PERTHITIC FELDSPARS. 131 



microscope in thin section. The albite is usually well twinned, 

 though sometimes so minutely as to be hardly discernible. This 

 feature renders the exact measurement of the extinctions difficult. 



Remarks on the individual specimens: No. 1, from Perth, Ontario, 

 Canada. — This specimen, from the original locality, is the richest 

 in albite of any of the feldspars studied. It consists of a rather dark 

 reddish-brown microcHne intergrown with about an equal amount 

 of a light red to almost white albite. The red color is due to the pres- 

 ence of exceedingly minute crystal scales of hematite which are chiefly 

 contained to the microcline. They are usually arranged along defi- 

 nite crystallographic directions. Some hematite is found along frac- 

 tures in the albite, or is more irregularly placed. It is estimated that 

 there is about one-third as much of this material in the albite as in 

 the microcline. The albite lamellae seldom exceed 1 mm. in width 

 while a commonly observed width is 0.5 mm. The microcline bands 

 will in general average broader than the albite. The orientation of 

 the bands is fairly uniform parallel to the usual direction, but there 

 is a common tendency to bend off toward the direction of the prism 

 and often many short bands coalesce along this same direction. 

 Pinching and swelling, branching and coalescing, often in a very com- 

 plex fashion, are common particularly in certain areas. The albite 

 forms minutely pointed surfaces of contact with the microcline as a 

 rule. The microcline sometimes shows very distinct polj^synthetic 

 twinning but more often this is faintly developed and is often not 

 visible at all. There is a slight alteration of the microcline to a fine 

 opaque dust but on the whole it is quite fresh. 



The extinction angles for the two feldspars are as follows : — 



Sipecinien, No. 2, Westficld, Mass., U. S. A. — Color white; perthitic 

 structure not conspicuous. The microcline is practically devoid of 

 structure as seen under the microscope. The albite lamellae rarely 

 exceed 0.8 mm. in width, the majority running from 0.1 mm. to 0.3 

 mm. ; they vary much in width along their length, pinching out and 

 frequently forking at the ends. Besides the usual very minute 

 lamellae running parallel to the general direction of intergrowth there 

 is a set which makes an angle of about 12° with the basal clea\age in 



