h the
limestone, it contains little diffused and no concentrated ore. It is
scarcely more previous than the underlying limestones, and why a
solution that could penetrate and leach ores from it should be stopped
at the upper surface of the blue limestone is not obvious; nor why the
plane of junction between the porphyry and the _blue limestone_ should
be the special place of deposit of the ore.

If the assays of the porphyry reported by Mr. Emmons were accurately
made, and they shall be confirmed by the more numerous ones necessary to
settle the question, and the estimates he makes of the richness of that
rock be corroborated, an unexpected result will be reached, and, as I
think, a remarkable and exceptional case of the diffusion of silver and
lead through an igneous rock be established.

It is of course possible that the Leadville porphyries are only phases
of rocks rich in silver, lead, and iron, which underlie this region, and
which have been fused and forced to the surface by an ascending mass of
deeper seated igneous rock; but even if the argentiferous character of
the porphyry shall be proved, it will not be proved that such portions
of it as here lie upon the limestone have furnished the ore by the
descending percolation of cold surface waters. Deeper lying masses of
this same silver, lead, and iron bearing rock, digested in and leached
by _hot_ waters and steam under great pressure, would seem to be a more
likely source of the ore. If the surface porphyry is as rich in silver
as Mr. Emmous reports it to be, it is too rich, for the rock that has
furnished so large a quantity of ores as that which formed the ore
bodies which I saw in the Little Chief and Highland Chief mines,
respectively 90 feet and 162 feet thick, should be poor in silver and
iron and lead, and should be rotten from the leaching it had suffered,
but except near the ore-bearing contact it is compact and normal.

Such a digested, kaolinized, desilicated rock as we would naturally look
for we find in the porphyry _near the contact_; and its condition there,
so different from what it is remote from the contact, seems to indicate
an exposure to local and decomposing influences, such indeed as a hot
chemical solution forced up from below along the plane of contact would
furnish.

It is difficult to understand why the upper portions of the porphyry
sheet should be so different in character, so solid and homogeneous,
with no local concentrations or