344 



University of California. 



[Vol. i. 



tion the occasional patches of fine-grained black' rock, which prob- 

 ably represent local more basic secretions. 



The idiomorphic constituents only occasionally exhibit the 

 rectilinear outlines of the ideally perfect crystals. Their constant 

 approximation to these outlines is, however, characteristic of them. 



Mineralogical Details. — The orthoclase is characterized through- 

 out the rock by a molecular tension which manifests itself in 

 undulatory extinctions. This is very probably a direct conse- 

 quence of the conditions under which it crystallized. The orthoclase 

 being the last product of the crystallization of the magma and 

 forming large individuals inclosing all the other constituents, it 

 would naturally result that there would not be opportunity for the 

 adjustments due to change of density at the point of crystalliza- 

 tion, or during cooling after crystallization. The orthoclase also 

 presents under low powers a very dusty appearance, due to the 

 abundance of minute interpositions. These are of two kinds: (i) 

 Slender opaque rods frequently arranged in parallel position resem- 

 bling schillerization products, and (2) minute opaque rounded bodies 

 scattered irregularly throughout the mineral. There are, also, 

 rarely occurring transparent microlites inclosed in the orthoclase, 

 but no liquid inclusions were observed. No twinning structure was 

 observed in any of the slides. For the most part the orthoclase is 

 fresh, but it is cloudy in places as a result of decomposition. Both 

 cleavages, parallel to OP (001) and to ccPoo (010) are distinctly 

 observable. In cleavage flakes parallel to OP, it was determined 

 by aid of the quartz wedge that a lies parallel to the trace of the 

 clinopinacoidal cleavage and that c lies normal to the same direc- 

 tion. The refractive power and double refraction are those of 

 orthoclase. 



The chemical character of the orthoclase was tested by subject- 

 ing small grains of it to the action of hydrofluosilicic acid, the 

 result being an abundant crop of cubes of fluosilicate of potassium, 

 with an exceedingly few more minute hexagonal crystals of fluo- 

 silicate of sodium, and occasional prismatic crystals, with rhombo- 

 hedral terminations, which represent probably the corresponding 

 compound of iron derived from the interpositions in the feldspar. 

 The result of the test shows that sodium is present in the feldspar 



