400 GEOLOGY OF THE COMSTOCK LODE. 



be conveniently divided into two portions. In the first of these it was 



found that 



5rz + l°.5±0°.l; 

 in the second 



Hence it appears that the variation of a alone was observed. The 

 values of B are to be regarded as an index of the eiTors incident to the 

 method in its present form, and it is moreover probable that the effect of 

 kaolinization is negligible in comparison. 



THE ELECTRICAL ACTIVITY OF ORE BODIES. 



Preliminary statement. — It is wcll kuowu that Fox, Rclch, and otlicrs made 

 experiments of great interest upon the electrical phenomena of ore bodies. 

 Bernhard von Cotta earnestly recommended that these experiments should 

 be further pursued, as they seemed to him likely to lead to results of prac- 

 tical importance in the discovery of ore bodies. If this recommendation 

 has ever been followed out, no account of the investigation has been 

 published. It was my earnest desire to see the subject pursued, and Dr. 

 Barns was invited to join the Survey on account of his special fitness 

 for this inquiry. All the plans and details of the electrical surveys made 

 are due to Dr. Barus, the general scope of the work and the localities only 

 being prescribed; and a resume of his results is given below. Neither of 

 the localities chosen was the best possible for the purpose. It was evi- 

 dently necessary in such an inquiry to begin by the examination of ore 

 bodies already exposed. At the date of the examination there was very 

 little ore in sight on the Comstock. At Eureka large bodies of ore were 

 exposed, but being in an oxidized condition would be likely to give weaker 

 currents than sulphides of similar quantity and distribution. These two 

 localities, however, were the only ones practically available, and at the same 

 time accessible through extensive workings. The results are nevertheless 

 of great interest, and a considei'able advance has been made towards a solu- 

 tion. It is one of the plans for the future to repeat these experiments under 



