Professor T. G. Bonney — Pavent-rock of the Diamond. 313 



act on polarised lij^ht, I regard them as empty cavities, and 

 attribute this to diffraction. 



(6) Chrome-diopside. — 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 examining jElakes, obtained by crushing, 

 I find the strong cleavage to be clinopinacoidal and the other 

 probably basal, and obtain on a clinopinacoid an extinction of 35° 

 with a prism edge. It is in fact identical with the pyroxene 

 described by Professor Lewis ^ as chrome-diopside. In it (though 

 rarely) small rounded enclosures of a greenish mineral aggregate 

 much blackened with opacite. I regard them as alteration products 

 of a ferriferous olivine. This diopside, at the exterior and along 

 cracks, is often converted into a minutely granular to fibrous 

 mineral, which gives a ' dusty ' aspect to that part of the crystal, 

 when viewed with transmitted light, and a whitish-green one with 

 reflected light. This often terminates in a minutely acicular fringe, 

 piercing the original diopside. Its grains occasionally are a little 

 larger, showing a cleavage, dull green in colour, fairly pleochroic, 

 and having the extinction of hornblende. A process of secondary 

 change, as in uralite, is no doubt indicated. Now and then a tiny 

 film of brown mica occurs in this part or even in a crack in the 

 diopside. 



It is this alteration product which gives the mottled aspect 

 mentioned above as visible to the unaided eye, so this is not 

 indicative of a third important constituent in the original rock. In 

 one of the slices the mica just named attains a larger size (about 

 '0'03 in. across), has a fairly idiomorphic (hexagonal prism) outline, 

 and is not restricted to the margin of the garnet. In this case it is 

 generally associated with calcite,^ which it tends to surround, and 

 that in one place encloses a radiating acicular mineral (? a zeolite), 

 in another the calcite, or some other carbonate, is mixed with 

 a serpentinous material. Distinct granules of iron oxide are 

 practically absent from the slices, though here and there it may be 

 indicated by some opacite. I have not found spinel, or rutile, or 

 zircon, or pseudobrookite. In fact, putting aside the diamonds, the 

 rock in its unaltered condition was a coarsely holocrystalline mixture 

 of chrome-diopside and garnet, with a few small enclosures of olivine ; 

 in other words, it was a variety of eclogite and of igneous origin.* 



' One may give a general idea of their relative imjjortance by comparing them to 

 the columns and cross-joints in some basalts. 



^ Loc. cit., p. 21. 



^ From the facts I think it probably of secondary origin. It reminds me some- 

 times of the brown mica produced by contact metamorphism. 



■* I am, of course, aware that eclogite, in the past, has been regarded by some 

 geologists as a metamorphic rock. Apart from the fact that several rocks once 

 assigned to this class are now, with good reason, regarded as igneous, I have had 



