The structural state of phenocryst and groundmass plagioclase from a thick prophyritic flow of the Columbia River Basalt ranges from high to intermediate disorder. Two groups have been differentiated: (1) Groundmass plagioclase in a lower structural state than phenocrysts; the groundmass plagioclase is more calcic than An60 and of small size, (2) Groundmass plagioclase in a higher structural state than phenocrysts; groundmass plagioclase generally less calcic than An50 and of large size. In addition, the latter group contains smaller amounts of glass than those of group one. The reversal in structural state relationship between the phenocryst and groundmass plagioclase of (2) is probably due to the sluggishness of the transformation from disorder to order in plagioclase less calcic than An50.

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