ELECTRICAL ACTIVITY OF ORE BODIES. 



341 



Table XII. 



Discussion of the results obtained on the 400 and 500-foot levels. FrOni a COmparisOIl of tllC 



resistances of circuits between different holes, as contained in Tables VII. 

 and IX., we find that in cases of fissured, of tough and impervious, or of 

 dry rock or earth, this quantity inclines toward a maximum ; whereas, on 

 the other hand, wherever the material is porous or moist minimal values 

 are obtained. It is to be remembered that under ground, from the exceed- 

 ingly damp atmosphere, as well as from infiltration of water, the rock form- 

 ing the walls of the drifts is throughout very moist, and at the surfaces of the 

 latter, at least, nearly saturated. Hence it follows that the conductivity of 

 the rock is largely, if not wholly, due to the presence of moisture in its 

 pores, and is therefore electrolytic. This important fact will be repeatedly 

 referred to hereafter. 



Intensities. — lu Tablcs YIII. aud X. the intensities of the currents ob- 

 served in the different circuits have been very fully given, both because the 

 present measurements are the first of the kind made, and because the char- 

 acter of these data furnishes an important criterion of the validity of the 

 results subsequently derived from them. From an inspection of the tables, it 

 is moreover obvious that an exchange of terminals in measurements of this 

 kind, however tedious and laborious in case of long circuits, is indispen- 

 sable. The intensities i' and i'", which are measured with the bags in the 



