“ Specimens of a brilliant sky-blue colour, such as those found formerly in Wheal Hamblyn, near Bridestowe in Devonshire, and in Sardinia, are specially attractive in appearance; the colour is here due to the presence of the copper mineral chrysocolla. The hardness is 3, and the specific gravity 1⋅9. Chemically, it is a hydrous aluminium silicate, Al2SiO5. 5H2O. Allophane is always of secondary origin, resulting from the decomposition of various albuminous silicates, such as felspar. It is often found encrusting fissures and cavities in mines, especially those of copper and iron. ”