Although so enlarged, the total actually understates potentialities by being exclusive of possible discoveries beyond these limits, and also by the estimates being framed on the older ratios of gold recovery to ore tonnage, thus ignoring the application of the latest scientific methods to the treatment of the poorest ore, which would tend to enhance the results considerably. Apart from these limitations, this estimate of 2871 millions is made in the most systematic manner, from careful calculations, area by area, according to the thickness of the reefs, the tonnage per claim, and the value per ton as they have been shown from past working. The total tonnage of payable ore available is estimated at 1,378,000,000. The average gold value per ton of ore in the figures works out at 41s. 7d., but in the actuality this varies from 78s. in the richest mines down to actual loss in the least paying of low-grade ores. In these stupendous figures are included the contributions of the great deep-level mines, the growing proportions of whose contributions to the aggregate output is already becoming a noteworthy feature, while the extent of their development cannot be foreseen. Mining engineers are indeed already considering the specifications of equipments for negotiating depths deeper than 7000 feet, and even in 1899 the possibility of mining at 12,000 feet was considered. Certainly the mining of minerals in other parts of the world has shown the feasibility of operations at much greater depths than those mentioned. [Illustration: MINES ON THE LINE OF REEF AT JOHANNESBURG Photo by Wilson, Aberdeen]