PETROLEUM AND NATURAL GAS. 63 
and gas, but as explaining their occurrence to a considerable extent. 
The distribution of the pools is dependent also on the presence or 
absence of water in a given region or a given sand ; on the continuity 
and shape of the anticlines, whether they are ascending or plunging 
in a given direction; on the porosity of the oil rock, its capacity to 
hold oil, the directions of its variation, etc. 
APPLICATION OF ANTICLINAL THEORY IN PENNSYLVANIA. 
Thus far the Pennsylvania folios which have been published 
show a certain degree of correspondence of distribution of these 
deposits to geologic structure. The gas fields occur generally on 
the anticlines; the oil fields part way down the slope if water is 
present, in the bottom of the synclines if water is absent. To take 
the Pennsylvania and northern West Virginia fields as a, group, the 
evidence at present seems to warrant the following generalizations 
regarding structural distribution: 
(1) When not affected by other conditions, accumulations of oil 
and gas show a definite relation to the structure of the region. 
(2) The greatest elongation of the pools is generally in a direction 
approximately parallel with the axes of the folds. 
(3) Where both oil and gas occur they are distributed according 
to their densities, the oil in the lower and the gas in the higher por- 
tion of a stratum. 
(4) When salt water is present oil may occur in that part of the 
stratum lying directly "above the water level. 
(5) Oil may occur on the crests of anticlinal folds below water 
level. 
(6) When salt water is absent the occurrence of oil is more irregu- 
lar and more affected by other conditions; it may occur along the 
synclinal axes or at many points scattered over the slope. 
(7) Oil may occur on a structural slope at points where the dip 
changes from gentle to steep. 
(8) Gas occurs most commonly on the higher portions of the 
anticlinal arches, above the upper level of the oil. 
(9) Gas also occurs in widely scattered localities, owing to small 
local folds or changes in porosity. 
Structure is not the only condition determining the occurrence of 
gas and oil. The structure may be favorable, yet neither oil nor 
gas occur. The chief condition other than those given above is the 
existence of rock of such character as to act as a reservoir. 
RELATION OF OIL AND GAS RESERVOIRS IN THE AMITY QUADRANGLE TO GEOLOGIC 
STRUCTURE. 
Washington field. — A study of the geologic map of the Amity 
quadrangle with reference to the distribution of oil and gas sho\\b 
a Geologic Atlas U. S., folios 82, 92, 94, 102, 110, 115, 121, 123, 125, 134. 
