COSMIC PHILOSOPHY 



that to which we had reached with the Silurian 

 rocks ; and it is now generally admitted that 

 even the Laurentian strata are modern com- 

 pared with the beginnings of life upon our 

 globe. 



But this is not all. Along with the im- 

 mensely long geologic rhythms, which have 

 thus entailed the periodic metamorphosis of 

 strata, there have been going on minor rhythms, 

 resulting in the alternate deposit and denuda- 

 tion of fossil-bearing strata. Each of the sedi- 

 mentary strata now surviving was deposited 

 during an epoch of subsidence, and since its 

 elevation to its present position has been more 

 or less denuded. Now it is only during epochs 

 of subsidence that permanent fossil - bearing 

 strata can be deposited. During epochs of ele- 

 vation the newly formed sedimentary deposit 

 is rapidly disintegrated by the action of coast- 

 waves ; and even those thin deposits which are 

 made during an epoch of subsidence are in the 

 next-recurring epoch of elevation soon worn 

 away. It is thus only the thicker strata de- 

 posited during an epoch of subsidence which 

 have preserved for our inspection a few speci- 

 mens of the organisms living at the time when 

 they were deposited. 



But in close juxtaposition to this comes the 

 remarkable fact that the most rapid variation 

 among specific forms must take place during 



56 



