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 with 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.