On the Cambrian Conglomerates resting upon and in the vicinity of some Pre-Cambrian Rocks (the so-called Intrusive Masses) in Anglesey and Caernarvonshire
In papers which I communicated to the Geological Society in 1877 and 1879, I brought forward evidence to show that some rocks in Caernarvonshire and Anglesey which were indicated on the maps of the Geological Survey as great eruptive masses intruded into strata of Cambrian and Silurian age, were really rocks of Pre-Cambrian age. I pointed out that there was the clearest evidence to show that the lowest Cambrian conglomerates known in those areas, instead of having been altered by these so-called eruptive masses, as would be inferred from the maps of the Geological Survey, repose unconformably upon the rocks composing these masses, and were mainly built up of materials derived from them by denudation. In the paper of 1877 I stated that the pebbles in the conglomerates were “usually distinctly rounded and generally imbedded in either an unaltered or semicrystalline matrix, from which they can be easily removed,” and that “they were evidently in their present state, as regards consolidation, before they were cemented together to form the conglomerates, and must have been derived from rocks highly metamorphosed at that time, such rocks, indeed, as now occur immediately under them, and which, we venture to believe, belong to a Pre-Cambrian series.” In the same paper I mentioned that the false appearance of being intrusive masses exhibited by these rocks and “the passage by gradual alteration mentioned by various observers, are mainly due to the fact that the matrix in the conglomerates has been derived from rocks immediately below or