1004 



SCIENCE. 



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



restrial unit of duration ; and as the day is 

 man's standard for the minute division of 

 time, so the year is his standard for larger 

 divisions, and the decade, the century and 

 the millenium are its multiples. 



But the rhythms of day and night, of 

 summer and winter, are not the only tides 

 in the affairs of men. At birth we are 

 small, weak and dependent, we grow larger 

 and stronger, we become mature and inde- 

 pendent, and then by reproducing our kind 

 we complete the cycle, which begins again 

 with our children. The cycle of human 

 life is the generation, a time unit of some- 

 what indefinite length and varying in phase 

 from family to family, but holding a place, 

 nevertheless, in human chronology. 



Still less definite is the rhythm of heredi- 

 tary rulership, progressing from vigor 

 throughluxury to degeneracy, and closing its 

 cycle in usurpation ; yet it makes an epoch 

 in the life of a nation or empire, and so the 

 dynasty is one of the units of the historian. 



The generation and the dynasty are of 

 waning importance in human chronology, 

 and they can claim no connection with the 

 problem of geologic time ; but here again I 

 have turned aside for a moment in order to 

 illustrate a principle of classification. The 

 daily rhythm of waking and sleeping, of 

 activity and rest, does not originate with 

 man but is imposed on him by the rhythm 

 of light and darkness, and that in turn 

 springs from the turning of the earth in 

 relation to the shining sun. 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 rela- 

 tion to the glowing sun. Bat the rhythm 

 of the generation and the rhythm of the 

 dynasty have origin in the nature of man 

 himself. The rhythms of human chronol- 

 ogy 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 regu- 

 lar excursion of the chronograph 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 ap- 

 proached, I shall mention 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 geo- 

 logic 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 



