BY W. G. WOOLNOUGH. 507 



The commonest type of felspar in the rock is certcainly a basic 

 variety of andesine, or an acid variet}^ of labradorite. This is 

 always fairly idiomorphic and strongly zoned. The decomposition 

 products are approximately centrally grouped. In sections per- 

 pendicular to (010) the maximum extinction angle is 19^. In 

 sections parallel to (010) the extinction varies from -16" to 

 - 25*^ for the different zones. The refractive index is in all 

 cases higher than that of quartz. 



The second type of felspar is far less abundant than that above 

 described. It occurs in small allotriomorphic grains entirely free 

 from decomposition products. In these the maximum extinction 

 in the zone perpendicular to (010) is 6*^, in section parallel to 

 (010) it is - 4^'. The refractive index is higher or lower than 

 that of quartz according to the relative orientation of the two 

 minerals. These sections must be rather basic oligoclase. 



The third type of felspar is untwinned or twinned only accord- 

 ing to the Carlsbad law. It is interstitial in character and very 

 sparsely distributed. The decomposition products are abundant, 

 and are evenly, not zonally, distributed. The refractive index 

 is distinctly lower than that of quartz or Canada balsam. This 

 mineral may be orthoclase, but is more probably anorthoclase, 

 since other sections whose habit is exactly similar show traces of 

 excessively fine lamellar twinning. 



Except for the arrangement of the decomposition products, 

 they do not call for very special description, as they consist prac- 

 tically entirely of kaolin. A little calcite and epidote are 

 developed locally. The more basic felspars are decomposed 

 centrally, but to a much smaller extent towards the periphery; 

 while those felspars which have been referred to anorthoclase are 

 pretty evenly affected. In all cases the products are more 

 thickly grouped in certain planes than in others, and these planes 

 are 7iot the cleavage planes. 



As inclusions, we have occasional prisms of colourless apatite 

 with a marked tendency to arrangement pai-allel to faces of the 

 host. Locally magnetite in minute crystals is very abundant. 

 Here and there very minute hair-like crystals are quite plenti- 



