A. Holmes & H. F. Harwood — Picrite, Mozambique. 151 



rock of the district, and is intruded along a line of fault, for in two 

 cases pegmatite dykes seen on the eastern side are broken across 

 and reappear on the western side with a well-marked northerly- 

 displacement. 



Petrography. 



The specimens collected were from the margin — evidently chilled — 

 of the transverse dyke, and have a dark-grey colour, mottled with 

 nearly black glassy phenocrysts of olivine. Here and there are 

 minute white amygdales, the infilling consisting of an isotropic 

 material that is probably glass. The weathered surface is creamy 

 grey in colour with rusty patches corresponding to the phenocrysts. 

 The average specific gravity of three fragments of the fresh rock 

 is 3-08. 



In thin section the rock is found to consist of corroded phenocrysts 

 of olivine in a fine-grained groundmass composed mainly of elongated 

 grains of augite and enstatite, the former alternating with and 

 sometimes intergrown with laths of soda-lime felspar. In places 

 interstitial patches of pale brownish-grey glass appear, and where 

 a minute amygdale is seen it is found to be composed of the same 

 obscure material (PL XI, Fig. 1). 



The olivine phenocrysts are occasionally hyp-idiomorphic in 

 outline, but generally they are deeply corroded, the resorption having 

 sometimes divided a large crystal into a number of rounded 

 fragments. Except around the edges and along cracks and cleavage 

 planes, where serpentinization has begun, the crystals are still quite 

 fresh. The serpentine is mainly of the fibrous variety, chrysolite, 

 the fibres being arranged normally to the edges or cracks from which 

 the alteration has developed. The only inclusions present are 

 sparsely scattered grains of magnetite. In one section it was noticed 

 that a shred of biotite had developed at the junction of an olivine 

 crystal with interstitial glass. 



The most abundant mineral of the groundmass, and indeed of the 

 whole rock, is a pale 3^ellow-green augite occurring in granules that 

 are generally slightly elongated along the c axis. The average 

 refractive index is 1*7 ; the optic-axial angle is low; and the specific 

 gravity is nearer 3 - 2 than 3*1 . These characters, when considered 

 in relation to the chemical composition of the rock, indicate that 

 the pyroxene approximates to the enstatite-augite variety. 1 



Among the augite granules there occur a few colourless grains 

 generally of similar average dimensions, but occasionally slightly 

 larger, having noticeably lower refractive index and double refraction, 

 and giving straight extinction. They were at first thought to be 

 wollastonite, but further optical examination showed that the optical 

 character is positive and that the average refractive index is about 

 l - 67. These properties lead to the conclusion that the mineral is 

 enstatite. It is clear from its relations to the surrounding minerals 

 that it crystallized after olivine and before augite. In PL XI, Fig. 2, 

 a good example of enstatite can be seen adjoining the dark space on 

 the right-hand side. 



1 See J. V. Elsden, Q.J.G.S., vol. lav, p. 287, 1908. 



