191-'-] STEVENSON— THE FORMATION OF COAL BEDS. 533 



Drew's^^^ description of conditions along the upper Indus are 

 equally illustrative. Alluvial fans of tributary streams issuing 

 from the highlands have a radius of about a mile and a slope of 

 about 5 or 6 degrees, the extreme limits being 3 and 8 degrees. 

 There are, however, other fans with steeper slope, but they are not 

 alluvial — they originated as talus. The streams subdivide on the 

 fans, which increase with regular form as each stream yields its con- 

 tribution. The fans, originally independent, become united. Drew 

 gives a figure representing the conditions along 30 miles, where the 

 fans have become continuous and extend two miles into the valley. 

 The boundaries of the original fans are still recognizable. Rivers 

 cut across the deposits and the tributaries, in lowering the channels, 

 form new fans at their outlets. 



The conditions described by Drew resemble those seen along the 

 upper Rhone. There one finds some cones with steep slopes at their 

 head near the wall, clearly of talus origin, for they were formed by 

 streams issuing from hanging gorges, like the very steep deltas in 

 some Italian lakes, described by Taylor.^-'^ The high angle of slope 

 reported by some authors must be due to this mode of origin. Along 

 the upper Rhone as well as along the Adige, just as in the western 

 states, the slopes of the greater alluvial fans are usually gentle 

 almost throughout. Even the gigantic landslide, on the Adige near 

 Rovereto, has a gentle slope where cut by the railroad, though cov- 

 ered with huge blocks. The areal extent of the fans depends on the 

 width of the valley and the transporting power of the streams. 

 There would be notable variations in a slowly subsiding area, espe- 

 cially if the subsidence were not continuous. 



One may link this type of deposit with that of the great river- 

 plains by a reference to conditions observed on the upper Nile. 

 Falconer^^^ cites Russeger, who says that between Khartoum and 

 Sennaar, not less than 200 miles, the deposits are : 



^^^ F. Drew, " Alluvial and Lacustrine Deposits and Glacial Records of 

 the Upper Indus Basin," Quart. Joiirn. Geo!. Soc, Vol. XXIX., 1873, pp. 

 441-471. 



"" F. B. Taylor, " Post-Glacial changes of Altitude in the Italian and 

 Swiss Lakes," Bull. Geol. Soc. Amer., Vol. 15, 1904, pp. 369-378. 



^^ H. Falconer, " On the asserted Occurrence of Human Bones in the 

 ancient Fluviatile Deposits of the Nile and the Ganges," Quart. Journ. Geol 

 Soc, Vol. 21, 1865, pp. 372-379- 



