ANNIVERSARY ADDRESS OF THE PRESIDENT. I35 



vents from which some of the later tuffs were discharged ; for beyond 

 and above the limestones and black shales of Cemmaes, other volcanic 

 breccias and ashes, with limestone, (juartzite, conglomerate, and 

 thin seams of black shale, continue to the extreme northern head- 

 lands. The amount of fine volcanic detritus distributed through 

 these strata is very great. We can clearly make out that while 

 ordinary sedimentation was in progress, an almost constant but 

 variable discharge of fragmental materials took place from the vents 

 in the neighbourhood. Sometimes a special paroxj'sm of explosion 

 would give rise to a distinct band of breccia or of tuff, but even 

 where, during a time of comparative quiescence, the ordinary sand 

 or mud predominated, it was generally mingled with more or less 

 volcanic dust. 



Some bands of conglomerate in this group of strata deserve par- 

 ticular notice. The most conspicuous of these, seen at Perth Wen, 

 is made up of quartz and quartzite blocks, imbedded in a reddish 

 matrix largely composed of ashy material, and recalling the red 

 spotted tuffs of Llyn Padarn. The occurrence of strong conglome- 

 rates 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. There is reason to suspect that con- 

 siderable 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 little vents of 



VOL. XLVII. Tc 



