RHYTHMS AND GEOLOGIC TIME. 343 



in turn springs from the turning of the earth in relation to the shining 

 snn. The yearly rhythm of sowing and harvesting, of the fan and the 

 furnace, does not originate with man, but is imposed on him by the 

 rhythm of the seasons, and that in turn springs, from certain motions 

 of the earth in relation to the glowing sun. But the rhythm of the 

 generation and the rhythm of the dynasty have origin in the nature of 

 man himself. The rhythms of human chronology may thus be grouped 

 according to source in two classes, the imposed and the original; and 

 the same distinction holds for other rhythms. The lunar day is an 

 original rhythm of the earth as seen from the moon; the ground swell 

 is an original rhythm of the ocean; but the tide is an imposed rhythm 

 of the ocean, being derived from the lunar day. The swing of the pen- 

 dulum is an original rhythm, but the regular excursion of the chrono- 

 graph pen, being caused by the swing of the pendulum, is an imposed 

 rhythm. 



In giving brief consideration to each of the more important ways by 

 which the problem of the earth's age has been approached, I shall men- 

 tion first those which follow the action of some continuous process, and 

 afterward those which depend on the recognition of rhythms. 



The earliest computations of geologic time, as well as the majority 

 of all such computations, have followed the line of the most familiar 

 and fundamental of geologic processes. All through the ages the rains, 

 the rivers and the waves have been eating away the land, and the 

 product of their gnawing has been received by the sea and spread out 

 in layers of sediment. These layers have been hardened into rocky 

 strata, and from time to time portions have been upraised and made 

 part of the land. The record they contain makes the chief part of 

 geologic history, and the groups into which they are divided correspond 

 to the ages and periods of that history. In order to make use of these 

 old sediments as measures of time it is necessary to know either their 

 thickness or their volume, and also the rate at which they were laid 

 down. As the actual process of sedimentation is concealed from view, 

 advantage is taken of the fact that the whole quantity deposited in a 

 year is exactly equalled by the whole quantity washed from the land in 

 the same time, and measurements and estimates are made of the 

 amounts brought to the sea by rivers and torn from the cliffs of the 

 shore by waves. After an estimate has been obtained of the total annual 

 sedimentation at the present time, it is necessary to assume either that 

 the average rate in past ages has been the same or that it has differed 

 in some definite way. 



At this point the course of procedure divides. The computer may 

 consider the aggregate amount of the sedimentary rocks, irrespective of 

 their subdivisions, or he may consider the thicknesses of the various 

 groups as exhibited in different localities. If he views the rocks col- 



