176 PETROLOGICAL NOTES ON SOME OF THE ERRATICS 



are decidedly green, others are clouded and opaque through deposition of limonite. 

 A very little magnetite and apatite are present, and a few small grains of fluorite. 



The order of crystallization is somewhat peculiar in this rock: of the essential 

 minerals biotite certainly began to form first, but continued to form for a long time, 

 as it is moulded on all the other minerals, including even quartz. The oligoclase 

 crystals ceased to form comparatively early. 



613. Felspar Porphyry 



Microscopic Character s. — The phenocrysts appear to include two species of 

 felspar both occurring in idiomorphic crystals. 



Some of these are completely clouded so that measurements are not possible ; they 

 appear to be orthoclase. The others are mostly clear, but are decomposed in patches 

 and sometimes zonally. They exhibit well-defined Manebach twinning as well as 

 that after the Carlsbad and albite types. They are mostly slightly more acid 

 than Abi An u but in some instances there appears to be a definite intergrowth with 

 oligoclase in discontinuous strips parallel to the trace of the vertical crystallographic 

 axis. 



The base is finely panidiomorphic granular. It consists chiefly of untwinned 

 columnar felspar, with square cross- sections. The extinction of these laths is never 

 more than 8° from their long axes, so that the felspar is probably soda- orthoclase. 

 In many instances such prisms have a " core " formed by a flake of biotite. 

 Round many of them there is a fringe or halo of very fine granophyric intergrowth 

 with quartz. These felspars are a little decomposed. There is a small amount of 

 free quartz, both interstitial and in subidiomorphic grains, and also a fair amount 

 of biotite in thin, bent flakes with greenish-yellow to dark green pleochroism. Minor 

 constituents are magnetite, sphene, and calcite-epidote aggregates representing the 

 alteration products of a pyroxene or amphibole. 



7. Minette (Plate I, Fig. 6) 



Minette (approaching Camjrtonite) with a xenolith of altered quartzite. 



Microscopic Characters. — The minette is a fine-grained panidiomorphic 

 granular rock without porphyritic structure, composed essentially of prismatic horn- 

 blende, plates of biotite, interstitial orthoclase, a little epidote and sphene, and very 

 little ilmenite. 



Hornblende is in sharply idiomorphic prisms 0*1 mm. by - 01 mm.; cross- 

 sections are bounded by (110) with or without (100), but with no trace of (010). 



Pleochroism is slight, ft colourless, tj and C faint green, extinction angle 15°. 



Biotite is in thick hexagonal plates, *06 mm. in diameter; strongly idiomorphic. 

 These are pleochroic from light brown to dark brown. 



Away from the contact there is no augite. 



The felspar, whose refractive index is less than that of Canada balsam, forms a not 

 very abundant paste. 



On the other side of the contact the rock consists essentially of quartz and augite 

 (with, possibly, some epidote). The quartz amounts to about 40 per cent of the rock. 

 It is in fair-sized grains, showing considerable undulose extinction. The augite is 

 finely granular and colourless. 



The contact zone, which is about 2 mm. in width, is rather finer in grain than the 

 rock on either side. On the minette side there is an admixture of augite in fine grains, 



