312 



SEE— THE CAUSE OF EARTHQUAKES. 



[October 19, 



mountains involved. On the contrary, the ranges of North America 

 exceed in length those of South America. The Rocky Mountains 

 are indeed farther from the coast than the Andes, but the shore line 

 has receded since they were formed; while the Sierra Nevada and 

 Coast Ranges are nearer the present shore. When one considers 

 all these circumstances, including the greater length of the chains*, 



Fig. 7. 



and the greater number of parallel ranges, notwithstanding their 

 distance from the seacoast, it will be found that in North America 

 P' is certainly not larger than P as found for South America. 

 Thus we may put them approximately equal to each other, and write 



I I 



P' ^'=7 — v^ • 7 — ^^= I ■ (decillion)^ 

 (10)^® (10) , 



In Africa the mountain ranges are not high, but they run quite 



