324 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 19 3 



in groups of years of varying length. Glaciers are conditioned by 

 opposing factors, nourishment, and depletion. These, in turn, are 

 governed by changes in the weather and in the amount of solar 

 heat. The intensity of solar radiation reaching the earth is affected 

 by the amount of water vapor, carbon dioxide, ozone, and dust in 

 the air; also by irregular variations in the sun itself. The varia- 

 tions in the sun fluctuate slightly about a mean of 1.938 calories 

 per minute per square centimeter. Although these solar variations 

 are moderate, they are important, for a real relation between them 

 and weather changes, as well as glacial conditions on the earth, is 

 strongly suggested. The records preserved in living and fossil 

 trees and varved glacial clays show not only seasonal and annual 

 variations, but also fluctuations for irregular groups of years, which 

 seem difficult to explain on any other grounds than a real relation 

 between them and the changes in weather and solar radiation. 



The graphs of solar constant and varved clay variations appear- 

 ing in this paper show that they are not unlike; in fact, they bear 

 close resemblances. Short cycles, averaging four years in length, 

 are prominent, as shown by the smoothed curves. Longer periods 

 as noted in Figure 14 are not wanting. 



A special study of Haverstraw and New Plaven varved clays of 

 the last glaciation (fig. 15) shows that their fluctuations in thickness 

 were due not to local causes, but to primary factors embracing a strict 

 periodicity in the melting of the ice, summer by summer. The fact 

 that these fluctuations vary from year to .year, and for short as well 

 as for long irregular cycles, shows that these variations were gov- 

 erned by changes in the weather and solar radiation extending over 

 a period of more than seven and one-third centuries. 



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