Some bands of conglomerate in this group of strata deserve particular notice. The most conspicuous of these, already referred to as seen at Porth Wen, is made up of quartz and quartzite blocks, embedded in a reddish matrix largely composed of ashy material, and recalling the red spotted tuffs of Llyn Padarn. The occurrence of strong conglomerates near the top of a volcanic series has been noted at St. David's, Llyn Padarn and Bangor. In none of these localities, as I have tried to show, do the conglomerates mark an unconformability or serious break between two widely-separated groups of rock. The Anglesey section entirely supports this view, for the conglomerates are there merely intercalations in a continuous sequence of deposits; they are succeeded by tuffs and shales like those which underlie them. The interposition of such coarse materials, however, may undoubtedly indicate local disturbance, connected, perhaps, in this and the other localities, with terrestrial readjustments consequent upon the waning of volcanic energy. The detailed geological structure of Anglesey is still far from being completely understood. Besides the serious crushing here referred to, there is reason to suspect that considerable plication, perhaps even inversion, of the strata has taken place, and that, by denudation, detached portions of some of the higher groups have been left in different parts of the island. The occurrence of Upper Silurian fossils in several localities adds to the perplexity of the problem by indicating that, among the folds and hardly distinguishable from the older slates, portions of Upper Silurian formations may have been caught and preserved. These difficulties, moreover, involve in some obscurity the closing phases of volcanic activity in Wales; for until they are, to some extent at least, removed, we shall be left in doubt whether the vents in the north of Anglesey, which were in eruption probably during Bala time, were the last of the long succession of Welsh volcanoes. If the black shales of Parys Mountain are really referable to the horizon of the Mayhill Sandstone, the two great igneous bands between which they lie would seem to mark an outbreak of volcanic energy during Upper Silurian time. No other indications, however, of eruptions of that age having been met with in Great Britain (though they occur in the south-west of Ireland and possibly in Gloucestershire), more careful investigation is required before such a position can be safely assigned to any rocks in Anglesey.