20 BEREA GRIT OIL SAND IN CADIZ QUADRANGLE, OHIO. [bull. 198. 
be subtracted from the elevation of the Pittsburg coal to determine 
the elevation of the Berea sand at any point. The elevation of the 
Berea sand at every point where it was determined was then marked 
on the map and the points of equal elevation were again connected, 
resulting in a contour map of the oil-bearing sand (PL I). a The map 
was drawn in 10-foot contours, as this seems the most desirable inter- 
val for the use of the oil operator, though it is hardly to be expected 
that the drill will always find this degree of accuracy in the work. 
In making such a map in the manner above outlined it is evident 
that there are several sources of error which can not be entirely pro- 
vided against. Thus, a bend or change of dip may occur at a point 
at which there is no outcrop of any one of the guide horizons. 
The difference of elevation having been determined at two points, the 
slope of the oil sand between them is represented as uniform, while, 
in fact, it may be horizontal for a part of the distance and have a steep 
dip for the remainder. The records of test wells, although carefully 
taken at the time of the drilling, can sometimes be procured only from 
the memory of the driller, and the only check upon their correctness 
is from the resulting appearance of the divergence sheet. Again, a 
sudden change in the rate of divergence between the key horizon and 
the oil sand may occur where no test well has been drilled, in which 
case the divergence sheet will show an erroneous uniformit} T . All of 
these conditions probably do happen, making the map more generalized 
than the contour interval would indicate. 
The true elevation of the top of the sand, although very desirable 
to know, is not absolutely necessary to the usefulness of the map. If 
there is an error of 20 to 30 feet in the elevation of the sand — the 
probable maximum error— there will be the same or nearly the same 
error in the elevation of adjacent portions of the bed, and the relative 
dip or slope between the two places will be approximately as repre- 
sented upon the map. Since this is the realty important feature of 
the map, the errors in absolute elevation, due to the causes above 
enumerated, are less serious than they might at first sight appear. 
RELATION OF PRODUCTIVE TERRITORY TO STRUCTURE 
IX THE CADIZ QUADRANGLE. 
Description of the structure.— The accompanying contour map of 
the Berea oil sand shows a system of parallel folds, forming elongated 
domes and canoe-shaped basins, with an indication of cross folding 
about at right angles to the principal system. The most prominent 
feature is the main anticlinal arch, which extends from near the south- 
west corner of the quadrangle in a northeasterly direction, passing 
just east of the town of Salem, where it attains its greatest height. 
"As the key horizon map and the convergence sheet represent merely steps in the process of mak- 
ing the map of the oil sand, it is not deemed necessary to publish them. 
