288 ECONOMIC GEOLOGY 
after nine years unsuccessful drilling, the Lucas * gusher ” 
in January, 1901, discharged 700,000 barrels of oil before it 
could be controlled ; it yielded 17 million barrels in 1902 ; 
it is also historically important as it provided the clue to 
the Mexican oilfields. 
The Rocky Mountain oilfields extend from Montana through 
Wyoming into Colorado. The rocks range from the Cambrian 
to the Cretaceous or occasionally Eocene, and have been 
bent into broad folds. Oil is sometimes found in an anti. 
cline, as in the Labarge field in Wyoming, although it 
contains no water and the oil is therefore not forced upward 
by water-pressure (cf. Schultz, U.S.G.S., Bull. 340, 1908, 
P. 369). In some of the fields, as in the Cretaceous of Mon- 
tana, the oil is found on the margins of synclinals, The 
Colorado fields are also in the main synclinal. The Florence 
oilfield (cf. Fig. 64, b), which by 1892 had yielded 95 per cent. 
of the Colorado oil, was discovered in the search for water. 
It is in a great elliptical syncline of Jurassic and Cretaceous 
rocks, the centre of which must be 8000 feet deep. A little 
oil comes from the Jurassic, but the bulk is from the Cre- 
taceous Pierre clays. The deepest well is 3650 feet, and the 
oil does not appear to occur lower as the slope below is too 
gentle to maintain the flow of the oil ; it collects at the foot 
of the steeper part of the syncline, in sandstone lenticles, 
which are separate as wells even 25 feet apart are fed in- 
dependently. The oil is obtained by pumping and wells 
paid with the yield of about 7 to8barrelsa day. The Range- 
ley field, also in Colorado, occurs in Jurassic or Cretaceous 
beds in an anticline and the wells occur in pockets in the 
Mancos Shale, Cretaceous (H. S. Gale, U.S.G.S., Bull. 350, 
1908, pp. 44-6). 
California has made a sensational but anticipated addition 
to the American oil supply, the output being raised to 263 
million barrels in 1923. Wells have been sunk to the depths 
of over 8300 feet. The Kainozoic rocks range from Eocene 
to Pleistocene, and are 34,000 feet in thickness, of which 
20,000 feet are Miocene ; their oil is derived from the distilla- 
tion of the soft tissues of foraminifera, diatoms, etc. Some 
of the oil comes from the underlying Cretaceous beds. The 
structures of the Californian fields differ greatly. The central 
field near Coalinga yields oil from lenticles of sandstones