14 E. Wynne Hughes — Geology of 



II. Previous Literature. 



Concerning the geology of these two ridges, the controversy as to 

 the age of the rocks and their relation to the neighbouring formations 

 lias produced a voluminous literature. It is unnecessary to dwell in 

 great detail upon this literature, and only the main conclusions, in as 

 far as they appear to affect the area to be described, need be con- 

 sidered here. 



1. General. 



In 1866 the Geological Survey memoir of North "Wales was 

 published. From this it is seen that Ramsay maintained that the 

 base of the Cambrian system in Carnarvonshire is not seen. 1 He 

 further considered that the Cambrian Slates and Grits pass down- 

 wards into a conglomerate which in turn passes by insensible 

 gradations into a quartz-porphyry which forms the core of the 

 St. Annes-Llanllyfni ridge. 2 This gradual transition of the con- 

 glomerate into a quartz-porphyry was explained as a metamorphic 

 change brought about by pressure, and the quartz-porphyry itself 

 was supposed to be the extreme stage of the transition. 3 



This somewhat interesting interpretation of the relation between 

 the conglomerate and the quartz-porphyry resulted in an examination 

 of the area by several geologists, amongst whose number were Blake, 

 Bonney, Hicks, and McKenny Hughes, and at a later period Tawney 

 and Geikie. All agreed that the theory of the metamorphic origin of 

 the quartz-porphyry was erroneous, and that in reality the mass was 

 composed of an original igneous rock. There was, however, con- 

 siderable divergence of opinion as to whether the rocks were of 

 intrusive or extrusive character. Hicks, 4 McKenny Hughes, 5 and 

 Bonney 6 considered that this so-called quartz-porphyry was an 

 eruptive mass composed of lavas, tuffs, and breccias, showing in 

 places both fluxion and spherulitic structures. Tawney maintained 

 that these rocks were intrusive. 7 Blake also at first maintained 

 that they were intrusive, but later he modified this view and admitted 

 the possibility that they were contemporaneous lava-flows. B 



A further controversy arose as to the significance of the con- 

 glomerate lying upon this igneous series. It was demonstrated by 

 Hicks, 9 McKenny Hughes, 10 and Bonney 11 that the pebbles in the 

 conglomerate were mainly composed of fragments from the under- 

 lying volcanic rocks, and accordingly they concluded that a geological 

 break existed here, and that the conglomerate marked the base of 

 the Cambrian system in Carnarvonshire. The series was thus 

 implied to be of Pre-Cambrian age. On the other hand, Tawney 12 



1 A. C. Bamsay, Geology of North Wales (Mem. Geol. Surv.), vol. iii, p. 2, 

 1866. 



2 Op. cit., p. 142. 3 Op. eit., p. 140. 



4 Q.J.G.S., vol. xxxiv, p. 132, 1878. 5 Q.J.G.S., vol. xxxiv, p. 142, 1878. 



6 Q.J.G.S., vol. xxxv, p. 310, 1879. 7 Geol. Mag., Vol. IX, p. 552, 1882. 



8 Q.J.G.S., vol. xliv, p. 282, 1888. 9 Q.J.G.S., vol. xxxiv, p. 148, 1878. 



10 Q.J.G.S., vol. xxxiv, p. 142, 1878. n Q.J.G.S., vol.xxxv, p. 311, 1879 . 

 12 Geol. Mag., Vol. X, p. 70, 1883. 



