Parent-Rock of the Diamond in South Africa. 101 



closures of a pale brown filmy mineral, which is rather 

 irregular in outline, very feebly pleochroic, and gives 

 with crossed nicols fairly bright polarisation tints. 

 Similar minerals sometimes have formed along the cracks. 

 They are probably mica, or possibly chlorite, and indicate 

 incipient decomposition. The garnets towards the out- 

 side of the boulder, as already said, are enveloped in a 

 " skin," and the microscope shows that it usually exists 

 inside, though there it is thinner. In the former case it 

 is generally browner in colour and more distinctly crystal- 

 line, corresponding in cleavage, pleochroism, &c., with a 

 mica of the biotite group ; in the latter it is greener and 

 more filmy with an aggregate habit, and seems to project 

 into the garnet. I regard it as due to decomposition, a 

 form of the well-known kelyphite rim, sometimes a mica, 

 sometimes a chlorite, possibly now and then associated 

 with a little minute hornblende. In a few cases a " rim " 

 is brown in the outer part and green within. The con- 

 stituents tend to a parallel rather than a radial grouping. 

 The garnets occasionally contain minute branching root- 

 like enclosures grouped in bands. Though these act on 

 polarised light, I regard them empty cavities, and attri- 

 bute this to difi'raction. 



(b) Chrome-diojjsicle. — The mineral described under that 

 name by Professor Lewis, and referred to by others as 

 omphacite or sahlite. The individuals are sometimes 

 about a quarter of an inch long. In thin slices it is a 

 pale dullish green colour, inclining to olive ; under the 

 microscope, a pale sea-green, with a trace of pleochroism. 

 It has one strongly marked cleavage, not, however, nearly 

 so close as in ordinary diallage, and a second weaker, 

 sometimes approximately at right angles to it.^ On ex- 

 amining flakes, obtained by crushing, I find the strong 

 cleavage to be clinopinacoidal and the other probably basal, 



i One umy give a general idea of their relative importance by comparing them to 

 the columns and cross-joints in some basalts. 



