172 OO! H~’Gisi 
this the data from a single test hole, the structure of the region covered 
by the survey should be determinable in many cases. 
Interpretation of some aspects of geoelectric surveys has often 
been made with the aid of empirical rules, for which no satisfactory 
theoretical support has yet been found. A few remarks on one of 
these, for which the writer bears some responsibility, may be of inter- 
est. In that form of development of the resistivity method where four 
electrodes are set in line at equal intervals, it was at the outset tenta- 
tively considered that the distance between adjacent electrodes was 
a rough measure of the depth of earth involved in the measurement. 
This rule seemed to do better service than was expected. Such checks 
as it was possible to make strengthened confidence in it, and even to 
the present time some geophysicists with extensive experience in 
geoelectric work continue to use it even though they are aware that 
the theoretical considerations of recent years give no general support 
to such a rule. The observational data now available to most workers 
are probably inadequate to determine whether there is a conflict 
here between fact and theory. The theory seems sound, except for 
the assumption that earth materials are isotropic as regards the 
property of resistivity. There seems to be considerable evidence that 
this assumption is not justified, but that the resistance to electric 
flow in a horizontal direction is different from that in a vertical direc- 
tion. A more general theory, embracing eolotropic materials, is, be- 
cause of its difficulty, not likely to be developed unless overwhelming 
evidence is adduced to show that the simpler theory is inadequate. 
Hence at present it would seem best to use the theory now developed 
as a guide in the interpretation of resistivity surveys. 
One who is confronted with the task of selecting an electric method 
for use in the search for oil will find his chief difficulty in making a 
choice between electromagnetic and resistivity methods. If he has 
had experience with one of these, he will probably do well to select 
that method, for in ‘‘the present state of the art’’ success depends to 
a considerable extent upon familiarity with the method and ap- 
paratus, and this is not gained in a day. The remarks which follow 
may help to elucidate this and, at the same time, call attention to 
some systematic errors which have not always been avoided in re- 
sistivity measurements. 
In 1922 the writer gave consideration to various possibilities of 
obtaining a measure of the resistivity of large masses of undisturbed 
earth. Such data were desired in connection with investigations of the 
502 
