776 .). BAKRELL MEA8T1REMENTS OF GEOLOGIC TIME 



factors, but it may be conceded to have been somewhat higher at recent 

 times in the Pleistocene. Going farther back, it is possible that during 

 epochs of very rapid retreat of seas the wash, of sediment may for a brief 

 time have equaled or exceeded the present, rate, bui^ such an effect Avould 

 have l^een very iemporar} . it is doubtless true that in other revolutions 

 broad areas of land have been uplifted, the older sediments removed with- 

 out leaving a trace, and deep denudation cut into tJie foundations. The 

 rate of denudation must have been locally as higli as it is in regions of 

 similar attitude at present. 



Nevertheless, although granting such approximations toward the pres- 

 ent topographic attitudes, it does seem probable that the present mean 

 rate may be twice the mean for the whole of the Cenozoic and ten or fif- 

 teen times the rate for all of geologic time since the opening of the 

 Paleozoic. 



To sum up the causes of this contrast of the present with the mean of 

 past time, they are to ])o found in a notable irregularly progressive in- 

 crease in volume of the ocean basins, resulting in greater mean conti- 

 nental elevation, in greater isostatic strains brought on the continents as 

 a whole by the d(>eper erosion, in the existence of a great period of revolu- 

 tion, the Cenozoic. whose culmination in the Pleistocene and Eecent is 

 marked by glaciation, and, lastly, the rate has been raised by man through 

 deforestation and agri(julture. The mean rate of denudation is a factor 

 which, then, in tlie very nature of thhigs. is subject to great variation 

 through geologic time and is therefore wliolly unsuited to serve as a 

 method of measurement. Qualitatively, however, it is clear that time is 

 far longer than those estimates which have been based on a hypothesis 

 that the present rate is a mean which applies to the geologic past. 



Part 11. — liHYTFOis jy Sedit^ihn^tation 



GENERALITY OF ACCUMULATION IN SHALLOW WATER 



Since the early days of geologic science it has been noted that the 

 greater part of sediments, even Aviiere accumulated in a geoscyncline, bear 

 the maj'ks of dei)osition in shallo^\' water. This is cons])iciious on noting 

 Die significance of the features which are found alniudantly in each ty])e 

 of deposit — conglomerates and sandsioiics, shales and limestones. 



Conglomerates and sandstones show transportation l)y ri\-er current^, 

 waves, or undertow. Only rarely, as i]i h)rr(^ntial deposits jjourcd in lake-, 

 do they exhibit the marks of rolling down a fore-set slope. On the con- 

 trary, most conglomerates are sti-eam deposits, a lesser amount ])eAno; made 

 and deposited by wave actiori along shores. The mai'in(> sands arc s]>rea^l 



