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University of California. 



[Vol. i. 



holocrystalline matrix. These idiomorphic orthoclases are tabular, 

 parallel to the clinopinacoid, and commonly attain a length of from 

 4 to 6 cm., and a thickness of from 3 to 5 mm. They are in many 

 cases twinned on the Carlsbad law. The basal cleavages are in all 

 cases well developed. In addition to these large crystals there are 

 also smaller irregular grains of the same feldspar scattered through 

 the dark green matrix of the rock, but still showing a more or less 

 pronounced tendency to assume tabular forms and thus grade into 

 the large porphyritic crystals. It is evident from an inspection of 

 the rock that the porphyritic orthoclases antedate the matrix in 

 which they are imbedded, but that the crystallization of the latter 

 set in before the orthoclase molecule was completely precipitated 

 from the magma, so that the residual orthoclase crystallized with 

 and became involved in the aggregate of minerals which constitute 

 the matrix. There is thus no hard and fast line between the bulk 

 of the orthoclase which appears as large crystals and the moiety 

 having the smaller and less regular forms. See Plate 18. 



Microscopic Characters. — Under the microscope the orthoclase 

 shows a constant and well-developed microperthitic intergrowth 

 .with albite. The latter mineral is readily distinguished from the 

 orthoclase by its somewhat stronger double refraction, its difference 

 of extinction, and in many cases by its lamellar twinning. It is 

 identified by its extinction angle against the basal cleavage in cleav- 

 age flakes parallel to the clinopinacoid of the orthoclase. In these 

 flakes the orthoclase has an extinction angle of about 4 , and the 

 plagioclase an angle of 16 to 18 , with an average of \j° 22', so 

 that there can be little doubt of its being nearly pure albite. 



The albite is intergrown with the orthoclase in thin lenticular 

 plates, which lie parallel to the orthopinacoid of the latter and give 

 striped sections in the zone P : M as of overlapping spindle-shaped 

 bodies. In these striped sections the orthoclase is generally some- 

 what cloudy, while the albite is clear, so that the striped appearance 

 is apparent without the use of crossed nicols. In sections parallel 

 to the orthopinacoid the orthoclase gives an interference figure in 

 which the plane of the optic axes lies parallel to the trace of the 

 basal cleavage. Sometimes the albite of the microperthite appears 

 to be in optical continuity with grains of albite on the periphery of 



