<)0 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



as another axis of the Uriconiaii series. It is very difficult, 

 however, hj any combination of geological structures to bring up 

 a portion of the very bottom of the Longmynd series and place it 

 apparently at the top. This is a feat which a detailed study of the 

 region and the detection of unconformabilities in the Longmynd 

 may possibly accomplish. In the meantime, however, I would 

 venture to suggest whether it is not more probable that we have 

 here a detached area of much younger volcanic rocks, like those 

 which, in various districts, may be included in the Cambrian system. 



lY. CAMBEIAN. 



Under this name I class all the known Palseozoic rocks which lie 

 on each other conformably below the bottom of the Arenig group. 

 It was maintained by Sir Andrew Eamsay and his colleagues on 

 the Geological Survey that on the mainland of Wales no base 

 is ever found to the Cambrian system. More recently certain con- 

 glomerates have been fixed upon as the true Cambrian base, both in 

 South and ISTorth Wales, and endeavours have been made to trace 

 an unconformability at that line, all rocks below it being treated as 

 pre-Cambrian. I have formerly insisted that conglomerates do not 

 necessarily mark a stratigraphical discordance, and that in South 

 Wales there is no trace of any unconformability between the strata 

 above and below the supposed line of break *. Professor Bonney 

 has shown that in ISTorth Wales several zones of conglomerate have 

 been erroneously identified as the supposed basal platform of the 

 Cambrian series, and more recently Mr. Blake has pointed out 

 that some of these conglomerates are unquestionably Lower Silurian. 

 My own examination so far confirms the conclusions arrived at by 

 these observers. Like my predecessors in the Survey, however, I have 

 been unable to detect anywhere in Caernarvonshire or Merioneth- 

 shire a base to the Cambrian system, and I am compelled to agree 

 with them in regarding as Cambiian (partly even as Lower Silurian) 

 all the rocks from Bangor to Llanllyfni, which have more recently 

 been classed as pre-Cambrian. But though thus supporting their 

 general stratigraphy, I am bound to acknowledge that they failed 

 to recognize the existence of a great volcanic series below the 

 Arenig horizon. The existence of this series, noticed by Sedg- 

 wick, was first definitely stated by Professor Hughes t, and his 



=*= Quart,. Journ. Geol. Soc. vol. xxxix. (1883) p. 305. 

 t Proc. Oarnb. Phil. Soc. vol. iii. (1877) p. 89. 



