«9i2.] STEVENSON— THE FORMATION OF COAL BEDS. 531 



of the Bonneville area show that the dips of layers are from 

 15 to 20 feet degrees near the top but they diminish downward, the 

 layers being disposed in sweeping parallel curves. 



Dejection cones are merely deltas formed in the air. They are 

 all due to stream transportation, but they differ greatly in form. 

 More than 35 years ago, Gilbert advised that the broad deposits 

 with gentle slopes be termed alluvial fans, and that the term alluvial 

 cones be restricted to forms with steep slopes and formed by the 

 smaller rapid streams. The distinction is important, as confusion 

 of the types in descriptions seems to have caused some miscon- 

 ception, and the steep slopes described by some writers are seem- 

 ingly regarded as typical of all. Hogard^-^ long ago found a slope 

 of 35 degrees in dejection cones composed of solid and angular 

 fragments. The greater slope is always shown by the less friable 

 material and cones made in air have much steeper slope than those 

 made in water. Surell's^-^ studies in alpine areas were made in 

 search of means for protecting mountain areas from ravage by rain- 

 fall. He found that the slope of a dejection cone depended greatly 

 on the material of which it is composed. Mud usually accompanies 

 torrents and, where abundant, it is the first material gathered. If 

 the flow be thick, the mud surmounts obstacles and in drying, espe- 

 cially if calcareous, it becomes tough, preventing access of air and 

 destroying vegetation. If it carry blocks or pebbles, it cements them 

 and in this way many breccias were formed in the areas examined. 

 The steepness of the slope, on which a deposit may be laid down, 

 depends on fluidity of the mass. Gravels are deposited on slopes 

 not exceeding two and a half per cent. ; fragments, 25 centimeters 

 in diameter or side, may be checked on slopes of two and a half to 

 5 per cent.; while blocks of half a cubic meter come to rest on a 

 slope of 5 to 8 per cent. If the blocks be very large, the current 

 drops them on rapids ; in any event they are dropped at the head of 



^^ H. Hogard, " Quelques observations sur les nappes et cones d'eboule- 

 ment et sur les lits de dejection des torrents," Bull. Soc. Geol. de France, 

 II., Vol. VII., 1850, p. 186. 



'^'A. Surell, " fitude sur les torrents des Hautes-Alpes," 2d ed., 1870, 

 Vol. I., pp. 37-39. 



