1913] Lawson: Designation of Alluvial Fan Formations 327 



its descent from the mountains to the valley, and the result is 

 in general a fairly regular gradation ; although large blocks are 

 of common occurrence far down the slopes of the fans, due to 

 exceptional rushes of water from the mountain canons. 



The material in general is but little rounded by attrition, 

 being prevailingly angular or at best sub-angular. There may, 

 however, be admixtures of well water-worn pebbles derived from 

 old conglomerates in the moiintains whence all the detritus comes. 

 Some of the larger blocks may have a rounded or boulder-like 

 shape, but this is due to exfoliation rather than attrition. 



Use of the Term Conglomerate. — Such alluvial fan deposits 

 when they become cemented, and so become coherent rocks, may 

 in accordance with current usage be called breccia. But this 

 word, as I have pointed out, is so overburdened with meanings 

 that it has no particular connotation suggestive of what kind of 

 rock we are speaking of, other than that it is composed of 

 angular fragments ; and an aggregate of angular fragments may 

 originate in several different ways. The insufficiency of the term 

 breccia as a designation for the kind of rock here considered is 

 well illustrated in the use of the term Gila Conglomerate for the 

 Quaternary alluvial fan deposits of Arizona. This term was 

 first used for this formation by Gilbert 1 in 1873, and has more 

 recently been adopted by Ransome. 2 But Ransome's descrip- 

 tion of the formation makes it clear that it is different from the 

 rocks to which the term conglomerate is usually and properly 

 applied. Indeed, he says that "the formation might appropri- 

 ately be termed a breccia.'" Geologic usage and formal defini- 

 tions are fairly consistent in applying the term conglomerate to 

 an aggregate of well rounded water worn pebbles, and there can 

 be little doubt of the advantage of adhering to this practice. 



It is the purpose of this paper to raise the question from a 

 general point of view of the advantage or disadvantage of con- 

 tinuing the use of the term conglomerate for formations having 

 an origin geologically distinctive from that of ordinary con- 

 glomerate, and physical characteristics which are those of a 

 breccia; (2) to call attention to the insufficiency for geological 



i Wheeler Survey, vol. Ill, 1875, p. 540. 



•! IT. S. G. S. Prof. Paper no. 12, 1903, p. 47. U. S. G. S., folio 111, p. 5. 



