234 EAETHQUAKES. 



CHAPTER XITI. 



DISTRIBUTION OF EARTHQUAKES IN TIME {continued). 



Seismic energy in relation to geological time. — If 

 we admit that seismic energy is only a form of volcanic 

 energy, it must also be admitted that any cause tending 

 to produce a general decrease in the amount of the latter 

 will also produce an alteration in the amount of the 

 former. 



The nebular hypothesis of Laplace tells us that the 

 solar system is the result of the whirling of a heated 

 gaseous mass, which as it cooled continually contracted 

 and consequently whirls the faster. With this hypothesis 

 before us, we understand why all the planets and their 

 satellites have a similarity in the directions of their move- 

 ments, why they revolve nearly in the same plane, in 

 orbits nearly circular, why some have a flattened figure 

 and are surrounded by rings or belts, why the exterior 

 planets should have a greater velocity of rotation, a 

 greater number of satellites, and a less density as com- 

 pared with the interior planets, the similarity of the 

 elements in meteoric stones, the sun, the stars, and those 

 found upon our earth, and lastly why there should be an 

 increase in temperature as we descend into our earth. ^ 

 This increase in temperature as we descend into the earth 



* Six Lectures on Physical Geography, by Rev. S. Haughton, F.R.S., 

 chap. i. 



