ESTIMATES OF TIME 827 



Depression of snow 



-line 



Alpine stages 



Scandinavian morj 



1,200 meters 





Wiirm 





900 " 





Blihl 



Baltic 



600 " 





Geschnitz 



Ra 



300 " 





Daun 



Ghristiania Valley 



Time from the Biilil stage has been estimated at from 16,000 to 24,000 

 years, with the probabilities in favor of the longer period. Time from 

 the outer Ea moraine has been more closely estimated at 17,000 to 18,000 

 years. Time since the last stage is as mu'ch as 7,000 years. Minor re- 

 treatal moraines lie between these stages. There seem to be here a com- 

 bination of two cycles, perhaps both irregular, the cycle discussed by 

 Taylor having a mean period of 2,000 or 3,000 years, the other with a 

 mean period perhaps as large as 10,000 years. In the interior of the 

 North American continent the shorter of these cycles has left a more 

 prominent record; on the northwestern shores of Enrasia the longer is 

 the more conspicuous. Such a difference in emphasis of cycles of differ- 

 ent periods in different regions is paralleled by what is known of the ex- 

 pression of the shorter climatic rhythms acting in recent times. The 

 climate of the interior of Worth America is controlled by continental con- 

 ditions; that of northwestern Eurasia by oceanic, especially by the nature 

 of the Gulf Stream. 



Changes up to this order magnitude are more readily ascribed to cycles 

 in solar radiation than to other causes. Volcanic dust in the terrestrial 

 atmosphere acts as a screen, but it is probably too sporadic and tem- 

 porary in occuTrerice to lead to such periodic changes in climate. Fur- 

 thermore, certain highly volcanic epochs, such as the Eocene and Oligo- 

 cene, have been marked by warm and equable climates over high latitudes. 

 In addition, the rather regular and pronounced climatic oscillations in 

 the Pleistocene are not known to have been related to rhythmic outbursts 

 of volcanic energy. 



Changes in atmospheric composition or in crust movement are too 

 slow and massive to be invoked for such changes as have been discussed, 

 but are the probable causes in determining the character of geologic 

 periods, as is shown by the concurrence of glacial climates at the culmi- 

 nation of periods of continental movement and revolution. The sharp 

 oscillations of decades, centuries, and thousands of years in length may 

 be classified as solar climatic rhythms ; the long, slow, and massive move- 

 ments, rhythmic in their character, as diastrophic climatic changes. 



There is another class of recurrent changes most important for the 

 measurement of time. These may be called orbital rhythms, depending 

 on the motions in the solar system. They are familiar as the rhythms 



