126 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



brian strata, or partake more of the nature of lenticular sheets or 

 laccolites which have been thrust between the planes of bedding. 

 There is usually an observable alteration of the surrounding rocks 

 .along the line of contact. 



The material that now fills these vents is sometimes thoroughly 

 acid, as is the granophyre of Y-foel-fras, the microgranite of 

 Mynydd-mawr with its riebeckite crystals, the augite-granite 

 porphyry of Clynog-fawr, and the granophyric and rhyolitic quartz- 

 porphyries of the Eivals. In other cases the rock is of an inter- 

 mediate grade, as in the enstatite-diorite of Penmaen-mawr, the 

 pyroxene-andesito of Carn Eoduan, and the quartz- augite-syenite of 

 Llanfoglen*. A few bosses of still more basic material occur in the 

 Sarn district, including hornblende- diabase and hornblende-picrite. 

 Sometimes both the acid and the more basic rocks are found in the 

 same boss, as in the large mass of Y-foel-fras. 



It must be confessed that there is no absolute proof that any of 

 these masses mark the actual sites of eruptive vents, except pro- 

 bably the boss of Y-foel-frtxs. Some of them may have been 

 intruded without establishing any outlet to the sarfacef. But that 

 a few of them really represent orifices from which the Bala 

 volcanic group was erupted may be plausibly inferred from their 

 neck-like forms, from their positions with reference to the volcanic 

 district, from the obvious thickening of the lavas and tuffs in the 

 direction of these bosses, and from the petrographical relation that 

 exists between their component materials and rocks that were dis- 

 charged at the surface. This last-named feature has been well 

 pointed out by Mr. Harker, who has established, by a study of 

 microscopic slides, a gradation from the granophyric material of the 

 bosses into structures greatly resembling those of the bedded 

 felsites, and likewise a close similarity between the intermediate 

 rocks of the other bosses and the porphyrites which have elsewhere 

 been poured out at the surface J. But perhaps the most impressive 

 evidence as to the sites of the chief centres of eruption is supplied by 

 the lavas and tuffs themselves as they thicken in certain directions 

 and ihin away in others. This feature of their distribution has been 

 well expressed in the maps and sections of the Survey, and has been 

 clearly summarized by Mr. Harker §. The oldest lavas now visible 



* The geological relations and petrographical characters of these various 

 rocks are given by Mr. Harker in the fourth and fifth sections of his Essay, 

 t Mr, Hai*ker speaks of some of them as laccolites. 

 t Op. cit. pp. 57, 72. 

 § See especially pp. 9, 120, ct seq., and fig. 6 of his Essay. 



