312 Professor T. G. Bonney — Parent-rocli of the Diamond. 



of the cross fracture. On the latter surface, nearly an inch below 

 the last-named, three small diamonds appear to lie in a line touching 

 one another, and near them are two others,^ all four within a space 

 about three-quarters of an inch square ; an eighth diamond is about 

 an inch and a half away (on the same face) ; a ninth, about one- 

 fifth of an inch from the top edge ; and a tenth occurs on the larger 

 cross-fractured surface, but near to the edge of the other one. These 

 diamonds are octahedra in form, generally with stepped faces — one, 

 at least, apparently twinned — perfectly colourless, with brilliant 

 lustre ; the largest being quite 0'15 inch from apex to apex ; the 

 smallest not exceeding 0'05 inch. All seem to be embedded in 

 the green part of the rock. As the outer part of the boulder looks 

 rather more decomposed than the inner, I had a piece removed from 

 one end, thus enabling me to study the mass to a depth of more than 

 an inch from the surface, and examined a strip, about 4 inches 

 long, in a series of five slices. 



The late Professor Lewis has given, in the volume already 

 mentioned, so full an account of the minerals which occur in the 

 ' blue ground,' that it will be needless on the present occasion to 

 do more than refer to his descriptions,- only calling attention to 

 any variations in the mineral constituents and their association 

 in these eclogites. The constituents are : — 



1. (a) Garnet (Pyrope). — In the slice these appear a light tawny 

 or yellowish red tint, retaining this tint (though much lighter) 

 under the microscope.^ They are generally clear, with frequent 

 and irregular cracks, but are occasionally traversed by wavy bands 

 of minute enclosures of a pale brown filmy mineral, which is rather 

 irregular in outline, very feebly pleochroic, and gives with crossed 

 nicols fairly bright polarization tints. Similar minerals sometimes 

 have formed along the cracks. They are probably mica, or possibly 

 chlorite, and indicate incipient decomposition. The garnets towards 

 the outside 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 crystalline, corresponding in cleavage, 

 pleochroism, etc., 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 constituents tend to a parallel rather than 

 a radial grouping. The garnets occasionally contain minute 

 branching root-like enclosures grouped in bands. Though these 



' It is possible that these two form a twin crystal, but I think they are separate. 

 As the point is unimportant, I have not attempted to clear away the matrix. 



^ We must also not forget the paper by Professor Maskelyne and Dr. Flight 

 (Quart. Journ. Geol. Soc, vol. xxx, p. 406), in which several of these minerals are 

 described, analyzed, and identified. In fact, the authors ascertained everything that 

 "was possible with the materials then obtainable. 



3 Unless it is expressly stated, the use of a 1 in. objective may be assumed. 



