1893. | NEW YORK ACADEMY OF SCIENCES. 205 
quartz and orthoclase is filled by secondary plagioclase, which 
sends out branches into the orthoclase, forming incipient 
microperthite. The plagioclase, represented by shading, gives 
a blue interference color, and all the areas extinguish simultan- 
eously. The original orthoclase gives a yellow interference 
color. 
Fia. 1. Secondary microperthite, 
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 feldspar, perceptible only with a high 
power. As the bands increase in width they 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. Much 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 
