4 ON A DIAMOND-BEARING PERIDOTITE AND
Prof.
T. Eupert Jones and Mr. Thos. Davies had identified a number of the
minerals occurring associated with the diamond. Mr. Dunn has described
the occurrence so well that I cannot do better than quote some extracts
from his paper.1
'
The conditions under which diamonds occur in South Africa are quite
different from those of ever}" other known locality, and are so unusual
as to deserve the earnest attention of all geologists.
' At the junction, and back for a distance of from one to several feet, the edges of the shale are bent sharply upwards. The
contents of these "pipes'' in the shale are the same in all cases, and
show distinctly that they are of igneous origin. The base is more or
less decomposed gabbro (?) or euphotide (?), through which are
scattered particles, fragments, and huge masses of shale, nodules of
dolerite, occasional fragments of chloritic schist, micaceous schist
and gneiss. The principal foreign ingredient is the shale, which in many places, particularly at Colesberg Kopje, is thoroughly comminuted, forming a breccia with
euphotide (?) as a base. Where large masses of shale occur, the lines
of the bedding, as might be expected, are not horizontal, but lie in
all directions.
'
For a depth of from 30 to 40 feet, cracks, joints, and irregular
cavities filled with red sand from the surface penetrate; with the
sand, and showing that it has come from the surface, are fragments of
ostrich egg-shell, small rounded grains of chalcedony, agate, &c, identical with the same substances mixed with the surface soil.
'
At 130 feet, the greatest depth so far attained, the rock becomes
compact, tough, and shows the original texture, though the ingredients
are altered, notably the pyroxene or augite into bronzite.2
' The entangled blocks of shale and sandstone arc frequently altered, the latter sometimes into quartz rock.
1 Quart. Journ. Geol. Soc, xxx. 1874, p. 54.
2
Probably diallage is here meant; the name bronzite, which formerly was
used somewhat vaguely, being now restricted to a rhombic pyroxene.— T.
G. B.
