U. S. GEO'L SVR VEY OF THE LEAD VILLE MINING DISTRICT. 281 
In regard to the mineral deposit under and west of the City of Leadville he 
■says: " The determination of the existence or non-existence of the blue lime- 
stone beneath the City of Leadville, is of prime importance, for the reason that 
so many rich bonanzas have already been developed at that horizon on its eastern 
borders, which it is reasonable to suppose once extended farther west, and that 
thus far the richness of the horizon seems to increase with its distance from the 
crest of the range. The evidence gathered upon this point will therefore be 
given in considerable detail." 
" It is sufficiently well proved by the general geological structure of the 
region that the blue limestone originally extended to the west of Leadville, its 
probable limits being a line drawn from the mouth of the east fork of the Arkansas 
in a southeast direction to a point just west of Weston's Pass." 
The value of this report is greatly enhanced by reason of its being done by 
and under the personal supervision of a government official. 
The ore of Leadville is described by Mr. Emmons in his previous report, 
for 1 88 1, as principally argentiferous galena and its secondary products, lead 
carbonate, silver chloride, and, less abundantly, lead sulphate or anglesite, 
pyromorphite, minium, zinc blende and calamine. The gangue, or material 
mixed with or holding the ore, consists of hydrated iron oxides or manganese 
oxides, silica and clay, all secondary products, the clay coming from the decom- 
posed porphyry. The cavities in the limestone were made by the eroding solutions 
which introduce the ores; the action commenced at the top of the limestone 
adjoining the sheet of porphyry, and from this plane worked downward into the 
limestone. The materials of the ores were taken from "circulating waters, 
which, in their passage through the various bodies of eruptive rocks, took up 
certain metals in solution, and, concentrating along bedding planes, by a meta- 
morphic or pseudomorphic action of replacement, deposited these metals as sul- 
phides along the contact or upper surface, and to greater or less depth below that 
surface, of beds generally of limestone or dolomite but sometimes also of siliceous 
rocks." Dikes intersecting the ore-bearing formation "seem to favor the con- 
centration of rich ore-bodies or bonanzas in their vicinity ;" but the planes of 
faults afford no deposits of importance, and evidently for the reason that "their 
origin is later than that of the original ore-deposits." Thus the intrusion of the 
igneous sheets preceded the production of the ore-deposits and of the cavities 
containing them ; and the production of the ores antedated the era of great dis- 
turbance which closed the Lignitic period or the Cretaceous, and which has con- 
tinued to be followed by feeble movements until the present time; even since the 
opening, according to some evidence, of the Leadville mines. 
These ores occur, according to the same authority, underneath a porphyry 
sheet and chiefly in cavities penetrating the lowest member of the carboniferous 
formation, the blue limestone — but occasionally also underneath the same por- 
phyry in the white or silurian limestone and the Cambrian quartzite. The ore 
deposits penetrate into the limestone to varying depths from its plane of contact 
