A SPHERULITIC DOLERITE FROM VRYHEID, NATAL. 38-3 



Anotliei- interesting phenomenon displayed l)y these angite 

 phenociysts is that they are frequently surrounded by a 

 secondary zone of augite. In ci'oss-sections the secondary 

 augite is seen to develop principally at the corners, and 

 whereas the phenocrysts are bounded by four prism-faces 

 alone, the secondary growth usually develops pinacoidal faces, 

 the prism becoming subordinate : this gives rise to a cruciform 

 appearance of the whole crystal. This secondary zone is of 

 quite small dimensions, but there is always a definite boundary 

 between it and the original crystal. In cross-sections the 

 outer zone is much more birefringent than the phenocryst, 

 but the two extinguish simultaneously. In sections giving 

 oblique extinction the difference in double refraction is less 

 marked, but the outer zone has a slightly higher extinction- 

 angle, which usually exceeds that of the phenocryst by 7°. 

 Very rarely this secondary growth of augite occurs in parallel 

 position on the enstatite phenocrysts. 



Olivine occurs sparingly as phenocrysts, and usually has 

 undergone considerable resorption. 



The felspars occur in long, very thin, lath-shaped crystals. 

 Longitudinal sections occasionally give extinction-angles of 

 11°, but the majority give quite small angles. In certain 

 sections, however, the felspar appears as small rectangles 

 showing albite- or more probably Carlsbad-twinning, and 

 giving symmetrical extinction-angles up to 32°, which corre- 

 spond to labradorite. 



Some of these cross-sections of felspar show inclusions in 

 the form of negative crystals now occupied by augite and 

 magnetite in minute grains. These cores can also be made 

 out in longitudinal sections running down the centres of the 

 crystals, and often it is almost impossible to distinguish 

 between the material inside the felspars and that occupying 

 the interspaces between the laths. The latter is composed of 

 crystals of pale yellowish augite, enclosing minute grains and 

 cubes of magnetite, and, as we shall show later, precisely 

 identical with the augite forming a secondary growth round 

 the phenocrysts. 



