ANNIVEKBAKY ADDUIJSS 01-' TJli: PRESIDENT. II9 



3100 feet. But this includes only the higher part of the whole 

 volcanic group. ]ielow it come the lavas of Y Glyder Fach, wliich, 

 according to the Survey measurements, are about 1500 feet thick, 

 while still lower lie the ancient coulees of Carnedd Dafydd and 

 those that run north from the vent of Y-foel-frus, which must reach 

 a united thickness of many hundred feet. We can thus hardly put 

 the total depth of volcanic material at a maximum of less than 

 0000 to 8000 feet. The pile is of course thickest round the vents 

 of discharge, so that no measurement, however carefully made at 

 one locality, would be found to hold good for more than a short 

 distance. 



Though little is said in the Survey Memoir of the vents from 

 which this vast amount of volcanic material was erupted, the pro- 

 bable positions of a number of these orifices may be inferred from 

 the map. From the shore west of Conway a series of remarkable 

 eminences miay be traced south-westwards for a distance of nearly 

 40 miles into the peninsula of Lleyn. At the northern extremity 

 of this line stands the prominent boss of Penmaen-mawr, while 

 southward beyond the large mass of Y-foel-fras, with the smaller 

 knobs west of Nant Francon, and the great dome of Mynydd-mawr, 

 the eye ranges as far as the striking group of ^uy-lUke cones 

 that rise from the sea around Yr Eifl and Nevin. Some of these 

 hills, particularly Y"- f oel-fras, were recognized by the Survey as 

 vents *. But the first connected account of them and of their 

 probable relation to the volcanic district in which they occur has 

 been given by Mr. Harker in his exceedingly able essay on " The 

 Bala Volcanic Series of Caernarvonshire," t — the most important 

 contribution to the volcanic history of Wales which has been mad« 

 since the publications of the Geological Survey appeared. I shall 

 refer to these vents more specially in the sequel. I allude to them 

 here for the purpose of showing at the outset the marvellous com- 

 pleteness of the volcanic records of Caernarvonshire. So great has 

 been the denudation of the region that the pile of lavas and tuffs 

 which accumulated around and above these orifices has been entirely 

 swept away. N"© trace of any portion of that pile has survived to 

 the west of the line of bosses ; while to the east, owing to 

 curvature and subsequent denudation, the rocks have been dissected 

 from top to bottom until almost every phase of the volcanic activity 

 is revealed. 



* Op. cit. pp. 137, 220. 



t This was the Sedgwick Prize Essay for 1888, and was published in 1889. 

 VOL. XLVII. i 



