Although mining operations have been carried on in the Mendip Hills for a period of about 2000 years—probably since pre-Roman times, very little is known respecting the minerals that occur in the district. The reason for this is that most of the mines were exhausted previous to the development of mineralogy as an exact science; and furthermore only few specimens have been preserved in collections. During the nineteenth century operations were mainly confined to re-working the vast accumulations of refuse left by the earlier miners. This work was continued at the St. Cuthbert's Lead Works (1 mile ESE. of Priddy) up to the year 1908, since when all work has ceased on the Hill. The latest mining was for iron and manganese ores at Higher Pitts farm (1 mile SSE. of Priddy); here work ceased in 1893.

The optical dispersion phenomena of felspars at the alkalic and calic ends of the plagioclase series have been investigated by C. Viola and S. Kôzu, but for those of intermediate composition the data have been lacking. In view of the usefulness of the dispersion data of felspars for their determination in cleavage-flakes by the method to be described in the next paper, the present writer has made a study of the dispersion of certain intermediate plagioelases.

Recently H. E. Merwin employed an improved immersion method for determining refractive indices to identify certain salts of the system Fe2O3–SO3–H2O. In the present paper is described an application of the principle of his method to the determination of plagioclases in cleavage-flakes.

The existence of rotatory polarization is comparatively easy to detect in isotropic crystals and in uniaxial crystals in the direction of the optic axis ; but in a biaxial crystal the problem presents far greater difficulties, since in general the rotatory polarization is obscured by the double refraction. The possibility of its existence in a biaxial crystal was denied by É. Mallard (Traité de Cristallographie, 1884, vol. 2, p. 818). However, the persistence of rotatory polarization in a quartz crystal , made biaxial by pressure, brought this conclusion into doubt. Later work, both experimental and theoretical, by H. C. Pocklington (Phil. Mag., 1901, ser. 6, vol. 2, p. 861) established its presence in certain biaxial crystals.