284 SKETCHES OF CREATION. 



miles. We can form but little conception of the circum- 

 stances under which such an enormous volume of gas can 

 be confined at the depth of six hundred feet beneath the 

 surface. 



The escape of oil at the surface of a well is caused some- 

 times by mere hydrostatic pressure, as water rises in com- 

 mon Artesian wells. More frequently, perhaps, the oil is 

 forced up by the elastic reaction of confined gases. An 

 open cavity, or a porous portion of rock bounded on all 

 sides by impervious walls — which constitutes a virtual cav- 

 ity — may be partly filled with oil, while gases occupy the 

 higher portions of the cavity. Such a cavity, whether 

 actual or virtual, may possess any form or extent — or may 

 consist of a number of cavities connected by narrow pas- 

 sages or mere fissures. In nearly all cases, more or less gas 

 accompanies the oil, and subsists under a very high degree 

 of pressure. The pressure in such cases is not the hydro- 

 static pressure of water, but a consequence of the continued 

 generation of gas and oil long after the cavity had been 

 filled. If a boring happens to penetrate the higher portion 

 of such a cavity (Fig. 89), the gas at once rushes forth with 

 greater or less violence and persistence. As soon, however, 

 as the tension is relieved, the escape ceases. No oil will be 

 obtained in such a case without applying suction, since 

 there is no hydrostatic pressure exerted from behind, and 

 the reaction of the gas tends rather to confine the oil in the 

 lowest ramifications of the cavity. 



Suppose, however, on boring a hole for oil, we happen to 

 penetrate some of the lower portions of the cavity occupied 

 by the oil (Fig. 89, b). The elastic pressure of the confined 

 gas above will at once force the oil up, and produce a spout- 

 ing or blowing well. The flow must necessarily subside 

 by degrees as the confined gas, by the escape of the oil, 

 acquires more space for its accommodation. It may con- 



