48 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



accordance with the custom now prevalent among German geologists, the 

 term pegmatite has been given. These pegmatites consist of large inter- 

 grown crystals of white orthoclase, microcline, and quartz, with irregular 

 masses of muscovite, and are evidently of later formation, probably the 

 filling in, by secretion from the surrounding rocks, of fissures and irregular 

 openings formed in the mass by contraction or dynamic movement. 



Microscopic constitution. Besides the normal components, which are easily 

 detected macroscopically viz, quartz, orthoclase and plagioclase feldspars, 

 potash and magnesia micas the only constituent of importance revealed 

 by the microscope is niicrocline, which occurs in all rocks examined except 

 those of the type from Democrat Mountain This is often quite abundant, 

 and seems to have been the last feldspar formed, which may be the reason 

 for its superior freshness and freedom from particles of limonite and hema- 

 tite, the abundance of which in the other feldspars causes their reddish 

 color. The quartz grains are often full of fluid inclusions and hair-like 

 needles. A few of the fluid inclusions were observed to be double, the 

 inner substance being probably carbonic acid. 



GNEISS. 



The gneisses, which are next in importance to the granites, are more 

 generally micaceous than those of the Archean along the Fortieth Parallel, 

 among which the distinctly hornblende gneisses were the more prevalent 

 They are much contorted and seldom exhibit very distinct bedding over 

 large areas. In structure they present a great variety of forms, prevailingly 

 the typical gneiss structure with fine, even grain and constant composition 

 in the different layers, aside from the flat lenses of quartz or feldspar which 

 are inserted between them. At other times a banded appearance is pro- 

 duced by the alternation of layers in which biotite or hornblende prevail 

 over quartz and feldspar. A porphyroidal structure is very marked in a 

 variety from the South Platte amphitheater, caused by the development of 

 large white orthoclase crystals, usually Carlsbad twins, reaching two to three 

 inches in length, in a matrix of ordinary gneiss. The tendency to a granitic 

 structure is locally noticeable, especially in the Twelve-Mile amphitheater. 

 In composition the gneisses are prevailingly micaceous, hornblende being 



