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. Albite [NcV AP Si' O'']. 



This feldspar is found in the great granite veins such as exist in Graf- 

 ton, Ac worth, 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 triclinia 

 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 70.83 



Alumina, 21.20 



Soda and impurities, 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. Orthoclase [ K^ AF Si' O^']. 



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. 



