Most of the measurements reported here were made on samples of a macroscopically isotropic basalt collected from a roadstone quarry located south-east of Reykjavík, Iceland. Microscopically it has an aphyric texture, comprising euhedral laths of unaltered plagioclase averaging 0.2 mm in length, and anhedral augite microphenocrysts averaging 0.1 mm in diameter, with accessory anhedral oxides up to 0.1 mm in diameter. No free quartz was visible under either optical or SEM microscopy.