ANNIVERSARY ADDRESS OF THE PRESIDENT. IO7 



great epochs of volcanic activity during the Silurian period in Wales, 

 one belonging to the time of the Arenig, the other to that of the 

 JBala rocks, and ho pointed out that the records of these two periods 

 are separated by a thick accumulation of sedimentary strata which, 

 being free from interstratificatious of contemporaneous igneous 

 rocks, mark a long interval of quiescence among the subterranean 

 forces *. 



The lower limit of the Arenig rocks has been fixed at a band or 

 bands of grit or conglomerate which can be followed with some slight 

 interruptions all round the great dome of Cambrian strata from 

 Llanegrin on the south to the shore at Criccieth on the north. The 

 volcanic group doubtless lies, generally speaking, above that base- 

 ment platform. But, besides the sections at Ehobell Fawr just 

 referred to, where the volcanic materials lie on the Lingula Flags, 

 the same relation may, I think, be observed on the north flank ci 

 Cader Idris. Messrs. Cole, Jennings, and Holland have come to the 

 conclusion that the eruptions began at a rather earlier date than 

 that assigned to them in the Survey Memoirs, and my own exami- 

 nation of the ground leads me to accept their conclusion t. I 

 believe that the earliest discharges in the southern part of the region 

 took place at the close of the deposition of the Lingula Flags, and 

 that intermittent outbursts occurred at many intervals during the 

 time when the Tremadoc and Arenig rocks were deposited. 



An upper limit to the volcanic group is less easily traceable, 

 partly, no doubt, from the gradual cessation of the eruptions and 

 partly from the want of any marked and persistent stratigraphical 

 horizon near the top of the group. Sir Andrew Eamsay, indeed, 

 refers to the well-known band of pisolitic iron-ore as lying at or 

 near to the top of the Arenig rocks J. There can be no doubt, 

 however, that the volcanic intercalations continue far above that 

 horizon in the southern part of the district. 



In spite of the extent to which the volcanic masses of this ancient 

 period have been covered by later Palaeozoic formations, it is still 

 possible to fix approximately the northern, western, and southern 

 limits of the district over which the ashes and lavas were distributed. 

 These materials die out as they are traced southwards from Cader 

 Idris and north-westwards from Tremadoc §. The greatest diameter 

 of ground across which they are now continuously traceable is about 



* Op. cit. pp. 71, 9(>, 105. 



t Quart. Journ. Geol. See. vol. xlv. (1880) p. 436 ; Geol. Mag. (1890) p. 447. 



\ Mem. Greol. Survey, vol. iii. liud edit. pp. 249, 'Zbd. 



§ Op. cif. p. 96. 



