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Pitchstones and obsidians from Arran, Eigg, and Iceland, are shown to have small but distinct differences in their major-and trace-element composition, and it is concluded that acid magma in the British Tertiary as represented by the pitchstones from Arran and Eigg maybe more variable in composition than that of the Icelandic subprovince. Fourteen new analyses are represented.
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I. S. E. Carmichael (1962) studied this question.
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