My own examination so far confirms the conclusions arrived at by these observers. Like my predecessors in the Geological Survey, however, I have been unable to detect anywhere in Caernarvonshire or Merionethshire a base to the Cambrian system, and I am compelled to agree with them in regarding as Cambrian (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 Sedgwick, was first definitely stated by Professor Hughes,[84] and his statements have been confirmed and extended by subsequent observers, notably by Professor Bonney and Mr. Blake. The Cambrian period is thus proved to have been perhaps even more continuously volcanic than the Lower Silurian period was in Wales. [Footnote 84: _Proc. Camb. Phil. Soc._ vol. iii. (1877), p. 89. The Cambrian volcanic areas of North Wales are represented in Map II.] The following table shows the subdivisions of the Cambrian system now recognized in Britain:-- +----------------------+----------------------+-------------------------+ | Wales. | | | | (Ranging up to | Western England. | N.W. Scotland. | | 12,000 feet | (About 3000 feet.) | (About 2000 feet.) | | or more.) | | | +----------------------+----------------------+-------------------------+ |Upper or _Olenus_ Zones. | | | Tremadoc Slates | Shineton Shales | Limestones, about 1500 | | Lingula Flags | (_Dictyograptus_ | feet thick, divisible | | (_Lingulella_, | or _Dictyonema_, | into seven groups | | _Olenus_, etc.). | _Olenus_, etc.). | (_Archæocyathus_, | | | | _Maclurea_, _Ophileta_,| |Middle or _Paradoxides_ Zones. | _Murchisonia_, | | Menevian group | Conglomerates and | _Orthoceras_, | | (_Paradoxides_). | limestones (Comley),| and vast quantities of | | | (Comley), with | annelid castings). | | | _Paradoxides_, etc. | | | | | | |Lower or _Olenellus_ Zones. | | | Harlech and | Thin quartzite | Shales ("fucoid beds"), | | Llanberis | passing up into | with _Olenellus_, | | group with | green flags, grits, | _Salterella_, etc. | | basement volcanic | shales | Quartzites with annelid |