80 Biology in America 



lies only fifty miles east of the region where the trees were 

 measured. The general climatic fluctuations of both districts 

 are the same. The uppermost strand, the huge gravel beach 

 at the level of the outlet, must date from about the time of 

 Christ, for both the chemical evidence and the trees point to 

 this conclusion. A series of similar, but much smaller 

 beaches at lower levels record the approach of a dry period 

 during which the lake fell to a low level whose exact position 

 cannot be determined. Judging by the trees this must have 

 culminated about 650 A.D. During this period gravels were 

 washed in by mountain streams and deposited in what are 

 known as fans, or low, flattened cones, which may be several 

 miles long. These covered the old strands in many places, 

 and extended far below their level to the diminished lake. 



"Next the waters rose again, but not halfway to their 

 former level. They formed two small strands, not gravelly 

 like their predecessors, but faint and sandy as if the winds 

 were weak. They must date from about 1000 A.D., when the 

 trees indicate a wet period, for they are younger than the 

 gravel fans of the preceding dry time. The next phase of 

 the lake was a dry period, which was most extreme about 

 1250 A.D. More gravels were then deposited, and the fact 

 that they cover the preceding strands and extend to a much 

 lower level shows that the lake then stood low, as would be 

 expected from the trees. 



"The next high period of the lake, about 1350 A.D. accord- 

 ing to the trees, is unusually interesting. The water did not 

 reach so high a level as formerly, because the rainy period 

 was short, but it formed a large, high beach of gravel quite 

 different from the preceding beaches. This seems to indicate 

 great storminess, a condition which is also suggested by the 

 fact that the growth of the trees at this time increased more 

 rapidly than at any other period for nearly 3000 years. In 

 Europe during the same century, unprecedented storms 

 caused great floods in France, while the severity of the waves 

 was so intense as to break through beaches and sand dunes, 

 and convert large marshy areas into portions of the sea along 

 the coasts of Holland and Lincolnshire. During the winters 

 the rivers froze to an unheard-of degree, and three or four 

 times men and animals passed from Germany to Sweden on 

 the solid ice of the Baltic Sea, an occurrence unknown in our 

 day. In England the summers were so rainy that the average 

 yield of grain diminished disastrously. In self-defense many 

 landowners gave up grain-raising, and turned their attention 

 to sheep and cattle. Distress and discontent were the inevit- 

 able result among the peasants. Far away in central Asia 

 the Caspian Sea and the lake of Lop Nor both rose with great 



