Appendix—On the Microscopic Study of some of the Rocks of Northern Anglesey

Publisher Website
Google Scholar
Abstract
The following notes are based on the examination of about 100 slides made from rocks collected by Mr. Matley during his work in Northern Anglesey, and examined by me at his request. These rocks may be classed under the following heads:— (1) The Green Series. (2) The Ordovician Rocks. (3) The Quartzites and ‘Quartz-knobs.’ (4) The Results of Earth-movement; Crush-Conglomerates. (1) The Green Series. These rocks usually show their original clastic structure distinctly, although they are often considerably altered. They vary from coarse quartzose grits, through finer grits, to close-grained phyllites. The fragments consist of quartz-grains, clastic mica, and felspar, the first constituent, being present in greatest quantity. Secondary chlorite and sericite are usually developed, the latter becoming more abundant as the evidences of earth-movement increase, and certain bands in the fine grits are schists consisting of little but sericite. More usually the groundmass is made up of fine chlorite-flakes embedded in granulitic quartz. Epidoto in veins and isolated crystals is usually present. Secondary chalcedonic silica has often been deposited along planes and lenticles produced by the movement. In the finer-grained rocks strain-slip is present, and not infrequently the rock is brecciated and contains fragments derived from itself; one example (ST.A. 43, from the Craig Wen ‘quartz-knob’)has the structure of a mylonite. One rock (N.A. 18, near Taldrws, Cemlyn) has been detected in the Green Series which contains, in addition to angular grains of quartz, several fragments of

This publication has 0 references indexed in Scilit: