470 A TREATISE ON METAMORPHISM. 



position of the parent rocks. Another illustrative region, described by 

 Hayes, is that of Nicaragua east of the continental divide. The red color 

 in this belt is from 3 to 10 meters deep." Both of these regions are those 

 of rather high temperature and abundant precipitation throughout the year, 

 and therefore abundant vegetation. They therefore well illustrate the con- 

 ditions under which decomposition of the rocks, including oxidation, takes 

 place. The very abundant transpiration by the luxuriant foliage (see p. 422) 

 doubtless is a very important factor in preventing the soil from becoming 

 saturated with moisture for any considerable period, and thus the conditions 

 for oxidation are maintained. 



One would suppose that in the soil, where oxidation of organic and 

 inorganic matter is active, the amount of oxygen in the gases would be less 

 than in the atmosphere, and observations by Fleck, Letts and Blake and 

 others confirm this conclusion. The amount in the atmosphere is 20.92 per 

 cent by volume, whereas Fleck, as cited by Letts and Blake, finds that at 

 a depth of 2 meters the minimum is 16.33 per cent and the maximum 19.39 

 per cent; at 4 meters the minimum is 15.67 per cent and the maximum 

 16.79 per cent; at 6 meters the minimum is 14.94 per cent and the maximum 

 14.85 per cent.'' These last numbers are only a little more than two-thirds 

 the full amount of oxygen of the atmosphere. It appears that as depth 

 increases, and therefore as the air of the soil is further removed from its 

 source of supply — the oxygen of the atmosphere — there is a steadily 

 decreasing- amount, and this fact must be explained by the consumption of 

 oxyg-en by the oxidation of organic and inorganic matter. 



In some. cases the amount of oxygen is little more than half as great 

 as that in the air. The deficiency varies directly, although not in simple 

 ratio, with rapidity of oxidation and with the depth below the surface where 

 the process takes place." 



While oxidation is the normal process in the belt of weathering, under 

 certain conditions deoxidation may take place. Under such circumstances 

 gray or white soils and subsoils are formed. This is illustrated by the 

 western division of Nicaragua, where Hayes states that the clays are blue, 



"Hayes, C. W., Report of the Nicaragua Canal Commission, Appendix II, Geologic Report, 

 1899, pp. 128-129. 



''The last number must be a misprint. Letts, E. A., and Blake, R. F., The carbonic anhydride 

 of the atmosphere: Scientific Proc. Royal Dublin Soc, vol. 9, new ser., pt. 2, p. 215. 

 "Aikman, C. M., Manures and manuring, Blackwood & Sons, London, 1894, p. 100. 



