OTTRELITE PRISMS. 



151 



Although twinning with composition face parallel to is not uncom- 

 mon in the ottrelite, a large part of what seems to be twinning is seen to 

 be clue to overlapping plates. As the stage is revolved the wavy extinc- 

 tion caused by this may be observed, — the usual spherulitic structure. 



Parallel Grouping of ottrelite Prisms. — Examples of crystal growth set up 

 in several places at the same time occur in the rock where each part is 

 controlled by every other, resulting in an irregularly outlined prism com- 

 ix »sed of many individuals separated by areas of the grounclmass and 

 yet all oriented together (see figure 1 ). This phenomenon is unlike that 



Figure 1. — Thin Section of ottrelite Schist. X 25. 



Showing formation of ottrelite at many different points. Each small area is oriented with all the 

 others, forming a large area showing a general prismatic outline. The prism has been developed 

 transverse to the sehistosity of the rock. The background is largely gneissic quartz, with some 

 secondary feldspar. (Drawing from mierqphotograph.) 



of andalusite, which occurs so frequently grown into large individuals, 

 enwrapping all other minerals of the background, but is another exam- 

 ple of independent parallel growth analogous to that of quartz in pegma- 

 tite, although in no way determined by the crystallographic position of 

 the minerals of the groundraass. Such growths are commonly developed 

 nearly at right angles to the layers of quartz and feldspar that make up 

 the sehistosity, and are usually freer from inclusions than the bundles. 

 They occur between the main areas of ottrelite and may represent a 



