140 GEOLOGY 



A comparison of the composition of rain-water with that of 

 springs and rivers gives some idea of the solvent work of water. 

 From a stud}' of the water of 19 of the principal rivers of the world 

 Murray has compiled the following table 1 showing the amount of 

 mineral matter in average river water: 



Tons in a 

 Constituents Cubic Milo 



Calcium carbonate (CaCOj 326,710 



Magnesium carbonate (MgCO a ) 1 12,870 



Calcium phosphate (Ca 3 P 2 O s ) 2,913 



Calcium sulphate (CaSO.,) 34,361 



Sodium sulphate (Na 2 SO 4 ) 31,805 



Potassium sulphate (K 2 SO 4 ) 20,358 



Sodium nitrate (NaNO 3 ) 26,800 



Sodium chloride (NaCl) 16,657 



Lithium chloride (LiCl) 2,462 



Ammonium chloride (NH 4 C1) 1,030 



Silica (SiO 2 ) 7 I. .177 



Ferric oxide (Fe 2 O 3 ) 13,008 



Alumina (A1 2 O 3 ) 14,315 



Manganese oxide (Mn 2 O 3 ) 5,703 



Organic matter 79.020 



Total dissolved matter 762. ">S7 



The 6,500 cubic miles of river water, which Murray estimates 

 flows to the sea annually, would carry about 4,975,000,000 tons 

 of mineral matter in solution. 



ANALYSIS OP EROSION 2 



Erosion is the term applied to all the processes by which earthy 

 matter or rock is loosened or removed from one place to another. 

 It consists of several sub-processes, namely, weathering, transporta- 

 tion, corrasion, and corrosion. 



Weathering 



Weathering is the term applied to nearly all those natural pnx-- 

 esses which tend to loosen or change the exposed surfaces of rock. 



1 Scot. Geol. Mag., Vol. Ill, p. 76. 1887. 



2 An excellent discussion of this subject is given by Gilbert in The Henry I 

 Mountains, pp. 99 et seq., and more briefly in the Am. Jour. Sci., Vol. XII 

 p. 85, et seq., 1876. 



