Granophyre from the roof zone of the Muskox intrusion, Northwest Territories, Canada, with a skeletal quartz crystal (violet, ˜0.17 millimeter long) surrounded by optically continuous vermicular quartz intergrown with alkali feldspar (blue). The intergrowths reflect the final crystallization of a magma rich in silica and alkalis that coexisted with an underlying silica-poor magma. See page 708. [Photograph by Brian W. Stewart, California Institute of Technology, with cross-polarized light and a 575-nanometer retardation plate]

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