GEOLOGICAL CHANGE, AND TIME. 127 



me ob\ ioiisly imicli more crt'cctix »■ in iiiomilaiiioiis rciiioiis t liaii on plains- 

 and their resnlts must constantly \ary, not only in (liflcicnt hasins 

 of (hainajio, but oven, and sonictimcs widely, ^\itllin the same basin. 

 Actual nieasuienient of tlu' proportion of sedinuMit in river water 

 sliows that while in some cases the lowerin.i;- of the surface of the land 

 may be as much as -.j„ of a foot in a year, in others it falls as low- 

 as ,. J,,,,. In other words, tlio rate of dejiosition of new sedimentary 

 formations, over an aica of sea Hoor eipiix alent to tliat wliieh has 

 yielded the sediment, may vary fr<»m one foot in se\en hundred ani' 

 thirty years to one foot in six thonsand eiiiht hundred years. 



I f now we take these lesults and apidy them as measurcvs of the leu'itu 

 of time recjuircd fortiie dejXKsition of the \arions sedimentary masse.s 

 that form the outer ])art of the earth's crust, we obtain some indication 

 of the duration of geoh)gica] history. On a reasonabk' com])utation 

 these stratilied masses, wliere most fully developed, attain a united 

 thickness of not less than 100,()()() feet. If they were all laid down at 

 the most rapid recorded rate of denudation, they would icquire a 

 ])eriod of seventy-three nullions of years for their completion. If they 

 were laid down at the slowest rate they would denuind a jieriod of not 

 less than six hundred and eij^hty millions. 



But it may be arjiued that all kinds of terrestrial energy are grow- 

 iuii' feeble, that the most active denudation now in ])io,<;ress is nnich 

 less vigorous than that of bygone ages, and hence that the stratified 

 part of the earth's crust may have been put together in a. much briefer 

 space of time than modern events might lead us to su])pose. Such 

 arguments are easily adduced and look sufficiently si)ecious, but no 

 contirmation of tliem can be gathered from the rocks. On the contrary, 

 no one can thoughtfully study the various systems of stratitied forma- 

 tions without being impressed by the fullness of their evidence that, on 

 the whole, the accunndation of sediment has been extremely slow. 

 Again and again Ave encounter groups of strata comi)osed of thin paj)er- 

 like lamina' of the finest silt, which evidently settled down (juietly and 

 at intervals on the sea bottom. We lind successive layers covered 

 Avith rii)})le-marks and sun-cracks, and we recognize in them memorials 

 of ancient shores where sand and nuid trauipiilly gathered as they do 

 in sheltered estuaries at the ])resent(lay. >\'e can see no proof what- 

 ever — nor e\en any evidence which suggests — that (mi the whole the rate 

 of waste aiul sedimentation was more rapid during Mesozoic and l*ala'- 

 ozoic time than it is to-day. Had there been any marked difference in 

 this rate from ancient to modeiii times, it would be incredible that no 

 clear proof of it should have licen ri'corded in the crust of the eaith. 



iJut in actual fact the testimony in faxor of the slow accumulation 



and high anticpiity of the geological lecord is much stiongei' than might 



be inferred from the mere thickiu'ss of the stratified formations. These 



I sedimentary depositshave not been laid down in one unbroken se(pience, 



but have had their continuity interrupted again and again by upheaval 



