1893.] 



NEW YORK ACADEMY OF SCIENCES. 



205 



quartz and ortlioclase is filled by secondary plaj^^ioclase, which 

 sends out branches into the ortlioclase, forininj:;' iiiciinent 

 microperthite. The plagioclase, represented by shadinj^', gives 

 a blue interference color, and all the areas extinguish simultan- 

 eously. The original orthoclase gives a yellow interference 

 color. 



Fig. 1. Secondary mictoperLhiu-. 



Secondary feldspar (shaded) filling crack between quartz and 

 orthoclase and with the latter forming microperthite. 

 Diameter of field, 0.7 min. 



Along many of the cleavage cracks there are very narrow 

 bands of the secondary feldsjiar, perceptible only with a high 

 power. As the bands increase in width thej' naturally show 

 less dependence upon the cleavage cracks, though a general 

 parallelism remains. 



From such facts it is evident that the microperthite results 

 from the development of plagioclase along planes of solution in 

 orthoclase, these planes of solution being determined chiefly by 

 cleavage. INEuch of the material of the secondary feldspar is 

 doubtless derived from the orthoclase, while a certain portion 

 is brought from without. Similar developments of secondary 



