100 MINERALOGY AND LITHOLOGY. 



mental stone. These colors in orthoclase have been shown by Reiisch 

 to be due to a cleavage of extreme delicacy parallel to the plane of the 

 clino-diagonal, producing interference colors like those of thin films. 



It is to be regretted that the various analyses that have been made on 

 New Hampshire orthoclase have been unaccompanied by optical micro- 

 scopic work, since triclinic feldspars are so often thus seen to be asso- 

 ciated with it. Enough has been done to show, however, that our ortho- 

 clase usually has a part of its potash replaced by soda. Jackson found 

 this so in the orthoclase from the granite veins, which may be assumed 

 to be pure. As typical of many analyses of the orthoclase of our rocks, 

 the following by C. A. Seely, of a specimen from Bartlett, is selected : 



Silica, 66.06 



Alumina, 20.04 



Iron oxide, trace. 



Lime, 2.08 



Soda, 7.28 



Potash, 5.47 



100.93 



Typical orthoclase contains silica, 64.6, alumina, 18.5, potash, 16.9. 

 The analysis indicates the large replacement of the potash ; and all the 

 analyses that have been made show the same thing. Des Cloizeaux 

 regards such analyses as indicating the existence of a soda feldspar 

 isomorphous with orthoclase. 



In thin sections, orthoclase is commonly seen as grains which show 

 more or less of an effort at crystallization. It is a fact of lithological 

 importance that the fusible feldspar formed its crystals in these common 

 acidic rocks before the infusible quartz did. At times these crystals are 

 quite perfectly developed, and then they have the quadratic, rhombic, 

 and hexagonal forms that would be expected from common crystals of 

 orthoclase. In old rocks like ours, orthoclase usually appears more or 

 less impure on account of the minute particles of foreign substances, and 

 pores, and cleavage planes that exist in it. It often contains mineral 

 enclosures, such as little scales of mica, particles of hornblende, etc., 

 but the rare occurrence of the cavities containing fluids, which are so 

 uniformly present in the quartz, is very noticeable. Cavities often exist 

 in it, but they are commonly empty, as is shown by their sharply defined 



