268 Arthur Holmes — Petrology of North-Western Angola. 



with the surrounding mosaic, and in one case it was noticed that 

 a long tongue of the latter, heavily charged with vermicular limonite 

 and minute zircons, had cut transversely across the crystal. The 

 porphyritic crystals are twinned according to the Carlsbad law, and 

 are made up of orthoclase with microperthitic streaks of albite. 

 Inclusions of riebeckite needles, and patches and blebs of limonite 

 follow the lines of perthitic intergrowth, and are commonly, but not 

 invariably, arranged in linear groups. Here and there irregular 

 aggregates of an indeterminable mineral, which may be kaolin or 

 sericite, can be seen. Similar inclusions occur in the microcline and 

 albite of the mosaic, but they are less abundant than in the larger 

 perthites. 



Intermediate in size between the porphyritic crystals and those of 

 the mosaic are a few allotriomorphic individuals of microcline perthite. 

 The ' cross-hatching ' of the microcline is somewhat patchy, and may 

 be absent altogether. Feeble strain shadows are seen in some cases, 

 but in others the extinction is sharp. 



The felspars of the mosaic are exclusively microcline and albite. 

 An analysis of the rock indicates that the albite molecule may be 

 accompanied by a very small proportion of anorthite. Making allowance 

 for the fact that more than half of the Ca in the rock is probably 

 present in riebeckite and segirine, the albite can scarcely be more 

 calcic than Ab 96 An 4 . Both orthoclase and microperthite are entirely 

 absent from the mosaic, and microcline is much less abundant than 

 albite. The felspars are generally allotriomorphic, but sometimes 

 they exhibit crystal planes in contact with quartz. 



The larger felspar crystals are predominantly of orthoclase, 

 accompanied by a little perthitic albite ; those of intermediate size are 

 chiefly of microcline, again accompanied by perthitic albite. Among 

 the smaller individuals albite is predominant, microcline is subsidiary, 

 and perthitic intergrowths are absent. It would be unwise to assume 

 too much from a single specimen of the rock, but the restriction of 

 strain phenomena to the larger crystals suggests that the mosaic 

 felspars may be partly the result of the granulation, recrystallization, 

 and albitization of the older orthoclase perthites. A similar 

 explanation of the structures and composition of the Quincy riebeckite 

 granite has been given by Warren. 1 



Quartz is the most abundant mineral in the rock, making up about 

 60 per cent of the colourless constituents of the mosaic. It is present 

 in allotriomorphic equidimensional grains which exhibit no unusual 

 features. 



Riebeckite is present to the extent of about 1 6 per cent of the whole 

 rock, an unusually high proportion for a rock of this type. It varies 

 in size from tiny microscopic needles and irregular shreds to larger 

 crystals which tend towards a prismatic habit, but which generally 

 have ragged edges. In transverse sections the average area is much 

 the same as that of- the quartz and felspars of the mosaic, but it is 

 subject to greater variation. The prism and clinopinacoid edges can 

 sometimes be identified. Longitudinal sections are elongated, the 



1 C. H. Warren, Proc. Am. Acad. Arts and Sci., vol. xlix, No. 5, p. 215, 1913. 



