178 T. G. BONNEY [SEPTEMBER 
covered by matrix) possibly may be in reality a twin;’ five are 
exposed within a space about three-quarters of an inch square, three 
of them apparently in linear contact. These diamonds are octahedra 
(stepped faces), with an excellent lustre, perfectly colourless and clear. 
They vary in diameter from nearly 0°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, ach 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 stréngly 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°. 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 tlus 
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 identification of the mineral. 
