Abstract
The object of this paper is to demonstrate the widespread occurrence of zoned olivines in igneous rocks, to show that the zoning is continuous, that the composition varies from a magnesium-rich centre to an iron-enriched margin, and that these findings have an important bearing on the question of petrogenesis.I propose to express the composition of olivine by stating only the percentage of fayalite, as the rock-forming olivines belong mainly to the forsterite-fayalite series. There are, however, a number of names in use for the members of this series. Assuming that the two names forsterite and fayalite are insufficient for the needs of classification, it yet seems to me that all but four of the existing names are superfluous.

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