286 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



closer relation to the number of sunspots than to rainfall; in general, 

 the fewer the sunspots the lower the lake level. Lake Uinta at this 

 stage had no outlet and lost much of its water by evaporation — 

 therefore it must have had such a cyclic fluctuation of level. Next, 

 in general the temperature of lake water rises as the lake goes down, 

 and the higher temperature favors the growth of the minute surface- 

 dwelling organisms and also the precipitation of particles of lime 

 carbonate. This gives a further check on the ancient conditions, for 

 in the deposits of Lake Uinta the layers of organic substance and 

 lime carbonate differ in thickness from year to year and show maxima 

 at intervals that average about 11 years. Similar cyclic variations 

 have been observed in the thickness of annual layers formed in modem 

 lakes. It is also a suggestive fact that the annual rings of trees that 

 grew around Lake Uinta show even better the same 11-year cycle, 

 just like the growth rings in modern trees. 



Much longer cycles, whose average length was about 21,000 years, 

 are also recorded in the deposits of Lake Uinta. By a somewhat 

 more involved line of reasoning we are led to think that these observed 

 variations in the lake deposits may be correlated ^\'ith the resultant 

 of two astronomic cycles — ^the change in eccentricity of the earth's 

 orbit and the precession of the equinoxes. It remains to be seen 

 whether these and other comparable cyclic variations in the climate 

 of the past can ever be used by meteorologists in their researches 

 into secular changes of cUmate. 



Ill 



Lake Uinta and the surrounding countryside did not always present 

 a picture of smiUng beauty, with forests and green meadows. Instead 

 during the later half of its existence death and starvation laid heavy 

 hands upon the community. From time to time pallid blankets of 

 volcanic ash descended upon it and snuffed out the hfe. Animals 

 and plants alike were smothered, and the streams were clogged with 

 the harsh mud. Gradually, as rains washed off the slopes, the forest 

 renewed its growth and animals again sought its shelter. But it 

 was to no purpose, for again and yet again at long intervals the 

 volcanoes in the neighboring mountain chains belched forth devas- 

 tating clouds of pumiceous ash. 



As if these recurrent disasters were not enough, the rains came less 

 frequently: the very Hfe-giving source of moisture began gradually 

 but surely to dry up. Under the pitiless summer sun the more lush 

 plants withered and finally gave up, weary of waiting for the rain. 

 Animals wandered away in search of water. 



The lake, too, suffered. For a long time it overflowed only during 

 the cooler rainy season, but as the years passed the thirsty air drew 

 more and more greedily from its surface until finally even at the 



