644 DYNAMICAL GEOLOGY. 



The bulk may be calculated by taking 1.9 as the specific gravity of the material. 

 The total annual discharge of sediment from the Ganges has been estimated 

 at 6,368,000,000 cubic feet. 



Besides the material held in suspension, as these authors observe, 

 the Mississippi pushes along into the Gulf large quantities of earthy- 

 matter ; and, from observations made by them, they estimate the 

 annual amount thus contributed to the Gulf to be about 750,000,000 

 cubic feet, — which would cover a square mile 27 feet deep ; and 

 this, added to the 241 feet above, makes the total 268 feet. 



The quantity of wood brought down by some American rivers 

 is very great. The well-known natural "raft" obstructing Ked Eiver 

 had a length, in 1854, of thirteen miles, and was increasing at the 

 rate of one and a half to two miles a year, from the annual 

 accessions. The lower end, which was then fifty-three miles above 

 Shreveport, had been gradually moving up stream from the decay 

 of the logs, and formerly was at Natchitoches, if not still farther 

 down the stream. Both this stream and others carry great num 

 bers of logs to the delta. 



3. Distribution of transported material. 



1. Alluvial formations in river-valleys. — Alluvial formations cover 

 usually a broad area on one or both sides of a river. They are in 

 general the basis of the flood-plain ; and the features of this plain, 

 as already described, are the exterior characteristics of the alluvium. 

 They are made from the material brought down by the stream, 

 especially during freshets, and consist of earth and clay, sometimes 

 thinly laminated, with some beds of pebbles, and occasionally 

 stones. These coarser beds are most abundant along the upper 

 portions of the stream, while towards the mouth — particularly in 

 the case of large rivers — the material may be wholly a fine silt. 



Logs and leaves are in some cases distributed through alluvial 

 deposits, but always sparingly ; for they are mostly destroyed 

 by wear or by decay. They rarely, if ever, accumulate in beds 

 fitted for making coal, being widely scattered by the currents. 

 Fresh-water and land shells are occasionally found in the beds. 

 Remains of other animals seldom escape destruction, unless buried 

 in a lagoon-portion of the flood-plain. 



As the range of height within which river-waters can work has 

 narrow limits, the thickness of the alluvial formations made by a 

 stream, in any given condition of it, is necessarily small. Even the 

 whole of the river-flat above the level of its bottom may not have 

 been deposited by the river in its existing state ; for the channel 



