50 GEOLOGY AND MINING INDUSTRY OF LEADVILLB. 



from the Twelve -Mile amphitheater presents a confused intergrowth of 

 different feldspars, a part being undoubtedly microcline. 



AMPHIBOLITE. 



The amphibolites are the next in importance to the gneisses among- 

 the crystalline schists, and occur interstnitified with them in layers of 

 varying thickness, and sometimes in large lenticular bodies. Under the 

 name amphibolite are here understood rocks of com])aratively coarse grain, 

 with less marked schistose structure than is common in hornl)lende schists 

 proper, and also differing from these in that other minerals, particularly 

 feldspar and (piartz, occupy prominent [)ositions beside the hornblende. 

 They are of frequent occurrence throughout the Archeau formation of this 

 district and have a comparatively uniform structure, although sometimes 

 showing a mottled appearance, from the concentration of hornblende in 

 patches. Biotite and magnetite are often (piite jH-ominent in them. Pyrite 

 is frequently visiljle macroscopically. 



Microscopic constitution. — The uiicroscopc shows that orthoclasc and pla- 

 gioclase are present in about equal (piantities, but that microcline, which 

 was found in many gneisses, does not appear in the associated amphibolites. 

 Hornblende occurs in stout, irregular individuals, and often contains inclu- 

 sions of a clear, colorless mineral in minute rounded })articles, which are 

 probably quartz, although too small for certain deternnnation. Amphibolite 

 from Weston's pass contains hornblende which is so full of black ore-grains 

 as to be opaque in certain cases. A fine striation parallel to the plane Px ^ 

 was observed on the same hornblende. Apatite in its usual form is common 

 to all. Titanite, as formed through the alteration of a titanium mineral, 

 probablv nigrine or rutile containing titanic iron,'- is present in two cases 

 in most tvpical form. The rutile has a dull-reddish hue by reflected light 

 and is surrounded In' titanite in clear oval grains. Two occurrences, viz, 

 from Buckskin gulch and from Twelve-Mile amphitheater, show the mode 

 of formation of titanite with exceptional clearness. 



•C. W. Cross, SUidien iiber bretonische Gesteiue; Min. und petro. Mitth. von G. Tschermak. 

 Xeiio Folnje, III., p. 386. 



' Ramtnelsberg, Mineralchemie, Iler Theil, 216 Aiillage, p. 1G9. 



