MINERALOGY. 107 
plane 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, .go— 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. Fiprouite [Al Si 0%]. 
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 Pl. 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. 
