Foliation and its Relations to Folding in the Mona Complex at Rhoscolyn (Anglesey)

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Abstract
In ‘The Geology of Anglesey’ it was pointed out that little had really been ascertained concerning the laws and chronology of the foliation of the Mona Complex. A chronology was given, but the foliation of Holy Isle was therein treated as a unit, whereas it has proved to be much more than a unit. I found in 1922 that the most favourable sections for further investigation were those of the South Stack Series on the sea-cliffs of Rhoscolyn, especially under Rhoscolyn Hill, opposite the Maen-y-fran islets (G. of A., folding-plate ii). For, in the core of that major anticline, the beds are, as wholes, nearly horizontal for a quarter of a mile, which eliminates complication due to dip on major folding. The present paper is the outcome of some four weeks' study of the section in 1922 and 1923, followed by reexamination of the thermal aureoles of the basic intrusions of Holy Isle. The original character of the rocks, their anamorphic condition, and their tectonics, have already been described. Here, therefore, it need merely be recalled that they were originally grits and shales; and that, as Rhoscolyn lies within one of the four anamorphic maxima of the Mona Complex, there are no catamorphic textures, but that all are now holocrystalline schists. Considered with regard to capacity for foliation, there are seven principal types :—(1) The massive green grits of the Stack Moor division of the South Stack Series (which are sometimes 30 feet thick); (2) the somewhat similar green grits

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