MINERALOGY. 10/ 



l^lane of vibration of the light, which indicates that they are probably 

 triclinic. The specific gravity of andalusite is 3.1, while the specific 

 gravity of these altered crystals is 3.56, as proved by several determina- 

 tions by Prof. Brush. My analysis of these crystals was as follows : 

 Silica, 37.90, alumina, 62.12, water, .90= 100.92. Now all these charac- 

 ters belong to cyanite, and demonstrate that a tendency exists in an- 

 dalusite to alter the arrangement of its particles from the form of anda- 

 lusite into the triclinic cyanite, which has a higher specific gravity, but 

 the same chemical composition. The natural outer angles of these 

 crystals are often much distorted by this change, and on examining a 

 number of crystals wide variations were found in those angles that 

 should be uniform. A still further process of alteration converts anda- 

 lusite into mica, kaolin, etc., making it opaque and lustreless. There- 

 fore the microscopic study of andalusite becomes a study of aggregates 

 of other minerals, 



66. FiBROLITE [AP Si O']. 



This mineral is the same in composition as andalusite, and differs from 

 it only in the greater angle of its prism and the smaller angle of its optic 

 axes, neither of which is a consideration with which we can deal, since 

 it is only found in fibres in the schistose rocks. That it really has some 

 specific difference is evident from the circumstance that it does not de- 

 compose as does andalusite, but, although in small crystals, is usually 

 clear and fresh. 



Fibrolite exists in some of the schists of the White Mountains in such 

 amounts as to give a character to the rock. In thin sections it is recog- 

 nized as orthorhombic, by noting that the fibres all become black when 

 they are parallel to the plane of vibration of either one of the crossed 

 Nicols. The interference colors that are obtained when it is in any other 

 position are very bright. In Fig. 3 on PI. 8 a section of fibrolite schist 

 from the Notch is represented. The fibrolite pierces the biotite and the 

 quartz in such a way that it is plain it was the mineral first formed in 

 this schist, with the exception of the little magnetite grains, a few of 

 which are included in the fibrolite. Winchester is another locality where 

 fibrolite abounds. 



