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University of California. 



[Vol. 3. 



the older and probably the more basic form, as in the majority 

 of cases the light mineral is moulded around it. 



The minerals included in the hornblende are magnetite in 

 irregular grains and well-formed crystals, and apatite in small 

 prisms. Some secondary epidote, with its very high birefring- 

 ence, has been formed, as well as some chlorite. The epidote is 

 often interlocked with fibers of undecomposed hornblende in an 

 intricate manner. 



The feldspars under the microscope are very clear, and in 

 most cases free from secondary products. What appears to be 

 single crystals to the unaided eye are found to consist of a 

 number of smaller individuals, with a few traces of idiomorphic 

 boundaries. In size the crystals range from 3.09 mm. at a 

 maximum to less than 1 mm., averaging 1.10 mm. Cleavages 

 parallel to base and brachypinacoid are developed sparingly. The 

 twinning most commonly observed is that on the albite law; 

 often pericline lamellae are seen, and more rarely carlsbad twins. 

 The measurement of the maximum symmetrical extinction angles 

 on the albite lamellae indicate that the feldspars range from 

 andesine, with an extinction angle of 20°, to a medium basic 

 labradorite. with an extinction angle of 41? However, most of 

 the feldspar corresponds to an acid labradorite, with an 

 extinction angle of 35? This determination of the feldspars was 

 checked by specific gravity tests, using Klein's solvation. A 

 small amount of feldspar came down at 2.64, and a somewhat 

 larger amount at 2.65. The major portion, a little over half, 

 came down at from 2.66 to 2.68, while the remainder did not 

 fall until a specific gravity of 2.70 was reached. Thus the 

 plagioclase is mostly an acid labradorite, with some basic 

 labradorite and considerable andesine. 



Zonal structure is common, the zones being usually divided 

 by sharp lines. The most basic feldspar is at the center, and 

 shows a greater number of twinning lamellae than the more acid 

 border. In a few cases the change from basic to acid is a 

 gradual one, a progessive wave of extinction crossing the crystals 

 as the stage of the microscope is revolved. These zonal crystals 

 show the best approach to idiomorphism, except in the cases 

 where quartz is in contact with the feldspar. Inclusions are few, 



