4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 56 



F. The Uruguay. Mean of two analyses by Kyle and Schoeller.^ Denu- 

 dation computed from Murray's figures for the discharge and 

 drainage area. 



Gr. The Nile. Annual average as shown by Letheby's analyses.^ Drain- 

 age area from Murray. Total dissolved matter, 20,772,400 metric 

 tons per annum according to Chelu.' 



COs 



so, 



CI 



N03 



Ca 



Mg 



Na 



K 



SiOa 



AI2O3, FezOs. 



:} 



100.00 100.00 100.00 100.00 100.00 100.00 100.00 

 Denudation factor 105. 98. 22.7 46.2 53. 50.4 16. 



These analyses tell a consistent story. The St. Lawrence represents a 

 humid area, and every square mile of it contributes something to the 

 salinity of the waters. Hence a solvent activity of 105 tons per square 

 mile. The Mississippi is a blend of waters from humid regions on the 

 east and arid or semi-arid plains to the west, and its load is, therefore, 

 proportionately lowered. The Colorado and Eio Grande are typical of 

 areas which are largely arid, and many square miles of their nominal 

 drainage basins add nothing to the saline burdens. The Amazon and the 

 Uruguay show the low salinity of the tropical forest regions, and the Nile 

 flows for a thousand miles of its course through a desert. The saline 

 matter of the Nile comes in great part from the regions south of Khar- 

 toum. It has no tributaries worth considering north of the Atbara. 



Even on the purely chemical side the analyses are instructive. The water 

 of the St. Lawrence is essentially a calcium carbonate solution ; in the Colo- 

 rado and Eio Grande sulphates and chlorides predominate, and calcium 

 is relatively low; the Mississippi has an intermediate character. As a 

 rule the waters from arid regions resemble the Colorado; those from 

 humid areas are more like the St. Lawrence. This, of course, is not an 

 absolute rule, but it holds fairly well in a broad general way. It is a 

 distinct aid in discussing the larger problems of chemical denudation. 



1 See " Data of Geochemistry," Bull. 330 U. S. Geol. Surv., p. 75. = Ibid., p. 82. 



2 Le Nil, I'Egypt, le Soudan. Paris, 1891. 



