Ore basalt, forming a sill or dike. To the eye and under a low power lens the rock appears dark and almost dense because of its unusually high percentage of ore. It is very fine grained with some odd, small phenocrysts of plagioclase (optical constants corresponding to An60), and a very few, small phenocrysts of olivine that are almost wholly resorbed and often obscured by tiny grains of ore (hematite?) which have grown along closely spaced cleavage planes. Some olivines have been effectively replaced by ore. The profusion of ore gives the rock its distinctive character.
Geology and petrology of the Pribilof Islands, Alaska