MINERAL OIL

287

Drake struck oil at Titusville beside Oil Creek in Pennsylvania.
 It was the first of the 680,000 wells that have been
drilled for oil in the United States up to 1926, and it led to
the opening of the great field which extends west of the
Appalachian Mountains through Pennsylvania, Ohio, Indiana,
and Illinois. The rocks range from the Ordovician to the
Carboniferous. Ordovician rocks are raised to the surface
in the W. by the broad Cincinatti anticline, and the Lima-Indiana
 field obtains oil from depths down to 1000 feet from
the Trenton Limestone which underlies the Hudson River
shales. The Silurian beds outcrop further E. and the central
Ohio field is fed from the Clinton Sandstone and Niagara
Limestone. These porous beds are capped by the Devonian
Ohio Shale. Further E,, in the geological centre of the field,
the synclinal of West Virginia and Pennsylvania consists
of Carboniferous rocks; pools of oil occur along many
secondary folds, especially the anticlines; but some were
along synclines while the adjacent anticlines were barren
(e.g. the Whiteley and Waynesburg synclines, Stone, U.S.G.S.,
Bull. 225, 1904, pp. 409-10, and the Hinton Syncline, Kentucky,
 #bid., 688, 1919, p. 60).
The mid-Continental field, in Oklahoma, Kansas, and Texas,
includes rocks ranging from the Carboniferous to the Cretaceous.
 The oil appears to have originated from the Carboniferous
 and some has accumulated in Permian sandstones.
The oil is sometimes found in domes; but most comes from
homoclinal beds; some of the pools are in almost horizontal
beds, and occur in patches of porous rock surrounded by shale
(Taff and Shaler, U.S.G.S., Bull. 260, 19035, pp. 441-5). The
Oklahoma field has been the greatest producing field in the
United States until for a time surpassed by California.
Northern Texas is the continuation of the Oklahoma field.
In Southern Texas the oil is found associated with salt domes,
of which 62 were known by 1922; others have since been
found by earth-waves due to explosions and by the torsion
balance. The distribution of the oil in these domes is apparently
 capricious, and a high proportion of the wells has
pro ed barren. The formation of salt domes is considered,
page 213. They have probably been impregnated with oil
from the shales through which the salt block has arisen.
The most famous of these salt domes is Spindletop from which,