ANNIVERSARY ADDRESS OF THE PRESIDENT. Ol 



forty years hence. The maps of the Geological Survey are no 

 exception to this rule. In criticizing and correcting them, however, 

 let us judge tliem not hy the standard of knowledge which we have 

 now reached, but by that of the time when they were prepared. It 

 is easy to criticize ; it is not so easy to recognize how much we owe 

 to the very w'ork which we pronounce to be imperfect. 



The ancient volcanoes of AVales, thanks mainl}' to the admirable 

 labours of my old friend and former chief. Sir Andrew C. Ivamsay, 

 have taken a familiar place in geological literature. But a good 

 deal has been learnt regarding them since he mapped and wrote. 

 The volcanic history, as he viewed it, began in the Arenig period. 

 The progress of subsequent enquiry, however, has shown that there 

 are volcanic rocks older than the Llanberis Slates. I hope to be 

 able to show in this Address that the Cambrian period in North 

 "Wales was eminently volcanic, and that there was probably a still 

 earlier era of eruption. 



I begin my narrative with Anglesey, which has been the subject 

 of much discussion in reference to what has been called " the pre- 

 Cambrian question." On the Geological-Survey map of that island * 

 four areas are coloured as altered Cambrian (and partly Silurian) 

 rocks, and an account of these areas is given in the Memoir on the 

 Geology of North Wales f. The surveyors clearly recognized the 

 metamorphosed character of the rocks in most parts of these four 

 areas, and they saw that the crystalline schistose masses must be of 

 far higher antiquity than the Arenig strata which they found to 

 cover them unconformably, and to contain abundant fragments 

 derived from their waste. Mr. Selwyn seems to have looked on the 

 metamorphic rocks as of higher antiquity than any others in the 

 region, and he wrote of them as " old green siliceous schists '"' and 

 "old chloritic slates "i. Sir Andrew Ramsay, however, having 

 formed certain hypothetical opinions regarding metamorphism, and 

 having traced what he regarded as somewhat metamorphosed 

 Cambrian strata from Caernarvonshire into Anglesey, came to the 

 conclusion that the schists and gneisses of the latter district were 

 only a further stage in the metamorphism of sedimentary rocks 

 belonging mainly to some early part of the Cambrian system. 

 Moreover, he recognized that dark shales containing Lower-Silurian 



* Sheet 78 of the Geol. Survey Map. 



t Memoirs of the Geol. Surrey, vol. iii.. 2nd cditiou, 1881. 



t Op. cit. ])p. 2*23, 227. 



