98 MINERALOGY AND LITHOLOGY. 
the other direction to make the other set dark. The yellow mineral to 
the right, without bands of color, is orthoclase. 
61. AuBiTE [Na? Al’ Sif O*}]. 
This feldspar is found in the great granite veins such as exist in Graf- 
ton, Acworth, Alstead, etc.; and in mining for mica very large amounts 
of it are extracted and thrown away. It occurs in tabular, white crystals 
belonging to the variety called clevelandite. It is whiter than the ortho- 
clase with which it is associated. It is often crystalline; and in places 
where the veins are cavernous, fine bright crystals of the ordinary form 
are found. The crystals are twinned according to the rule for triclinic 
feldspars. The optical properties of sections have already been given. 
Our albite from Alstead* was analyzed by Prof. J. D. Whitney, with 
the following. result : 
Silica, . F 5 - ‘ 5 3 ‘ ‘ 3 3 3 70.83 
Alumina, . 3 F 2 c 2 é ‘ e ' - , 21.20 
Soda and impurities, F ‘ ‘ a 4 s . ‘ . 7.97 
100.00 
Albite is not an important mineral in our crystalline rocks. G. Rose 
stated that albite was never present as a constituent of rocks. This has 
been shown to be otherwise. It does exist in small amounts in some of 
our granites, where it is associated with orthoclase and characterized 
by an excessively fine striation. 
62. OrtTHocLAsE [ K? Al Si’ OF]. 
This, the most common feldspar, is monoclinic. Its basal and brachy- 
diagonal cleavages make a right angle with one another, and basal sec- 
tions are consequently black between crossed Nicol prisms when the 
sharp edge formed by these two cleavages is parallel or perpendicular 
to the plane of vibration of the light. It is subject to twinning; but 
one crystal is rarely composed of more than two parts,—hence in the 
rocks it is never confounded with any other species of feldspar. 
The orthoclase of the most mineralogical interest is found in the mica 
quarries at Acworth, Grafton, etc., where very large crystals from eight 
* Geology of New Hampshire, Dr. C. T. Jackson, p. 178. 
