226 



Prof. T. G-. Bonney. 



Two small diamonds are exposed on the curved outer surface, one 

 about half the other about one-fifth of an inch from the edge 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 015 

 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. These 

 constituents are : — 



1. (ft) 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 en- 

 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 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, &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 



* 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. 



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

 (*' Quart. Journ. Geol. Soc.,' vol. 30, p. 406), in which several of these minerals are 

 described, analysed, and identified. In fact the authors ascertained everything 

 that was possib]e -with the materials then obtainable. 



X Unless it is expressly stated, the use of a 1-inch objective may be assumed. 



