324 GEOLOGY AXD MINING INDCTSTEY OF LEADVILLE. 



form, aud is usually much altered. The three accessory- minerals are sparingly present, 

 apatite especially so. In none of the sections examined is there any finer-grained 

 interstitial matter. 



Alteration — The decomposing agencies acting upon the Mount Ziou Porphyry seem 

 to have been particularly fovorable to the formation of muscovite, which is the end prod- 

 uct of the alteration of the biotite, as well as the immediate one of that of orthoclase 

 and plagioclasj. In the latter two minerals the process takes place in the usual way, 

 and in the extreme decomiiosed state each grain aud microlite not wholly inclosed in 

 quartz is replaced by a brilliantly polarizing aggregate of minute, colorless, but 

 lustrous leaves. In the case of the biotite there are visible transition stages. Ore 

 particles and yellow needles (rutile ?) are first formed, aud the biotite passes into a 

 yellowish-brown, faintly-polaiizing, unknown substance, which soon gives way to a 

 mica indistinguishable from the product of the adjoining feldspars. Occasionally 

 pure leaves of muscovite ai'e found in quite fresh rock, but, as they always iuci'ease in 

 quantity in more decomposed specimens, their secondary origin is probable. No other 

 secondary product of importance remains, in the advanced stages of decomposition. 

 Specimens of Mount Zion Porphyry which are bleached through the disapi^earauce 

 of the biotite become indistinguishable from White Porphyry. (See p. 7C.) 



WHITE OR LEADVILLE PORPHYKY. 



On account of its relation to the ore bodies, its peculiar mode of occurrence, the 

 large area in which it is found, aud its petrographical interest, the White Porphyry 

 must be regarded as the most important eruptive of the district, and it will be described 

 in considerable detail. 



Macroscopicai — In its most typical form it is a nearly white, compact or finely 

 granular rock, which at first glance seems to be homogeneous, but under close exami- 

 nation usually discloses a number of small feldspar crystals, and, scattered irreg- 

 ularly through the mass, not unfrequeutly, double pyramids of quartz. Hexagonal 

 crystals of dark brilliant muscovite may occasionally be seen, but this is probably 

 secondary, as are, very certainly, the clusters of pearly leaves of the same mineral, 

 which are characteristic of the rock in some places, as in California gulch, on Lamb 

 Mountain, and in the intermediate region. The total absence of biotite and bisilicates 

 makes the rock seem dull white, except when stained by secondary infiltration prod- 

 ucts, and decomposition in the ordinary way only makes the rock seem more homo- 

 geneous and compact than before. Upon the contact with the wallrock or in some 

 of the more narrow dikes the White Porpyhry is found to contain more numerous 

 crystals of quartz aud feldspar, imbedded in a very compact groundmass [235].' 



Through decomposition the rock assumes in some places a granular appearance, 

 as if composed of small, worn grains,- but no corresponding microscopical structure 

 can be seen. 



' Tlio collection uumbers of particular specimeas will be inclosed in brackets. 

 ^ The structure referred to is illustrated by specimens horn tbe .Siiainus O'Brien [:i;l], Robert Euimet 

 [33a], Little Pittsburgh [r-iSa], and Katie [;«c] claims. 



