Page:South African Geology - Schwarz - 1912.djvu/20

 of 1° F. for every 30 ft. to 1° F. for 254 ft. In the British Isles there is an increment of 1° F. for every 55 ft. at London, 1° F. for 34 ft. near the Scottish border, and 1° F. for 92 ft. in the south of Ireland, with every gradation between the two latter in different localities. In one shaft alone, at the Rose Bridge Colliery, at Wigan, between the levels 1674-2445 ft., the rate of increase varies from 1° F. for 24 ft. to 1° F. for 110 ft. In South Africa data are very scarce; in the Karroo the rate of increase is approximately 1° F. for every 100 ft., and on the Witwatersrand 1° F. for 254 ft. On the Nebular Hypothesis all these variations would be due to the varying nature of the rock, which in some cases allowed the heat from the interior to leak out quickly, in others obstructed the outflow of heat; an average, therefore, of all the variations would give approximately the average leak and from this average temperature increment the depth at which any temperature could be obtained could be calculated. Thus at 30 ml. a temperature of something like 2500 F. would be obtained, taking the increment to be 1° F. for 60 ft.; at this temperature steel melts and most rocks would be fused. Again, with this average increment of temperature it is possible to calculate the time when the earth first cooled sufficiently for a crust to form on the surface, the consistentior status of Lord Kelvin, because the temperature increment gives an estimate of the rate at which the heat of the interior is being dissipated. The over-crusting of the liquid globe has been calculated to have happened from 60 to 100 million years ago.