INSIDE OF THE EARTH 



53 



sometimes with sufficient force to form a natural fountain. 

 This is because the well-boring has tapped water which is 

 under pressure, due to its intake being at a greater altitude 

 than the mouth of the well (see Figs. 12 and 13). Or, in 

 some cases, the pressure is due to the confinement of the 

 liquid between two deep-placed layers which it cannot 



FIG. 2i. Oil-wells. 



penetrate. This is the explanation of gushing oil-wells 

 (see Fig. 21). When formations of oil and natural gas are 

 tapped, the gas naturally comes to the surface first. Gas 

 and oil are found in the earth both together and separately. 

 Both are believed to have an organic origin, that is, they 

 have resulted from extensive chemical and physical changes 

 in accumulations of organic substances. By organic sub- 

 stances we mean those derived from the bodies of living 

 things (plants and animals); all other substances are 

 inorganic. 



