328 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



Microscopical Zircon, ilmenite, pyrite, specular hematite, and probably magnetite 

 are present in small quantity, a diversity in such constituents seldom seen in rocks of 

 this region. Apatite, noticeable even macroscopically, is developed in stout prisms, 

 with many minute inclusions, producing the dusty appearance often described. No 

 other rock of the range exhibits a similar development of this mineral. 



Biotite is shown in various stages of decomposition, chlorite being the first product, 

 which sometimes gives way to epidote, or, as is clear in many cases, to a micaceous min- 

 eral apparently identical with the muscovite which is formed from adjacent orthoclase. 

 Accompaniments of this change are yellow needles, presumably rutile, while the iron 

 of the chlorite either is carried away or separates out in glistening black ore particles, 

 thought to be specular hematite. 



Of the feldspars, orthoclase seems to predominate slightly. Plagioclase is pres- 

 ent both in crystals and in the groundmass, where its small microlites are much more 

 prominent than usual. Quartz is regularly but rather sparingly present in large 

 grains, seldom showing crystal outline and containing numerous small fluid inclusions, 

 while none of glass was observed. A microcrystalline, granular mixture of quartz 

 and two feldspars, with but very little primary mica, makes up the groundmass. 



Chemical analysis shows 68.01 per cent, of silica, 4.36 per cent, of potash, and 

 4.26 per cent, of soda. The alkalies are rather more nearly balanced than one would 

 suppose them to be from the microsc >pical examination. 



LINCOLN PORPHYRY. 



This rock is the most important of the varieties belonging to the second division 

 of the quartz-porphyries of the district, namely, those in which the porphyritic structure 

 is macroscopically very plain. It has been called the Lincoln Porphyry from the fact 

 that it is best developed in and about the mass of Mount Lincoln, forming the extreme 

 summit of that peak, and in this once important mining district bearing approximately 

 the same relation to the ore deposits which near Leadville is assumed by the White 

 Porphyry. As will be shown later, it is very closely allied to the Leadville Gray Por- 

 phyry and has intimate connection with the Eagle River Porphyry and other rocks 

 of the adjoining district upon the north. In the following description will be con- 

 densed the observations upon twenty specimens collected at different places. Devi 

 ations from the type rock of Mount Lincoln will be specially noted. 



Macroscopicai. The essential constituents are quartz, orthoclase, plagioclase, and 

 biotite, all occurring in distinct crystals and imbedded in a compact groundmass of 

 varying importance. A part of the orthoclase appears in large, stout crystals, fre- 

 quently two inches in length, usually pinkish in color, and so fresh and glassy as to 

 resemble markedly the sanidine of younger rocks. They are often Carlsbad twins 

 and contain noticeable inclusions of biotite leaves. For most occurrences of the por- 

 phyry these large orthoclase crystals are eminently characteristic, though their (level 

 oprnent has been hindered in some cases, particularly in dikes and small masses. In 

 some of these instances small crystals of pinkish color are plainly more numerous- 



