382 'W. CAMPEELL SMITH. 



ill mineral composition the rock is very similar to the enstatite- 

 dolerites intruded in the Karoo beds of Natal and Znluland, 

 recently described by Dr. Prior. ^ 



Small phenocrysts of enstatite and aug'ite with very occa- 

 sional olivines lie in a ground-mass composed of long lath- 

 shaped felspars with interstitial augite and magnetite. 



The enstatite occurs in well-formed crystals showing the 

 forms a (100), b (010) and m (110). Rectangular sections 

 bounded by the two pinacoids and having their corners trun- 

 cated by the prism-faces are frequent, but very often prism- 

 faces appear only on one pair of opposite corners and not on all 

 four corners. These cross-sections measure about 1 mm. by 

 0*4 mm. and the shorter side is invariably parallel to the])laiie 

 of the optic axes. The prismatic cleavage is well developed. 

 The crystals are colourless in thin section and show the usual 

 optical properties of enstatite. 



The augite phenocrysts show a very striking development, 

 exhibiting a strong tendency to form skeletal crystals. They 

 occur as prismatic crystals 3 to 6 mm. long and about 0*2 mm. 

 broad. Longitudinal sections of these crystals have a forked 

 appearance, but where cut nearly normal to their long axes 

 they give square sections with hollow centres, showing that 

 the crystals are not forked but are actually hollow for the 

 greater part of their length. Most crystals are bounded by 

 the prism-faces alone, the clino-pinacoid occurring as a sub- 

 ordinate form. The prismatic cleavage is not always evident, 

 but there is always present an irregular fracture roughly 

 perpendicular to the length of the crystals. Twinning about 

 the ortho-pinacoid is very common. The crystals are quite 

 colourless in thin sections. Longitudinal sections give high 

 interference tints with extinction-angles ranging up to 40°, but, 

 as a rule, averaging about 32°. Square cross-sections give 

 very low interference tints and frequently show one optic 

 axis emerging in the field of view. 



1 G. T. Prior, " Petrograpliical Notes on the Dolei'ites and Rhyolites 

 of Natal and Zululand," ' Annals of the Natal Museum,' vol. ii, pt. 2 

 (1910), p. 141. 



