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



Between the grains of augite another colourless mineral, with 

 prismatic habit, occurs. The optical characters indicate that it is 

 a soda-lime felspar, approaching bytownite in composition. The 

 average refractive index is about 1 "57, and the maximum extinction 

 angle is 40° measured from the direction of elongation of the 

 microliths. As the specific gravity is estimated to be about 2 - 73, the 

 composition of the felspar is approximately Abso An 70 , i.e. that of 

 a calcic labradorite. 



The relations between augite and felspar are illustrated in PI. XI, 

 Fig. 2. A casual glance would suggest that the augite crystallized 

 before the felspar, but this appearance is due to the superior relief of 

 the augite, and a careful examination shows that the two minerals 

 crystallized together. This conclusion is corroborated by the fact 

 that in places a micrographic intergrowth can be detected. The 

 augite grains become more angular, and between them the minute 

 interstitial labradorite can be traced in optical continuity from place 

 to place. On the borders of the micrographic areas the felspar 

 frequently becomes fibrous and tends to radiate. 1 Becoming gradually 

 more obscure the fibrous felspar dies away and an interstitial clearing 

 of glass appears. The refractive index of the glass is 1*55, and its 

 specific gravity is 2*5, indicating approximately the composition of 

 syenite. 2 



The relative proportions of the minerals were measured by the 

 Kosiwal method. Under a 1 in. objective the proportions of olivine and 

 groundmass were determined, and afterwards the groundmass was 

 analysed under a |- in. objective. As it was not found possible to- 

 distinguish in every case between augite and enstatite, the two 

 pyroxenes wese estimated together. 



In order to determine the specific gravities of the various minerals, 

 a diffusion column of methylene iodide and methylene iodide diluted 

 with benzine was prepared so as to give a range from 3 - 3 to 2*3. 

 Olivine and magnetite sank. A well-marked band of pyroxene 

 formed between 3 - l and 3 - 2. Another band formed at 2 - 73 

 (labradorite), tailing out above and below owing to the difficulty of 

 obtaining a clean separation from such fine-grained material. Finally, 

 another layer formed at the level corresponding to 2*50. 



The results obtained are as follows : — 



Mineral. 



Glass 



Labradorite, Aiito 

 Pyroxene . 

 Olivine 



Magnetite . 



Total . . 100 Average . 3-20 



The actual specific gravity of the rock is 3 - 08, so that the figure 

 assumed for olivine, 3-45, is probably a little too high. As the 



1 See Iddings, Rock Minerals, 2nd ed., fig. 28, p. 215, 1911. 



2 See J. A. Douglas, Q.J.G.S., vol. lxiii, p. 153, 1907, for relations between 

 refractive indices and specific gravities of glasses. 



