Page:Popular Science Monthly Volume 30.djvu/486

466 a large vertical shaft through the reef at the northeast corner some two hundred yards back from the limit of the claim-ground. The shaft, however, was stopped as soon as the hard rock was reached at two hundred and eighty-six feet below the surface. Tramways were laid to take the place of the carts that had been used, and the capacity of all the machinery for removing material was greatly increased. Early in 1878 one quarter of the claims were covered by reef, which was hauled out at an expense of four shillings per load of sixteen cubic feet. In 1879-'80 the board expended £300,000 on the removal of reef, dibris, and water; in 1881, £200,000; and in 1882, more than £500,000, while claims were still covered, and other slips were impending. Although at the beginning of 1883 three shafts, one of them four hundred and fifty and another six hundred and fifty feet deep, had been sunk to assist the process, the removal of reef did not keep pace with the constantly recurring slips; and the board having spent £650,000 in eighteen months, and laboring under a deficit of £250,000, could not get its bills discounted. Consequently a "Black Friday" came to the mines, and the value of their shares fell by fifty, seventy-five, and ninety per cent; and holders of shares on which calls were liable to be made "would have been only too glad to have paid men to take them over, so as to free themselves from the liability which they were incurring."

The discovery of the hard rock, says Mr. Reunert in his report, practically confined the reef difficulty within manageable limits, as there is little doubt that this rock will stand without disintegrating, even when exposed to a great depth. But the experience of successive reef-slips has considerably increased the estimate of the shale to be removed to render the mine safe for working on the open or quarry principle. It was at first supposed that the reef would stand if cut back to an angle of 60° receding from the mine; then 45° was spoken of as the angle of repose; but it has now been found that an angle of 30° will be needful. The cost of removing this obstruction has been vastly increased by the failure to apply comprehensive and systematic plans in the beginning; but this failure is apologized for by saying that no one knew what the future of the mine would be, or was ready to venture at once upon so large a permanent outlay as would be required.

The area of the Kimberley mine, originally inclosed within the reef, was about eleven acres. Successive slips and removal of reef have widened the area till it is now twenty-five or thirty acres. The inclosing rocks which form the walls of the diamond-bearing "pipe" converge inward from the surface downward, so that the area of claim-ground is constantly reduced as the mine deepens; but in some of the lowest sinkings, which have gone to more than six hundred feet below the surface, the rock has been found to recede from the mine, and thus to permit the regaining of a certain area of rich ground.