RHYTHMS AND GEOLOGIC TIME. 349 



It is quite conceivable that the bottom of a quiet bay may receive at 

 each tide a thin deposit of mud which could be distinguished in the 

 resulting rock as a papery layer or lamina. If one could in some way 

 identify a rock thus formed, he might learn how many half-days its 

 making required by counting its laminas, just as the years of a tree's age 

 are learned by counting its rings of growth. 



The next imposed rhythm of geologic importance is the year. There 

 are rivers, like the Nile, having but one notable flood in each year, and 

 so depositing annual layers of sediment on their alluvial plains and on 

 the sea beds near their mouths. Where oceanic currents are annually 

 reversed by monsoons, sedimentation may be regularly varied, or inter- 

 rupted, once a year. Streams from a glacier cease to run in winter, and 

 this annual interruption may give a definite structure to resulting de- 

 posits. It is therefore probable that some of the laminae or strata of 

 rocks represent years, but the circumstances are rarely such that the 

 investigator can bar out the possibility that part of the markings or 

 separations were caused by original rhythms of unknown period. 



The number of rhythms existing in the solar system is very large, 

 but there are only two, in addition to the two just mentioned, which 

 seem competent to write themselves in a legible way in the geologic 

 record. These are the rhythms of precession and eccentricity. 



Because the earth's orbit is not quite circular and the sun's position 

 is a little out of the center, or eccentric, the two hemispheres into which 

 the earth is divided by the equator do not receive their heat in the same 

 way. The northern summer, or the period during which the northern 

 hemisphere is inclined toward the sun, occurs when the earth is farthest 

 from the sun, and the northern winter occurs when the earth is nearest 

 to the sun, or in that part of the orbit called perihelion. These relations 

 are exactly reversed for the southern hemisphere. The general effect of 

 this is that the southern summer is hotter than the northern, and the 

 southern winter is colder than the northern. In the southern part of the 

 planet there is more contrast between summer and winter than in the 

 northern. The sun sends to each half the same total quantity of heat in 

 the course of a year, but the difference in distribution makes the climates 

 different. The physics of the atmosphere is so intricate a subject that 

 meteorologists are not fully agreed as to the theoretic consequences of 

 these differences of solar heating, but it is generally believed that they 

 are important, involving differences in the force of the winds, in the 

 velocity and course of ocean currents, in vegetation, and in the extent of 

 glaciers. 



Now, the point of interest in the present connection is that the 

 astronomic relations which occasion these peculiarities are not constant, 

 but undergo a slow periodic change. The relation of the seasons to the 

 orbit is gradually shifting, so that each season in turn coincides with 



