ANNIVEllSARY ADDRESS OF THE TKESIDENT. 12 7 



lie at the northern end of the district, and the vents from which they 

 proceeded may, with considerable probability, be placed somewhere 

 in the tract which includes the chain of bosses of Penmaen-mawr, 

 Y-foel-fras, and Y Drosgl. The main mass of the eruptions no 

 doubt took place somewhere in the Huowdon tract, where the lavas 

 and tuffs attain their greatest thickness, and whence they thin away 

 in all directions. The Mynydd-mawr boss may be presumed to have 

 been one of the main vents. 13ut there were not improbably others, 

 now concealed under the deep cover of their own ejections. 



More diligent search, with a special eye to the discovery of such 

 vents, might indeed be rewarded, even in the midst of the volcanic 

 district itself. To the north-east of Capel Curig, for example, there is 

 a prominent knob of agglomerate *, Avhich I visited with Mr. B. N. 

 Peach, and which we regarded as probably marking one of the minor 

 vents. The material of this eminence has a base which by itself would 

 probably be regarded by the field-geologist as a felsite. liut through 

 this compact matrix are dispersed abundant stones of all sizes up to 

 six inches or more in diameter. They are mostly subangular or 

 somewhat rounded-off at the edges, and generally markedly- cellular. 

 Among them may be observed pieces of trachyte, felsite, and a rock 

 that is probably a devitrified pitchstone or obsidian. The vesicles 

 in these stones are sometimes lined with an acicular zeolite. Traces 

 of rude bedding can be detected, dipping at high angles. On the 

 north-east side of the hill finer agglomerate is seen to alternate with 

 ashy grits and grey shales, which, dipping E.N.E. at 20°-30°, pass 

 under a group of felsites, one at least of which retains a very fine 

 perlitic structure and evidently flowed as a true glass. Some of 

 these lavas are full of enclosed pieces of various flinty cellular and 

 porphyritic felsites and andesites or trachytes, like the stones which 

 occur abundantly in the agglomerate. The connexion of these 

 bedded lavas and tuffs with the agglomerate-neck seems obvious. 



The Caernarvonshire volcanic area furnishes another admirable 

 example of the intrusion of basic sills as a final phase of eruptivity. 

 These masses have been carefully separated out on the maps of the 

 Geological Survey, which present a striking picture of their distri- 

 bution and their relation to the other igneous rocks. An examina- 

 tion of the maps shows at once that the basic sheets tend to lie 

 parallel with the bedding along certain horizons. In the southern 



* This rock is referred to in the Geological Survey Memoir as ' a short thick 

 band of conglomeratic ash, which strikes northwards about lialf a mile and then 

 disappears' (p. 134). 



