84 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



carefully studied as the other varieties. It is an extremely fine-grained, 

 greenish-gray rock, which in the hand specimen is characterized by fine 

 needles of what is apparently decomposed hornblende. It carries quartz in 

 small amount, two feldspars whose relative proportions are not readily 

 apparent, with hornblende and biotite. These constituents are so very 

 small as not to be readily distinguished. The microscope discloses the usual 

 accessory minerals, including allanite and pyrite. The groundmass is 

 holocrystalline and contains no glass. A porphyritic rock found on a south- 

 ern spur of Mount Silverheels, at the forks of Crooked Creek, although of 

 much coarser grain and more distinctly porphyritic habit, has essentially 

 the same elements as the Silverheels Porphyry. 



DIORITE. 



Only three occurrences of granular plagioclastic rocks were found in 

 the region, each of which was in the form of a dike cutting through the 

 Archean in Buckskin gulch. The rock of each of these occurrences repre- 

 sents a distinct variety of the type. 



Hornblende diorite. The normal diorite, which forms a broad dike cross- 

 ing the head of the gulch, is a fine-grained, gray rock, in which the prom- 

 inent constituents are plagioclase feldspar and hornblende, while a little 

 quartz, brown biotite, yellow titanite, and dark ore grains can be detected 

 by the naked eye. The microscope discloses also zircon and apatite, with 

 chlorite and epidote as alteration products of the hornblende and biotite, 

 and muscovite formed from orthoclase. A similar rock i's found in French 

 gulch, on the west side of the range 



Quartz-mica diorite. This rock occurs on the south side of Buckskin gulch, 

 opposite the Red Amphitheater. It is a dark, even-grained rock, in which 

 quartz and feldspar are more prominent than the small irregular leaves of 

 biotite; hornblende is wanting. The microscope shows zircon, magnetite, 

 apatite, biotite, plagioclase, orthoclase, and quartz as original constituents. 



Augitic diorite. This rock, which is darker and finer grained than either 

 of the preceding, occurs in the Red Amphitheater, cutting up through the 

 Archean into the base of the Cambrian. In the hand specimen only horn- 

 blende, biotite, plagioclase, and a little quartz can be distinguished, tmt the 



