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Indiana University Studies 



is derived from the original blue as a result, primarily, of 

 the oxidation of the ferrous oxide to the ferric, and, seconda- 

 rily, of the oxidation of the organic matter to some volatile 

 form, furnishes a problem worthy of further consideration. 

 The analyses, referred to above, do not support the theory 

 in regard to the iron oxide, since each sample of stone con- 

 tains the same amount. Thus, the quantity of Fe203 could 

 not account for the difference in color of the two samples. 

 However, the expected relation in organic matter is noted, 

 the blue stone containing 0.60 per cent, and the buff, 0.43 

 per cent. With a view to determine the cause of the colora- 

 tion, Hopkins submitted three sets of analyses, of the two 

 varieties of Salem limestone, which were not as satisfactory 

 as might be desired (Hopkins and Siebenthal, 1897, p. 309; 

 1908, p. 315). In all cases the amount of ferric oxide was 

 very small, less than 0.20 per cent, and did not substantially 

 support the theory with regard to the coloration. In two 

 of the three sets of analyses, the buff stone showed a slightly 

 higher percentage of FCsOg; but in one case the blue stone 

 contained the greater amount. Hopkins pointed out that "the 

 percentages are so small that it is doubtful whether the dif- 

 ferences are due to more than the possible errors incident to 

 manipulation". The analyses showed further, however, that 

 the organic matter in each case was only half as much in the 

 buff as in the blue stone. It should be borne in mind that the 

 difference in color of the blue and buff stone is not great, 

 sometimes scarcely perceptible in a hand specimen, but is 

 often quite distinct on a large block or quarry face. In face 

 of the limited evidence, the writer is inclined to believe the 

 slight difference in color is due, primarily, to variations in 

 the amount of organic matter. 



There is always a definite relationship between the color 

 of the clay partings of stylolite-seams and that of the asso- 

 ciated limestone. Stylolites of the blue stone bear black caps ; 

 those of the buff stone, brown caps. From the analyses 

 (Tables No. 4 and 5), one finds that the black residual clay 

 from the blue stone contains 19.14 per cent organic matter 

 and 8.22 per cent ferric oxide; whereas the brown clay of 

 the buff stone has but 7.94 per cent combustible matter, and 

 14.41 per cent ferric oxide. In either clay the ferric oxide 

 exists in more than sufficient quantity to produce a reddish 



