On Varieties of Serpentine and Associated Rocks in Anglesey

Abstract

I. G eneral D escription. (1) Mainland of Anglesey—south of Valley Station, O n the eastern side of the ‘strait’ is the gabbro a t Graig-fawr described in a former paper by one of the present authors. A serpentine, there indicated as ‘about 70 yards to the north’ of this mass, has been quarried further, and parts of the rock exhibit aslightly variolitic character. The junction of schist with serpentine or ‘ophicalcite’ noticed and figured ( op. cit. p. 41, fig.1) is in a low cliff by the shore almost due south of Graig-fawr and near a boss of variolitic serpentine, described on p. 281. This junction is not easy to interpret ; but further study of the rocks affords some evidence favourable to the hypothesis of a fault. The green schists have a squeezed appearance, but the breeeiation and crushing of the rock adjoining them are still more marked. In it are angular fl'agments coloured by haematite, the whole being permeated by carbonates~; hence the rock is one of the so-called ‘ophiealcites.’ Near by are foliated masses, including a squeezed diallage-enstatite-rock associated with serpentine and gabbro. Lenticles of t, ho rock have escaped, while surrounding parts have been crushed. Bosses of gabbro occur in the fields between here and Tyddyn-y-cob Inlet, where the rock becomes schistose. The gabbro reappears inland beyond the sluice, and extends as tkr as the outcrops of schist near Glan-rhyd-uchaf and east of the railway, but is interrupted south of Cruglas by serpentine and ‘ophicalcite,’ a the latter rock