414 SEE— THE CAUSE OF EARTHQUAKES. [October 19, 



which gives Hfe to animals and plants, and in the form of steam is 

 the greatest mechanical agent of man, when sunk into the crust be- 

 comes also one of his worst destroyers, on account of the explosive 

 vapor generated beneath by the internal heat of the globe. 



Finally, it may not be without interest to note that, so far as our 

 knowledge extends, the earth alone among the encrusted planets 

 of the solar system has an abundance of water and mountains, with 

 volcanoes and earthquakes. Mars seems to have only a trace of 

 water and no sensible mountains or volcanoes. And while Venus 

 is largely obscured from our view, the chances are that its veil is 

 due to clouds, indicating an abundance of water, and hence that its 

 conditions of evolution may approach those of the earth. 



Blue Ridge on Loutre, Montgomery City, Missouri, 

 September 23, 1906. 



Note. — For kind permission to use Figs. 6, 7, 8, and 9 we are 

 indebted to the courtesy of Rand, McNally and Co., of Chicago. 



