H. C. Sargent — The Penmaenmawr Intrusions. 25 



basic rock than the one in question, which is clearly of intermediate 

 character. Moreover, the structure is hardly doleritic, seeing that 

 there is nothing in the rock in the nature of the ophitic habit, or to 

 show that the felspars have on the whole crystallized out before the 

 augite. In view of the wide structural variations shown to exist 

 in the Penmaenmawr mass, ' markfieldite ' does not seem a verv 

 suitable name for a rock, none of whose varieties agrees closely with 

 the type-occurrence, and some of which depart from it widely. 



The decks being thus cleared, it remains, to suggest that since the 

 Penmaenmawr rock is porphyritic in structure, hypabyssal in 

 occurrence, and intermediate in composition, its essential constituents 

 being bronzite, augite, and soda-lime felspars, it may be safely classed 

 with the porphyrites, as has been done by Hatch, 1 and there can be 

 no more appropriate name for it than bronzite-porphyrite. 2 



The different varieties can be distinguished by suitable prefixes. 

 Thus at the margin we have an andesitic-bronzite-porphyrite ; 

 further inwards a quartz-bronzite porphyrite ; and in the most acid 

 variety, which clearly grades in the direction of the true granophyres, 

 a granophyric-bronzite-porphyrite. The rock of Dinas and Carregfawr, 

 so far as existing exposures enable us to examine it, may be termed 

 a quartz-porphyrite. 



It is to be observed, however, that in the case of Penmaenmawr 

 a laccolitic origin is by no means established. The flow-arrangement, 

 so pronounced in the andesitic type of the margin, and the phenomena 

 noticed above which have attended the consolidation of the mass, are 

 features which appear to be opposed to the view of an interbedded 

 injection of homogeneous magma. The simultaneous injection of 

 bodies of magma of heterogeneous composition can hardly be invoked 

 to explain the latter feature, since the variation from margin to 

 centre appears to be a continuous one, and there is nothing in the 

 shape of banding or the streaked structure that would be present if 

 such were the case. 



The writer has already been perhaps somewhat too fertile in 

 suggestions, but he will venture on one more before closing 

 this paper. 



Harker, in the fascinating chapter entitled " Review of Vulcanicity 

 in Caernarvonshire", in the work referred to above, has pointed out 

 the probability that volcanic action in this region manifested itself in 

 Bala times as a result of the pressure from the south-east ; " the 

 fluid lava . . . being lifted by the pressure into the sphere of action 



1 Text-booh of Petrology, 5th ed., p. 219, 1909. 



2 Since the above was written the writer finds that Eosenbusch was latterly 

 inclined to take the same view. " Doch kann man es wegen seiner ausge- 

 sprochen porphyrischen Struktur nicht wohl zu. den Noriten stellen . Ob man 

 es nicht besser seiner chemischen Konstitution nach, zumal wegen seines hohen 

 Gehaltes au Kieselsaure zu den Enstatit-porphyriten stellen sollte, dariiber 

 liesse sich streiten " (Mikr. Phys., 4th ed., vol. ii, p. 1258, 1908). [One 

 cannot, in consequence of its pronounced porphyritic structure, very well class 

 it with norites. It is an open question if it would not be better, having 

 regard to its chemical composition, and particularly considering the amount of 

 silica it contains, to class it with the enstatite-porphyrites. — Ed.] 



