Abstract
There is a rough parallelism between the course of variation of composition of successive fractions of magma as shown by (1) successive stages in the crystallization of certain basalts (phenocrysts, groundmass, interstitial glass), (2) successive superposed layers of certain stratiform sheets such as the Skaergaard, Bushveld, Duluth, etc., (3) successive members of certain related series of successive intrusions. If these are interpreted as due to differentiation the trend of variation in composition is from gabbroic or noritic in the direction of concentration of iron, titanium, and phosphorous up to a certain maximum (melanocratic gabbro facies) followed by a sharp decrease of these constituents and the development of a salic residuum. This is suggested as possibly one line of magmatic differentiation though it is recognized that there are others as for example by way of augite pyroxenites to end in ilmenite‐magnetite deposits without phosphorous, and so forth.
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