METHOD OF INVESTIGATION. 
23 
latter method was used to eliminate any error which might result 
from the failure to make allowance for the dip of the rocks. 
Level lines were run to the mouths of a large number of oil and gas 
wells in the area, so that the elevations of the oil sands and other 
beds of rock whose positions are recorded in the logs of the wells 
could be determined, and from these elevations the distances and 
degrees of parallelism that existed between the outcropping and sub¬ 
surface rocks could be established. 
CONSTRUCTION OF MAPS. 
The work of making a map of a particular stratum lying at a 
considerable depth below the surface consists of three distinct steps— 
first, careful contour mapping of some prominent surface bed, called 
the “key horizon; ’ second, the more difficult task of ascertaining 
the distance between this key horizon and the producing oil sand 
below and the amount and direction of the variation in this dis¬ 
tance; third, the application of a correction to the surface mapping 
equal to this convergence, so that lines drawn on this map connecting 
points of equal elevation above the sea (contour lines) will show 
the true shape of the surface of the oil sand. 
STRUCTURAL MAP OF TIIE KEY HORIZON. 
On the completion of the field work, as previously described, 
the geologist had a topographic map of the area, on which the 
horizontal location and the elevation of the outcrops of different 
marking strata are shown at hundreds of points. By a comparison 
of these outcrops, the intervals between different marking beds were 
obtained. One bed was selected as the key horizon, usually that out- 
cropping over the greatest area. 'By adding to or subtracting from 
the elevation of*outcrops of other known beds the distance they have 
been found to be below or above the key horizon, the elevation of 
that stratum was obtained at a great many points. By drawing 
lines connecting the points of equal elevation, a contour map of 
the key horizon was produced. 
CONVERGENCE MAP. 
A knowledge of the variation in distance between the key horizon 
and the oil sand was gained from the records of wells in different 
parts of the area, and without these records it would be impossible 
to make any illustration that would show the form and position 
of the sand, unless it were exactly parallel with the key horizon. 
To make use of the well records and construct an actual map of 
the oil-bearing sand the following method was employed: On the 
