242 THE OCEAN 



borings show that the temperature of the 

 superficial crust increases with depth, the 

 increase ranging from one degree Fahren- 

 heit for every 100 feet to one degree for 

 every 300 feet. Should these rates of in- 

 crease continue for six or seven miles of 

 depth, a very high temperature would soon 

 be reached. Indeed, all observations go 

 to show that there is a very high temperature 

 in the interior of the earth. The increase of 

 temperature is, of course, accompanied by an 

 increase of pressure due to the overlying 

 rocks ; at a depth of 13,000 feet it has been 

 estimated at 1000 tons to the square foot. 

 At such pressures the strongest rocks are 

 strained beyond their limit of elasticity. In 

 the Mont Cenis tunnel a bed of soft granite 

 was met with that continued to swell with 

 irresistible force for some months — suffi- 

 cient to crush an arched lining of two-feet 

 granite blocks. The origin of the earth's 

 high internal heat has been attributed to the 

 residue of the original heat of the nebula 

 from which the globe shaped itself, and also 

 to the effects of the gradual gravitational 

 compression of the earth's mass, and its con- 

 densation during growth. Although radium 

 is not likely to be present in the deep-seated 

 interior of the earth, possibly because of 

 the enormous pressure there, still it has 



