56 



LITHOLOGICAL GEOLOGY. 



Crystals, 



melts with difficulty. The lustrous cleavage-surface serves to 

 distinguish it from quartz when the two occur together, as in 

 granite. 



In the feldspars the ratio of the alumina to the protoxyd bases (potash, soda, 

 or lime) is uniformly 1 : 1. 



a. Orthoclase, or potash-feldspar, is the most common, and is often called 

 common feldspar. Crystals as in the annexed 

 figures (monoclinic). The two cleavage- 

 planes are at right angles with one another. 

 Colors, usually white or flesh-red. Composi- 

 tion: Silica, 64.8, alumina, 18.4, potash, 16.8 

 = 100. Ratio of the potash, alumina, and 

 silica, 1:1:4. 



b. Albite, or soda-feldspar, is also common. 

 Color, generally white, whence the name (from the Latin albus, white). 

 triclinic; the two cleavage-planes incline to one another at the angle 93° 36'. 

 Plane of crystals often striated in only one direction. Composition: Silica, 



68.7, alumina," 19.5, soda, 11.8. Ratio of the soda, alumina, and silica, 1:1:4. 

 Some albites contain a little potash, and some orthoclases a little soda. 



c. Oligoclase, a lime-and-soda feldspar. Resembles albite in crystallization 

 and appearance. Ratio of the protoxyds (lime and soda), alumina, and silica, 

 1:1:3. Andesine is another lime-and-soda feldspar. Ratio of the protoxyds, 

 alumina, and silica, 1:1: 2§. They are distinguished from albite with difficult}*" 

 without chemical analysis. 



d. Labradorite, or lime-feldspar. Colorless to grayish and smoky brown, 

 and usually with beautiful internal reflections. Crystallization nearly as in 

 albite. Composition : Silica, 53.1, alumina, 30.1, lime, 12.3, soda, 4.5. Ratio of 

 protoxyd bases, alumina, and silica, 1:1:2. 



e. Anorthite is another lime-feldspar, but of less common occurrence, being 

 mostly confined to certain volcanic rocks. Composition : Silica, 43.2, alumina, 



36.8, lime, 20.0. Ratio of lime, alumina, and silica, 1:1: 1£. 



Anortliite and labradorite differ from the other feldspars in containing pro- 

 portionally less of silica and being decomposable easily by acids. 



56. (3.) Mica. — Readily distinguished by its splitting easily into 

 very thin elastic leaves or scales, — even thinner than paper, — and its 

 brilliant lustre. It is colorless to brown, green, reddish, and black. 

 It may occur in small scales, — as common in granite as one of its 

 constituents, — or in plates a yard in diameter. 



As with the feldspars, mica is a silicate of alumina and different bases with 

 usually some fluorine, and is of several kinds, which differ in composition and 

 optical characters more than in appearance. Some of the varieties resemble 

 talc and chlorite, from which they differ in being elastic (unless weathered). 



a. Muscovite, or common mica, is a, potash-mica. Its crystals, when distinct, 

 are oblique prisms, and by polarized light it affords two sets of rings, with the 

 angle between the two axes 60° to 75°. Composition : Silica, 48, alumina, 



