THE MATRIX OF THE DIAMOND 49
The
extreme basicity of the rock, the abundance of the calcite as a
decomposition product, the high magnesia and low alkali, the presence
of biotite, and more especially of perovskite, point to the possible
presence of nepheline or melilite in the ground-mass. All these
characters occur in melilite-basalt.1 Traces of a mineral
giving rectangular sections occur in the ground-mass, which possibly
are to be referred to melilite or nepheline, but its altered state
prevents this suggestion from being confirmed with the present
material. In melilite-basalt, as in the Kimberley rock, the pyroxene is
chrome-bearing, the perovskites surround the olivine ' einsprenglinge,'
garnets may occur, and the augite may be replaced 2 by biotite.3
The
composition of the whole rock, characterised by a low amount of silica
and a high amount of magnesia, together with much lime and little
alkalies, may be compared with that of certain garnetiferous olivine
serpentines.' Among unaltered rocks the only ones that show similar
relations between the silica and magnesia are dunite,5 olivinfels,6
