EARTH RESISTIVITIES fs 
sees no other explanation for the lowering of the resistivity than the 
effect of the low resistivity alluvium. The rock was the same, the 
stratigraphic horizon the same, the current lines parallel, and the 
separation of instrument locations less than the distance of maximum 
current path spread so that there must have been overlap of current 
paths of the two set-ups; the only difference between the two locations 
was the 3 feet of alluvium at 76. Other determinations, some of which 
will be shown later, substantiate this view. The writer believes that 
the effect due to the thickness and nature of the mantle rock is sig- 
nificant and important, and can not be ignored in careful work, 
especially at shallow depths. 
In marked contrast to 76 is 43, taken over the same formation andl 
at about the same stratigraphic horizon, but where the limestone is 
exposed in approximately a 75-foot vertical river bluff. The soil was 
thin and the hill well drained on all sides. The resistivity has reached 
a maximum of 475 field units. 
SANDSTONE 
The St. Peter sandstone was studied under similar conditions to 
those of the Kimmswick limestone. Curve 47 of Graph No. 6 indicates 
the resistivity of the St. Peter in a creek bed where the electrodes 
were laid directly upon the sandstone in shallow water. Curve 46 
indicates the resistivity of the same rock about 25 feet north of 47, 
where 4-5 feet of alluvium covers the sandstone. Electrodes were 
parallel in the two set-ups and conditions made as near identical as 
possible with the exception of the alluvium. The lowering of the 
resistivity in this case is similar to that with limestone excepting that 
it is more pronounced here. In fact, curve 46 could as well be taken for 
shale. In three other instances the writer found low resistivity values 
for sandstone, due once to clay impurities and for the other two pre- 
sumably to a mantle of fine wet soil. 
The same formation (St. Peter) at approximately the same strati- 
graphic horizon was run on a high bluff covered with a thin layer of 
sandy soil. The resistivity was high, as shown in curve 58. The writer 
again attributes the increase to the drainage of the sandstone, that is, 
lowering of the water table. 
A 50-foot thickness of La Motte sandstone (Cambrian) resting on 
porphyry was run and its results shown in curve 152. It was located 
on the side of a moderately sloping stream valley mantled with 
about 4 feet of soil. 
The location of 46 and 47 is on Big Sandy Creek, one mile west of 
Burr Oak, Lincoln County; 58 is half a mile west of Bethany Church, 
near Foley, Missouri; 152 is 5 miles west of Fredericktown, Missouri. 
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