The paper argues that Roger Bacon adhered to a unique form of geometrical atomism, according to which elemental matter can be analysed into cubic (when at rest) or pyramidal (when in motion) portions. Bacon addressed geometrical atomism from the perspective of the Aristotelian review, using his interpretation of Aristotelian principles to render the theory plausible. He was mostly concerned with solving the contradiction between the angular shapes of the portions and the shape of the elemental spheres. His motivation for doing (...) so, I argue, was his conviction in the applicability of geometry onto natural philosophy, and his goal was to render matter capable of geometrical analysis. In this way, he was convinced, geometrical properties can be considered efficient causes of matter’s spatial formation and motion. (shrink)