102 MINERALOGY AND LITHOLOGY. 
63. MicrociinE [K’ Al Sif QO”). 
Microcline is a feldspar of the same composition as orthoclase, but 
differs from it in being triclinic. Its position as a species has been fixed 
by Des Cloizeaux. The difference in the angle of its planes that deter- 
mines it to be triclinic is very slight, for the angle between the base and 
the clinopinnacoid varies but 16’ from a right angle,—a result obtained 
as the mean of many measurements. This difference is of course imper- 
ceptible to an ordinary observer; but the optical distinctions are so 
marked that its determination becomes very easy. As already explained, 
a basal section of orthoclase is black between crossed Nicols when the 
edge of the clino-pinnacoid is parallel to the plane of vibration of the 
light. If, now, a section of microcline is placed in the same position, it 
will be brightly colored, and will not be dark until it is revolved 15° from 
this position. This indicates that a plane of elasticity normal to the 
plane of the lateral axes cuts the base, making an angle with an axis,—a 
property of triclinic crystals. 
Microcline is most commonly green, forming what is called Amazon 
stone. At the Notch very pretty crystals of green orthoclase occur, and 
also a more massive variety. I have examined this feldspar, and, as might 
be anticipated, it proves to be microcline. As is the case with microcline 
in general, it possesses a complicated structure induced by twinning, 
which takes places in planes parallel and perpendicular to the clino-pinna- 
coid, and accompanied with this twinning there is also an interlamina- 
tion of orthoclase in the same planes. Therefore a basal section of our 
Amazon stone, when examined in polarized light, appears as made up of 
innumerable laminz running at right angles to one another. Its appear- 
ance between crossed Nicols is illustrated in Fig. 2 on Pl. 8, which is drawn 
from a section of this Notch microcline. It is placed in the figure in such 
a position that the edge of the brachy-pinnacoid is parallel to the plane 
of vibration of the light, as indicated by the lines. Now, in this position 
the orthoclase will be black like the field to the left, and it is plainly seen 
in bands of various lengths and breadths, while the larger part, being 
microcline, is highly colored, and one part of it becomes dark on turn- 
ing 15° to the right, and the rest, on turning the same amount to the left. 
Now in sections of our granites and gneisses, crystals with this struct- 
