THE VALUE OF CERTAIN CRITERIA FOR THE DETER- 
MINATION OF THE ORIGIN OF FOLIATED 
CRYSTALLINE ROCKS. II 
J. D. TRUEMAN 
University of Wisconsin 
PART II 
CHEMICAL COMPOSITION AS A CRITERION FOR THE DETER- 
MINATION OF THE IGNEOUS OR SEDIMENTARY 
ORIGIN OF FOLIATED ROCKS 
VIEWS OF OTHER WRITERS 
It has long been well known that igneous rocks exhibit certain 
characteristic regularities in the amounts and proportions of their 
oxides, such as are not observed among sedimentary rocks. The 
composition of the latter is in part dependent upon the relative 
solubilities of minerals under weathering conditions, but also to a 
large extent upon the somewhat erratic redistribution of material 
during sedimentation. H. Rosenbusch' was the first to point out 
clearly the possibility of the application of chemical composition as 
- a means of determining the original character of metamorphosed 
rocks. He considered that no important changes take place during 
dynamometamorphism because, first, many altered rocks possess 
a composition similar to that of certain normal igneous types and, 
second, on account of the preservation of clean-cut divisions 
between altered sedimentary strata of different composition. Asa 
distinguishing feature between altered igneous and sedimentary 
rocks he pointed out that in the latter the molecular ratio of ALO, 
to Na,O, K,O, and CaO was greater than 1. 
E. S. Bastin? has published the latest and most complete study 
regarding chemical composition as a criterion for the recognition 
of the original character of metamorphic rocks. From an examina- 
tion of available analyses Bastin has determined certain distinguish- 
1 Tschermak’s Min. petrog. Mitt., XII (1891), 40. 
2 Jour. Geol., XVII (1909), 445. 
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