[alcock] 



NORITE ROCKS 



27 



The feldspar shows both striated and unstriated varieties. Com- 

 parison of the index of refraction, however, of the unstriated crystals 

 along the borders of the section shows that they all consist of plagio- 

 clase. The twinned plagioclase is a labradorite. The labradorite 

 shows many combinations of albite and pericline twinning. One 

 crystal ^ cut perpendicular to the albite twinning extinguishes at an 

 angle of 37 degrees. Crossing the albite lamellae is a broad band 

 which represents a pericline twin, since it extinguishes at the same 

 time as the smaller bands on either side of it, which are of known 

 pericline twinning. On this large pericline twin faint albite lamellae 

 are clearly distinguishable and these in turn are crossed by pericline 

 twins on the large pericline twin. 



Quartz is present in the rock filling around the other minerals. 

 It shows undulating extinction showing that it was present before 

 the rock was strained. 



* The crystal is shown in figure 1, where it fills the whole central part of the plate. 



