The pillow-laves are a group of basic igneous rocks that occur, in our experience, only as submarine flows, and very frequently exhibit ‘pillow-structure’. A lava-flow of this type is composed of sack-shaped masses, globular or elongated, and varying in size. The external surface of the pillows may be compact, but in their interior there are numerous cavities often concentrically arranged. First described in Britain by Nicholas Whitley (1), and figured also by De la Beche (2), they have become generally known through the work of Teall (3), Raisin (4), Reid and Dewey (5), and their importance among the Palæozoic eruptive rocks of Britain is now well established.