I0Oo MINERALOGY AND LITHOLOGY. 
mental stone. These colors in orthoclase have been shown by Reusch 
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, ‘j < : 3 : : : F ‘ ‘ 5 . 66.06 
Alumina, ‘ - f 3 ‘ 5 : ‘ , 3 20.04 
Tron oxide, F - 5 . ‘ A Re 5 3 : trace. 
Lime, - s . : is . - s Fi 2 7 2.08 
Soda, . : 4 3 é - . ‘ a ° : ‘ 7.28 
Potash, . . F . é tan . . fi ‘ : 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 
