[Footnote 64: In accordance with the custom usually observed in Britain, the Tremadoc slates are placed in the Cambrian system; most continental geologists place them in the succeeding Ordovician system. The matter is not an important one, as the fauna is an intermediate one between that of the Lingula Flags and that of the Arenig series of the Ordovician system, and the beds are true beds of passage. As the lithological classification is essentially British, it will be as well to retain the Tremadoc Slates in the Cambrian system.] The original lithological classification was essentially the result of Prof. Sedgwick's work in North Wales, while the classification according to faunas is the outcome of the researches of Dr Hicks in South Wales. _Description of the Strata._ The Cambrian rocks of North Wales occur in two complex anticlines, separated by an intermediate syncline of Ordovician strata occupying the Snowdonian hills. The southerly or Harlech anticline forms a part of Merionethshire to the east of Harlech, whilst the northern one is developed around Bangor and Llanberis. The South Welsh Cambrian rocks are chiefly found on either side of the Pembrokeshire axis of Precambrian rocks which runs through St David's. As the corresponding rocks of the two regions were deposited in bathymetrical zones of much the same depth, it will be convenient to give a general account of the rocks of the two regions at the same time, leaving the student to acquire information of the detailed variations in the larger text-books and in special memoirs[65]. [Footnote 65: A general account of the Cambrian, Ordovician and Silurian rocks will be found in the Sedgwick Essay for 1883, _A Classification of the Cambrian and Silurian Rocks_, though the use of a cumbrous nomenclature therein will tend to confuse the reader. For a detailed account of the Cambrian rocks of North Wales the reader is referred to the Geological Survey Memoir, _The Geology of North Wales_, by Sir A. Ramsay (2nd edition), he may also consult Belt, T., "On the Lingula Flags or Festiniog Group of the Dolgelly district," _Geol. Mag._, Dec I. vol. IV. pp. 493, 536, vol. V. p. 5. The geology of the Cambrian rocks is described in a series of Memoirs in the _Quarterly Journal of the Geological Society_ by Dr H. Hicks; the following should be consulted: Harkness, R. and Hicks, H., "On the Ancient Rocks of the St David's Promontory, South Wales, and their Fossil Contents," vol. XXVII. p. 384; Hicks, H., "On some Undescribed Fossils from the Menevian Group," vol. XXVIII. p. 173; and "On the Tremadoc Rocks in the neighbourhood of St David's, South Wales, and their Fossil Contents," vol. XXIX. p. 39. See also Hicks, "The Classification of the Eozoic and Lower Palæozoic Rocks of the British Isles," _Popular Science Review_, New Series, vol. V., and Hicks, "Life-zones in the Lower Palæozoic Rocks," _Geol. Mag._ Dec IV. vol. I. pp. 368, 399 and 441.]