On the Origin of some of the Gneisses of Anglesey

Abstract

The comparative ages of the subdivisions of the schistose and gneissic rocks of Anglesey have been discussed with very varied conclusions, but these discrepancies have been in great part the result of the unsettled state of opinion on the subject of metamorphism. The former belief that the granite was of metamorphic origin, and that a parallel-structure necessarily indicated an original sedimentation, could not fail to mislead the stratigraphical geologist. It is therefore requisite that the genesis of the respective rocks should be determined before attempting to revise the old classifications. Before entering upon my task, I wish to indicate briefly the present state of our knowledge of the Archæan (pre-Cambrian) rocks of Anglesey. Dr. H. Hicks, F.R.S., divided them into three groups:—Dimetian (granite), Arvonian (hälleflinta), and Pebidian (schistose), these divisions being named in ascending order. I have never been able to define more than two systems, an older one, which, to avoid theory, I simply designated as ‘Gneissic’ and a newer one, which I described as ‘Slaty,’ and provisionally identified with the Pebidian. The Rev. J. F. Blake, F.G.S., admits the pre-Cambrian age of all these rocks, but he describes them as one continuous series, which he calls ‘Monian.’ Prof. T. G. Bonney, F.R.S., also regards the crystalline schists as Archæan, but Sir A. Geikie, F.R.S., and Prof. T. McK. Hughes, F.R.S., place some of the less altered schists at a higher horizon. The pre-Cambrian age of the bulk of the granitic, gneissic, and schistose rocks of Anglesey may therefore