356 STUDIES FOR STUDENTS 



frequently take place by subsidence of land areas, even the highest 

 of such land deposits stand a chance of preserval. Therefore others 

 of a similar nature should be expected to occur buried in older portions 

 of the geological record, but not so abundantly as those originally 

 formed at lower levels. Continental deposits depend, however, so 

 largely upon the climate and geography that their areal extent and 

 importance must have varied largely through geological time. 



