THEORETICAL DISCUSSION. 
21 
tions. In later periods when this surface was again submerged it 
received other sediments, generally in equal amounts, and the upper 
formations were laid down across it in approximately parallel layers, 
but not necessarily parallel to the earlier-deposited formations. The 
relation between the upper and the lower formations, under these con¬ 
ditions, depends on the uniformity of the erosion and the absence of 
any folding prior to the time of second submergence. This relation 
is called an unconformity. In areas where it exists it is necessary 
to have positive data with reference to the distance of the deep-lying 
beds from the surface strata at many points in order to be able to 
determine accurately the true structure of the lower formations. 
SUMMARY. 
From the foregoing considerations it becomes evident that certain 
general geologic conditions should be known before undertaking the 
determination of the geologic structure of the oil-bearing sands from 
strata upon the surface, the first and most important being the possi¬ 
bility of an unconformity between the surface strata and the oil¬ 
bearing stratum. 
from the nature of the.se deposits the coal and limestone beds 
seem to be the most reliable surface strata from which to determine 
the geologic structure. 
