S6 PRINCIPLES OF STRATIGRAPHY 



ther be oxidized into hydrated sesquioxide of iron or limonite and 

 sulphuric acid, thus : 



4FeSO,+20+7H,0=2Fe,03+3FLO+4H,SO,. 



In each of these reactions heat is hberated. Where 11,8 is 

 Hberated, this may be further oxidized, especially in the presence of 

 bacteria, and sulphur may result thus: HoS-f-O^HoO+S. The 

 sulphur may later be oxidized to sulphuric acid. 



The change from sulphides to oxides without hydration involves 

 a diminution in volume. A change to magnetite means a decrease in 

 volume of 24 to 39 per cent., oxidation and hydration of pyrite, how- 

 ever, result in a total increase in volume of 3 per cent., while the 

 increase in the case of pyrrhotite is 25 per cent. 



Red and Ycllozv Colors of Soil Due to Oxidation. Within the 

 zone of active rock weathering, the iron compounds are transformed 

 to ferric oxide or hydrate, if the amount of oxygen is sufficient. 

 These iron compounds will impart their characteristic color to the 

 soil, which will be yellow or red, according to the simultaneous 

 occurrence or non-occurrence of hydration. Dehydration of ferric 

 hydrates may likewise change the color of yellow soil to red, as is 

 shown in brick-making, when the yellow clay on burning changes to 

 a red brick. Crosby has attributed the contrast of the color of soils 

 of high and low latitudes to the process of dehydration of the latter, 

 partly due to the greater heat of the southern latitudes and partly to 

 the greater age of the products of decomposition. He calls at- 

 tention to the fact that the red color is mostly a superficial feature, 

 varying usually from two to five feet in depth, and rarely exceeding 

 ten feet. The color is "usually reddest at, or near, the surface, 

 changing downward gradually, more rarely abruptly, through vari- 

 ous shades of orange to yellow ; while occasional complete sections 

 show the yellow changing through paler tints to gray or the color 

 of the underlying hard rocks." (Crosby-2o:77.) Russell has 

 shown that the grains of the rock are encrusted by a ferruginous 

 clay which contains both ferric oxide and alumina. 



In Nicaragua, east of the continental divide, the red color is 

 from 3 to 10 meters deep. (Hayes-45 r/i'c?, i^Q.) The abundant 

 vegetation here causes a transpiration of the moisture so that the 

 soil is not saturated with water. On the western side of Nicaragua 

 the color of the clay soil is blue, the iron being in the ferrous form. 

 Here dry seasons alternate with wet ones, and the deeply cracked 

 soil of the dry season is filled with the remains of vegetation, which 

 thus become a powerful reducing agent within the zone of weather- 

 ing. 



