A SPHERULITIC DOLERITK FROM YKYHEID, NATAL. 389 



splierulites are frequently formed of a micropegmatitic iiiter- 

 g-rowtli of quartz and felspar.^ Teall- has pointed out that 

 in alloys of eutectic composition spherulitic structures are 

 developed by rapid cooling. Indeed, just as a magma of 

 eutectic composition seems to lend itself to the formation of 

 glass,-'' so, under other conditions, it seems to favour tlie 

 production of spherulites. In this connection the occurrence 

 of small spherulites in glassy rocks may be not without 

 significance. 



A study of the order of crystallisation in the Vryheid 

 dolerite leads to some further conclusions on the conditions 

 favourable to the growth of spherulites. From the extreme 

 regularity of their development it appears that the enstatite 

 crystals grew comparatively slowly, and were among the first 

 products of crystallisation. The augites, on the other hand, 

 must have crystallised out with extraordinary rapidity, as 

 their skeletal and long-prismatic habit suggests. These 

 phenocrysts represent the excess of augite in the original 

 magma above the eutectic composition. The next mineral to 

 commence sepai-ating out was the labradoi-ite. We have seen 

 that it is closely bound up with the interstitial augite, but 

 the way in which its crystals control the radial structure 

 clearly shows that it started to crystallise just before the 

 augite; The shape of the curved boundaries between 

 spherulites of unequal size shows that the growth of the 

 radius- — that is, of the felspar crystals — ^was uniform,'^ so that 

 the fact that most of the boundaries lie midway between two 

 centres is evidence that crystallisation of the felspar com- 

 menced simultaneously at a great number of points. The 

 frequent inclusions of ground-mass in negative crystals and 

 the lath-shaped habit show that the growth of the labradorite 

 took place with great rapidity, while the small extent to 



I Whitman Cross. ' Bull. Phil. Soc. Washington,' xi (1891), p. 430. 

 " J. J. H. Teall. ' Quart. Journ. Geol. Soc..' Ivii (1901), p. Ixxv. 

 3 Harker. • Natiu-al History of Igneous Rocks,' 1909. p. 224. 

 ■* Popoft', '• Eine neue Untersuchungsweise sphiirolitischer Bildungen," 

 • Min. Petr. Mitt. Wein," xxiii (1904), p. 153. 



