EARTH RESISTIVITIES 47 
IGNEOUS ROCK 
Curve 153 of Graph No. 7 shows the resistivity of a diabase dike 
4 miles west of Fredericktown, Missouri. The instrument was then 
moved over to within 50 feet of the contact with the rhyolite por- 
phyry and curve 154 run. The short flat parts occur in this curve where 
the current and potential electrodes moved over the contact, but the 
break is not sufficiently pronounced to be used to differentiate the 
rocks in an unknown area. The curves are given merely as examples 
of igneous rock resistivities. 
Curve 161 of Graph No. 8 illustrates the data taken over bare 
rhyolite in Stouts Creek above Lake Killarney in Iron County. The 
resistivity is extremely high and variable. 
Insufficient data were accumulated on igneous rock to justify 
drawing any definite conclusions, but the indication appears to be 
that the Gish-Rooney method of electrical prospecting can be used 
more satisfactorily in the stratified sediments than in the igneous. 
RESISTIVITY AT A QUARRY FACE 
The occasion arises at times when one wishes to obtain, for the 
purpose of a standard or a known, the resistivity of a deposit which is 
exposed in a vertical face in a quarry or open mine. If a set-up is 
made at the edge of the face the figures obtained should theoretically 
not be the same as those from a set-up 150 or more feet back because 
half of the earth conductor has been removed at the quarry face. It 
would seem that the resistivity should be higher at the face because 
the cross sectional area of the conductor has been reduced. That this 
is actually true is shown by curves 53 and 54 on Graph No. g. Curve 
53 is the resistivity record of a deposit of Joachim dolomite-Plattin 
limestone 175 feet back of the face of a vertical bluff roo feet high 
half a mile north of Foley, Missouri. Set-up 54 was next made within 
a few feet of the edge of the bluff and the resistivities found to be con- 
sistently too—150 units higher than a normal set-up. 
It is logical to assume that an ore deposit in this bluff would show 
a higher resistivity than the same ore would when wholly buried. A 
correction factor derived in the same manner as the foregoing should 
be obtained and applied wherever the same conditions are met in 
practice. 
ADJOINING HIGH AND LOW RESISTIVITY FORMATIONS 
There may be a necessity to make a set-up in some regions on the 
edges of dipping beds that differ electrically, and if the line of elec- 
trodes is run across the strike it is possible that more than one forma- 
569 
