178 T. G. BONNE Y [September 



covered by matrix) possibly may be in reality a twin ; 1 five are 

 exposed within a space abont three-quarters of an inch square, three 

 of them appareutly in linear contact. These diamonds are octahedra 

 (stepped faces), with an excellent lustre, perfectly colourless and clear. 

 They vary in diameter from nearly - 15 inch to 0'05 inch, and all 

 apparently are embedded in the green part of the rock. In the 

 second specimen only one diamond is visible, and this has been 

 exposed by a slight flaking away from the outer surface. It is in all 

 respects similar to those just mentioned. Each of these boulders, on 

 microscopic examination, is found to be holocrystalline and to consist 

 almost entirely of pyrope and a chrome-diopside. In a thin slice the 

 former mineral is a light tawny red colour, is generally clear, but is 

 much and irregularly cracked, and is occasionally traversed by wavy 

 bands of minute enclosures, one set being branching and root-like, 

 probably cavities, the other filmy, apparently a variety of brown mica, 

 and indicative of incipient decomposition. The " skin " enveloping 

 many of the garnets, especially towards the exterior of the boulder, is 

 mainly composed of a mica of the biotite group, which in the latter 

 case appears to be associated with a chlorite (by passage) and perhaps 

 with a little fibrous hornblende. It is, in fact, a variety of the 

 kelyphite rim, to which attention has often been called, but the 

 radial structure is less marked than usual (so far as my experience 

 goes), the mica flakes showing a tendency to parallelism. The 

 chrome-diopside is the mineral described under that name by Professor 

 Lewis ; by others as omphacite or sahlite. In these slices it is a pale, 

 dullish green colour, inclining to olive. The crystals are sometimes 

 partially converted (at the exterior and along cracks) into a mineral, 

 generally in minute matted fibres, but occasionally in grains large 

 enough to show cleavage ; these give the extinction of hornblende, and 

 are no doubt the result of secondary change. The unaltered pyroxene 

 shows one strongly marked cleavage (not so close as is usual in 

 diallage), and a second less developed, sometimes almost at right angles 

 to it. The former, as already noticed by Professor Lewis, is 

 parallel to the clinopinacoid, and by measuring some flakes I obtained 

 extinction angles up to quite 35°. 2 This diopside occasionally 

 encloses a small rounded spot, consisting apparently of a serpentinous 

 mineral, much blackened by opacite. I presume that a very few 

 small grains of a ferriferous olivine were originally present, being 

 among the first minerals to separate from the magma. In one of my 

 slices the brown mica attains a larger size (about 0'03 inch in 

 diameter) than at the margin of a garnet (from which it is dissociated), 

 and exhibits a fairly idiomorphic outline (hexagonal prism). In this 



1 The point, of course, could easily be settled, but as it is unimportant I have preferred 

 to leave things as they were. 



2 Professor Lewis obtained an angle of 39°. My measurements were rough, intended 

 only for identilication of the mineral. 



