GEOLOGICAL IMPORTANCE OF SEDIMENTATION 345 



deposited the shallower the waters, since the bottom velocity 

 becomes less for the same surface gradient.^ A broad delta, unre- 

 strained by valley walls, unless these valley walls themselves furnish 

 sediment, is therefore advantageous for surface deposition. The 

 same may be said of deltas covered with vegetation, since Lyell 

 long since pointed out from observations on the Mississippi delta the 

 effectiveness with which vegetation entangles the sediment of flood 

 waters. Coarseness and abundance of material, signifying rapid ero- 

 sion at the headwaters of a river, are likewise favorable for subaerial 

 deposition on a steeper gradient. 



Summing up the foregoing discussion, it may be said that moderate 

 subsidence, originally shallow and quiet seas, broad and long delta 

 areas, and the presence of not far-distant mountain uplifts are all 

 favorable to a large proportion of subaerial delta deposition. In 

 addition, periodical floods over an arid delta region, and dense vegeta- 

 tion over a humid one, work to the same end. 



While there is probably not sufhcient data at hand to give quanti- 

 tative expression to these statements, and indeed they must vary for 

 every example, it may still give definiteness to thought to attempt to ex- 

 press in figures the ratios for the several classes of delta deposits. James 

 Ferguson, quoting F. Prestage, states that in 1861 careful simul- 

 taneous experiments were made as to the quantity of solid particles 

 held in suspension in the waters of the Matabangah (one of the dis- 

 tributaries of the Ganges) : first on leaving the Ganges, where it was 

 found to be i in 294 parts; while nearly at its junction with the Hoogly 

 the quantity was i in 884, proving that two-thirds had been deposited 

 en route in that short distance of not much over 50 miles, in a straight 

 line, though much longer by the meandering river. 



In answer to the question as to how much of the entire Ganges 

 alluvium is deposited upon the delta, and how much is carried to sea, 

 Ferguson points out that during the cold weather, when the rivers are 

 low, almost all of their silt will be carried to sea; but then the quan- 

 tity of water is small, and that little comparatively clear. At the 

 height of the inundation, when the river is overflowing its banks, at 

 least one-half is deposited inland.'' It is also pointed out that up to 



' J. J. Revy, Hydraulics 0} Great Rivers (London, 1874), p. 147. 

 2 " On Recent Changes on the Delta of the Ganges," Quarterly Journal of the 

 Geological Society, Vol. XIX (1863), pp. 350, 351. 



