3887.] 417 [Shaler. 



tide mark have been exempt from such invasions of swiftly moving 

 water as have been impelled against the west coast of South Amer- 

 ica several times within the present century. 



This hypothesis furthermore affords us an explanation of one of 

 the most puzzling features in the history of sedimentary forma- 

 tions. It is a fact well known to geologists that in almost all the 

 great formations there exist considerable sections of shales or de- 

 posits of clayey materials which are destitute of organic matter, al- 

 though the beds above and below them contain an abundance of 

 fossils. It seems not improbable that the frequent occurrence of 

 earthquake shocks may serve to prevent the establishment of or- 

 ganic life on certain portions of the sea floor and so leave work of 

 sedimentation altogether to the gradual down-showering of the' 

 inorganic sediments which the sea is constantly depositing on its- 

 floor. 



The destruction accomplished by an earthquake shock is likely 

 to extend over a very large area. In the greater of these disturb- 

 ances the field of a whole ocean such as the Atlantic may possibly 

 be involved. In lesser shocks the surface affected is likely to be 

 several thousand square miles in area. The reestablishment of or- 

 ganic conditions on such a desolated sea floor could not be imme- 

 diately effected. In the depths of the sea where the currents are 

 inconsiderable such a reestablishment of organic relations might 

 well require a period considerable even in a geological sense. We 

 can often see evidence of this slowness of return on the part of ani- 

 mals and of the difficulty with which the reestablishment is effected 

 in the considerable thickness of the clay layer which intervenes 

 between the limestones. There is hardly any doubt that the lime- 

 stones of the Cincinnati series in the central portion of the Ohio 

 valley were deposited at a considerable distance from, shore lines 

 which could have furnished sediments, and yet we often find shales 

 a foot or more in thickness which were deposited between two 

 layers of limestone. The time required for the accumulation of 

 such a stratum in the thallassal seas is probably great ;. it certainly 

 must be reckoned by thousands of years. Therefore we may pre- 

 sume that the impoverished condition of the life continued for a 

 long time and that repeated shocks of great violence might retain 

 a sea floor in the condition in which it could not be occupied by 

 life even if the shocks came at intervals of several thousand years. 



It is evident that such a disturbing agent as would prevent the 



PROCEEDINGS B. S. N. H. VOL. XXIII. 27" APRIL, 1888. 



