INTRODUCTION 379 



do the record of earth history and organic evolution, a fundamental 

 question is therefore whether the strata were laid down in contact with 

 tlie air or beneath the level of the sea. If fossils are present they com- 

 monly give an answer, but the absence of fossils from many formations 

 leaves the problems of origin to be solved by other methods of attack. 

 It is important that the criteria which are used for such j^urposes should 

 be always subject to scrutiny in order that inherited errors may be de- 

 tected and further progress made toward refinements of discrimination — 

 refinements which though concerning snuiU details may yet result in a 

 disproj^ortionately large increase of knowledge in tiie interpretation of 

 the sediments. It is in delta deposits especially that the line of the 

 ancient strand is difficult to draw, since it is not here coincident with 

 the limits between erosion and sedimentation and the same formation is 

 made in part above, in part below the level of permanent water. It is 

 the purpose of this paper, therefore, to review the validity of the criteria 

 which have been employed to discriminate between terrestrial and sub- 

 aqueous sediments — criteria whose use is preliminary to a knowledge of 

 the physiography or climate of those former periods where knowledge is 

 drawn from the stratigraphic record ; since building on an insecure 

 foundation results in danger to the whole superstructure of knowledge. 

 But before a discussion of such criteria is given it is important that 

 the view should be accepted that at certain geologic times deltas may 

 have been of widely diiferent character and have varied greatly in im- 

 portance from the rather limited place which they now hold in the 

 physiography of the lands. The field of j^rogress in science is hedged in 

 by the limits of the mental angle of vision, and the range of hypotheses 

 should therefore be made broad before examining in detail the possibili- 

 ties of a problem. Tlie rise of geology as a science depended on the 

 establishment of the principle of uniformitarianism — that the present is 

 the key to the past. But applied too rigidly it narrows the visual angle, 

 since the scale or rate at which various processes work may vary greatly 

 in successive periods and their changing dominance is now believed to 

 have resulted at times in great climatic and geographic contrasts. The 

 highly variable importance of glaciation in geologic history may be used 

 as an illustration of a similarly possible wide variation from period to 

 period in delta-building. No a priori limit can therefore be set safely to 

 the maximum area and thickness which under favoring conditions deltas 

 may attain. In fact, as the successive geologic periods differ from one 

 another in character, it should follow that the importance of delta-build- 

 ing will vary from age to age according to the physiographic stage-set- 

 ting of the continents. A direct application of the growth of glacial 



