NO. 5 STUDY OF CHEMICAL DENUDATION — CLARKE 3 



position of a river water shows marked climatic features, and the magni- 

 tude of the load carried is affected in the same way. In the Temperate 

 Zone, alternations of freezing and thawing tend to break up the rocks and 

 so to render them more easily decomposed by percolating waters. With 

 even moderate humidity the activity of tbe waters is great, and large 

 amounts of material are transported by them. The rivers of Europe and 

 of the eastern United States are marked by more than the average salinity 

 of flowing waters, and estimates based upon them exclusively are sure to 

 be excessive. In an arid region, although salinity may be high, the run- 

 off is small, and much of an ostensible drainage area will contribute little 

 or nothing to the mineral matter of a stream. Arctic rivers flow to a con- 

 siderable extent over tundra, which is frozen during the greater part of 

 the year. They, therefore, have comparatively small influence in rock 

 solution, and much of their flow must be mere surface run-off. So also • 

 tropical rivers like the Amazon carry relatively light loads in solution, for 

 their courses are largely through swamps and forests where the soil, unre- 

 plenished by fresh rock decomposition, has been leached for centuries 

 and so deprived in great part of its soluble matter. 



To illustrate some of these differences in the composition of stream 

 waters a few typical examples may be taken. The analyses represent, in 

 ionic form, the percentage composition of the dissolved mineral matter, 

 and under the caption of denudation factor I give the number of metric 

 tons annually removed in solution from each square mile of a drainage 

 basin. The following waters have been chosen for this purpose : 



A. The St. Lawrence at Ogdensburg. Annual average as given by Dole, 



the denudation factor by Dole and Stabler." 



B. The Mississippi at New Orleans. Annual average; Dole and Stabler. 



Analyses by J. L. Porter. 



C. The Rio Grande at Laredo, Texas. Annual average as given by Dole 



from analyses by W. H. Heileman. Denudation factor from Dole 

 and Stabler. 



D. The Colorado at Yuma. Mean annual composition as given by Forbes 



and Skinner. Bull. 44, iVrizona Agric. Exper. Station, 1902. 

 Denudation factor by Dole and Stabler. 



E. The Amazon. A single analysis by P. F. Frankland, used by Reade. 



Two analyses by Katzer give lower salinities, and, therefore, a 

 smaller transport of material. For the total discharge of the 

 river I have taken the data given by Murray in the memoir already 

 cited, 



1 Dole and Stabler's figures for denudation are given in tons of 20<)0 lbs. They are here reduced 

 to metric tons of 2205 lbs. The analyses are recalculated to two decimal places. 



