330 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



seemed to lie approximately parallel to oo P v. extiuction took place at 35° to 38° fiom 

 the vertical axis. Pleocbroism distiuct, tbe color varying from light to dark shades 

 of yellowish browu. 



Zircon is abundaut in minute clear crystals. Fig. 3, Plate XXI, shows two zircon 

 crystals of characteristic form included iu a quartz graiu of a Lincoln Porphyry. 

 Titanite was seen in but one or two specimens, and then very sparingly. Magnetite 

 and apatite occur as usual in such rocks. Biotite frequently includes apatite and zir- 

 con and may be peuetrated by allanite. It is otherwise iuterestiug from its altera- 

 tion products, which will be discussed below. 



The plagioclase, which is so prominently developed iu crystals, is probably an 

 oligoclase, judging from the extinction iu the zone perpendicular to the laminfe, the 

 direction being always within the limits of oligoclase. Orthoclase is seldom met with 

 among the crystals of medium size, being present in larger individuals or in the irreg- 

 ular grains of the groundmass, where it presents nothing noteworthy. The signifi- 

 cance of this development is pointed out later. 



The large C[uartz grains and crystals contain a few fluid inclusions of irregular 

 shape, and bays of granular groundmass penetrate them without any very maiked 

 change in texture of the mass. Glass inclusions are very rare in any specimens of the 

 Lincoln Porphyry and never Lave been noticed iu the type rock of Mount Lincoln. 

 Quartz crystals have frequently exerted an influence upon grains of the same mineral 

 iu the adjoining groundmass, which have within a narrow zone the same optical orien- 

 tation as the crystal. There is no regular relation of the quartz to the orthoclase within 

 this zone. 



Alteration. — Biotite is usually more or less altered and presents different products 

 under difi'erent circumstances. In a specimen from the head of Clinton gulch, Summit 

 County, the chief product is a micaceous mineral, seemingly muscovite, which con- 

 tains uumeroils needles of rutile. In other cases chlorite is first formed, and this is 

 also accompanied by yellowish needles, or by irregular paler grains of undeterminable 

 nature, which resemble titanite or at times auatase. Epidote seems to rei)lace the 

 chlorite, or iu other cases to come directly from the biotite without any intermediate 

 stage. The felds;)ars give place to an aggregate of muscovite leaves iu most cases, but 

 calcite is frequently seen as a product from jjlagioclase and epidote, also, may be often 

 found resulting from the alteration of the triclinic feldspar. As in some of the other 

 types to be described epidote is very comuionly a result of alteration of i)ure feldspar, 

 there appears no good reason for regarding it as induced by the presence of assumed 

 inclusions in the case of the Lincoln Porphyry. Secondary chlorite is sometimes 

 deposited throughout the groundnuiss, giving a greeu color to the rock. 



GRAY PORPUYKY. 



This rock, which occurs in the vicinity of Leadville, is the nearest relative of the 

 Lincoln type. It is, however, directly connected with a porphyry which has its chief 

 vent of eruption and largest masses in the adjoining region to the north, at the head- 

 waters of the Eagle Eiver This latter type will be fully treated iu the report upon 

 the geology of the Ten-Mile district, and, as other allied rocks can there be drawn into 

 the discussion, the present description will not go deeply into a comparison of types. 



