Summary: A new copper arsenate, dimorphous with cornwallite, has been found on specimens from five localities in Cornwall, one in Devon, and one in Cumberland. The name cornubite (from Cornubia, the medieval Latin name for Cornwall) is proposed for the new mineral. Chemical analyses of cornubite and cornwallite agree well with Cu₅(AsO₄)₂(OH)₄; sp. gr. cornubite 4·64, cornwallite 4·52. X-ray study suggests that the unit-cell of cornubite has a volume of 228 Å.³ or a simple multiple thereof. New X-ray data for cornwallite (a 17·33, b 5·82, c 4·60 Å., β 92° 13′) agree well with L. G. Berry's except for a. Powder data for both are given.