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OBSTACLES AND PERILS
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25
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in
the mines," and in view of this danger and loss of life, the inspector
urgently recommended the limitation by the government of the height
and width of the underground workings.
" Main tunnels to be used only for traffic not to exceed 8 feet in width and 8 feet in height.
"
Working chambers or stalls from which the blue or dia-mantiferous
ground is excavated in bulk, not to exceed 18 feet in width by 20 feet
in height to the highest point.
" Partitions or pillars not to be of less thickness than half the width of the contiguous chambers or stalls.
"
The roof of ceiling between one level and the next above to be not less
than 20 feet in thickness at the highest point of the lower workings."
This
recommendation had in view obviously the precautions enforced in the
working of coal mines, and would doubtless have afforded an increased
measure of protection, but the method of working proposed was not well
suited to the development of the diamond-bearing ground, as was later
conclusively determined. The slaking and crumbling of the
diamond-bearing breccia upon exposure to air and moisture make roof
falls and slips from the sides especially frequent and disastrous. The
ground is full of soapy seams, and pieces of considerable size drop
without a moment's warning, so that it is necessary, in places, to keep
the tunnels timbered as near the working face as possible. Risk from
this cause cannot be wholly obviated in such mining, but the
introduction of the new system adopted for the working of the mines,
shortly after they came under my management, has greatly diminished
this peril, and the resultant loss of life or injury to the workmen. By
the new system the levels are worked back from the surrounding hard
rock or reef in sections, formerly 30 feet, now 40 feet apart, as
before particularly described, in a series of terraces, extracting the
ground from the uppermost level downward in succession. This method did
away with any danger of collapse in the underground works, and by
successively robbing out the roof and sides of the tunnels on each
descending terrace, the caving of
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