60 W. D. KELLER 
shows the cross section and resistivity profile along a traverse crossing 
the fault there. The resistivities of the dolomite and the limestone are 
so nearly alike that no direct indication of the fault is given by this 
method. There was no evidence of clay gouge in the fault plane or 
variation in resistivity due to mineralization. The resistivity of the 
rocks is of little direct value in this case. 
However, if the observer keeps his geologic section in mind, he 
may be able to approximate the position of the fault or at least be 
aided in his search by the location of the Maquoketa shale. He may 
be aided indirectly, although given no direct evidence. 
This study serves to indicate that earth resistivity methods of 
prospecting are no panaceas for all geologic ills but simply additional 
tools useful in prying loose the sometimes tightly held secrets of 
Mother Earth, and that to gain most information from the electrical 
data of the rocks their geological relations must ever be kept in view. 
A’ consideration of the three resistivity profiles across the fault 
plane leads to the following conclusions. 
1. The mere presence of a fault plane does not necessarily cause a 
break in the resistivity profile. The resistivity at the plane is not 
necessarily a convenient and opportune variation from those of its 
flanks, regardless of what their values may be. 
2. However, the location of the position of a fault plane is to be 
expected more usually as a result of fundamental differences in the 
resistivity of the two adjoining formations. 
ALNOITE PIPE IN DOLOMITE 
The writer’s study of the resistivity of igneous rocks was too 
brief to be conclusive, as has been said before, but one set of curves, 
taken where an alnoite pipe is intruded into dolomite, is believed to 
be significant enough to be included because of the possible applica- 
tion to the location of intrusives into sediments. The pipe described 
by Singewald and Milton? is exposed for about 120 feet in length and 
15 feet in width. It occurs in a small ravine eroded in a wide, gently 
sloping hillside which affords an excellent surface for earth resistivity 
tests. 
The traverse was run from the dolomite on one side, across the 
pipe, and onto the dolomite on the other side. The line of electrodes 
was run in a north-south line which was fairly level and in a direction 
transverse to the long axis of the outcrop. 
® Joseph T. Singewald, Jr., and Charles Milton, ‘An Alnoite Pipe, Its Contact 
ena and Ore Deposition Near Avon, Missouri,” Jour. Geol., Vol. 38 (1930), 
582 
