Natural gas, in conjunction with hydraulic pressure, is the cause of what is known to oil operators as a “gusher” or flowing well. It is the compression and volatility of the gas imprisoned for ages in the rock that sends the oil spouting into the air and has been known to create a flow of 170,000 barrels in a single day. As a general practice, and probably due to the weight of overlying strata, the pressure of gas encountered in drilling into oil formations is proportional to the depth. This pressure is generally known as rock pressure and the flow of the wells is in part due to it. A principal factor in the production of oil or gas is the nature of the formations from which the production is derived--their thickness and porosity. In some cases, notably in Mexico, the flow seems to be caused by the action of water. Here the formations are very porous, opposing little obstacle to the flow of the oil and gas through the formation. The production from the wells under these conditions is very great and, unlike most wells, a gradual decline in the yield is unusual, there being little sign of exhaustion until the moment when the well begins producing salt water in increasing proportions. After the appearance of the salt water the production of oil diminishes rapidly and for practical purposes soon ceases, due to the small production of oil and the fact that it comes out as an emulsion with the water, which is very difficult to utilize. A characteristic of the Mexican wells is that the oil, and finally the salt water which follows it, are generally produced at a high temperature--from 115 to 145 degrees. Such gushers originally produced another fallacious belief that oil exists in subterranean pools or reservoirs; but investigation has shown that oil has been preserved in the rocks in a way somewhat similar to that in which water is retained in a sponge. A typical piece of oil rock examined under the microscope reveals millions of tiny interstices between different grains of sand. Porous, oil-bearing sandstone may contain one-tenth or one-eighth of its bulk in petroleum. The term “oil sands” is common in the oil industry and refers to the type of coarse grained porous rock which forms the best reservoir for petroleum; but limestone and some of the rocks described by geologists as conglomerates sometimes serve the same purpose. In every instance the oil-bearing stratum has been covered by a layer of non-porous rock, whose impervious qualities keep the oil and gas imprisoned until penetrated by the drill. Surface deposits are also a well-known phenomenon; and were the only type of deposits known to the world until modern times. About them has grown up much interesting history and legend which will be dealt with in a subsequent chapter.