.
"Rim rock" is the boundary line of the banks of the old channel, and, like
the bottom, is well worn and corrugated by the running water into cavities
and "pot holes," where the force of the stream eddied. The width of these
channels varies from 60 to 400 feet, and the cement near the rim and
bottom is always richer than elsewhere. The wider and deeper channels
generally course from N. to N.W. The richest and most explored belt of
gold-bearing alluvium in California lies between the South and Middle Yuba
Rivers, commencing near Eureka, in Nevada county, and extends downwards to
Smartsville and Timbuctoo, in Yuba county, a distance of 40 miles; and
from among snowy mountains the country falls gradually from where the
ravines or canons are cut by the actual rivers, which are 2,000 feet
beneath the auriferous gravel and region near Smartsville, and 2,000 feet
above the Yuba River, where snow is unknown, and near its terminus the
ancient river bed courses more westerly than it does above it, and crosses
Yuba below Timbuctoo, where the auriferous depositions disappear.
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