EARTHQ TJAKE-PHENOMENA. 



517 



quakes to be the dynamic result of action of the earth's heated in- 

 terior upon its cooled exterior. Whether the central portions of the 

 earth be tiuid or not, it is quite certain that heat increases as we de- 

 scend ; and it is estimated by Sir Charles Lyell that the heat at a depth 

 of 25 miles would be sufficient to melt granite, and at 34 miles to render 

 fluid or incandescent every known substance. We have no means of 

 knowing the condition of matter under the enormous pressure which 

 prevails at a depth of 34 miles, and are most concerned with the fact 

 that the heat of fusion exists at no very great depth beneath the sur- 

 face. The earth's crust is, therefore, its cooled exterior. 



Fig. 3. 



Ciboular Hollows in the Plain of Rosarno. 



It is found that nearly all rocks contract by cooling and expand 

 by heat. Lyell estimates that sandstone a mile in thickness, and 

 heated to 200 Fahr., would expand so as to lift a mass of rock upon 

 it 10 feet above its former level ; and if a mass of the earth's crust 

 equally expansible, 50 miles in thickness, be heated to 800, it would 

 rise 1,500 feet. From cooling we have the reverse effect shrinkage, 

 contraction, lateral pressure, and ultimately bending of the strata. 



The strain thus produced will at last cause fracture, and the vibra- 

 tion that results is an earthquake. In Fig. 5 we have an illustration 

 of fracture and displacement. 



This form of tension is being continually and everywhere produced 

 in the earth's crust, and there is probably no instant of time when that 

 crust is entirely free from vibrations. 



" There is nothing," observes Darwin, " not even the wind that 

 blows, so unstable as the level of the crust of the globe." 



