178 PRINCIPLES OF STRATIGRAPHY 



ther. Thus in the Lake Superior mining region it has gone to a 

 depth of lOO meters on an extended scale, while in a few exceptional 

 cases the depth affected has been 500 to 700 meters. In the San 

 Juan district of Colorado it is marked at a depth of 600 meters, and 

 occasionally is noted at 1,000 meters. In the Missouri-Kansas lead 

 and zinc district, on the other hand, oxidation scarcely extends be- 

 neath the level of ground water. 



In the belt of weathering, and to some extent in that of cementa- 

 tion, hematite and limonite are the common products of oxidation of 

 iron compounds. Throughout the remainder of the belt where oxi- 

 dation occurs, magnetite is the common product of oxidation of iron, 

 since this requires a smaller amount of oxygen in its production. 

 Oxidation of the sulphur of pyrite and marcasite produces a ferrous 

 sulphate of ferric oxide, and sulphuric acid, in the belts where oxy- 

 gen is abundant. In the major part of the belt of cementation, 

 where oxygen is not abundant, magnetite and sulphurous acid are 

 more likely to be formed. 



The oxidation of organic matter produces carbon dioxide and 

 water, which join the circulating ground-water, producing carbon- 

 ates in the belt of cementation. When the oxygen is all exhausted, 

 organic and other compounds may be taken into solution, and so 

 give the waters a reducing quality. Waters which have made long 

 underground journeys are especially likely to be in this state. Oxi- 

 dation of carbonates, and the liberation of COo, may cause a consid- 

 erable decrease in volume, amounting in some instances to 50 per 

 cent. This may counterbalance the increase from oxidation of inor- 

 ganic compounds, which, in some cases, is as much as 64 per cent. 

 (Van Hise-41 : 608.) 



CarbOxVATion. Carbon dioxide is produced in abundance in 

 regions of luxuriant vegetation, through the oxidation of the carbon, 

 and hence the waters entering the soil here will be rich in CO2, but 

 poor in oxygen. Conversely, in regions of little vegetation carbon 

 dioxide will be lacking, but oxygen may be carried in considerable 

 quantity by the water sinking into the ground. If this water en- 

 counters buried carbon in the form of coal beds or of other types, 

 the oxygen will be used up and carbon dioxide produced. The same 

 result is caused by the decomposition of carbonates in the zone of 

 cementation. Carbon dioxide is also produced in enormous quanti- 

 ties in the zone beneath the ground water (the zone of anamorphism, 

 see Chapter XIX) by silication or the union of silicic acid with the 

 bases of the carbonate rocks, and the simultaneous liberation of the 

 CO. (Van Hise-41). 



The process of carbonation, or the union of COo with bases, 



