COLLECTED AT CAPE ROYDS 177 



and a very well-marked development of " Schlieren." On the quartzite side there is 

 a noticeable development of biotite and a little hornblende. 



The altered rock is probably derived from an argillaceous sandstone rather than 

 from an eruptive rock, as no felspar is present in it. 



534. Minette 



Macroscopic Charade r s. — Dark grey granular rock with occasional irregular 

 light-coloured patches. The apparent grainsize is about "25 mm. The most abundant 

 constituent is a greenish-black micaceous mineral. Interstitial felspar is not abundant. 

 The larger white patches are of quartz and calcite up to 3 mm. in diameter. 



Microscopic Character s. — Grainsize even, fabric panidiomorphic granular. 



The most abundant constituent is orthoclase in idiomorphic crystals, untwinned or 

 twinned after the Carlsbad law. Occasionally a multiple- twinned plagioclase crystal 

 can be seen whose extinction angles and refractive index point to a composition of 

 AbpAiij about. 



All these felspars are very much clouded by decomposition, and are crowded with 

 flakes and tufts of light-yellowish chlorite or mica. Many such clouded crystals have 

 outgrowths of clear orthoclase in crystal continuity. In both nucleus and outgrowth 

 the extinction angles, measured from the long axis, are very small ; but a difference 

 of 1° or 2° between inner and outer portions can be detected. Many of the cloudy 

 crystals are enclosed in extensive optically continuous areas of somewhat cloudy 

 orthoclase. 



Some of the felspars are more or less replaced by quartz in a manner so irregular 

 as to point to metasomatic action. 



There is much biotite with very light yellow to reddish- brown pleochroism, and 

 showing bright polarisation tints. This mineral is considerably altered to green 

 penninite with separation of the titanium content in the form of "sagenite" webs 

 and granular leucoxene. Much ilmenite is present, slightly decomposed to leucoxene. 



Apatite in relatively large needles is very plentiful, but is almost confined to the 

 quartz, and does not enter much into the felspar or biotite. 



There is not much calcite, but what there is is in large patches mixed with quartz 

 in such a way as to suggest progressive replacement by the latter mineral. 



There is quite abundant clear quartz, mostly interstitial in character, and probably 

 secondary. The graphic intergrowth with what appears to be secondary orthoclase 

 has been noted above, as has also the fact that the quartz contains inclusions of apatite. 

 The appearance is strongly suggestive of secondary action having introduced quartz, 

 orthoclase, and apatite. Such a process, if it happened, is somewhat remarkable. 



1066. VOGESITE 



Macroscopic Character s. — Dark-greenish rock of very fine grain. The only 

 noticeable feature is the development of abundant irregular flecks of dark- coloured 

 material up to 2 mm. diameter. 



Microscopic Characters. — The rock, which is much decomposed, shows a 

 holocrystalline porphyritic structure with a panidiomorphic granular base. It is com- 

 posed essentially of felspar, hornblende, uralitised augite, and chlorite (apparently after 

 biotite), in about equal proportions. The felspars average about 0*25 mm. by 01 mm. 

 in size, and are very cloudy from secondary material. All are apparently ortho- 

 clase. 



