85 



crysts, whereas in others an indistinct cleavage is shown 

 parallel to the (010). The refractive index and double re- 

 fraction are characteristic of olivine. The traces of the 

 (010), (Oil), and (100) are seen in different crystals. In 

 convergent polarized light, a section perpendicular to the 

 optic axis gives a slightly curved brush. The dispersion 

 shows p < V. The (010) face in some phenocrysts is largely 

 developed. Spheroidal cracking has gone on to a marked 

 degree. The principal inclusions are glass and magnetite 

 grains, and in one or two instances inclusions of liquid and 

 gas occur. 



The augite phenocrysts show rather well-defined outlines. 

 Sections parallel to the (100) and (010) are the most com- 

 mon, and occasionally sections showing the two cleavages in- 

 tersecting at about 90°. Frequently, sections parallel to the 

 (100) show the trace of the (Til) face. Twinning has taken 

 place parallel to the (100), and in a few instances the effect 

 on the outline of the crystal is seen in sections parallel to 

 the (010). In addition to this, a few interpenetration twins 

 are noticeable in sections parallel to the (100), and more 

 rarely a cruciform twin is to be seen, in which case twinning 

 has taken place on the (101). A prismatic cleavage is fairly 

 well developed in sections parallel to the (100), (010), and 

 (110), the plane of the cleavage being parallel to the (HO) 

 and (110). Only a few sections show the (110) and (HO) 

 cleavages intersecting at 90°. Besides the isolated pheno- 

 crysts it is not uncommon to find that the augite crystals 

 have arranged themselves into rosettes, some of which in- 

 terpenetrate. These rosettes are about half the size of the 

 olivine phenocrysts, and usually have a fair to imperfect 

 cleavage developed, which is probably the trace of the (110) 

 or (110). Such rosettes are usually found to be sections 

 parallel to the (100). The augite phenocrysts, unlike the 

 olivine phenocrysts, have a pale-brownish-yellow colour, and 

 although the colour is so feeble a pleochroic scheme was 

 with difficulty made out. 



R = Very light-brownish-yellow. 



b=Very light-brownish-green. 



C = Very light-greenish-yellow. 



From this it is evident that the absorption is very slight 



Tlie principal inclusion in the augite is magnetite, and 

 occasionally a few crystals with rather a low double refrac- 

 tion, which is probably apatite. No apparent decomposition 

 has taken place in the augite. 



