I06 PROCEEDINGS OF THE GEOLOGICAL SOCIETT. 



prepared the Cambrian and Lower-Silurian rocks had nof- been sub- 

 divided into the various palacontological groups which are now 

 recognized. Nor had any attempt been made to separate the various 

 kinds of contemporaneous igneous masses from each other and from 

 the tuffs in so extensive and complicated a mountain-region. The 

 task undertaken by the Survey was beset with difficulties, some of 

 which geologists, furnished with the advantages of a later time, 

 can hardly perhaps realize. The imperfections of the mapping were 

 long ago recognized by the original surveyors, and various correc- 

 tions of them were made from time to time. Pirst of all, the vol- 

 canic rocks which originally had been all massed under one colour 

 were traced out separately on the ground according to their struc- 

 ture and mode of origin, and were distinguished from each other on 

 the map *. Subsequently divisional lines were followed out between 

 some of the larger stratigraphical groups, the maps and sections 

 were still further modified, and the results were summed up in the 

 second edition of the volume on the Geology of North Wales f. But 

 short of actually re-surveying the whole of that rugged tract, it was 

 impossible to bring the maps abreast of the onward march of science. 

 They consequently remain, as a whole, very much as they were some 

 thirty or forty years ago. 



Sir j^ndrew Eamsay, in his great Monograph, has described the 

 Merionethshire volcanic district in considerable detail. He seems 

 finally to have come to the conclusion that the eruptions of that 

 area were included within the Arenig period t- He shows, indeed, 

 that on Khobell Pawr the ejected materials lie directly on disturbed 

 Lingula Flags without the intervention of the Tremadoc group, 

 which is nevertheless present in full development in the near 

 neighbourhood. And in trying to account for this remarkable fact 

 he evidently had in his mind the possibility that volcanic eruptions 

 had taken place long before as well as after the beginning of the 

 deposition of the Arenig grit and slates §. He seems eventually, 

 however, to have looked on the Rhobell-Fawr sections as exceptional 

 and possibly to be accounted for by some local disturbance and 

 intrusion of eruptive rock. He clearly recognized that there were two 



* Mem. Geol. Surv. vol. iii. 2nd edit. p. 95, note. 



t Some of the modifications introducecl are, I think, to be regretted, for the 

 earlier editions of the maps and sections are in certain respects more accurate 

 than the later. On this point I concur with the criticism made by Messrs 

 Cole and Jennings, Quart. Journ. Geol. Soc. vol. xlv. (1889) p. 436. 



J Mem. Geol. Survey, vol. iii. 2nd edit. p. 96. 



§ Op. cit. p. 72. 



