IV.—Note on the Keratophyres of the Breidden and Berwyn Hills

Extract

It may be of interest to note the occurrence of a somewhat rare and interesting rock, named keratophyre, at two easily accessible localities in a district where it has been hitherto unknown—at Moel-y-Golfa, in the Breidden Hills, and in the Berwyn Hills.

The Breidden Hills.

Moel-y-Golfa was visited by me in 1899, and I collected a few specimens, which, however, remained unexamined until a few days ago when I needed one for teaching purposes. On examination of a thin section the plagioclase proved to be albite, idiomorphic and tabular in form, set in a matrix of chloritic decomposition products, doubtless the remains of pyroxene. The ratio of albite to pyroxene must have been about two or three to one. This composition is sufficient to place the rock in the keratophyre group, but confirmation was obtained by comparison with a slide of the well-known keratophyre of Hüttenrode, in the Harz, the rocks proving almost identical both in structure and composition. The determination of the felspar was made by Becke's bright line method, which proved its refractive index to be everywhere slightly lower than that of balsam.