EARTH RESISTIVITIES 57 
in the confirmation of that fact. The writer does, however, consider 
that the location of the plane between the sandstone and limestone 
because of the low resistivity of the fault zone is worth mentioning. 
Another item of importance in practical prospecting, that having 
to do with the determination of the optimum spacing of electrodes 
for rapid traversing, can be derived, tentatively at least, from this 
graph. For instance, when a party is doing commercial traversing or 
reconnoitering, the element of time and expense enters in. It would 
usually be too costly to make detailed determinations at each station. 
Instead, only one depth reading or perhaps two which are most 
diagnostic would be made. 
A glance at the profiles of the different depths, although of the 
same traverse, shows that they are not the same. Their variation, 
however, is not such that one contradicts the other, as might be 
argued, but instead, if the geology involved is studied, their inter- 
pretation is entirely logical. 
‘Inspection of the graph shows that a 20-foot depth determination 
is hardly satisfactory because of the relatively great influence of local 
variations in the soil or mantle rock. At the other extreme, a 100-foot 
potential electrode spacing takes in 300 feet between current elec- 
trodes, which is so large that it usually includes more rocks in the cir- 
cuit than is desirable. The 80-foot depth, as shown on the graph, is 
not much better than that of 100 feet. The 60- and 4o-foot depths 
appear to be almost equal in their diagnostic values, perhaps the 60- 
foot one being a bit more pronounced. On the other hand, if one is 
prospecting for small ore deposits, the 40-foot spacing would be pre- 
ferred because its current circuit takes in 60 feet less of rock that might 
alter the effect of the ore, than does that of the 60-foot spacing. The 
choice of the two should really be determined by the use to which the 
prospecting is to be put. 
The section shown on Graph No. 14 is located on the C. T. Cannon 
farm, 3.5 miles southwest of Foley, Missouri. 
SANDSTONE FAULTED AGAINST SHALE 
A fault where sandstone is brought against shale is sharply in- 
dicated by earth-resistivity methods. The curve showing the extremes 
of high resistivity of sandstone and adjacent low resistivity of shale 
has an abrupt steep slope between the two that permits ready location 
of the contact of the two formations. 
The resistivity curves on Graph No. 15 demonstrate nicely such 
a fault condition found near McClain’s Creek Crossing, 5 miles west 
579 
