1008 



SCIENCE. 



[N. S. Vol. XI. No. 287. 



acterized by a series of halts or partial re- 

 advances, recorded in concentric belts of 

 ice-brought drift. Of these belts, called 

 moraines of recession, Taylor enumerates 

 seventeen in a single system. 



In explanation of these and other repeti- 

 tive series incorporated in the structure of 

 the earth's crust, a variety of rhythmic 

 causes have been adduced; and mention 

 will be made of the more important, begin- 

 ning with those which have the character of 

 original rhj'thms. 



A river flowing through its delta clogs 

 its channel with sediment and from time to 

 time shifts its course to a new line, reach- 

 ing the sea by anew mouth. Such changes 

 interrupt and vary sedimentation in neigh- 

 boring parts of the sea. Storms of rain 

 make floods, and each flood may cause a 

 separate stratum of sediment. Storms of 

 wind give destructive force to the waves 

 that beat the shore, and each storm may 

 cause the deposit of an individual layer of 

 sediment. Varying winds may drive cur- 

 rents this way and that, causing alterna- 

 tions in sedimentation. 



To explain the forest beds buried in the 

 Mississippi silts it has been suggested that 

 the soft deposits of the delta from time to 

 time settled and spread out under their own 

 weight. Various alternations of strata, and 

 epeciallj^ those of the coal measures, have 

 been ascribed to successive local subsidences 

 of the earth's crust, caused by the addition 

 of loads of deposit. It has been suggested 

 also that land undergoing erosion may rise 

 up from time to time because relieved of 

 load, and the character of sediment might 

 be changed by such rising. Subterranean 

 forces, of whatever origin, seemingly slum- 

 ber while strains are accumulating, and 

 then become suddenly manifest in disloca- 

 tions and eruptions, and such catastrophes 

 affect sedimentation. 



A more general rhythm has been ascribed 

 to the tidal retardation of rotation and the 



resulting change of the earth's form. If 

 the body of the earth has a rather high 

 rigidity, we should expect that it would for 

 a time resist the tendency to become more 

 nearly spherical, while the water of the 

 ocean would accommodate itself to the 

 changing conditions of equilibrium by seek- 

 ing the higher latitudes. Eventually, how- 

 ever, the solid earth would yield to the 

 strain and its figure become adjusted to the 

 slower rotation, and then the mobile water 

 would return. Thus would be caused peri- 

 odic transgressions by the sea, occurring 

 alternately in high and low latitudes. 



Another general rhythm has been re- 

 cently suggested by Chamberlin in connec- 

 tion with the hypothesis that secular vari- 

 ations of climate are chiefly due to varia- 

 tions of the quantity of carbon dioxide in 

 the atmosphere. * The sj'stem of interde- 

 pendent factors he works out is too com- 

 plex for presentation at this time, and I 

 must content myself with saying that his 

 explanation of the moraines of recession in- 

 volves the interaction of a peculiar atmos- 

 pheric condition with a condition of glacia- 

 tion, each condition tending to aggravate 

 the other, until the cumulative results 

 brought about a reaction and the climatic 

 pendulum swung in the opposite direction. 

 With each successive oscillation the mo- 

 mentum was less, and an equilibrium was 

 finally reached. 



Few of these original rhythms have been 

 used in computations of geologic time, and 

 it is not believed that they have any posi- 

 tive value for that purpose. Nevertheless, 

 account must be taken of them, because 

 they compete with imposed rhythms for the 

 explanation of many phenomena, and the 

 imposed rhythms, wherever established, 

 yield estimates of time. 



The tidal period, or the half of the lunar 



* An attempt to frame a workiug hypothesis of the 

 cause of glacial periods on an atmospheric basis. 

 Journ. Geo!., Vol. VII., 1899. 



