CHAPTEB I. 
THEORETICAL! DISCUSSION OF THE OCCURRENCE OF 
PETROLEUM AND NATURAL GAS. 
GENERAL CONDITIONS IN THE APPALACHIAN OIL FIELDS. 
The rocks from which the oil and gas of the Appalachian fields 
are derived are of sedimentary origin. They are porous rocks, prin¬ 
cipally sandstones, embedded in and underlain by a great thickness 
of shale. Below the shale are probably heavy limestone beds. The 
sandstones are numerous; they lie approximately parallel to one 
another and occupy a section in the geologic column of more than 
2,000 feet, extending from the Allegheny formation of the Penn¬ 
sylvanian series nearly to the base of the Devonian system. Gen¬ 
erally the rocks show evidence of fairly continuous sedimentation, 
but in the early stage of Pennsylvanian time the surface was raised 
above water level, and the greater part of the Mauch Chunk forma¬ 
tion was eroded before the later sediments were deposited, leaving 
an unconformity at the base of the Pottsville formation. 
The oil-bearing sandstones vary greatly in composition and tex¬ 
ture. The upper or younger sands are usually white, some being 
of uniform texture and others containing lens-shaped bodies of con¬ 
glomerate in which the separate pebbles are of considerable size. 
The older or lower beds are of brown or reddish sandstone and are 
usuallv more uniform in texture. 
In general the Appalachian oil fields occupy the bottom and west¬ 
ern side of a large spoon-shaped structural trough. The rim of this 
trough may be considered as passing through central Ohio, swinging 
eastward south of the Great Lakes, and thence southward along the 
western base of the Appalachian Mountains. The sandstones which 
show in outcrop in northern Ohio and New York are 2,000 to 3,000 
feet underground in the southwest corner of Pennsylvania and in 
West Virginia. The dip of the formations is not regular, but is the 
result of two periods of folding. The main folds have a northeast- 
southwest trend, the secondary folds crossing these at right angles. 
On the northwestern side of the main trough the secondary folds 
give rise in places to northwest dips, but these are of minor sig¬ 
nificance in the general southeast slope of this side of the trough. 
Each important sandstone bed underlies many square miles of 
territory, usually including a number of counties. They have been 
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