EARTH RESISTIVITIES 49 
tion is included in the circuit. In other instances the width of outcrop 
of a formation may be so small that the current circuit and even the 
_ potential circuit extends beyond its limits and on other formations. 
One would not expect the measured resistivity to be that character- 
istic of one formation alone. 
An actual test case of this type was run by making a depth deter- 
mination from a set-up at the contact of two electrically different 
formations, the Grassy Creek shale and the Chouteau-Burlington 











































































































































































































» 
DWN 
SX > 
~ 

limestones, shown in section below Graph No. ro. The results of the 
measurement are shown in curve 69, which occupies a position es- 
sentially midway between what would be expected for limestone, lire 
AA, and for shale, line BB. In other words, the resistivity at the con- 
tact of two formations should be an average of their resistivities in 
proportion to the volume of each functioning as a conductor. 
It is interesting to note that the increase in resistivity is not in 
proportion to the increase in depth at this particular station, but that 
it flattens out. The explanation is that as the depth of current penetra- 
tion increases, the volume of shale included in the expanding circuit 
increases relatively at the cost of volume of limestone because of the 
571 
