In a reexamination of microlites and arborescent growths in pitchstones from Arran, off the west coast of Scotland, it is shown that biotite is a common constituent (or as chlorite pseudomorphs) and that clinopyroxene is often present, as well as hornblende. In discussing the chemical composition and experimental work on crystallization, the 4-4.5 percent H 2 O content shown by the pitchstones is taken as the equilibrium amount in the magma at 800 degrees C + or - and a depth of about 2 km. The compositions of several pitchstones have been plotted in the system NaAlSi 3 O 8 -KAlSi 3 O 8 - SiO 2 at 1000 kg/cm 2 water vapor pressure, and it is shown that the Arran pitchstones and several others are in the central portion of the quartz-feldspar boundary curve. Those lying outside are suspect, either as inferior or unrepresentative of the liquid from which they formed.

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