Igneous and sedimentary rocks of Precambrian age in Southern Anglesey are in a low grade of metamorphism over most of the area. The history of the basic igneous rocks has been traced from the practically unmetamorphosed state, through the greenschist stage, to low grade epidote-amphibole-schist stage. The associated semipelites have been converted to muscovite-epidote-garnet schist. The occurrence of glaucophane and lawsonite within a restricted part of the area is of particular interest. Not all the epidote-amphibole schists have necessarily passed through all the stages of increasing metamorphism. The very fine grained nature of these rocks suggests that some of them may have arisen direct from lavas, without having had to pass through the chlorite-schist stage But along the transitional zone between glaucophane schists and greenschists there is evidence of amphibole growing at the expense of chlorite. The chemistry of the glaucophane-epidote schists is essentially the same as the chemistry of other basic rocks that do not contain glaucophane. The formation of glaucophane appears to depend on physical rather than chemical factors. It is suggested that glaucophane was formed in a restricted zone where the mean pressure was elevated, compared to the pressures in the rest of the area, as a result of tectonic overpressures
