498 Professor T. G. Bonnet/ — Rocks from Kimherley. 



grains of quartz, with the usual somewhat irregular outline. Certain 

 of these, under crossed nicols, exhibit a slightly radial structure. 

 This might be the result of strains in an exceptional direction, but 

 it seems more likely to have been assumed during the formation of 

 the grain. The outer part of the fragment, about one-eightieth of 

 an inch wide, exhibits a chalcedonic structure, which seems to cut 

 across the contiguous grains of quartz, and continues successively 

 the dominant tint of each of these. I think this chalcedonic zone 

 most probably the result of an external alteration of the original 

 fragment and a phenomenon of contact metamorphism. There are 

 a few enclosures and cavities in the quartz grains themselves, but 

 these generally do not call for remark, except that one or two grains 

 are crowded with very minute rounded spots of a light-brown 

 material which appears to be anisotropic. I think these fragments 

 come from the quartzite or quartz-grit, which is occasionally inter- 

 bedded with the dark shales, and that they have been caught up by 

 the " melaphyre." The latter jDerhaps is rather a diabase than 

 a basalt, and is almost certainly a flow. 



A specimen labelled " Basalt," without number or depth, looks 

 as if it might be part of a rudely spheroidal mass ; it appears to 

 be a very fine-grained dolerite, and under the circumstances I have 

 thought a microscopic examination needless. 



"Igneous dyke No. 8, 1,200-feet level" : is an irregular fragment 

 of a dark-coloured, very fine-grained dolerite, its s.g. being 2'989. 

 Microscopic examination shows that it consists mainly of four 

 minerals : (a) FelsjDar, ill preserved, replaced by microliths, probably 

 once plagioclase. (6) Augite, fairly idiomorphic, with characteristic 

 cleavage, slightly brown in colour ; showing at the exterior incipient 

 replacement by a filmy green hornblende, (c) Prisms of a dull- 

 greenish mineral enclosing flattish granules of opacite. This 

 mineral (clearly secondary) occurs in parallel fibres or needles with 

 slightly oblique extinction, and so probably is hornblende. The 

 original mineral possibly might have been an enstatite, but I think 

 some unstable form of augite is more probable, {d) Grains and 

 occasional rods of iron-oxide sometimes passing externally into 

 limonite. We also find small flakes of biotite ; clear acicular 

 microliths (? actinolite) piercing the felspars, and two or three 

 grains, similarly pierced, of a water-clear mineral, like quartz, but 

 probably a secondary felspar. Hence the rock is a rather altered 

 dolerite, approaching a diabase. 



" South of dyke No. 10, 1,200-feet level." This is a rather irregu- 

 larly-shaped fragment, partly bounded by rude joint faces, a little 

 more than six inches in length, of a dark, slightly speckled basaltic 

 rock with a few irregular grains of pyrite. The slice exhibits the 

 following minerals : (a) plagioclastic felspar, more or less replaced 

 by secondary products ; (5) brownish augite of dirty aspect, due to 

 decomposition ; (c) rather elongated prisms of a pale-green serpen- 

 tinous mineral retaining indications of a platy cleavage, probably an 

 altered enstatite; [d) iron-oxide in crystalline grains, sometimes 

 also in rods or plates ; (e) pyrite, occasionally forming an aggregate 



