In this paper is described an apparently hitherto scientifically unnoticed mode of alteration of olivine-dolerite in the intrusive sill at Ible near Matlock.

The igneous rocks of Derbyshire, locally called ‘toadstones’, constitute a well-known basic series of both contemporaneous and intrusive rocks. They comprise lava-flows, sills, a few thin dikes, agglomerates, and tufts. All occur in beds of Lower Carhoniferous age, and are confined to the upper half of the known thickness of the Carboniferous Limestone, with, in the south of the county, the overlying Limestone Shales. They have been described, in recent years, by Sir A. Geikie (‘Ancient Volcanoes of Great Britain’, 1897, vol. 2); II. H. Arnold-Bemrose (Quart. Journ. Geol. Sot., 1894, vol. 50, pp. 603-644; 1907, vol. 63, p. 241, et seq.); and H. C. Sargent (ibid., 1918, vol. 73, pp. 11-25).