446 J. BARRELL RECOGNITION OF ANCIENT DELTA DEPOSITS 



ing these limitations, it is thought that the criteria are sufficiently varied 

 and numerous to determine the conditions of origin of the great majority 

 of delta deposits. Finally, there should be emphasized the need of much, 

 broad study of a quantitative nature regarding modern conditions of 

 sedimentation to determine minutely the characteristics which become a 

 recognizable part of the buried formation as distinct from the passing 

 surface features. The development of distinctive criteria must be 

 studied, furthermore, in relation to the physiographic and climatic con- 

 ditions of orio-iu. This is a line of progress bea'un in the earlv davs of 

 geology, but then essentially of a qualitative nature, and by Lyell made 

 the basis of the interpretation of earth history. HaAing grasped this 

 idea the centers of scientific interest were transferred to the geologic 

 record, and the interpretations made by the generation of Lyell were 

 carried forward without material improvement till near the close of the 

 nineteenth century. It is clear, however, that a more accurate and 

 quantitative knowledge of that earth history which is now being recorded 

 is needed in order to obtain in turn a more accurate knowledge of the. 

 past. Many of the criteria which in this paper are considered somewhat 

 indefinite may become definite through a wide and more discriminative 

 study of the sedimentation now in progress. 



