EARTH RESISTIVITIES 61 
Graph No. 17 carries the resistivity profiles along with the prob- 
able section sketched below it. Curve 159, which was taken on the 
north end of the traverse, 600 feet north of the pipe, is a normal 
dolomite curve. After advancing to 157 we find the resistivity to rise 
rapidly at lower levels and then flatten. Apparently a highly resistive 
body lies at the south. 
Station 155 is directly over the pipe. Its entire resistivity determin- 
ation, and particularly that for the 40-foot depth, is somewhat higher 
than that for the dolomite. Apparently alnoite is more resistive than 
dolomite. 
Normal dolomite resistivities are resumed by moving over 160 and 
161. 
It is questionable whether the earth-resistivity method could be 
relied on to locate similar pipes or dikes in a similar dolomite. The 
increase in resistivity over this pipe is relatively small, and that 
amount of variation can be caused as well by such other factors as 
variations in soil and topography. In this case, however, there is 
hardly any doubt that the rise in resistivity is due to the igneous rock. 
If no other evidence of the existence of an intrusion had been at 
hand, the profile on Graph No. 17 would probably not have been 
interpreted as it has been. 
However, the method could be used to supplement core drilling 
or to follow buried dikes from their exposures. 
In view of the resistivity values obtained from these set-ups there 
is hardly any doubt that intrusions in shales could be detected 
readily. 
CONCLUSIONS 
The data obtained by the writer, using a modified Gish-Rooney 
earth-resistivity apparatus while working at depths less than roo 
feet, lead to the following conclusions. 
1. Damp alluvial and residual soil, shale, clay, and other argil- 
laceous materials show a relatively low resistivity. 
2. Gravelly deposits and soil, sandstone, limestone, and igneous 
rocks show a high resistivity. 
3. The same formation shows a higher resistivity where located in 
a well drained place than where saturated with water. 
4. The resistivity of a formation taken through an overlying one is 
roughly equal to an average of the resistivities of the two in the 
proportion to their thicknesses involved in the test. 
5. If both current electrodes or potential electrodes are not set in 
the same formation, or if a foreign body lies between the electrodes, 
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