. Cassier's magazine . region like this just de-scribed the chances of success infinding petroleum are far better thanin any other way in which oil accu-mulates. In the United States the oil fieldswhere the petroleum occurs in rockswhich have been folded into wavesare: (a) The Appalachian field, ex-tending from Western New York ina southwest direction along the west-ern side of the Alleghany Mountainsthrough Pennsylvania, Eastern Ohio,West Virginia, into Kentucky andTennessee. The rocks containing oilin this field are principally sand-stones. (b) The Lima-Indiana field in-cludes the northweste
. Cassier's magazine . region like this just de-scribed the chances of success infinding petroleum are far better thanin any other way in which oil accu-mulates. In the United States the oil fieldswhere the petroleum occurs in rockswhich have been folded into wavesare: (a) The Appalachian field, ex-tending from Western New York ina southwest direction along the west-ern side of the Alleghany Mountainsthrough Pennsylvania, Eastern Ohio,West Virginia, into Kentucky andTennessee. The rocks containing oilin this field are principally sand-stones. (b) The Lima-Indiana field in-cludes the northwestern part of Ohioand a strip in the middle of petroleum occurs mainly indolomitic limestone. (c) The oil field of SoutheasternIllinois is a third instance of thestructure under consideration. Theoil rocks are generally sandstones orsandy shales. (d) In the California petroleumfields one of the modes of accumu-lation is in the above-mentioned rock-wave structure. But the California PETROLEUM ACCUMULATIONS 601. FIG. 2. SECTION SHOWING OIL IN THE MINOR FOLDS OF A LARGE ROCK WAVE AT WELLS A AND B Well A missed the gas and oil, striking water; well B obtained gas and oil. Near well C the gas andoil have accumulated in the porous sandstone, which is surrounded by shaly sandstone having less porespace. Well C missed the oil, which it would not have done had the sandstone been uniformly porous, asin Fig 1. fields have numerous other ways inwhich oil has lodged, and which areabsolutely hidden from man save bydrilling, and then the chances of find-ing the oil are unfavourable. (e) Other important examples ofthis manner of oil deposit are foundin Colorado, Alaska and the Wyom-ing oil fields. PETROLEUM IN ROCKS OF LENS-SHAPED STRUCTURE Oil is obtained in regions wherethe rocks are not folded. It has ac-cumulated in disconnected lenses orbeds of sandstone surrounded by shale (see Fig. 3). This form of oildeposit makes the striking of petro-leum extremely hazardous. Wellsdrilled a
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