Caledonite is a basic sulphate of copper and lead, concerning which there has long been doubt both as to its composition and its symmetry. It was first described by Brooke (1820) as a sulphato-carbonate of ortho-rhombic crystallization. The carbonate constituent was later thought to be derived from admixed cerussite, and Dana (1892) made it a sulphate. But the last published analysis of it by Berg (1901), discussed on a later page of this paper, seems to establish the correctness of the first determination. Although we have made no new analysis, we were able to make certain that crystals of the mineral dissolve with vigorous effervescence in nitric acid, depositing lead sulphate.