FRESH-WATER STREAMS. 657 



mated to be 143-5 tons. The Rhine, according to Reade's calculations, removes about 

 92-3 tons in solution per square mile; the Rhone, 232 tons; the Danube, 72*7 tons; the 

 Garonne, 142 tons; the Seine, 97 tons. From these data the conclusion is reached that 

 over the world the average annual amount of rock-material dissolved and carried off by- 

 rivers is about 100 tons per square mile, of which about one half is probably calcium 

 carbonate, one fifth calcium sulphate, 7 silica, 4 each of magnesium carbonate and sul- 

 phate and sodium chloride, and 6 of alkaline carbonates and sulphates. The annual 

 amount of detritus brought down by the Danube is about soVo tn °* tne water > or three 

 times the amount of solids in solution. Taking the amount of solids removed mechan- 

 ically at six times that in solution, the whole amount for the globe of denuded material 

 annually would be 600 tons per square mile. 



While the land loses through erosion, the gain of the oceanic depressions, or of its 

 borders, is exceedingly small. C. G. Forshey, after stating that the Gulf of Mexico has 

 an area of 600,000 square miles, an average depth of 4,920 feet, and is about 85,000,000,- 

 000,000,000 (or 85 quadrillions) of cubic feet in contents; that its whole drainage area 

 is 2,161,890 square miles, and the amount of fresh water it receives from this area is 

 37"78 trillions of cubic feet; adds that if empty, it would take its tributary rivers at this 

 rate 2,250 years to fill it with water, or the Mississippi alone, 4,000 years. Conse- 

 quently, if all the rivers contribute on an average l-2600th their bulk of detritus, it 

 would take nearly 6,000,000 years to grade the depression up to the sea-level, or for 

 the Mississippi alone, about 11,000,000 years. 



Mr. Forshey estimates, after two years of observation by him, the amount of silt 

 pushed along by the Mississippi river three times that held in suspension, instead of one- 

 ninth, the amount allowed by Humphreys and Abbot. 



The quantity of wood brought down by some American rivers is 

 very great. The well-known natural " raft," obstructing Red River, 

 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 numbers of logs to the delta. 



4. Distribution of the Transported Material. 



The transported material of rivers is distributed — 



(1.) Along the channel, in sand-flats, and mud-flats, and over the 

 bottom, and also in lakes. 



(2.) Over the flood-grounds, supplying what these may annually 

 lose during floods, and adding, in places, to their height, thus making 

 alluvial formations, and, about lakes, lacustrine deposits or formations. 



(3.) Along the shores of the ocean, or of interior seas, making del- 

 tas, and shore and off shore deposits, and contributing to the exten- 

 sion seaward of coast lines and the making of marine formations. 



1. Along the Channel. — Sand-flats and sand-bars are made in broad 

 channels, when the flow is not rapid enough throughout the breadth 

 to sweep all the transported material down stream. The chief cur- 

 rent or currents make their own deep passage-way ; either side, the 

 detritus drops because of the slower flow, and raises the bottom more 

 42 



