42 ECONOMIC GEOLOGY OF AMITY QUADRANGLE, PA. 
RELATION OF STRUCTURE OF PITTSBURG COAL TO STRUCTURE OF OTHER 
BEDS. 
LACK OF PARALLELISM BETWEEN BEDS. 
In using the contours represented on the map, it should be remem- 
bered that few beds are exactly parallel, and hence allowance must be 
made for the increase and diminution of intervals in various direc- 
tions.. For instance, the Upper Washington limestone, one of the 
most persistent outcropping beds, varies in this quadrangle from 630 
to 710 feet above the Pittsburg coal, and the Waynesburg coal varies 
from 290 to 360 feet above the same bed. Some of the formations 
below the surface vary even more than this, as shown in the table of 
oil and gas sands (pp. 70-87). The causes of variation are twofold — 
(1) the slight increase and decrease in thickness of various beds due 
to differences in sedimentation, and (2) the marked variations in the 
Mauch (/hunk formation, owing to an unconformity at its top. The 
thickness of the Mauch Chunk decreases in general from southeast to 
northwest. 
STRUCTURE OF GANTZ SAND. 
It has been said thai the Mauch ('hunk decreases in thickness from 
nearly 200 feet at Deemston until it feathers out northwest of Wash- 
ington. It is even probable that in the northwest corner of the quad] 
rangle, the Big Injun also may be eroded somewhat. Fig. 3 (p. 53) 
shows graphically the interval between the Pittsburg coal and the 
Gantz sand, decreasing from 1,1)60 to 1,800 feet. As determinations 
of this interval at many points are not based on steel-line measure- 
ments, the linos of equal interval may be somewhat in error. As a 
name for these lines the term isochore is suggested. The word is 
derived from the Greek isos (equal) and chora (space), and means 
lines of equal space, or equal interval. That is, at all points along a 
given line the interval between the Pittsburg coal and the Gantz sand 
is the same. In order to determine the elevation of the Gantz sand at 
any point it is only necessary to find the approximate interval on 
fig. 3 and subtract it from the figures given for the Pittsburg coal 
for the same point on the general map. 
MINERAL RESOURCES. 
PETROLEUM AND NATURAL GAS. 
DISTRIBUTION OF OIL AND (IAS FIELDS IN THE AMITY QUADRANGLE. 
Fig. 2 is a map of western Pennsylvania and portions of south- 
ern New York, eastern Ohio, and northern Maryland and West 
Virginia, illustrating the distribution of oil and gas fields in the 
northern Appalachians. The oil fields are represented by the dark 
shade, the gas fields by the lighter shade. The Amity quadrangle, 
