522 STEVENSON— FORMATION OF COAL BEDS. [November 3, 



McGee's- observations in the Sonoran district of Arizona and 

 Mexico are equally instructive, though dealing with a somewhat dif- 

 ferent type of phenomena. That district is within the arid region 

 and the valleys are sand wastes, with shallow channel ways which 

 are dry during all but, perhaps, five days in each year. The moun- 

 tains are scarred by barrancas or stream-worn valleys, which end 

 abruptly at the plain. The streams during the greater part of the 

 vear are mere threads ; but, during thunder gusts or cloudbursts, the 

 old channels are filled and new ones digged, though the flow may 

 last but a few minutes or at the most a few hours. The stream 

 gathers loosened rock masses in the mountains, hurls them down 

 slopes into the barrancas, dashing them to fragments and carrying 

 the debris to the edge of the plain. There the coarser materials are 

 dropped, but the finer stulT is transported beyond as a sheetflood 

 until the water disappears by absorption or evaporation. The incli- 

 nation of the channel ways is not far from 300 feet per mile in the 

 mountains, decreases to 200 feet at the edge of the plain and to 50 

 feet at the end of the torrential area, several miles away. The 

 flooding of the plain has, in some cases, a width of ten or more miles. 



McGee saw one of the floods in 1894. It came abruptly, a mass 

 of water thick with sand, foaming and loaded with twigs and dead 

 leaves. It advanced at first with " racehorse speed " but the velocity 

 diminished quickly owing to evaporation and the flood died out in 

 irregular lobes. The depth was not more than 18 inches on the 

 lower border, where the width of the muddy flood was about half 

 a mile. The noteworthy feature in this connection is that the mass 

 of slime, moving with so great speed, had no injurious eft'ect upon 

 the clumps of mesquitc bushes, scattered here and there over the 

 slope. When the flowing mud reached those clumps, its speed was 

 checked momentarily ; its course was diverted and it moved along- 

 side at twice to thrice the ordinary rate. After the flood was over, 

 the most striking observable efi^ect was the accumulation of twigs 

 and branches against the clumps of shrubbery and other obstacles. 



Russell's notes on the Yahtse river of Alaska, to be considered 



'W J McGce, "Sheetflood Erosion," Bull. Geol. Soc. America, Vol. 

 VIII.. i<S()7, pp. 87-112. 



120 



