Summary: 1. Ankerite and ankeritic calcite are described from veins in the Coal Measures on the coast at Hartley and from various seams in the Northumberland coal-field. Evidence is given that, in the portion of the coal-field examined, both minerals are of constant composition, and that the deposition of the ankerite preceded that of the ankeritic calcite.
2. The ankerites are members of an isomorphous series consisting of dolomite MgCO3.CaCO3 and ferrodolomite FeCO3.CaCO3, with small amounts of mangandolomite MnCO3.CaCO3, and they may hold up to 20 % of CaCO3 a in solid solution.
3. The refractive indices (ordinary ray) of the ankerites are examined, and the value deduced for FeCO3.CaCO3 is 1•765.