36 ECONOMIC GEOLOGY OF AMITY QUADRANGLE, PA. 
The following quotation from White's report is as definite a state- 
ment as can be made of the probable correlations for the rocks below 
the Catskill red beds: 
The hole stopped in a dark shale supposed to be the Marcellus, and probably not more 
than 100 feet above the horizon of the Corniferous limestone, although of course this is a 
mere inference based upon the fact that in the Conway deep well near Franklin, Pa., the 
top of the Corniferous was struck at 3,608 feet below the top of the Venango oil sand group! 
while the drill in the Bedell well stopped at 3,840 feet below the same horizon, and hence the 
Devonian shales could not extend touch deeper. The sand at 3,150 feet has been doubtfulli 
identified with the Speechley horizon, sinee it underlies the Pittsburg coal by an interval 
(3,280 feet) 200 feet greater than in Butler County. This, however, would agree with the 
general southeastward thickening, and is what would be expected. Messrs. Young and 
Crocker are responsible for the identification with the Bradford horizon of the sand struck 
at 3,500 feet. 
The Warren sand, which, according to Oliphant. lies 350 feet above the Speechley sand, 
or ."()() feet below the top of the Fourth sand, does not appear to have been represented 
by any distinct sand in this Bedell record. Its horizon belongs near the bottom of the 
200 feet of shells, the top of which was struck at 2,400 i'wi . 
GEOLOGIC STRUCTURE. 
METHOD OF MAPPING. 
Structun contours. The method of representing the structure or 
" lay " of I he beds is ns follows: The top or bottom of some persistent 
and easily recognizable stratum is selected as a datum surface, and its 
elevation above sea level determined at as many points as possible. 
In the Amity quadrangle the horizon selected is the bottom of the 
Pittsburg coal, the best known and most persistent bed in the region] 
The structure is shown on the geologic map, PL I (pocket), by means 
of red contour lines. These are drawn at uniform intervals above 
sea level, and all points on a given contour have the same elevation. 
In other words, a given structure contour is the line of intersection of 
the datum surface with a horizontal plane, all points of which have 
the same elevation above sea level. For instance, the Pittsburg coal 
at all points along the 650-foot contour has an elevation of 650 feel 
above sea level. It descends in the direction of the 600-foot contour 
and rises toward the 700-foot contour. 
The intersection of a surface contour with a structure contour of the 
same elevation marks a point on the outcrop of the Pittsburg coal. At 
points where the elevation of the surface is greater than that of the 
coal, the approximate depth of the coal below the surface can readily 
be found by subtracting the elevation of the structure contour from 
that of the surface contour. Where the elevation of the surface is less 
than the corresponding elevation of the coal, the latter has been 
removed by erosion and the contours simply show the structure. In 
case the depths of other beds than the Pittsburg are desired, their 
intervals above or below this must be subtracted or added to the depth 
of the Pittsburg coal. 
