35o POPULAR SCIENCE MONTHLY. 



the perihelion; and the climatic peculiarities of the two hemispheres, 

 so far as they depend on planetary motions, are periodically reversed. 

 The time in which the cycle of change is completed, or the period of 

 the rhythm, is not always the same, hut averages 21,000 years. It is 

 commonly called the precessional period.* 



Assuming that the climates of many parts of the earth are subject to 

 a secular cycle, with contrasted phases every 10,500 years, we should ex- 

 pect to find records of the cycle in the sediments. A moist climate 

 would tend to leach the calcareous matter from the rock, leaving an 

 earthy soil behind, and in a succeeding drier climate the soil would be 

 carried away; and thus the adjacent ocean would receive first calcareous 

 and then earthy sediments. The increase of glaciers in one hemisphere 

 would not only modify adjacent sediments directly, but, by adding mat- 

 ter on that side, would make a small difference in the position of the 

 earth's center of gravity. The ocean would move somewhat toward the 

 weighted hemisphere, encroaching on some coasts and drawing down on 

 others; and even a small change of that sort would modify the condi- 

 tions of erosion and deposition to an appreciable extent in many 

 localities. 



Blytt ascribed to this astronomic cause the alternations of bog and 

 forest in Scandinavia, as well as other sedimentary rhythms observed in 

 Europe; and it has seemed to me competent to account for certain alter- 

 nations of strata in the Cretaceous formations of Colorado. Croll used it 

 to explain interglacial epochs, and Taylor has recently applied it to the 

 moraines of recession. 



The remaining astronomic rhythm of geologic import is the varia- 

 tion of eccentricity. At the present time our greatest distance from the 

 sun exceeds our least distance by its thirtieth part, but the difference is 

 not usually so small as this. It may increase to the seventh part of the 

 whole distance, and it may fall to zero. Between these limits it fluc- 

 tuates in a somewhat irregular way, in which the property of periodicity 

 is not conspicuous. The effect of its fluctuation is inseparable from the 

 precessional effect, and is related to it as a modifying condition. When 

 the eccentricity is large the precessional rhythm is emphasized; when it 

 is small the precessional effect is weak. 



The variation of eccentricity is connected with the most celebrated 

 of all attempts to determine a limited portion of geologic time. In the 

 elaboration of the theory of the Ice age which bears his name, Croll 

 correlated two important epochs of glaciation with epochs of high eccen- 

 tricity computed to have occurred about 100,000 and 210,000 years ago. 

 As the analysis of the glacial history progresses, these correlations will 



♦Strictly speaking, 21,000 years is the period of the precession of the equinoxes as referred to 

 perihelion ; but the perihelion is itself in motion. As referred to a fixed star the precession of the 

 equinoxes has an average period of about 25,700 years. 



