38 PRINCIPLES OF STRATIGRAPHY 



or cla}-. This commonly takes place in the presence of COo, the 

 formula for the alteration of orthoclase into kaolin and c[uartz being 

 (Van Hise-iooijcVp) : 



2KAlSi308+2H,0+C02=H,ALSi,09+4SiOo+K,C03. 



The change to kaolin is accompanied by a volume decrease of 

 54.44 per cent. ; the change to kaolin and quartz, as in the above 

 reaction, by a decrease of 12.57 P^^ cent. The change by hy- 

 dration of hematite to limonite is accompanied by an increase in 

 volume of 60.72 per cent., and may be expressed by the formula : 

 2Fe203 + 3H2O = 2Fe203.3HoO. Hydration under the atmos- 

 phere often goes on so rapidly as to suggest slaking of quicklime. 

 This has been observed by Merrill in the granitic rocks of the Dis- 

 trict of Columbia (66:188), and by Derby in the sedimentary rocks 

 of railway cuttings in Brazil (Branner-6). In both cases the 

 rocks exposed at the surface soon break into powder, al- 

 though in fresh exposures they may be so strong as to require 

 blasting. 



Dehydration. The loss of combined water or dehydration is a 

 phenomenon of regions of high temperature and low humidity. It 

 probably occurs on the largest scale in regions of alternating wet 

 and dry climate combined with high temperature. Such dehydra- 

 tion tends to change the limonite or gothite of the soil to hematite, 

 with a corresponding change in color from ochery to red. Crosby, 

 Dana, and Russell have attributed the red color of the soil of south- 

 ern regions to this process of dehydration under the influence of high 

 temperature and, to some extent, under that of age. Partial dehy- 

 dration of such compounds as the zeolites, colloidal silicic acid, 

 aluminum hydroxide, etc., may take place on a considerable scale in 

 regions of high temperature and little moisture. 



Carbonation. The atmosphere furnishes much of the CO2 

 which combines with the bases to form carbonates, either directly or 

 indirectly through the mediation of plants. Like oxidation, it is 

 thus a superficial phenomenon. As we have seen, the oxidation 

 (decay) of the vegetable matter on the surface of the earth liber- 

 ates much COo, which may be taken up by the waters entering the 

 soil and used for carbonation. The air, as already noted, contains 

 about 3.4 parts by volume in 10,000 (4.5 by weight). Rain water 

 contains from 0.22 to 0.45 per cent. ( AIerrill-66:/79) of the vol- 

 ume of the water or 0.00044 to 0.00089 P^^ cent, by weight. In the 

 pore spaces of soils much more COo exists, as shown by the follow- 

 ing table by Boussingault and Lewy copied from Merrill (66:1/8) : 



