316 THE DIAMOND MINES OF SOUTH AFRICA
further
run of blue ground would follow. When the blue ground was worked back
toward the centre of the crater, larger boulders or fragments of
basalt, which had come down through the loose reef from the surface,
would be met with. This sysÂtem of working would be continued until
reef alone came down, the waste or reef removed being sent to the
surface by itself and dumped on the reef tips ; it formed, however,
only an inconsiderable proportion (one to four per cent) of the total
output. It will be remembered that, when the roof caved in, the gallery
was nearly full of blue ground. By the work which followed, only a part
of this ground was removed by the men working on that level, the miners
preferring to take it out on the next level below. This process of
mining was repeated from level to level until finally there was no more
loose ground to be recovered. The cost of extracting blue ground while
loose ground existed was very low.
