Chapter XII: The Mineral Depths

The chapter on "The Origin and Development of Modern Geology," vol. III., p. 116 _seq._, may be read in connection with the allied subjects here treated.

In preparing the section on the use of electricity in mining, the article by Thomas Commerford Martin, entitled _Electricity in Mining_, in the United States _Census Report_ of 1905, has been freely drawn upon. The quotations on pp. 262, 266, 268, and 270 are from that source.