274 DESCRIPTIONS OF MINERALS. 
and 7; the specific gravity, between 2°44 and 2°75 ; lustre 
vitreous, but often pearly on the face of perfect cleavage ; 
and each species transparent to subtranslucent. 
I]. Acipic AND Basic FELDSPARS. _ 
Oligoclase, albite, and orthoclase are called acidic feld- 
spars, because of the large amount of the acidic element, - 
silicon, in their constitution, analyses giving 60 to 70 per 
cent. of silica; and labradorite and anorthite are called 
basic feldspars, the amount of silica being 42 to 55 per 
cent. Correspondingly, ernptive, and metamorphic rocks 
in which either of the acidic feldspars is a prominent con- 
stituent—for example, granite, gneiss, trachyte, true por- 
phyry-—are called acidic rocks; while those rocks in which 
basic feldspars are constituents—like doleryte, and a large 
part of eruptive rocks—are called basic rocks. 
Ill. Distinctions oF THE TRICLINIC FELDSPARS. 
The triclinic feldspars are distinguished from the mono- 
clinic (e. g. orthoclase) by the occurrence of very fine stria- 
tions on the cleavage surface, sometimes too fine to be seen 
without a good pocket-lens. ‘These striations are due to 
multiple twinning parallel to the other cleavage face, as ex- 
plained on page 57. They are rarely absent from triclinic 
feldspar crystals. They are best brought out by transmitted 
polarized light, in which a transverse section of the erys- 
tal is seen banded with spectrum colors, each band corre- 
sponding to one plate of the twin structure. 
The triclinic feldspars, andesite excepted, may be dis- 
tinguished from one another by an optical method when 
the cleavage direction can be made out. For this purpose 
a plate is prepared parallel to the plane of easiest cleavage. 
In such a plate the multiple twinning is parallel to the other 
cleavage plane, or the shorter diagonal. When the plate is 
placed on the stage of a polariscope, between crossed Nicol- 
prisms, as the stage is revolved, the adjoining bands of color 
become dark alternately, and the angle through which the 
