1010 



ISCIENGK 



[N. S. V'OL. XI. No. 287. 



years, we should expect to find records of 

 the cycle in the sediments. A moist cli- 

 mate would tend to leach the calcareous 

 matter from the rock, leaving an earthy 

 soil behind, and in a succeeding drier cli- 

 mate 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 matter on that side 

 would make a small difference in the posi- 

 tion 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 conditions of erosion and deposition to 

 an appreciable extent in many localities. 



Blytt ascribed to this astronomic cause 

 the alternations of bog and forest in Scan- 

 dinavia, as well as other sedimentary 

 rhythms observed in Europe; and it has 

 seemed to me competent to account for cer- 

 tain alternations of strata in the Cretaceous 

 formations of Colorado. CroU used it to 

 explain interglacial epochs, and Taylor has 

 recently applied it to the moraines of re- 

 cession. 



The remaining astronomic rhythm of 

 geologic import is the variation of eccen- 

 tricity. At the present time our greatest 

 distance from the sun exceeds our least dis- 

 tance by its thirtieth part, but the differ- 

 ence 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 fluctuates in a somewhat ir- 

 regular way, in which the property of peri- 

 odicity is not conspicuous. The effect of 

 its fluctuation is inseparable from the pre- 

 cessional effect, and is related to it as a 

 modifj'ing condition. When the eccen- 

 tricity is large the precessional rhythm is 

 emphasized ; when it is small the preces- 

 sional effect is weak. 



The variation of eccentricitj' 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, CroU 

 correlated two important epochs of glacia- 

 tion with epochs of high eccentricity com- 

 puted to have occurred about 100,000 and 

 210,000 years ago. As the analysis of the 

 glacial history progresses, these correlations 

 will eventually be established, or disproved, 

 and should they be established it is possible 

 that similar correlations may be made be- 

 tween events far more remote. 



The studies of these several rhythms, 

 while they have led to the computation of 

 various epochs and stages of geologic time, 

 have not yet furnished an estimate either 

 of the entire age of the earth or of any 

 large part of it. Nevertheless, I believe 

 that thejr may profitably be followed with 

 that end in view. 



The system of rock layei's, great and 

 small, constituting the record of sedimen- 

 tation, may be compared to the scroll of a 

 chronograph. The geologic scroll bears 

 many separate lines, one for each dis- 

 trict where rocks are well displayed, but 

 these are not independent for they are 

 labelled by fossils and by means of these 

 labels can be arranged in proper relation. 

 In each time line are little jogs — changes in 

 kind of rock or breaks in continuity — and 

 these jogs record contemporary events. A 

 new mountain was uplifted, perhaps, on 

 the neighboring continent, or an old uplift 

 received a new impulse. Through what 

 Davis calls stream piracy a river gained or 

 lost the drainage of a tract of country. 

 Escaping lava threw a dam across the 

 course of a stream, or some Krakatoa 

 strewed ashes over the land and gave the 

 rivers a new material to work on. The 

 jogs may be faint or strong, manj' or few, 

 and for long distances the lines may run 

 smooth and straight; but so long as the 



