86 MINERALOGY AND LITHOLOGY. 
perfect crystals. This granite contains both muscovite and biotite in 
comparatively large scales, with no crystalline form; but some of the 
grains of quartz are filled with the finest little muscovite crystals. A 
section of this rock is represented in Fig. 2 on Pl. 5. On changing the 
focus, many more of these crystals are brought into view. The large 
mineral above is muscovite, and the black one on the right is biotite. 
FELDSPAR. 
In a state like ours, which is covered by crystalline rocks, feldspar is, 
next to quartz, the predominant mineral. All the feldspars of litholog- 
ical importance are well represented, the species being,— 
Anorthite, Ca Al? Si2 O8. 
Labradorite (Ca Na), Al? Si O19, 
Andesite (Ca Na2), Al? Sit O12, 
Oligoclase (Ca Na? K2), Al? Sid O14, 
Albite, Na? Al? Sis O18, 
Orthoclase, K2 Al2 Sis 018, 
Microcline, 
It is useless to give localities for specimens of the species; and hence 
the space will be given to a discussion of those properties that are of the 
most importance in the approaching study of the rocks. The feldspars 
are all triclinic, with the exception of orthoclase, which is monoclinic, 
and hence easily distinguished from the others by its optical properties. 
Microcline, though not very evidently monoclinic in external form, is very 
plainly so in its inner structure, as will be shown. These two species 
stand isolated from the others, which are triclinic, and which all in com- 
mon are subjected to a peculiar method of twinning. This twinning is not 
the simple revolution of one part of a crystal about the other, but is what 
is called polysynthetic twinning, which consists in the repetition of this 
process so many times that a small crystal may consist of many hundred 
laminz, each one of which is revolved 180° from the position occupied 
by the neighboring one. To make the effect of this twinning plain, Fig. 
8 on Pl. 3 is introduced. Let the figure represent a section of a feldspar 
crystal cut parallel to the plane of the macro-pinnacoid, and suppose that 
an axis of elasticity makes some given angle with the vertical axis; then, 
if this section is placed between crossed Nicol prisms, it will be colored, 
