68 



Indiana University Studies 



The writer wishes to consider (a) the decomposition of 

 Hmestones in general, showing the relationship between their 

 chemical constituents and those of their residual clays; and 

 {h) the chemical relations between the clay partings of stylo- 

 lite-seams and the associated limestones, showing that the 

 clays fulfil the requirements of a residual product of the lime- 

 stones in which they are found. 



It has already been pointed out that since limestones are 

 composed, for the most part, of calcium carbonate, their de- 

 composition is effected primarily by solution — especially in 

 humid climates (see p. 48). In the alteration of limestone 

 to residual clay, the soluble constituents of the parent rock 

 are leached out (in varying proportions, depending upon the 

 solubility) and the less soluble constituents are concentrated 

 in the form of a clay residue. By comparing the chemical 

 composition of an original rock with the composition of its 

 decomposed equivalent, one is able to obtain an approximate 

 idea of the loss of the various elements. In order to under- 

 stand best the changes which take place in limestones, refer- 

 ence should here be made to Tables No. 1, 2, and 3. In these 

 tables, column I is the analysis of the fresh limestone, while 

 column II is that of the residual product. Columns III, IV, 

 and V are calculated from I and II.' Column IV shows the 

 percentage of each constituent saved, assuming a certain sub- 

 stance to be constant (silica or alumina), while column V 

 gives the percentage of each constituent lost. Column III, 

 the percentage of loss for the entire rock, is derived by multi- 

 plying I by V. The supposition, however, that any element 

 is fixed in amount is erroneous, since the most resistant ma- 

 terials are attacked, to a limited extent, by carbonated waters 

 (Van Hise, 1904, p. 514) . In making calculations, the method 

 has been to choose the constituent in which the loss has been 

 least, and by this to gauge the loss of the other substances. 

 A study of the decomposition of limestone shows calcium car- 

 bonate to be removed in most appreciable quantities, while 



A 



'The formula employed in these calculations is as follows: = x; and 100 — 



B X C 



X = y, in which A = the percentage of any constituent in the residual material ; B = 

 the percentage of the same constituent in the fresh rock ; and C = the quotient obtained 

 by dividing the percentage of alumina (or silica, whichever is taken as a constant 

 factor) of the residual material by that in the fresh rock, the final quotient being 

 multiplied by 100. Then x equals the percentage of the original constituent saved, in 

 the residue, and y the percentage of the same constituent lost. (Merrill, G.P., 1921, 

 p. 188, footnote 2.) 



