174 PETROLOGICAL NOTES ON SOME OF THE ERRATICS 



makes comparison of refractive indices difficult. In most instances the refractive 

 index of the plagioclase is greater than that of Canada balsam, but an occasional section 

 of the peripheral portion of a crystal of the first period exhibits equality of refractive 

 index with the same standard. The refractive index of the peripheral zones of felspars 

 of the first period is very decidedly less than that of the grains of the second period 

 in which they are imbedded. Quartz is moderately abundant in large grams mostly 

 strained, and many of them showing signs of incipient granulation. Very abundant 

 fluid cavities with spontaneously moving bubbles are present. 



Hornblende is abundant in idiomorphic prisms. It is notably complex in character, 

 showing a decided zoning, the central portion being dark brown and the peripheral 

 part dark green. The pleochroism of the brown portion is : 



8 = bright golden yellow ; 

 b = very dark brown ; 

 C = dark greenish -brown. 



a<fj>c 



Sections parallel to (010) show twinning, and yield symmetrical extinctions of 17°. 

 The passage from brown nucleus to green rim is very gradual. The central portion 

 frequently shows no cleavage, but is strongly schillerised. 



Biotite is fairly abundant in subidiomorphic sections. The pleochroism is golden 

 yellow to dark reddish-brown. The crystals contain inclusions of apatite, plagioclase, 

 and zircon (the latter with pleochroic halos). 



Biotite and hornblende frequently show regular intergrowths with definite morpho- 

 logical relationship. Sometimes one is inside, sometimes the other, but mostly the 

 hornblende is the older. Frequently the biotite is completely altered into dark green 

 penninite. 



Apatite is extremely abundant in perfect crystals. 



448. Granophyric Granite-Porphyry 



Macroscopic Characters. — Grey mottled with dark and light spots. The 

 groundmass is fairly fine in grain and consists of felspar and biotite. The fighter 

 coloured phenocrysts consist of pink orthoclase (3 mm.) and grey plagioclase (8 mm. by 

 3 mm.). The dark patches are finely granular aggregates of greenish-black colour 

 up to 4 mm. by 2 mm. 



Microscopic Character s. — The fabric of the base is panidiomorphic granular 

 with the exception of fairly abundant patches which are distinctly granophyric. 



The base consists of felspar, quartz, and biotite, with a little accessory zircon 

 and magnetite. The felspars of the base are idiomorphic and subidiomorphic and 

 very even in size. Mostly they are untwinned, have a refractive index less than 

 that of Canada balsam, and give very nearly straight extinctions, so that they are 

 probably orthoclase. A few appear to possess very hazy albite twinning and may 

 possibly be anorthoclase. These felspars are considerably kaolinised so that satis- 

 factory determinations are difficult. 



Quartz is not so abundant as felspar; some of it is idiomorphic, some enters into 

 beautiful granophyric intergrowth with the felspar. 



The ferromagnesian constituent is not abundant. It occurs in small flakes whose 

 pleochroism is from yellow to dark brown. From this condition it is present in various 

 stages of bleaching, the ultimate product being an aggregate of green fibres. While 

 these aggregates have been described as an alteration of biotite it is possible that they 



