274 C. L. Whittle — Ottr elite-hearing Phase of the 



in the rock, where each part is controlled by every other, result- 

 ing in an irregularly-outlined prism composed of many indi- 

 viduals. These parts are separated by areas of the groundmass 

 and yet are all oriented together (see tig. 1). This phenomenon 

 is unlike that of andalusite. which occurs so frequently grown 

 into large individuals enwrapping all other minerals of the back- 

 ground ; but is another example of independent parallel growth 

 analogous to that of quartz in pegmatite. Such growths are com- 

 monly developed nearly at right angles to the layers of quartz 

 and feldspar that make up the schistosity, and are usually freer 

 from inclusions than the bundles. They occur between the 

 main areas of ottrelite and may represent a second generation ; 

 they were necessarily formed after the groundmass was con- 

 verted to a mosaic by granulation. 



The same phenomenon is noticed in the rutile. Little yel- 

 lowish-brown grains of this mineral developed in the inter- 

 spaces of the minerals composing the background tend, 

 although made up of separate and sometimes isolated grains, to 

 orient themselves parallel to one another, forming groups hav- 

 ing prismatic outlines. These groups are only sparingly 

 developed, but when observed they are generally parallel to 

 one another and to the schistosity of the rock and are restricted 

 in their occurrence, like the ottrelite individuals just described, 

 to the most quartzose parts of the rock which they enclose in 

 the same manner as the ottrelite. 



An interlamination of chlorite and ottrelite at first glance 

 was mistaken for either the contemporaneous formation of 

 these minerals or an infiltration of chlorite parallel to the 

 basal cleavage of the ottrelite. Further investigation, how- 

 ever, showed that the chlorite as often traversed the ottrelite 

 irregularly in bifurcating veins and enclosed parts of it (see fig. 

 2). A study of the nature of these veins convinced me that 

 the chlorite is an alteration product of the ottrelite. The 

 edges of the veins where they traverse the ottrelite transverse 

 to the basal cleavage are jagged, the saw-like teeth projecting 

 along the composition faces or basal cleavage. The chlorite in 

 such cases is distinctly made up of little fibers which have 

 arranged themselves parallel to one another and to one set of 

 twinning lamellae. Lines of inclusions once continuous in the 

 ottrelite now stop short against interlaminated areas of chlorite 

 showing the evident secondary nature of the latter mineral. 



In other places the chlorite is developed along the basal cleav- 

 age, leaves this and follows one of the prismatic cleavages and 

 then again follows the basal, making one continuous line, produc- 

 ing steps in much the same manner that garnet does when it 

 undergoes this alteration. Cores of unaltered ottrelite remain 

 in the chlorite and the pleochroic zones once in the parent are 



