CHAPTER XV. 



STRUCTURAL CHARACTERS AND LITHOGENESIS OF THE 

 MARINE HYDROCLASTICS. 



Marine hydroclastics are accumulating in nearly every portion 

 of the ocean to-day, and their fossil representatives are among the 

 most widespread of the geological formations. They are most abun- 

 dantly developed in the littoral portion of the seas, including the 

 epicontinental seas, but also occur in the abyssal regions. In gen- 

 eral, we may classify the material with reference to its source, either 

 as terrigenous or land-derived, or as oceanic or derived from purely 

 marine deposits. The latter group is essentially limited to the 

 regions around coral reefs or other organic deposits, and so has a 

 marked uniformity of petrographic character. Viewed as a whole, 

 marine elastics are nearly always well stratified, and they are as a 

 rule fossiliferous. Indeed, it may be seriously questioned if marine 

 elastics are ever wholly free from organic remains, though for con- 

 siderable distances off certain shores organisms may be so rare as 

 to escape detection. Thus Kindle (54) reports dredging off the 

 coast of Alaska for a hundred miles or more along the shore, with- 

 out finding any organic remains whatever. This of course does not 

 prove their absence, but only indicates their scarcity, and indeed at 

 another point of the same coast organisms were abundant. More- 

 over, such dredging affects only the surface layers of the sea floor, 

 and does not prove the absence of remains in somewhat deeper 

 layers. 



It is perfectly well known that marine organisms migrate with 

 the seasons, and that at a certain locality, where life was abundant 

 during one season, it is almost entirely absent in another, the or- 

 ganisms having migrated into deeper water. What is true of sea- 

 sons is also true of longer periods, some i-egions formerly well 

 stocked with organisms being barren for years at a time, after 

 which a return of the fauna takes place. Such migration up and 

 down the ocean floor is often determined by factors difficult to as- 



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