EARTH RESISTIVITIES 43 
Shale of still higher resistivity is shown by curve 42. However, it 
does not differ in composition from 65, but is the same Maquoketa 
measured on a high dry creek bank where little soil mantled the shale 
outcrop. The writer shows this as an excellent illustration of the effect 
of the position of a set-up with reference to the drainage and water 
table on the resistivity of the rocks. It seems unquestionably true 
that the higher resistivity of 42-65 is due to an unsaturated condition 
prevailing in the vertical high creek bank. As a precaution the meas- 
urement of 42 was taken sufficiently far back from the face of the 
bank to obviate any effects of a limited conductor, as would be the 
case if the set-up had been made on the brink. In that position half 
of the normal current conductor space would have been air instead of 
rock and a high resistivity would have resulted. 
The measurement of shale under ordinary topographic conditions 
is represented by 65 and that curve is taken as normal. The other 
should be kept in mind because it may occur under some conditions 
of prospecting. A similar effect was also shown by limestone. 
FIRE CLAY 
Thirty feet of Pennsylvanian fire clay (Cherokee formation) lying 
below 15 feet of reworked glacial sand and clay is represented by 
curve 53 D on Graph No. 4. Its electrical properties are so similar to 
those of shale that the two rocks can not be differentiated by resistiv- 
ity methods. Unfortunately also, from an economic standpoint, the 
overburden on this deposit had a resistivity similar to the clay, and 
the resistivity method was of no value in prospecting here. 
The resistivity of diaspore clay is shown in the flat part of curve 
V. H. 6. It is normally slightly higher than that for shale, but the 
difference is hardly great enough to enable one to differentiate 
between the two on this basis alone, The geology of the region and 
effects of the sandstone rim rock must be taken into consideration in 
the electrical prospecting for diaspore. 
The steep parts of curves V. H. 6 and 11 represent resistivities to 
depths of about 12 to 13 feet, the transition from overburden to solid 
rock. Both locations, V. H. 6 for diaspore, and 11, a hematite prospect, 
were drilled and found to check electrical predictions exactly as far 
as overburden and solid rock relationship were concerned. However, 
the flat part of 11 went into only slightly ocherous, sandy shale and 
not into hematite which the lessees desired. 
The high surface resistivities are due to the nature of the over- 
burden, a cherty gravelly soil that drains readily, especially in the 
hills of Osage County, where these locations were run. Similarly, 
glacial gravels that are fairly clean from clay show high resistivities. 
565 
