352 



GEOLOGY OF THE EUREKA DISTRICT. 



in the second column denote the degrees to be added to or subtracted from 90 to give 

 the required angle for labradorite in sections taken every 5 from the normal to the 

 edge OP, ooPdb. 



Table showing the angle between OP cleavage and the trace of ooP<X> in 

 planes in the zone perpendicular to ocP ob for labradorite. 



From this it will be seen that the variation in the angles made by the cleavage 

 is only 1 20' for 45 of rotation each side of the normal, or for a whole quadrant, but 

 for 70 on both sides the variation is only about 6. Applying this to Figures 7 and 

 8 it will be readily seen what combinations may occur. Thus in a section in ttis 

 zone 45 to the left of X the cleavage angle will be 94 42' in the half (a) and about 

 120 in the half (6), and whilst the extinction angle in the first half (a) is 20 that in 

 the second half (b) is 0. If in conjunction with this Carlsbad twinning we have the 

 polysynthetic twinning of albite, as generally happens, we shall find sections in the 

 zone under discussion one side of which will show striations having symmetrical 

 extinction angles differing from the symmetrical extinction angles of the striations in 

 the other side by as much as 20 in some cases, a phenomenon which might lead to 

 the erroneous conclusion that two species of plagioclase feldspar had formed together 

 along the plane of the brachypiuacoid. It is possible that instances of such an occur- 

 rence, which have been mentioned by other observers, may be sections of Carlsbad 

 twins of a single species. 



In the thin sections of this pyroxene-andesite occur many examples of twinned 

 feldspars, in nearly rectangular sections, that exhibit optical phenomena similar to 



