Page:South African Geology - Schwarz - 1912.djvu/58

 crust, which is spherical in shape, then the wave sutlers no change; but directly the chord or straight path enters a portion of the earth deeper than 30 ml, there is an immediate change. From this evidence we judge, therefore, that the rocks we find at the surface of the earth continue downwards for 30 ml., but that, beneath, there is a centrosphere or nucleus of different material, probably meteoric iron and various iron-magnesium silicates, such as we find in meteorites. The evidence from radium is similar. If the radium content of the rocks is taken as 5 parts in a million million, then the heat of the earth's crust will be accounted for by such rocks forming an envelope 45 ml. thick, lying on a nucleus, cold and con- taining no radium. As we saw that the Rhodesia granite contained over 9 parts of radium in a million million parts of rock substance, the thickness of radium-con- taining rocks may well be under 45 ml. thick. We may take it as a fair estimate that the crust of the earth is 30 ml. thick. The deepest bore hole is only a little over 6000 ft. thick, and the deepest mine, Morro Velho gold mine, in Brazil, 4920 ft., so that bore holes and mines are quite insignificant in depth as compared with the whole crust. Even the crust is only a very thin layer overlying the centrosphere, for the distance of the centre of the earth from the surface is nearly 4000 ml.

The greatest elevation on the crust is Mount Everest, 29,000 ft. The greatest depression in the earth's crust is off the Ladrone or Marianna Islands, in the Pacific, 5269 fathoms, or 31,632 ft.; the Kermadec deep, near the islands of that name, north-east of New Zealand, is 31,000 ft. (5156 fathoms); so that the crust has irregularities in the surface of a little less than 6 ml. above the