GEOLOGICAL IMPORTANCE OF SEDIMENTATION 325 



sented upon maps of the period; and thus in any restoration of 

 the relation of land and sea of past times the paleogeographer is called 

 upon to commit himself as to the origin of the deposit, with all of its 

 far-reaching implications. 



Even if the formation is considered as originating upon a conti- 

 nental surface, the problem of interpretation is not ended ; for several 

 alternatives remain to be considered ; whether the deposit has accumu- 

 lated within the confines of lakes, or has been laid down under 

 humid climates upon aggrading river plains, or washed upon arid 

 plains by intermittent floods, or, finally, accumulated by the action 

 of the desert wind as loess or dune sand. To extend the illustration 

 previously mentioned: Under the view that the Tertiary accumula- 

 tions of the Great Plains and Rocky Mountain region in general were 

 made in enormous fresh-water lakes, the Great Plains must have 

 acquired later their tilted character, implying a more recent westward 

 uplift of some thousands of feet. The mountainous plateau has con- 

 sequently often been regarded as at that time low-lying and the climate 

 moist. Under the contrary view, that the deposits are largely fluvia- 

 tile or iEolian, the region may, however, be regarded as having been 

 as greatly elevated and tilted then as now, and the climate on the 

 whole as always semi-arid. 



THE CLASSIFICATION OF CONTINENTAL, LITTORAL, 

 AND MARINE DEPOSITS 



The term ^^ continental,''^ as used by Penck and Walther, applies to 

 all deposits upon the land, whether made by talus, creepage, by 

 rivers, by lakes, or by the wind. The fact that they are made upon the 

 visible surface of the continents and can show no relation to the sea 

 is the only bond of union among these otherwise unrelated- deposits. 

 Each is marked by a certain assemblage of characteristics, which 

 cannot be given in detail here, the most certain being the presence 

 of fossils of an abundant fresh-water or land life, and an absence of 

 marine. Where the deposit is unfossiliferous, there may frequently 

 be doubt as to its continental or marine origin. 



To marine or estuarine deposits belong all formations deposited in 

 the ocean, or its outlying portions, below the level of the average of 

 the lowest tides. Within this area, covering about three-fourths of 



