into
ferric, with the result that the colour is yellow instead of bluish,
and this upper weathered portion is therefore known as "yellow-ground."
At the point of junction of yellow-ground and blue-ground there is
sometimes a transitional layer of " rusty-ground," marking a lower
stage of oxidation.
On
carefully examining this mass of Diamond-bearing material, we find it
is roughly in the shape of a tube. The Kimberley mine " pipe " has the
form of an inverted coaching horn. In other words, the plan on the
surface is circular in general outline, and at successively lower
levels the cross section becomes less, though it decreases at a slower
rate as the depth becomes greater. In all the mines the surrounding
rock abuts suddenly against the mass, often with the beds turned
upwards next the pipe, separated only by slight fissures filled with
secondary minerals, if separated at all. Thus the material is filling a
huge " pipe " of unknown depth and of a surface diameter varying from
200 to 700 yards in the different pipes. There seems little doubt that
the material filling these pipes consists of the larger fragments
ejected by an ancient volcano into the air, thence to fall back again
into the throat or vent along with a certain amount of finer volcanic
dust and probably large quantities of water condensed from the steam
accompanying the eruption ; this volcanic origin of the pipes was first
suggested by Cohen in 1872. The bell-mouth of the pipe in the
Kimberley mine suggests that no very great thickness of rock overlaid
the present surface at the time the volcano was active, so in that case
almost certainly all the "foreign" rocks contained in the pipes are
from below, and in the other cases there is a probability that they are
also rocks
