224 THE CRYSTAL FALLS IROX-BEARING DISTRICT. 



o-ives ratlier tliin sections, tliouiili tliev can hardly be correctly called lath- 

 er ■ ' O ^ c' J 



shaped. No other form of plagioclase, showing a uniformly better or 

 poorer development, or any other difference in chai-acter indicating the 

 presence of two kinds of lime-soda feldspar, was observed. 



The plagioclase sections almost invariably show polysynthetic twinning 

 according to the albite law, with twinning lamellai which vary from ^•ery 

 thin to moderately thick plates, the thinner being the more common form. 

 Very common is the combination of the albite and Carlsbad twinning laws 

 in one individual. Less commonly we find individuals twinned according 

 to the pericline as well as the albite law, and sometimes a Carlsbad twin is 

 made up of individuals twinned according to the albite and pericline laws. 



In determining the character of the feldspar, the Levy method was 

 followed.' A great number of measurements made on the zone perpen- 

 dicular to 010 gave equal extinction angles, varying chiefly around 15 

 degrees, but running as a maximum to 19 degrees. From this it appears 

 that the plagioclase is andesine, probably a somewhat basic kind. That 

 these andesines vary slightly in composition is shown by a very slight but 

 noticeable zonal structure, the more basic character of the center of the 

 individuals being most admirably brought out by the more advanced con- 

 dition of alteration of the center as compared with the periphery. 



The andesine is for the most part very much altered, to such an extent 

 that in many sections the boundaries of the twinning lamellte are so blurred 

 that measurements are rendered imjjossible. Muscovite, which appears in 

 minute rectangular sections showing good cleavage, is the chief secondary 

 product from the feldspar, with epidote-zoisite next in importance. Calcite 

 and biotite are present, but in comparatively small quantities. In some 

 cases muscovite almost replaces the feldspar; in others epidote-zoisite does 

 so. In such a case one sees in the center of the feldspar only a mass of 

 secondary mineral. As the examination is carried from the center toward 

 the outside, the original feldspar material is distinguished as a thin film 

 between the secondary minerals. This increases in mass until we reach 

 the outside narrow rim of practically unaltered feldsj^ar. 



orthociase. — Tliis Is prescut in large quantity in irregular plates which 

 form a part of the mesostasis for the plagioclase and bisilicates. Less com- 

 monly we find it in micropegmatitic intergrowth with the quartz. It is 



' fitucle sur la determination ilea feldspatlis, by A. Michel L6vy, Paris, 1894. 



