FUEL FOR GAS TRACTORS 115 



for the present, we usually think of some petroleum product 

 when we discuss fuel for internal-combustion engines. Crude oil 

 or petroleum is now generally thought to have been formed 

 from vegetable or animal matter deposited in sedimentary 

 rocks at the time of their formation. Oil and gas seem to have 

 been produced by some sort of distillation, natural gas being 

 a secondary product formed from the vaporization of petro- 

 leum. Petroleum is usually found at depths of from 300 to 

 2000 feet, or even more, under great pressure. The most 

 profitable reservoirs or pools are found in inclined but unbroken 

 strata of sand or porous rocks, covered with an impervious cap 

 layer of shale rock or fine-grained limestone. The basins in 

 which the oil accumulates are not underground caverns, but 

 masses of coarse-grained rock. Oil, gas, and salt water are 

 found together in nearly all oil fields. In fact, along our 

 Pacific coast, and elsewhere, oil wells are sunk in the sands of 

 the beach and sometimes in the ocean itself. The gas and 

 water separate from the oil in layers according to density. 

 In consequence the same field may have wells which tap the 

 reservoirs at different points and produce natural gas, pure 

 oil, and a mixture of oil and water, according to the layer 

 tapped. 



Crude oil, as it flows or is pumped from wells, is usually 

 rather thick, of medium weight, and ranges from a light yellow 

 or green to black in colour. Some oil, practically free from 

 color, has been obtained from older geological formations. 

 Some California oils are so heavy that they cannot be piped, 

 hence are transported in V-shaped troughs. Coal as it comes 

 from the mines is practically the same, except as to size, as 

 when it gets to the consumer, though it varies in quality 

 and composition according to the source from which it comes. 

 Crude oil likewise varies a great deal in its original character, 

 but it is different from coal in that the quality of its products 

 can be further varied to an enormous degree by the method 

 of refining. 



