METAMORPHOSED SEDIMENTS AND IGNEOUS ROCKS. 915 



rocks. Furthermore, it has been shown that in the zone of anamorphism 

 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 charac- 

 teristic 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 botli 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 meta- 

 morphosed 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 meta- 

 morphosed 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. This criterion has been successfully used by Adams in 

 discriminating between the Grenville gneisses of the Original Laurentian 

 area and the lower granitoid gneiss. The former have the composition, as 

 he has shown, of pelites, whereas the lower gneisses have the composition 

 of granites. 



While this criterion of chemical composition is one of the most impor- 

 tant and has a wide application, it is not infallible. It has already been 

 seen on pages 497, 870-874 that in such places as the Gulf of California, 

 and certain other localities, the sediments are little decomposed or sorted, and 

 thus there is produced a sedimentary rock which has essentially the same 

 composition as the original igneous rocks from which it was derived. 



(4) Closely connected with (3), in fact dependent upon it, is a fourth 

 criterion, that of mineral composition. On account of the difference in 

 chemical composition which has already been mentioned, when the rocks 

 are metamorphosed there results different mineral compositions. It has 

 been seen that the depletion of certain elements in the sediments and 

 the sorting of the material produce comparatively few kinds of original 



