ANNIVERSARY ADDRESS OF THE PRESIDENT. IO9 



The pyroclastic members of this volcanic scries present many 

 features of interest both to the field-geologist and thepetrographcr; 

 but they have as yet been only partially studied. At the southern 

 end of the district it is remarkable to what a large extent the earliest 

 eruptions must have been mere gaseous explosions, with the discharge 

 of comparatively little volcanic material. Many of the tuffs that 

 are intersfcratified with black slates (? Lingula Hags), at the loot of 

 the long northern slope of Cader Idi is, consist mainly of black-slate 

 fragments like the slate underneath, with a variable proportion of 

 grey volcanic dust. Some of these bands of tuff are leas than an 

 inch thick, and they follow each other at frequent intervals. The 

 first glimpse we thus get of the volcanic history of this part of Wales 

 shows us a continued series of feeble gaseous discharges from probably 

 many small vents, whereby the black clay on the sea-lloor was blown 

 out, the fragments falling back again to be covered up under the 

 gradual accumulation of similar dark mud. 



But elsewhere, and likewise at a later period, in this same southern 

 part of the district, the fragmental discharges consisted mainly of 

 volcanic material. Sir Andrew Hamsay has described the coarse 

 conglomerates composed of subangular and rounded blocks of different 

 ^' porphyries," sometimes twenty inches in diameter, embedded in a 

 fine matrix of similar materials. The true nature of the component 

 fragments in these and similar rocks has still to be worked out. 



Messrs. Cole and Jennings have noticed that the gre^^ volcanic 

 dust of th<3 older slate-tuff' of Cader Idris is seen under the micro- 

 scope " to abound in particles of scoriaceous andesite-glass, now con- 

 verted into a green palagonite ;" and they found abundant traces of 

 andesitic lavas among the tuffs and conglomerates of Rhobell Eawr *. 

 Their investigations show that while the same kinds of volcanic 

 rocks continue to be met with from the bottom to the top, never- 

 theless there is an increase in the acid character of the lapilli as the 

 section is traced upwards. Some of the fragments consist of colour- 

 less devitrified glass with pieces of pumice, as if derived from the 

 breaking up of previously-formed tuffs. Others resemble quartz- 

 andesites, rhyolites, or trachytes, while in at least one instance, some- 

 what low down in the section, quartz-grains with intruded material 

 point to the existence of some fairly acid and vitreous lava f. On 

 the south side of Llyn Can, that is towards the top of the volcanic 



* Quart. Journ. Geol. Soc. vol. xlv. (1889) p. 424 ; Geo]. Mag. (1890) p. 447. 

 t Op. cit. p. 429. A tuff lying below the ironstone near Cross Foxes, east of 

 Dolgellj, likewise contains fragments of trachytic lavas. 



