On the Metamorphism of a Series of Grits and Shales in Northern Anglesey

Abstract

In a recent paper read before this Society, I endeavoured to show that certain gneisses in Southern Anglesey were formed out of plutonic rocks. Fragments of these gneisses have been identified in younger pre-Cambrian strata lying to the north, and these, in their turn, yield an abundance of rounded derivatives to the basal Palaœzoic conglomerates of the island. These newer pre-Cambrians, which I have provisionally referred to the Pebidian system, display great variations in their degree of crystallization. Hence arises one of the chief difficulties in unravelling the geology of Anglesey. It has become increasingly evident that the questions still in dispute in this complicated area can be settled only by (1) carefully mapping the ground in minute detail, and (2) determining the nature and degree of alteration which many of the rocks have undergone. Mr. Edward Greenly has devoted himself with great enthusiasm to the former task, and the first-fruits of his work promise highly satisfactory results. The present paper is offered as a further contribution to the series of problems arising under the latter head. It is here proposed to examine a series of grits and shales in Northern Anglesey, and to show that they are transformed into true crystalline schists as we follow them in a southerly direction. I. T he A rea D escribed . North and north-west of Amlweh the ground is much disturbed and mixed up with faulted fragments of Palæozoic strata. To avoid controversial topics, I shall confine myself to a patch about 3 miles square, lying