the Comstock have simply opened the conduits of hot springs,
which are doing to-day what they have been doing in ages past, but much
less actively, i.e., bringing toward the surface the materials they have
taken into solution in a more highly heated zone below. Hence it seems
much more natural to suppose that the great sheets of ore-bearing quartz
now contained in the Comstock fissure were deposited by ascending
currents of hot alkaline waters, than by descending currents of those
which were cold and neutral The hot springs are there, though less
copious and less hot than formerly, and the natural deposits from hot
waters are there. Is it not more rational to suppose with Richthofen
that these are related as cause and effect, rather than that cold water
has leached the ore and the silica from the walls near the surface? Mr.
Becker's preference for the latter hypothesis seems to be due to the
discovery of gold and silver in the igneous rocks adjacent to the vein,
and yet, except in immediate contact with it, these rocks contain no
more of the precious metals than the mere trace which by refined tests
may be discovered everywhere. If, as we have supposed, the fissure was
for a long time filled with a hot solution charged with an unusual
quantity of the precious metals, nothing would be more natural than that
the wall rocks should be to some extent impregnated with them.

It will perhaps illuminate the question to inquire which of the springs
and water currents of this region are now making deposits that can be
compared with those which filled the Comstock and other veins. No one
who has visited that country will hesitate to say the hot and not the
cold waters. The immense silicious deposits, carrying the ores of
several metals, formed by the geysers of the Yellowstone, the Steamboat
Springs, etc., show what the hot waters are capable of doing; but we
shall search in vain for any evidence that the cold surface waters have
done or can do this kind of work.

At Leadville the case is not so plain, and yet no facts can be cited
which really _prove_ that the ore deposits have been formed by the
leaching of the overlying porphyry rather than by an outflow of heated
mineral solutions along the plane of junction between the porphyry and
the limestone. Near this plane the porphyry is often thoroughly
decomposed, is somewhat impregnated with ore, and even contains sheets
of ore within itself; but remote from the plane of contact wit