"If the rocks are dry, then the chief points of accumulation of the oil will be at or near the bottom of the syncline (downfold), or lowest portion of the porous bed. If the rocks are partially saturated with water, then the oil accumulates at the upper level of saturation. In a tilted bed, which is locally porous, and not so throughout, the oil, gas, and water may arrange themselves according to their gravity in this porous part." (Ries.) Although the term "oil pool" is commonly used, there is really no actual pool or underground lake of oil, but rather porous rock saturated with oil. It has been estimated that in an oil field of average productiveness a cubic foot of the porous rock contains from six to twelve pints of oil. The life of a well drilled into an "oil pool" varies from a few months to twenty or thirty years, or sometimes even more, but a heavy producer (especially a "gusher") almost invariably falls off very notably in production in a few months, or at most a few years. The typical Pennsylvanian oil well is said to last about seven years. The fact that the United States is still able to increase oil output is because new fields are found and developed, the most recent being in the interior and northern parts of Texas. It is practically certain, however, that the climax of oil production in the United States will be reached before many years--long before that of bituminous coal. It is a well-known fact that oil, as well as natural gas, is usually under more or less pressure within the earth. The pressure is so great in some cases that where, in the course of drilling, oil or gas accumulated under proper conditions, as for example those shown by Figure 79, are encountered the pressure may be hundreds of pounds per square inch, or enough to blow to pieces much of the drilling outfit. It is under such conditions that great "gushers" are struck. A wonderful case in point was the famous Lakeview gusher, struck in California in 1910. "Within a few days the well was far beyond control. It continued to flow (for a time shooting high into the air) for eighteen months, finally stopping after it had produced over 8,000,000 barrels of oil, about 6,000,000 of which had been saved. The daily production of the well varied greatly, reaching a maximum of 65,000 barrels." (Pack.) One very common cause of oil pressure is the expansive force of the associated imprisoned gas which steadily increases as the gas is generated. Another cause which is seemingly applicable in many cases is hydrostatic pressure, where under certain structural conditions the pressure of water in a long-tilted layer is exerted against oil accumulated toward the top of an anticline (or upbend) in the strata. The world's output of petroleum for 1917 was nearly 559,000,000 barrels, of which the United States produced nearly 338,000,000 barrels, Mexico 87,000,000 barrels, and Russia 34,000,000 barrels of 42 gallons.