78 GEOLOGY AND MINING INDUSTRY OF LBADVILLE. 



occurrence is most noticeable in the area of the Leadville map along an 

 imaginary northwest and southeast line, on one side of which it is found 

 both above and below the Blue Limestone, while on the other it occurs only 

 above it. 



The main sheet has an average thickness of several hundred feet and 

 varies in its extreme dimensions from 20 feet along the northeast edge of 

 the zone to 1,500 feet at White Ridge, on the east side of the range, the 

 point of its maximum development and supposed to be the locality of its 

 principal vent. 



Although all these masses must have been originally forced up from 

 below through the Archean, it is remarkable that no section has yet been 

 found which would show the actual passage from the Archean dike to the 

 interbedded sheet. The nearest approach to this has been at the head of 

 Iowa gulch, on Empire hill, and in a Lore-hole in South Evans gulch, 

 where White Porphyry has been found in the Archean in probable dike 

 form, and on White Ridge and Lamb Mountain, in Horse Shoe gulch, where 

 it is seen cutting up nearly vertically across Carboniferous strata. 



South of the zone above mentioned, White Porphyry is found as a 

 remarkably persistent sheet at the Blue Limestone horizon gradually thin- 

 ning out and extending to the southward as far as Weston's pass. North of 

 the zone it is found only in small sheets at Little Zion, Mosquito Peak, and 

 London hill, and in several small dikes in the Mount Lincoln massive, its 

 place being occupied by other varieties of porphyry. 



LINCOLN PORPHYRY. 



The other forms of porphyry found (and which on the Mosquito map 

 have been designated by one general color), though presenting a number of 

 varieties in the field, have essentially the same general composition, both 

 mineralogiccil and chemical. They consist mainly of quartz, two feldspars, 

 and biotite, hornblende occurring as an essential ingredient only in one 

 variety. The crystalline ingredients are easily distinguishable by the eye, 

 and there is therefore no danger of confounding them in the field with 

 White Porphyry, except in the conditions of extreme decomposition in 

 which they may be found near the ore bodies. This crystalline structure, 



