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24 THE MATRIX OF THE DIAMOND
double
refraction, positive character, parallel extinction, and with traces of
vertical and basal cleavage. The axis of greatest elasticity is at
right angles to the best cleavage (b= ). The high colours in polarised
light are identical with those of olivine, and serpentine sometimes
replaces it. All these characters point to olivine as the mineral
forming the greater part of the zone. These olivine prisms are all
rounded, and radiate irregularly from the central bronzite. They are
imbedded in a nearly amorphous glass-like brown serpentinous substance,
and are accompanied by small quantities of calcite, and of a mineral
with a high index of refraction, but very low double refraction, which
occurs in rectangular prisms, has traces of a cleavage in partings
parallel to the base, and is negative. This microlith, however, was
noticed only in a few cases. Olivine, it seems, is the main constituent
of these zones. The grains are generally so minute and crowded together
that they form a grey highly refracting fibrous border or fringe around
the enstatite, like leucoxene around titanic iron. Farther away from
the enstatite the grains become larger and more worm-like, so that they
can be separately studied. The structure of these zones resembles that
figured by Becke,1 as occurring around garnets in a
garnet-olivine rock from Karlstiitten, Lower Austria, and called by him
' centric' structure. He has figured a similar structure in the
omphacites of certain eclogites,2 and speaks of it as ' darmzottenahnlich.'3
It
has often been called a pegmatitic or micro-pegma-titic structure,
since, in many instances, a number of the grains have the same
orientation, extinguishing simultaneously, the two substances being
contemporaneously crystallised. This is also the case, to a limited
extent, in the zones under consideration. Perhaps the term
centro-pegmatitic would more nearly express it. From its
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