BHYTHMS AND GEOLOGIC TIME. 345 



The oldest known fauna, the Eocambrian, does not- represent the begin- 

 nings of life, but a well-advanced stage, characterized by development 

 along many divergent lines; and by comparing Eocambrian life with 

 existing life the paleontologist is able to make an estimate of the rela- 

 tive progress in evolution before and after the Eocambrian epoch. The 

 only absolute blank left by the time ratios pertains to an azoic age which 

 may have intervened between the development of a habitable earth 

 crust and the actual beginning of life. 



Erosion and deposition have been used also, in a variety of ways, to 

 compute the length of very recent geologic epochs. Thus, from the 

 accumulation of sand in beaches Andrews estimated the age of Lake 

 Michigan, and Upham the age of the glacial lake Agassiz; and from the 

 erosion of the Niagara gorge the age of the river flowing through it has 

 been estimated. But while these discussions have yielded conceptions 

 of the nature of geologic time, and have served to illustrate the extreme 

 complexity of the conditions which affect its measurement, they have 

 accomplished little toward the determination of the length of a geologic 

 period; for they have pertained only to a small fraction of what 

 geologists call a period, and that fraction was of a somewhat abnormal 

 character. 



Wholly independent avenues of approach are opened by the study 

 of processes pertaining to the earth as a planet, and with these the 

 name of Kelvin is prominently associated. 



As the rotation of the earth causes the tides, and as the tides expend 

 energy, the tides must act as a brake, checking the speed of rotation. 

 Therefore the earth has in the past spun faster than now, and its rate 

 of spinning at any remote point of time may be computed. Assuming 

 that the whole globe is solid and rigid, and that the geologic record 

 could not begin until that condition had been attained, there could not 

 have been great checking of rotation since consolidation. For if there 

 had been, it would have resulted in the gathering of the oceans about 

 the poles and the baring of the land near the equator, a condition very 

 different from what actually obtains. This line of reasoning yields an 

 obscure outer limit to the age of the earth. 



On the assumption that the globe lacks something of perfect rigidity, 

 G. H. Darwin has traced back the history of the earth and the moon to 

 an epoch when the two bodies were united, their separation having been 

 followed by the gradual enlargement of the moon's orbit and the gradual 

 retardation of the earth's rotation; and this line of inquiry has also 

 yielded an obscure outer limit to the antiquity of the earth as a habit- 

 able globe. 



One of the most elaborate of all the computations starts with the 

 assumption that at an initial epoch, when the outer part of the earth 

 was consolidated from a liquid condition, the whole body of the planet 



