The structure here called pillow lava has been variously described as cushion-like, sack-like, globular, spheroidal, ovoid, egg-shaped, ellipsoidal, lenticular, and concretionary. Once regarded as rare and peculiar, but now widely known in many countries, it has given rise to a great diversity of opinion among geologists concerning its real nature, and hence to an equal diversity of hypotheses as to the mode of its formation. Among the numerous causes to which it has been attributed are included: spheroidal weathering, spheroidal jointing, brecciation in situ, columnar jointing, with subsequent movement of the columns on each other, concretionary action, explosive eruption (bombs in agglomerate), normal flow of lava on land, viscous flow and fracturing of stiff lava, lava flow under water, intrusion into unconsolidated sediments, fracturing and partial remelting of lava crusts, and rapid cooling and parting into separate masses by the action of steam.
Sir Archibald . . .