348 POPULAR SCIENCE MONTHLY. 



drive currents this way and that, causing alternations in sedimenta- 

 tion. 



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 especially those of the coal measures, have been ascribed to succes- 

 sive 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 charac- 

 ter of sediment might be changed by such rising. Subterranean forces, 

 of whatever origin, seemingly slumber while strains are accumulating, 

 and then become suddenly manifest in dislocations 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 seeking the higher latitudes. Eventually, however, 

 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 periodic transgressions by the sea, occurring alternately 

 in high and low latitudes. 



Another general rhythm has been recently suggested by Chamberlin 

 in connection with the hypothesis that secular variations of climate are 

 chiefly due to variations of the quantity of carbon dioxide in the at- 

 mosphere.* The system of interdependent factors he works out is too 

 complex for presentation at this time, and I must content myself with 

 saying that his explanation of the moraines of recession involves the in- 

 teraction of a peculiar atmospheric condition with a condition of 

 glaciation, 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 

 momentum 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 positive value 

 for that purpose. Nevertheless, account must be taken of them, be- 

 cause they compete with imposed rhythms for the explanation of many 

 phenomena, and the imposed rhythms, wherever established, yield esti- 

 mates of time. 



The tidal period, or the half of the lunar day, is the shortest imposed 

 rhythm appealed to in the explanation of the features of sedimentation. 



* An attempt to frame a working hypothesis of the cause of glacial periods on an atmospheric 

 basis. Journ. Ge»l., Vol. VII., 1899. 



