ANNIVERSARY ADDRESS OF THE PRESIDENT. QI 



statements have been confirmed and extended by subsequent 

 observers, notably by Professor Bonney and Mr. Blake *. But 

 I venture to think that its real position and range have hitherto 

 escaped notice, and that it proves the Cambrian to have been a 

 period perhaps even more continuouslj^ volcanic than the Lower 

 Silurian was in Wales. 



In the vast pile of sedimentary material of the Harlech anticline, 

 estimated by the Geological Survej^ to be from 6000 to 7000 feet 

 thick, no trace of any contemporaneous volcanic rocks has been met 

 with t. Dr. Hicks, indeed, has referred to certain " highly felsitic 

 rocks, for the most part a metamorphic series of schists, alternating 

 with harder felsitic bands, probably originally felsitic ashes,'' lying 

 at the bottom of the whole pile, and he has claimed them as pre- 

 Cambrian J. But I could not find any evidence of such rocks, 

 nor any trace of igneous materials save dykes and sills, acid and 

 basic, such as are indicated on the Survey map. The purple 

 slates that rise along the centre of the anticline dip below 

 the grits and conglomerates on either side without allowing us 

 a glimpse of the base of the system. This enormous accumu- 

 lation of sedimentarj^ deposits seems to diminish in thickness as 

 it is traced northwards, for towards the Menai Strait it does not 

 reach more than a fourth part of the depth which it displays in the 

 Harlech anticline §. In the Pass of Llanberis the series of grits 

 that overlies the purple slates is estimated to be about 1300 feet 

 thick !1 . This gradual thinning away of the Cambrian series towards 

 the north was, in the opinion of Sir Andrew Itamsay, accompanied 

 by an increasing metamorphism of the lower portions of the system. 

 In his view, the long ridge of quartz-porphyry which crosses the lower 

 end of Llyn Padarn represents the result of the extreme altera- 

 tion of the stratified rocks. He believed that he could trace an 

 insensible passage from the slates, grits, and conglomerates into the 

 porphyry, and he was led to the " conviction that the solid porphyry 

 itself is nothing but the result of the alteration of the stratified 

 masses carried a stage further than the stage of porcellanite, into 

 the condition of that kind of absolute fusion that in many other 

 regions seems to have resulted in the formation of granites, syenites, 

 and other rocks, commonly called intrusive '' ^. Certain structural 



* In the papers already quoted on p. 82. 



t Mem, Geol. Surv. vol. iii. 2nd edit. Geology of North Wales., p. 21. 



; Geol. Mag. (1880) p. 519. 



§ ]Mem. Geol. Surv. vol. iii. 2nd edit. p. 24. || Op. cif. p. 173. 



5 Op. cit. p. 173. 



