THE NONMETALLIC MINERALS. 291 



ton and Jenny, 1 Blake, 2 and Vincent regard them as intrusive, while 

 Carpenter 3 and Crosby 4 hold the opposite view. 



According to Blake the mica occurs in granitic masses, remarkable 

 for the coarseness of their crystallization, the constituent minerals 

 being usually large and separately segregated. " Large masses of pure 

 quartz are found in one place and masses of feldspar in another, and 

 the mica is often accumulated together instead of being regularly dis- 

 seminated through the mass. It also occurs in large masses or crys- 

 tals, affording sheets broad enough for cutting into commercial sizes." 

 Associated with the mica at this point are the minerals quartz and 

 feldspar, mainty a lamellar albite (Clevelandite), which form the gangue, 

 and irregularly disseminated cassiterite (tinstone), gigantic spodumenes, 

 black tourmalines, and, in small quantities, block mica, beryls, garnets, 

 columbite, and a variety of phosphatic minerals, such as apatite, tri- 

 phylite, etc. 



In Nevada mines have been worked in the St. Thomas mining dis- 

 trict, Lincoln County, the mica occurring in hard, glassy quartz rock 

 forming an outcrop some 200 feet wide by 600 feet long in gneiss and 

 schists. At the Czarina Mine, located in May, 1891. near Rioville, the 

 mica occurs under similar conditions. The mineral seems to follow 

 the division plane of the stratification, along the line or axis of fold. 

 This line runs north and south, slightly east of north of the main trend 

 of the range, thus running into Arizona a few miles north of Rioville. 

 In fact, the mica belt forms the boundary line between Nevada and 

 Arizona for 50 miles The mica, mostly small, is abundant, but mar- 

 ketable sizes are rare, and not to be had without a great deal of hard 

 work. 5 



Merchantable mica has been reported on the Payette River and Bear 

 Creek, in the Coeur d'Alene region of Idaho, and also in Oregon and 

 Alaska. 



According to Mr. R. W. Ells 6 the Canadian micas of commercial 

 importance occur associated with eruptive dikes of pyroxenite and 

 pegmatite cutting the Laurentian gneisses. More rarely, as in the 

 Gatineau area, they are found where dikes of the pyroxenite cut the 

 limestone. This authority gives the condition of occurrence as below: 



1. In pyroxene intrusive rocks which either cut directly across the strike of grey- 

 ish or other colored gneisses or are intruded along the line of stratification. Some 

 of these deposits have been worked downward along the contact with the gneiss, 

 where the mica is most generally found, for 250 feet, as at the Lake Girard Mine, and 

 irregular masses of pink calcite are abundant. In certain places apatite crystals 



1 Geology of the Black Hills of Dakota, Monograph, U. S. Geological Survey, 1880. 



2 Engineering and Mining Journal, XXXVI, 1883, p. 145. 



3 Transactions of the American Institute Mining Engineers, XVII, 1889, p. 570. 

 Proceedings of the Boston Society of Natural History, XXIII, 1884-1888, p. 488. 

 ;> Mineral Resources of the United States, 1893. p. 754. 



"Bulletin of the Geological Society of America, V, 1894, p. 484. 



