558 



EXPLORATION GEOPHYSICS 



by a description of a survey conducted in Trinity County, California. 

 The property consisted of 10,000 acres of gravels which, on the surface 

 and in occasional test pits, panned uniformly in gold. The problem v^as 

 first to select, by a geological study and by preliminary geophysical work, 



Fig. 343. — Map of placer area in Trinity County, California, showing surface 

 geolog^y and topography. (^Engineering and Mining Journal, Feb. 1934.) 



what part of the total acreage should first be studied; then, by detailed 

 geological and geophysical work on that restricted acreage, to outline 

 the gravels, give their depth, the contour of bedrock and attitude of 

 bedrock surface, and if possible, locate the likely areas for gold concentra- 

 tion * As a result of the survey, recommendations for further develop- 

 ment and exploration were to be made. 



After careful geological reconnaissance of the whole property, between 

 500 and 600 acres were selected for detailed work. The lateral extent 

 of the gravels, the areal geology, and the topography of this area were 



*The likely areas for gold concentration would be indicated by "black sand" 

 concentrations. 



