LEADVILLE AND VICINITY. TTt 



Besides these there are eruptive rocks — porphyries and diorytes — mostly 

 Mesozoic in age. The common kind is the white porphyry, an evenly granular 

 rock, consisting of quartz (70 per cent.), feldspar {the latter occasionally in small 

 rectangular crystals), black mica or biotite, and some muscovite. The rock is 

 partly decomposed, and the muscovite " is the result of the decomposition of the 

 feldspar." Other kinds of porphyry, more granite-like, consist of quartz, two 

 feldspars and biotite, and in one variety horn-blende is present. The dioryte is a 

 porphyritic crystalline-granular variety. The white porphyry occurs to the south 

 of an east and west line through Leadville, and the other kind north of this line. 

 The main sheet of the former which lies upon the surface of the blue limestone 

 forms, at the Four-Mile Creek where is its principal vent, the larger portion of a 

 hill 2,000 feet high, and thence spreads southward reaching nearly to Buffalo 

 Peaks. On Iron and Carbonate Hills it has a possible thickness of over 1,000 

 feet; but along Evan's Gulch it will scarcely average 100, and even thins out en- 

 tirely. Other sheets occur between lower strata, and there is a local sheet in the 

 lower quartzite or Cambrian. The intrusive masses of the other porphyries have 

 a wider vertical distribution, "extending up to the Jurassic and possibly even to 

 the Cretaceous." A single section exhibits "fifteen sheets, many several hundred 

 feet thick, between the blue limestone and the top of the Carboniferous." The 

 various sheets of porphyry form an integral part of the sedimentary series ; they 

 never reached the surface, but were spread out and cooled between deep-lying 

 strata — laccolith-like, before the mountain-building epoch at the close of the Cre- 

 taceous, and therefore before the associated strata were folded or faulted. Ar- 

 chaean rocks make large parts of the Sawatch Range on the west, and of the Front 

 Range on the east, and their areas must have been great islands in the Paleozoic 

 seas. '■ ' The Paleozoic and Mesozoic beds are a littoral deposit around the Sawatch 

 Archaean Island and were consequently formed in comparatively shallow waters." 



Of later formations, the region contains only the Quaternary ; what have ex- 

 isted of Mesozoic strata — probably not less than 10,000 feet — having been re- 

 moved by erosion and abrasion. 



Several faults occur, the more prominent of which have the strike of the 

 rocks, or about N. 60° W. and the upthrow on the east side; and, of these, the 

 Mosquito fault, west of the main crest of the Mosquito Range, amounts on the 

 north to 5,000 feet or more. Besides these there are many cross faults." 



On the Pacific side of the Continental Divide the geological formations are 

 similar, but the mountain sides are more precipitous and more heavily covered 

 with loose material of every possible variety, while the upheaval and displacement 

 have been even more complete. The ores so far discovered are principally gold 

 in fissure veins rather than in pockets, but as comparatively little work has been 

 done yet all may take a different phase after more complete development. 



The ore of Leadville is described by Mr. Emmons (condensed ,as before by 

 the American Journal of Science) as principally argentiferous galena and its second- 

 ary products lead carbonate, silver chloride, and, less abundantly, lead sulphate 

 or anglesite, pyromorphite, minium, zinc blende and calamine. The gangue, or 



