CHEMICAL COMPOSITION IN SEDIMENTS 447 
becomes even more clearly defined and hence can be disclosed by the chemical 
analysis. Yet even this (chemical) method does not always give us the desired 
result, since there are rocks, such as arkoses, shaly sandstones, and sandy shales, 
which approach igneous rocks very closely in their chemical composition. 
In his chapter on the recognition of schists of igneous origin, 
he adds: 
. . as already mentioned, the chemical makeup of igneous rocks shows 
certain characteristic features in the proportions in which the constituent oxides 
aremmixeds 2.272): Moreover these features persist throughout the metamorphism 
so that the chemical analysis furnishes a second and usually a safer means of 
recognizing the igneous origin of a schist. 
Van Hise in his Tveatise on Metamor phism? says: 
A third criterion of great importance in the discrimination of metamorphosed 
sedimentary and igneous rocks is chemical composition. It has been shown that 
the materials for sedimentary rocks are sorted, that in general there is depletion 
in certain of the elements as compared with the igneous rocks, and that the pro- 
portions of the elements in the sedimentary rocks are therefore different from those 
in the igneous rocks. Furthermore, it has been shown that in the zone of ana- 
morphism the chemical composition of rocks is not greatly changed during the 
process of metamorphism, and it has already been seen that this is the only zone 
in which metamorphism is likely to result in the confusion of the two classes of 
rocks. ‘Therefore the metamorphosed sedimentary and igneous rocks which are 
likely to be confused have the compositions which are characteristic of their class: 
the metamorphosed sedimentary rocks, with minor modifications, have the 
chemical composition of muds, grits, sandstones, etc.; the metamorphosed igneous 
rocks have the compositions of granites, diorites, etc. For both sedimentary and 
igneous rocks there are wide variations in chemical composition, but in general 
the proportions of the elements are markedly different, in the two classes, as may 
be seen by comparison of the composition of the metamorphosed sedimentary 
rocks and that of the metamorphosed igneous rocks. The criterion has great value 
in some cases where the criterion of banding fails, for instance, in discriminating 
between metamorphosed sedimentary rocks and metamorphosed tuffs. The 
metamorphosed sediments have their characteristic compositions, while the 
metamorphosed tuffs, notwithstanding the fact that they may show banding, and 
thus closely resemble metamorphosed sediments, have the composition of igneous 
rocks. 
In discussing the composition of muds, he says:3 
. muds are likely to be deficient in the more readily soluble compounds. Of 
these the alkalies stand first, and of the alkalies sodium is more largely dissolved, 
t [bid., p. 13. 
2 Monograph XLVII, U. S. Geol. Survey, pp. 914, 915. 
3 Ibid., p. 889. 
