MENTAL EVOLUTION 131 



of evidence indicate that this was a period of unusual climatic 

 •stress. (See Figure 24.) It was like a glacial period on a small 

 scale, and like present periods of solar activity on a large scale. 

 This was especially evident in the two storm belts. In the southern 

 belt during the early part of the fourteenth century the Big Trees 

 of California, as we know from the rings of their stumps, suddenly 

 increased their rate of growth. They grew with a vigor un- 

 equalled at any other time since about 1000 A. D. This means 

 that the rainfall, especially in the late winter and spring, was at 

 a maximum. Not far from the Big Trees, but on the opposite or 

 eastern side of the Sierra Mountains, lies Owens Lake. This body 

 of salt water now has no outlet, but formerly it overflowed. When 

 the Owens River was diverted into the Los Angeles aqueduct to 

 be carried two hundred and fifty miles across the mountains, both 

 the lake and the river were most carefully measured and analysed. 

 From the amount of salt in the two, Gale has calculated the length 

 of time since the lake overflowed. This must have happened not 

 long before the time of Christ. The climate must then have been 

 so moist that the lake was two and a half times as large as at 

 present. From this fact and from our knowledge of the Big 

 Trees we can determine which of the various elevated beaches 

 surrounding the lake belongs to the fourteenth century. The 

 beach thus determined appears to have been formed by waves of 

 unusual intensity. This means that the storms of the early part 

 of the fourteenth century were very violent. Far away in western 

 Asia historic records show that at this same time the Caspian 

 Sea rose rapidly to a level many feet above that of today and 

 above the position it had previously held for several centuries. 

 Still farther to the east in the very heart of Asia the desert lake 

 of Lop-Nor in Chinese Turkestan likewise rose, and overwhelmed 

 the "Dragon Town." Thus at both ends of the southern storm 

 belt we find signs of unusual storminess culminating about 1325 

 A. D., while other evidence indicates that similar conditions 

 prevailed in the intervening portions. 



