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ABSTRACT

Ronov and Migdisov (1971) very effectively document and explain the apparent compositional evolution of the shield and platform rocks of the world in terms of secular changes in source composition, the mode and intensity of weathering (in turn a reflection of secular variations in the composition of the biosphere, atmosphere, and hydrosphere), and compositional differentiation resulting from repetitious episodes of granitization, metamorphism, erosion, and sedimentation. Based on modifications of my own analysis of geosynclinal sediment compositions (Schwab, 1971), I suggest some additional comparisons and interpretations (Fig.1, Table I).