nster--now
thoroughly oxidized as far as penetrated--forms a sheet from twenty to
forty feet in thickness, consisting of ferruginous, sandy, or talcose
soft material carrying from twenty to thirty dollars to the ton in gold
and silver. The ore of the Deer Trail forms a thinner sheet containing
considerable copper, and sometimes two hundred to three hundred dollars
to the ton in silver.

The rocks which hold these ore deposits are of Silurian age, but they
received their metalliferous impregnation much later, probably in the
Tertiary, and subsequent to the period of disturbance in which they were
elevated and metamorphosed. This is proved by the fact that in places
where the rock has been shattered, strings of ore are found running off
from the main body, crossing the bedding and filling the interstices
between the fragments, forming a coarse stock-work.

Bedded veins may be distinguished from fissure veins by the absence of
all traces of a fissure, the want of a banded structure, slickensides,
selvages, etc.; from gash veins and the floors of ore which often
accompany them, as well as from segregated veins, they are distinguished
by the nature of the inclosing rock and the foreign origin of the ore.
Sometimes the plane of junction between two contiguous sheets of rock
has been the channel through which has flowed a metalliferous solution,
and the zone where the ore has replaced by substitution portions of one
or both strata. These are often called blanket veins in the West, but
they belong rather to the category of contact deposits as I have
heretofore defined them. Where such sheets of ore occupy by preference
the planes of contact between adjacent strata, but sometimes desert such
planes, and show slickensided walls, and banded structure, like the
great veins of Bingham, Utah, these should be classed as true fissure
veins.

THEORIES OF ORE DEPOSIT.

The recently published theories of the formation of mineral veins, to
which I have alluded, are those of Prof. Von Groddek[1] and Dr.
Sandberger,[2] who attribute the filling of veins to exudations of
mineral solutions from the wall rocks (i.e., lateral secretions), and
those of Mr. S.F. Emmons,[3] and Mr. G.F. Becker,[4] who have been
studying, respectively, the ore deposits of Leadville and of the
Comstock, by whom the ores are credited to the leaching of adjacent
_igneous_ rocks.

[Footnote 1: Die Lehre von den Lagerstatten der Erze, von Dr. Albrecht
von Grodde