MINERAL OIL 
287 
Drake struck oil at Titusville beside Oil Creek in Pennsyl- 
vania. 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, Ken- 
tucky, #bid., 688, 1919, p. 60). 
The mid-Continental field, in Oklahoma, Kansas, and Texas, 
includes rocks ranging from the Carboniferous to the Creta- 
ceous. The oil appears to have originated from the Carboni- 
ferous 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 ap- 
parently 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,