Page:Popular Science Monthly Volume 30.djvu/489

Rh conducted. A shaft is sunk in the blue to a total depth of five hundred and twelve feet below the red soil, and two sets of workings are opened up—one at this lowest level in course of preparation, while a first level eighty feet higher is now being worked out. At each level two sets of tunnels are used, one above the other, at a vertical distance of fifty feet apart. The upper tunnels give the level at which the excavation of large chambers is begun; and these are gradually worked downward, the roof being left untouched, while the excavated ground is delivered down vertical passes to the trucks in the lower or main tunnel.

The De Beer's mine is similar in formation to the Kimberley mine, but about one fifth larger in area. The reef incasing it for a depth of one hundred feet is a yellow basalt, after which succeeds a layer of black shale, extending to a total depth of two hundred and ninety feet from the red sand, where the hard igneous rock is struck. Within the mine the diamondiferous soil is "yellow ground" to a depth of one hundred feet from the surface, after which follows an unknown depth of "blue ground." As in the Kimberley mine, the richness of the ground greatly varies in the different sections. Having the advantage of the experience of the Kimberley, this mine has been steered clear of serious difficulties from reef. But some falls of reef which took place in 1883 and 1884, and a fall of nearly half a million loads of top unpayable ground, forced the claim-holders to consider some alternative to continuous working in the open mine; and it has become apparent that the future excavations must proceed underground. Five out of the seven companies holding the mine have sunk shafts within it for the purpose of reaching the "blue ground." One company has a shaft outside of the reef-margin, with a tunnel leading into the mine at a depth of one hundred and fifty feet from the surface. Another company has sunk a shaft from the outside at an angle of thirty-five degrees from the vertical to a depth of five hundred feet from the surface, from which working-galleries open at depths of five hundred and three hundred and eighty feet. This mine, like the Kimberley, is reduced in size at the cutting in of the hard rock.

The Bultfontein mine is almost circular in shape, with a diameter of about three hundred yards. It does not present equal promise to the operator with the two mines already described, and a considerable proportion of its claims are still unworked, while others have been abandoned on account of the encroachment of a mass of shale upon them. No underground workings having been begun in it, it presents to the casual observer a better idea of the nature of the operations that have been carried on within the last ten years than any of the other three mines, and for that reason it has been chosen to furnish a model of diamond-mining for the Indian and Colonial Exhibition.

The Dutoitspan mine derives its name from the "pan," or small lake, which lies between it and the Bultfontein mine. This pan is filled