334 GEOLOGY OF THE COMSTOCK LODE. 



west country intersects the 400-foot level on a line approximately parallel 

 to the drift between P. C. and No. IV. 



Unfortunately, local circumstances rendered it absolutely impossible 

 to make this survey in a single continuous series, however desirable such a 

 method of procedure would have been. But the object was accomplished indi- 

 rectly by selecting a permanent contact both on the 400 and on the 500-foot 

 levels, and carrying the two lines of measurement onward to the same inter- 

 mediate point. The differences of potential thus obtained from two fixed 

 points, respectively, can then be converted by a simple method of reduction 

 into those which would have been obtained had all the electromotive forces 

 been measured from one and the same P. C. 



On the 500-foot level the permanent contact was placed in chamber 

 No. 12, in calcareous earth stained with iron, its position coinciding nearly 

 with the letter C in the plan of this chamber (Fig. 27, C. D.). The points 

 selected as T. C.^s are designated on the map by small circles, to which 

 Roman numerals are annexed, and extend from I., near the shaft m on the 500- 

 foot level, in a more or less broken line to XV., in chamber No. 15, about 

 30 feet below the 400-foot level. The following table will describe them 

 more completely. Column 2 in Table VII. contains the points, some of which, 

 to prevent confusion, were omitted on the map; column 3, the depth of each 

 below the mouth of the shaft, taken as zero. "Distance" refers to the length 

 of the lines joining consecutive points for which data are given.^ The 

 figures under "bearing" are to be similarly understood. (S. 81° W. refers 

 tothelinel.-IIL; S. 26° W.,toIII.-V.; N.67° W.,toV.-IX.,etc.) Itappeared 

 unnecessary to give more than the bearings of the main lines of direction 

 on which the points approximately lie. The figures included under "re- 

 sistance" are the means of two determinations of this quantity made for 

 each of tlie points. They express the sum of the resistances of the rock 

 surrounding P. C. and the T. C. specified. The original results were always 

 greater than those made at a subsequent time; this from the fact that the 

 rock in the neighborhood of P. C. and T. C. became, during the progress of 

 the experiments, gradually more saturated with moisture. 



' The points lor which no data are given are distributed through various parts of chambers 12 and 

 l!), in positions for which it was difficult to make measurements. 



