GEOLOGICAL IMPORTANCE OF SEDIMENTATION 447 



by broad continental uplift, orogenic movements, and variegated 

 climates, while the Jurassic was, on the contrary, characterized by a 

 spread of epicontinental seas, less rugged and elevated land-masses, 

 and warm, equable climates extending into the polar regions. This 

 conception of world-wide conditions characterizing the several ages, 

 and distinguishing them from each other must, according to Cham- 

 berlin, form the ulterior basis of time-divisions and the classification 

 of geologic history. But if the several ages have been characterized 

 by certain relations of mountains, plains, and seas, then they should 

 also be characterized by the kinds of deposits made under these 

 conditions. The most natural method of testing this question would 

 be to study all of the formations of the world belonging to a certain 

 age, and group them according to area and origin. This has been 

 done, and it is the method by which the conclusions in regard to the 

 broader features have been arrived at. But as the very subject under 

 discussion involves the possible confusion of certain unfossihferous 

 littoral and shallow- water marine deposits with those made upon the 

 land, this method must be reversed, and by basing the arguments 

 upon the known continental relations, as determined by formations 

 in regard to which there is no doubt, conclusions may be reached in 

 regard to the relative importance of deposits of those three classes 

 which should characterize the different ages. This may be of value 

 in suggesting discriminations not otherwise thought of, but it is not 

 the purpose to offer here criteria for finally and definitely testing the 

 origin of any particular formation. The latter question must, of 

 course, be finally settled by a detailed study of the formation in the 

 field and in all its aspects. 



As introductory to this discussion it may be well to restate briefly 

 certain of the principles which are found to govern the development 

 of several classes of deposits at the present time. 



Where young and lofty mountain ranges stand sharply above the 

 surrounding country there will be much local waste on the Piedmont 

 plains at their feet. If at least one slope faces the interior of a con- 

 tinent, this will result in extensive continental deposits, perhaps 

 accentuated by conditions of aridity. Down-warped interior basins 

 may also receive deposits. For effective delta building, however, 

 there must be a submerged continental platform, or epicontinental 



