the mine. It is also evident that the mines were not all filled
with the same material at one and the same time. Each mine
has
its distinctive characteristics, and even in the same mine all the
blue ground does not seem to have been forced up at one time. This is
particularly demonstrated by the striking fact that, in both De Beers
and Kimberley mines, the west side blue ground is wholly unlike the
other portions of the mines, and carries fewer diamonds, and these are
unlike the diamonds that are found in other parts. The
blue ground which filled the west ends of these mines must
have come up first, filling
the whole crater. Afterward
there was a second upheaval
which filled the eastern parts
of the craters with a richer
deposit. The reason why
the west end was not mixed
with the better blue ground
was because the west end
parts of the mines formed
benches, and were not vertically above the second
boiling mass. Mr. Rhodes
suggested this solution, and
I quite agree with him. This
peculiarity is noticeable in
the other mines.
The composition of the blue ground, which is the principal
filling of the volcanic pipes, has been carefully determined by
