INTRODUCTION AND SUMMARY 753 



or the dropping of the old mciidelian factors, yet the building up of the 

 organization of the metazoa implies the sifting out of favorable combina- 

 tions from chance variations by a process of natural selection. Only in 

 this way can organisms become organized and efficiently adjusted to their 

 environment. But this requires numberless generations living iu the rela- 

 tively brief times of changing environment and resulting organic stress. 

 Finally, the recognition of this larger magnitude of geologic time opens 

 up sharply another problem — the source of solar energy. But the sun is 

 only one of the numberless host of stars, and the source of its energy is 

 a far-reaching cosmic problem. To warm the earth through the vast 

 length of geologic time, gravitational condensation of the solar mass is 

 found to be totally inadequate. The energy supplied by the atomic de- 

 generation of uranium and thorium would have ample endurance in time ; 

 but, even if the sun were composed entirely of these elements, their decay 

 could not supply the quantity of energy which is daily expended. Geo- 

 logic time brings to light, consequently, the evidence of unknown sources 

 of energy, cosmic forces which must constitute a fundamental factor in 

 any satisfactory hypothesis of stellar evolution, a factor which has not as 

 yet been taken into full consideration in its bearings. Even if, as a lesser 

 difficulty, it should be sought to deny the validity of the radioactive meas- 

 urements of the earth\s age, escape can not behad from this conclusion, 

 for various lines of purely geological evidence indicate an age many times 

 greater than that which could be granted if the solar energy were due 

 simply to contraction of the sun's mass. The depths of geologic time 

 leave us face to face with the unknown. 



Part I. — Ehythms in Denudation 



PRESENT RATES OF DENUDATION ^ 



Sedimentation rests on the rate of denudation. It can not go forward 

 faster than material is supplied, but it may proceed slower, and the balance 

 of the w^aste be carried beyond. In studying the time values to be as- 

 signed to the components of the stratigraphic column, the first problem, 

 therefore, is to discuss the present rate of denudation. By noting the 

 factors which enter into it we may arrive at a truer conception of the 

 variations of denudation in the past. 



The mean rate of denudation — the lowering of the continental surfaces 

 by pluvial and fluvial erosion — has long been sought. The usual method 

 has been to measure the waste which representative rivers carry to the 

 sea in the course of a year. The drainage basin of the Mississippi was 

 thus estimated to he lowered one foot in 6,000 years, or adding the dis- 



