450 Professor T. G. Bonney — Rocks from Kimherley. 



seldom exceed ^' in diameter, the largest, a flake of brown 

 mica, being barely ^". A few of them represent the compact 

 darkish rock with a lighter border, already mentioned ; the rest 

 are minerals, most of these being olivine, more or less serpentinized ; 

 two or three are brown mica, one or two grains are garnet and 

 iron-oxide — all as usual. The s.g. of the rock is 2-695. 



The other fragment (16) is rudely wedge-shaped, the most 

 regular side being about 4" square. The ground-mass is as above, 

 but the purplish tinge is more variable. Enclosures are fairly 

 numerous, but seldom exceed ^" in diameter, the largest (olivine, 

 or possibly a peridotite) being about ^". Olivine, more or less 

 serpentinized, is very abundant in grains markedly variable in size. 

 Other minerals are inconspicuous. There are a few rock fragments, 

 most of them representing the usual compact dai'kish kind, but the 

 largest one resembles a fine-grained eclogite. The s.g. of the rock 

 is 2-658. 



Slices for microscopic examination have been prepared from the 

 specimen from the 1320-feet level and the two from 1400-feet. The 

 first shows a number of fragments, both of mineral and of rock, 

 scattei'ed in a ground-mass ; they are more or less rounded, 

 especially the former, which are the more numerous, though here 

 and there the sharp edge of a fracture can be seen. In size they 

 vary from about 46" downwards, though larger are present. The 

 minerals are : olivine, enstatite, augite, brown mica, and perofskite. 

 The first and second are partially serpentinized, often only as an 

 external zone, but occasionally to a considerable extent, and a small 

 grain may be entirely changed. ^ In size these olivine grains often 

 vary from about '0125" to •006", but smaller than the latter may 

 be found, and it is difficult to say where the fragments stop. There 

 are only a few grains of the brownish mica, with the usual external 

 zone. Grains of perofskite are numerous, varying in colour and 

 size ; some certainly are of secondary origin, and it is possible that 

 all may be.- Small black grains also occur, sometimes associated 

 with these, probably magnetite or ilmenite. The rock fragments, 

 which are very interesting, I leave for the moment. The ground-mass 

 seems to consist of about three 7ninerals, besides grains of opacite 

 and ferrite — (1) a rather flaky mineral, the tints of which, with 

 crossed nicols, do not rise above a dull lead-white ; (2) numerous, 

 generally well-defined, flakes, in length up to about -005", a very 

 pale-brown tint to colourless, with bright polarization colours — a 

 mica^; (3) an irregularly outlined clear granular mineral giving 

 bright tints, probably dolomite. 



The specimen from the 1400-feet level labelled "East Blue" 



^ For the structures of the serpe;itine aud the olivine, see Lewis, ut supra, p. 14. 



^ In our former paper we separated the dark-coloured varieties as possibly pseudo- 

 hrookite, because we thought sundry slight differences, not only dependent on size, 

 were perceptible. But study of these slices and of Professor Lewis's description 

 makes me doubt whether the latter specimens are more than impure perofskite, so I 

 follow him in placing all under the one name. 



3 A true white mica may be present among them, but in most cases a faint 

 yellowish tint with a trace of pleochroism is perceptible. I think this mineral must 

 be authigenous. 



