Sometimes the layer of sand is nearly level, as in Fig. 2, and oil wells can be sunk over a wide space. Oil is also found in pockets; and one well will sometimes yield a large output, while others, close about it, show little or no oil.
34. The origin of petroleum is a matter of some uncertainty, but the available facts of geology seem to support the supposition that the oil is of animal rather than of mineral origin. The accepted theory on the subject may be stated about as follows: Many thousands of years ago, the ocean covered, in addition to its present bed, a large portion of what is now dry land. Animal life in those seas was very abundant, and, from time to time, earthquakes, landslides, and strong currents in the ocean buried large numbers of animals, both on the present dry land and under the sea. Oxygen is necessary for the destruction of animal matter by decay; and the air and water not being in contact with these large quantities of animal remains, another sort of trans- formation took place. In the course of time, and under high pressures produced by circumstances that cannot be explained in a treatise of this nature, the softer portions of these animal remains were changed to petroleum, the bones being wholly or partially transformed into rock.
35. In original deposits (those places where the animal remains have been changed into petroleum), skeletons of fishes and various animals have been found, with petroleum surrounding them. It is known that the petroleum was formed there, and came from nowhere else, from the fact that these skeletons, with the surrounding oil, are enclosed in solid rock, through which nothing could have passed from the outside. There is scarcely any doubt of this being the true origin of the large quantities of petroleum found in all parts of the world.
EXTRACTION AND TRANSPORTATION
36. Extraction. — The extraction of petroleum from its deposits in the earth presents various degrees of difficulty, depending, of course, on the locality. Although in some
