THE MATRIX OF THE DIAMOND 25
resemhlance
to organic alga-like forms it might also he called ' eozoonal.' More
nearly allied, however, than any of these is a peculiar structure,
often found in meteorites, for it is a phase of the so-called
chondritic structure. As will subsequently appear, our rock has some
close resemblances, both in composition and structure, to certain
meteorites ; the structure of the zone that we are now attempting to
describe has frequently been observed in meteorites, and has given rise
to various conjectures.
In
the Kentucky rock I have noticed the same peg-matitic olivine zones
around the enstatite. The olivine grains in these zones are imbedded in
a brown, nearly inotropic, substance, which suggests a serpentinised
glass or base.
Mica.—The
mica is, next to the olivine, the most abundant mineral, and in hand
specimens it is the most conspicuous mineral. When the rock has been
exposed to the weather it appears sprinkled with glittering mica
crystals which, when weathered, form silvery spangles on the black
rock. The ' blue ground ' and the soil about the diamond diggings, both
at Kimberley and on the Vaal river, contain many fragments of mica,
which is thus regarded as the principal indication of diamonds. In the
rock which we are describing it is of a dark red-brown colour, like
biotite or phlogopite, and is apparently quite fresh and undecomposecl.
Plates half a centimetre in diameter are very common, and sometimes
pieces occur two centimetres in diameter. It is brittle when fresh.
Before the blowpipe this mica melts quietly to a dark glass—sometimes
slightly exfoliating.
Under
the microscope the mica is seen to be nearly pure and unaltered. It
occurs usually in thick isolated plates or crystals, lying like the
olivine porphyritically in the ground mass. These are frequently
polysynthetically twinned, according to the law so ably expounded by
Tscher-mak. It is indicated by the different absorption in the
alternate layers. The extinction angle between the two
