THOUGH silica (SiO2), either free or combined, is the commonest constituent of the earth's crust, and is estimated to constitute sixty per cent, of the first ten miles depth, there are no sources of silica sufficiently pure for fusion known in Great Britain. The highest grades of transparent fused silica “glass “are made from rock crystal, or from deposits of minute crystals, commercially known as geyserite, which occur in Germany, Sweden, and elsewhere. True geyserite, however, is a semi-hydrated silica of the opal type, deposited by water containing dissolved SiO2. The celebrated pink and blue terraces in New Zealand, which were destroyed by the volcanic eruption of Mount Tarawera in 1886, consisted of silica in the form of geyserite. Pure silica occurs also rarely in other crystalline forms known as tridymite and crystobalite. Opaque fused silica ware is made by the fusion of pure silica sand, of a silica content of about ninety-nine per cent.