488 H. E. Gregory — Formation and Distribution of 



ance of the problem thus presented was pointed out by Pro- 

 fessor Davis,* and at his suggestion an attempt was made to 

 analyze the physiographic features resulting from ocean and 

 river work, in so far as they are related to the deposition of 

 gravel, with the hope that some constant features character- 

 izing conglomerates of various modes of origin might be recog- 

 nized. Because of the lack of quantitative data obtained from 

 field investigations, the results of this study are unsatisfactory : 

 they have, however, been found useful in field work, in direct- 

 ing increased attention to thickness, extent, and stratigraphic 

 position of beds of gravel and conglomerate, and for this 

 reason the original manuscript has been revised and the conclu- 

 sion here presented as a contribution to stratigraphy from the 

 viewpoint of physiography. 



Fluviatile Gravels. 



Although the importance of rivers as agents of erosion and 

 deposition is widely recognized, the methods and results of 

 stream work as conditioned by changing environment have yet 

 to be formulated. The fluviatile deposits which have been 

 studied in detail are too few in number and too exceptional in 

 character to serve as a basis for generalizations founded on 

 induction. Until many streams and typical streams have been 

 examined with reference to habit induced by volume and gra- 

 dient and load as affected by temperature, precipitation, crustal 

 movements, regolith, bed rock, valley form, and physiographic 

 age, we must be content with an outline sketch drawn largely 

 by the blunt-pointed pen of deduction. 



Comparative rates of subaerial and marine erosion. — Even 

 a superficial examination indicates that in preparing and dis- 

 tributing waste, rivers, assisted by the atmosphere, are distinctly 

 more effective than marine agents. A narrow zone of water takes 

 part in marine erosion ; the atmosphere reaches all land per- 

 manently or temporarily above water. The atmosphere and 

 the rivers may arrange the order in which their work is to be 

 taken up ; the sea has little choice. In developing subsequents 

 on less resistant rocks, thus carving inner lowlands as well as 

 reducing highlands, rivers are unimpeded ; the sea on the other 

 hand has no opportunity to reach more easily eroded materials 

 except by cutting its way through intervening strata. For the 

 world as a whole, therefore, the estimates of rates of subaerial 

 erosion are many times greater than those for marine erosion. 



On the basis of study of the Mississippi drainage basin, 

 subaerial erosion for the United States is placed at 1/4500 of a 



* Davis: An Excursion to the Grand Canyon of the Colorado, Bull. Mus. 

 Comp. Zool., vol. xxxviii, 1901, pp. 107-201. 



