52 W. D. KELLER 
if it is steep and mark the contact of the two formations at that point. 
Since the slope and the apparent magnitude of break in the curve 
will vary with the relative scale in plotting values on the axes, the 
depth predictions might vary slightly if either scale is exaggerated. 
The writer finds that the highly conductive (low resistivity) beds 
apparently exert a greater effect on the current and show a thickness 
greater than is actually the case. Hence, it is usually more nearly cor- 
rect to favor slightly the size of the beds of high resistivity at the ex- 
pense of the low. An exception to this rule of thumb is in the case of 
overburden of high resistivity, in which case its thickness is usually 
slightly less than indicated electrically. 
Generalizing, where bed A underlies bed B its resistivity is neither 
that of normal A or B at A’s depth below the surface but is roughly 
equal to the average resistivity of the two taken in proportion to the 
thickness of the two involved.. 
Stations 44 and 45 were located half a mile west of Dameron, 
Missouri. 
LOW OVER HIGH RESISTIVITY 
The curves on Graph 12 indicate the resistivities of increasing 
thicknesses of shale over limestone. The series was made by setting 
up near the base and at points consecutively higher on the side of a 
gently sloping, large hill of Maquoketa shale underlain with Kimms- 
wick limestone. Electrode lines were parallel throughout, roughly fol- 
lowing contours. 
Curve 77, made with the electrodes on bare limestone in water, 
is repeated for reference. The resistivity rises with increasing depth 
of current to about 30 feet in depth, from which depth it remains 
fairly constant at 1oo-110 units. Curve 76, also repeated, is from 
measurements of the limestone under 3 feet of alluvium. Notice that. 
it almost parallels the mean of 77 but is about 40 resistivity units 
lower, due, no doubt, to the alluvium. 
Where 10 feet of shale covers the limestone its (the shale’s) higher 
conductivity lowers the whole curve as shown in 49. The minimum 
point on the curve checks the 1o-foot thickness nicely. 
The curve of 20 feet of shale over limestone is shown by 79. Note 
that the higher conductivity of the shale has lowered the resistivity 
of the entire remaining thickness of limestone. The agreement be- 
tween the actual thickness and that indicated electrically is excellent. 
A 35-foot thickness of shale over 65 feet of limestone lowered the 
resistivity of the whole section to that of ordinary shale, as shown in 
curve 78. The curve indicates limestone at a depth of 30 feet, which is 
574 
