;Shaler.J 418 . I Dec. 7, 



possession of the sea floor by organic forms for a considerable pe- 

 riod must have much effect upon the rate of deposition of sediment. 

 In many of our limestone sections limy matter is more than nine- 

 tenths of 'the whole contents of the strata. Thus the arrest in the 

 development of organic forms may reduce the rate of growth of 

 deposits to one-tenth their original amount. In this manner 

 through the recurrence of seismic accidents in particular areas for 

 long continued periods we may have a slow deposition of strata on 

 one portion of the sea floor while in other regions, exempt from 

 such disturbances, accumulation may proceed with great rapidity. 

 It seems likely that the movements of the earth's surface are con- 

 siderably affected by the rate of deposition upon the sea floor. In 

 some manner, as yet not well explained, rapid deposition appears 

 often to be accompanied by a subsidence of the surface on which the 

 deposit is formed while the neighboring land area may undergo a 

 corresponding elevation. Therefore a cause which interferes with 

 or which fosters the development of organic life on the sea floor may 

 have a considerable influence on the movements of the earth's crust. 



It is also easy to see that the effect of such destructions on the 

 history of organic species may be most important. Each time a re- 

 gion is depopulated it becomes a free field for the life which is 

 enabled to return to it. Wherever such a free field is afforded to 

 organic species an opportunity is given for a readjustment of rela- 

 tions between various forms and consequently for rapid diversifi- 

 cation. In an old settled region, whether on sea floor or on the 

 land, adjustments have been accomplished which tend to limit va- 

 riations. In the return of life to such a region an opportunity is 

 given for readjustment which is never afforded without the access 

 of cataclysms which laid the area open for resettlement. Thus 

 whether it be in reoccupying the field opened by the retreat of 

 glaciers on the land or in repossessing an area over which life has 

 been expelled on the sea floor, organisms find an admirable oppor- 

 tunity to effect variations when they work their way back into the 

 region whence they have been driven. 



It would be interesting to follow the considerations suggested in 

 the last paragraph, but it would lead us away from our subject 

 matter, the main point of which concerns the causes of the breaks 

 in strata. We may sum up the propositions of this paper in the 

 following manner. Geographic change, alterations in the level 

 of the sea floor or in the position of shores in relation to areas of 



