California Mines and Minerals. E-book. Formato PDF California Miners Association   -  Forgotten Books, 2017  - 

In the earlier history of gold mining in California, the main portion of the precious metal was procured from the placers or surface wash ings, in the gulches, canyons, and river bars and beds, and for many years an enormous yield was maintained from these sources. Gradu ally, however, as was to be expected, the area available for this kind of mining was narrowed, as ground was worked out, and then atten tion was turned to other sources of gold supply.