ESTIMATES OF TIME 829 



The oscillations of day and night, of summer and winter, have been 

 of fundamental importance, however, not in stratigraphic record, bnt in 

 the manner in which they have stamped their existence in the life 

 rhythms of organisms. In a little perceived, but even more fundamental 

 way, the diurnal and annual rhythms have been essential to evolution. 

 They have hardened the endurance of plants and animals to changes of 

 temperature and to the dangers of night and winter. From this ever- 

 recurring annealing process organisms have acquired the power of adapta- 

 tion to the longer rhythms of climatic change, and thus have been able 

 to emerge sifted and improved from the climatic stress of critical periods. 



The higher rhythm of precession has a mean period of 21,000 years, 

 from which it may var}^ by about 10 per cent. It brings in no climatic 

 change except by virtue of the eccentricity of the earth's orbit. Croll has 

 made it familiar by his attempt to apply it as a cause of glaciation, but 

 the lack of accord with the history of the glacial period, as it is now under- 

 stood, has caused it to be generally discarded. K"evertheless, at times of 

 high eccentricity of orbit it remains true that at one phase of the rhythm 

 the summers in one hemisphere will be somewhat shorter and hotter, the 

 winters somewhat longer and colder. The reverse effects will be felt at 

 the same time in the opposite hemisphere. Every 10,500 years the rela- 

 tions of the hemispheres will shift, and thus a change in the character 

 of the climate will be felt 21,000 years in length. The only question is : 

 Can it rise to visible expression above the various other solar and dias- 

 trophic rhythms which are seen to have entered into the climatic oscilla- 

 tions of the Pleistocene and presumably extend backward with various 

 expressions through the earlier periods ? 



The only rhythms which, because of their certain existence and un- 

 changing length, are adapted to the measurement of geologia time are 

 the regular orbital rhythms. The terrestrial and solar climatic rhythms 

 are apparently too numerous and too irregular in period to serve as safe 

 measures of time. Furthermore, in one region and at one time one 

 rhythm may dominate, as the 21-year cycle in Arizona, and in another 

 place or time another rhythm,- such as that of the 35-year cycle in glacial 

 advance and retreat. The oscillations which occur in centuries, in thou- 

 sands of years, and in tens or hundreds of thousands of years are best 

 adapted to be stratigraphically recorded; but their discrimination, their 

 persisteijce, their regularity, and their mean period are all open to serious 

 question. 



Even if the precessional rhythm can be distinguished it can not bridge 

 unconformities and in most formations surely does not appear; but if 

 from time to time we can detect in the strata under favored conditions 



