THE WORK OF STREAMS 83 



Pennsylvania, in 1889, and swept away large rocks, twenty-ton loco- 

 motives, and massive iron bridges as easily as, under ordinary cir- 

 cumstances, the river could move sand. A fall of one foot in a mile 

 is quite sufficient to carry a river steadily onward ; one foot in a 

 thousand feet will make a fairly rapid river; one in two hundred, a 

 torrent. 



Water Wear. — The pebbles and sand carried by the streams are 

 worn away by their impact against the bed rock and by striking 

 against each other. The result of such wear is the production of 

 rounded stones. In mountain streams the angular fragments from 

 the talus are rounded before they have been carried a mile. 



Solution. — In addition to the sediment carried by the force of the 

 current, the waters of every river contain a large amount of mineral 

 matter in solution. This is largely obtained from springs, but also 

 from the run-ofF and by the solution of the stream bed. The amount 

 in any stream varies with the season, being greater in proportion to 

 the volume of water in dry than in wet seasons, since in the former 

 the water is largely underground water. The small river Thames, 

 England, carries to the sea about 348,230 tons of dissolved minerals 

 a year, and the Mississippi River carries 113,000,000 tons. "The 

 Rhine carries enough carbonate of lime to the sea each year for the 

 annual formation of 3,320,000,000 oyster shells of the usual size." 

 (A. Geikie.) It is estimated that in every 5000 years rivers carry their 

 own weight of minerals in solution to the sea. The weight of the 

 dissolved matter carried to tidewater by the streams of the United 

 States (270,000,000 tons) is more than half that of the sediment 

 (513,000,000 tons). 1 " The tons per square mile per year removed 

 from different basins show interesting comparisons. In respect 

 to dissolved matter the southern Pacific basin heads the list with 

 177 tons, the northern Atlantic basin being next with 130 tons. 

 The rate for the Hudson Bay basin, 28 tons, is lowest; that for the 

 Colorado and western Gulf of Mexico basins is somewhat higher. 

 The denudation estimates for the southern Atlantic basin correspond 

 very closely to those for the entire United States." (Dole and 

 Stabler.) 



Vertical Erosion (Corrasion). 2 — By erosion (Latin, erodere, to gnaw 

 away) streams are able to cut down their valleys. This may be 



1 Water-Supply Paper, U. S. Geol. Surv. No. 234. 



2 The terms corrasion, abrasion, corrosion, erosion, and denudation have sometimes been used 

 rather loosely in geological and geographical literature. In this work corrasion (Latin, corra- 



