COLLECTED AT CAPE ROYDS 179 



a hypabyssal facies of which the other two are the plutonic and volcanic types 

 respectively. 



Biotite is idiomorphic to subidiomorphic in sections up to 1*2 mm. by 05 mm. 

 It shows pleochroism from golden yellow to very dark brown, some basal sections 

 being practically opaque. 



Apatite and zircon occur as inclusions in both hornblende and biotite. 



There appears to be considerable overlap in the crystallisation periods of the 

 porphyritic constituents. Hornblende is for the most part the earliest, but very 

 occasionally is moulded on felspar and hornblende ; but also occurs included in the 

 large felspars. 



284 (610). Sericitised Diabase -Porphyry 



Microscopic Character s. — The base of the rock is rather coarsely pilotaxitic 

 in fabric. It consists mostly of oligoclase-andesine in idiomorphic crystals, 0"3 mm. 

 byO'2 mm., which are fairly fresh and undecomposed. There is a good deal of strongly 

 prismatic uralite in crystals, 1 mm. by 0*1 mm., a little dark brown biotite hi flakes 

 or confused aggregates, and still smaller quantities of magnetite. Throughout the 

 base abundant leucoxene is scattered. 



The phenocrysts are large idiomorphic plagioclases up to 1 - 5 mm. by 0'5 mm., 

 almost completely sericitised. In the majority of instances the phenocrysts are con- 

 verted into pseudomorphs of interlacing fibres, the whole aggregate possessing strong 

 double refraction ; but there is always an outer zone of clear felspar, showing strongly 

 zonal extinction, but with low angles like those of the felspars of the second generation. 

 In a few instances patches of the original material are sufficiently unaltered to get 

 symmetrical extinctions up to about 32° (Ab 9 An n ). Under high powers the alteration 

 products of the felspar can be resolved into fine interlacing laths of sericitic mica 

 extending inwards from the sides of the original crystals. While there may be con- 

 siderable divergence of individual laths yet there is a very marked regularity in their 

 general arrangement. They are distributed along certain definite crystal planes whose 

 positions have not been identified. In any particular haphazard section the angles 

 between the different solution planes (if such they be) depends upon the orientation 

 of the section. In one instance, where two well-defined axes of arrangement made 

 an angle of 58° with one another, the traces of the (001) and (010) cleavages of the 

 felspar appeared symmetrically placed with respect to the said axes. This rather 

 suggests that a pah - of brachydome faces may lie parallel to the solution planes, but 

 of this I cannot feel at all certain. 



There is a good deal of secondary material throughout the rock. Leucoxene has 

 been mentioned already. In addition there is much quartz, calcite, and some chlorite. 

 The quartz and calcite often enclose or partly enclose the main constituents of the rock, 

 suggesting an original miarolitic texture. 



452. Solvsbergite (Plate II, Figs. 3 and 4) 



Macroscopic Character s. — Dark grey porphyritic rock with stony ground- 

 mass. There are abundant phenocrysts of colourless felspar, up to 2 mm. in diameter occa- 

 sionally. Hornblende is very abundant and conspicuous in sharply defined prisms 

 which occasionally reach 5 mm. by - 5 mm., but are mostly much smaller. Occasional 

 grains of pyrites are visible. 



Microscopic Character s. — The rock is porphyritic, with very fine textured 

 pilotaxitic base consisting of lath- shaped felspars apparently hornblende and quartz 

 granules. 



