the ore body, seems to be entirely barren; 2d, the Horn Silver ore “chimney,” perhaps fifty feet thick, five hundred feet wide, and of unknown depth, is the only mass of ore yet found in a mile of well marked fissure; and 3d, the Carbonate mine opened near by in a strong fissure with a bearing at right angles to that of the Horn Silver, and lying entirely within the trachyte, yields ore of a totally different kind.  Both are opened to the depth of seven hundred feet with no signs of change or exhaustion.  If the ore were derived from the trachyte, it should be at least somewhat alike in the two mines, should be more generally distributed in the Horn Silver fissure, and might be expected to give out at, no great depth.

If deposited by solutions coming from deep and different sources, the observed differences in character would be natural; it would accumulate as we find it in the channels of outflow, and would be as time will probably prove it, perhaps variable in quantity, but indefinitely continuous in depth.]

Another question which suggests itself in reference to the Leadville deposits is this:  If the Leadville ore was once a mass of sulphides derived from the overlying porphyry by the percolation of surface waters, why has the deposit ceased?  The deposition of galena, blende, and pyrite in the Galena lead mines still continues.  If the leaching of the Leadville porphyry has not resulted in the formation of alkaline sulphide solutions, and the ore has come from the porphyry in the condition of carbonate of lead, chloride of silver, etc., then the nature of the deposition was quite different from that of the similar ones of Tybo, Eureka, Bingham, etc., which are plainly gossans, and indeed is without precedent.  But if the process was similar to that in the Galena lead region, and the ores were originally sulphides, their formation should have continued and been detected in the Leadville mines.

For all these reasons the theory of Mr. Emmons will be felt to need further confirmation before it is universally adopted.

From what has gone before it must not be inferred that lateral secretion is excluded by the writer from the list of agencies which have filled mineral veins, for it is certain that the nature of the deposit made in the fissure has frequently been influenced by the nature of the adjacent wall rock.  Numerous cases may be cited where the ores have increased or decreased in quantity and richness, or have otherwise changed character in passing from one formation to another; but even here the proof is generally wanting that the vein materials have been furnished by the wall rocks opposite the places where they are found.

The varying conductivity of the different strata in relation to heat and electricity may have been an important factor.  Trap dikes frequently enrich veins where they approach or intersect them, and they have often been the primum mobile of vein formation, but chiefly, if not only, by supplying heat, the mainspring of chemical action.  The proximity of heated masses of rock has promoted chemical action in the same way as do the Bunsen burners or the sand baths in the laboratory; but no case has yet come under my observation where it was demonstrable that the filling of a fissure vein had been due to secretion from igneous or sedimentary wall rocks.