gkiswold] METHOD OF CONSTRUCTING THE CONTOUR MAP. 19 
Vertical distance from Pittsburg coal to coal No. 7 b. 
Feet. 
On Cadiz and Union vale pike near Cadiz 134 
On Cadiz and Hopedale road near Greenongh 127 
1 r ertical distance from Pittsburg coal to top of Ames limestone. 
From ridge south of Opossum Hollow into Opossum Hollow 218 
From ridge north of Opossum Hollow to Skelley road 213 
On road from Bloomfield to Bloomfield station 221 
On ridge south of Amsterdam 220 
On road from East Springfield to Shane post-office, via Town Fork 217 
Near Carmen station 205 
Near Richmond 230 
METHOD OF CONSTRUCTING THE CONTOUR MAP OF THE 
OIL SAND. 
From the permanent bench marks established, spirit-level lines were 
run over nearly every road and up a great many hollows, establishing 
the true elevation of about five hundred outcrops of recognizable geo- 
logic horizons. By then adding to or subtracting from each elevation 
thus determined an amount equal to the vertical distance between the 
particular bed located and the Pittsburg coal, the elevation of the Pitts- 
burg coal at these points was obtained. These points were then plotted 
and there was thus prepared a map having marked upon it the elevation 
of the key horizon, the Pittsburg coal, at 500 points. By connecting 
the points of equal elevation, a contour map showing the folds of this 
key horizon was made, the accuracy of this map depending upon the 
number of elevations obtained and their even distribution over the 
quadrangle. 
The vertical distance from the Pittsburg coal to the Berea grit at 
each test well was obtained by comparing the elevation of the mouth 
of the well with the elevation of the Pittsburg coal at that point and 
adding this difference of elevation to, or subtracting it from, the dis- 
tance from the mouth of the well to the Berea grit as given b}^ the 
well record. This distance from the Pittsburg coal to the Berea grit 
was found to vary, but with a quite remarkable regularity. The true 
position of each test well was then plotted and the vertical distance 
between the Pittsburg coal and the Berea grit marked upon the map. 
The positions of the different test wells were connected by straight 
lines and these lines were divided so that each subdivision represented 
the horizontal distance in which the vertical distance from the Pitts- 
burg coal to the Berea grit decreased 5 feet. The points of equidis- 
tance from coal to sand were then connected and a drawing was built 
up called the convergence sheet. This shows by a series of lines the 
points of equal distance between the Pittsburg coal and the Berea grit. 
The convergence sheet was then placed over the plot showing eleva- 
tions of the key horizon, and it showed at once the amount that should 
