June 29, 1900.] 



SCIENCE. 



1009 



day, is the shortest imposed rhythm ap- 

 pealed to in the explanation of the features 

 of sedimentation. 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 laminje, 

 just as the years of a tree's age are learned 

 by counting its rings of growth. 



The next imposed rhythm of geologic im- 

 portance 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 tiieir alluvial plains 

 and on the sea beds near their mouths. 

 Where oceanic currents are annually re- 

 versed by monsoons, sedimentation may be 

 regularly varied, or interrupted, 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 laminje or strata of rocks represent 

 years, but the circumstances are rarely such 

 that the investigator can bar out the possi- 

 bility that part of the markings or separa- 

 tions 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 men- 

 tioned, which seem competent to write 

 themselves in a legible way in the geologic 

 record. These are the rhythms of preces- 

 sion and eccentricity. 



Because the earth's orbit is not quite cir- 

 cular and the sun's position is a little out 

 of the center, or eccentric, the two hemi- 

 spheres 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 hemi- 

 sphere is inclined toward the sun, occurs 



when the earth is farthest from the sun, 

 and the uorthern winter occurs when the 

 earth is nearest to the sun, or in that part 

 of the orbit called perihelion. These rela- 

 tions 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 at- 

 mosphere is so intricate a subject that me- 

 teorologists 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 dififer- 

 ences 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 the peri- 

 helion ; and the climatic peculiarities of 

 the two hemispheres, so far as they depend 

 on planetary motions, are periodical!}' re- 

 versed. The time in which the cycle of 

 change is completed, or the period of the 

 rhythm, is not always the same, but aver- 

 ages 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 



* 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. 



