174 PRINCIPLES OF STRATIGRAPHY 



waters of springs and rivers, and in tilled soil, peat, decaying wood, 

 and bog ores ; it easily dissolves in water. The salts of the alkalies 

 and alkaline earths are soluble, the latter to a less degree. Some 

 of the apocrenates of the metals are soluble, others insoluble. 



The solvent action of these acids on siliceous rocks is of con- 

 siderable importance. Humic acid is said to decompose silicates. 

 The various humous acids absorb nitrogen from the air and form 

 azohumic acids, which, in the presence of alkaline carbonates, are 

 capable of dissolving silica, especially when it is in the amorphous 

 state, though even quartz is corroded. From this it follows that a 

 stream well supplied with these organic acids may acquire a high 

 percentage of silica in solution, unless the solvent power is neu- 

 tralized in some way. Since tropical streams carry the highest per 

 cent, of organic matter they are likely to be rich in dissolved silica. 

 Thus the Uruguay River, with 59.9% of the total solids as organic 

 matter contains 46.22% of the remainder, or 18.53% o^ the whole, 

 of SiOo, though the total salinity (inorganic) is only 0.04 permille. 

 In the St. Lawrence, along the borders of which "the granite of the 

 main and islands is almost everywhere covered with peat, full of 

 stagnant ponds of dark bog water" (Hunt), SiOo forms 23% of its 

 total dissolved inorganic matter, while the Ottawa, which "drains 

 a region occupied chiefly by crystalline rocks, covered by extensive 

 forests and marshes" (Hunt-i6: /^d; Julien-17 : jjj), has 33.7% 

 of its total dissolved inorganic matter as SiOg. 



Chemical Work of the Natural Waters. 



The chemical work of the natural waters is chiefly that of the 

 ground water, and is primarily confined to that zone or belt within 

 the earth's crust which is permanently occupied by this water, i. e., 

 that zone extending from the level of the ground water or the water 

 table to the greatest depth to which such water extends. This belt, 

 for which Van Hise estimates the great depth of 10,000 or 12,000 

 meters, but which others believe to be much less (see ante), con- 

 stitutes the belt of cementation, in contradistinction to the belt of 

 weathering, which forms the upper zone of the lithosphere above 

 the level of the ground water. In this latter belt the activities 

 are largely those of the atmosphere, but the ground water in passing 

 through it also performs a considerable amount of both chemical 

 and mechanical work, of which solution is the most pronounced. 



The chief chemical processes going on in the belt of cementa- 

 tion are solution and redeposition, hydration, carbonation and oxida- 



