All)ite. 



Oligoclase. 



latoadorite. 



144 



Ortlioclase is generally dull in appearance by reason of tlie 

 formation of a small quantity of kaolin, but when perfectly 

 fresh it may be as clear and glassy as quartz. A tabular form of 

 clear orthoclase, of common occurrence in the lava trachyte, is 

 known as sanidine. 7701, Wantialable Creek ; sanidine crystals 

 picked out from a trachyte tuff. 



Moonsione, is an opalescent variety.* 6107, from St. Lawrence 

 County, New York, 



JPerthite. Ortlioclase intergrown with alhite, the complex 

 nature of the mineral being indicated by its streaky appearance — • 

 Canada, Pennsylvania. 



Microcllne. A variety differing very slightly from true ortho- 

 clase in crystalline f orm t The beautiful green felspar from 

 Virginia, Hussia, and California, known as amazon stojie, is 

 microcline. 143, Eussia ; tliis shows the crystal-form better 

 than any of the specimens of true orthoclase exhibited. 

 8795, Coolgardie ; ortlioclase and microcline intergrown. J 

 3190 (in case 120), California; a group of crystals. 



The remaining felspars very commonly show a parallel striation 

 on certain faces, which is due to twinning. They are grouped 

 together as "plagioclase." This striation can frequently be seen 

 on the faces of felspar crystals in igneous rocks, and is of great 

 assistance in distinguishing them from orthoclase. 



Yirginia, Eussia. 



Ttterby (Sweden), Canada. 



Labrador. This felspar possesses a beautiful blue "play of 

 colours" on certain faces. It is found in great quantity in 

 Labrador. As a rock constituent it is common in basalt. 



Felspathoids. — A small group of minerals resembling felspar 

 in chemical composition, but differing in crystalline form. They 

 are found in the lavas of certain volcanic areas, such as A'^esuvius. 



* Some of the South Coast granites, particularly these from Mount Dromedary and 

 Jloruj-a, contain opalescent felspars, that approximate to moonstone. 



t A certain cr\ sfal ang-le is 9 i° in orthoclase, but not exactl}' 90° in microcline. 

 X This intergrowth can only be satisfactorily detected under the microscope. 



