ANNlVERS.VRr ADDUESS OF THE PKESIDENT. I 25 



will be observed that they lie outside the area of the bedded vol- 

 canic rocks and rise through parts of the Silurian system older than 

 these rocks. The largest and most important of them is unques- 

 tionably that formed by Y-foel-fnla and its neighbouring heights. 

 As mapped by the Geological Survey, this mass of igneous rock is 

 irregularly elliptical, measures about six square miles in area, 

 and consists mainly of intrusive " fclstone-porpliyry " passing into 

 " hornblendic greenstone " *. Mr. Harker, however, has made an 

 important correction of this petrography, by showing that a large 

 part of the area consists of augitic granophyre, while the so-called 

 " greenstone" is partly diabase and partly andesitic ashes and agglo- 

 merates. He suggests that an older vent has here been destroyed 

 by a later and larger protrusion of igneous matter f. This high 

 and somewhat inaccessible tract of ground is still in need of de- 

 tailed mapping and closer study, for undoubtedly it is the most 

 important volcanic vent now visible in jN"orth Wales. My colleague 

 in the Survey, Mr. E. Greenly, spent a week upon it a few years ago, 

 and he has been good enough to supply me with the following notes of 

 his observations : — The central and largest area of the neck is mainly 

 occupied with diabases and andesites, while the ashes and agglo- 

 merates which are intimately connected with them seem to run as 

 a belt or ring round them, and to occur in one or more patches in 

 the midst of them. Portions of green amygdaloid run through the 

 pyroclastic masses. Outside the ring of agglomerate and ashes an 

 interrupted border of felsite can be traced, which may be presumed 

 to be older than they, for a block of it was observed in them. The 

 granophyre, on the other hand, which is interposed between the 

 fragmental masses and the surrounding rocks on the western wall 

 of the vent, seems to be of later date. Dykes or small bosses of 

 diabase, like the material of the sills, pierce both the agglomerates 

 and the rocks of the centre X- 



No agglomerate appears to have been noticed by any observer 

 among the other supposed vents along the line that runs south- 

 westwards from Penmaeu-mawr, until we reach the south-western 

 end of the line in the promontory of Lleyn. The vents are rudely 

 circular bosses rising vertically out of the Lower-Silurian or Cam- 



* Mem. Geol. Survey, vol. iii. 2nd ed. pp. 137, 139. 



t 'Bala Volcanic Series,' pp. 41, 71, 72, 123. 



J Mr. Greenly has made a sketch map of this interesting locality, and I 

 trust he may soon find an opportunity of returning thither and completing his 

 investigations. 



