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Indiana University Studies 



silica and alumina are least attacked. The amount of lime 

 retained in the residual clays is quite variable. Analyses 

 often show an increase in the percentage of lime upon ap- 

 proaching- the parent rock (see Table No. 3, columns II, IV, 

 VI, and VIII). From the analyses of various limestones and 

 their residual clays one learns that in some cases alumina is 

 the most constant constituent, and in others, silica. If pyrite 

 is present in the limestone, the alumina may be removed in 

 part as aluminum sulfate. Where alkalies are almost wholly 

 lacking in the fresh rock, it is believed that one is safe in 

 saying that little or no silica is lost thru the action of alkaline 

 carbonates (Merrill, G.P., 1921, p. 217). As the Tables No. 

 1, 2, and 3 (column IV) show, most of the silica and alumina 

 of the parent rock is saved, and one is safe in assuming that 

 neither of these constituents is lost in appreciable quantities 

 in the decomposition of limestone. Magnesium carbonate is 

 generally more resistant than calcium carbonate, altho Van 

 Hise points out that ''in some instances more magnesia is dis- 

 solved than lime" (Van Hise, 1904, p. 516). A good portion 

 of the potash and soda remains in the clay, these materials 

 being less soluble than either magnesia or lime. C. H. Smyth, 

 Jr. (1913, Jour. Geol., Vol. 21, pp. 105-120) shows that the 

 solubility of potash is low as compared ^vith that of calcium 

 carbonate. The solubility of soda, however, is often fairly 

 high, altho variable. 



The amount of iron leached out varies greatly and irregu- 

 larly (see Tables No. 1, 2, and 3; column V). 



Doubtless this variability is dependent upon the fact that iron occurs 

 in both the ferrous and ferric forms — the former being more readily 

 soluble. Where the iron is mainly ferrous, one would expect that a 

 larger proportion would be dissolved; where ferric, a smaller proportion 

 (Van Hise, 1904, p. 517). 



In the complete decomposition of any rock, the ferrous oxide 

 (which has not been leached out) is almost entirely oxidized 

 to the ferric state. A striking characteristic of residual clays 

 is the high percentage of ferric oxide as compared with the 

 small amount of ferrous oxide (see analyses of residual clays: 

 Clarke, F. W., 1916, pp. 507-508; Merrill, G.P., 1921, p. 294). 

 In sedimentary clays and shales one usually finds a consider- 

 able portion of the iron as ferrous oxide. This is surprisingly 

 well showm in a composite analysis of fifty-one Paleozoic 



