Welsh Journals

times, blisters were formed in North Wales. As time went on, the newly generated acid magma increased in quantity, blocking the access of basic magma to the surface. Thus the late Caradoc volcanic rocks are essentially acid. In Wales most of the manifestations of vulcanicity died out with the Ordovician period. Presumably the sedimentary trough has got so choked with sediments and volcanic debris that igneous magma could not reach the surface of the crust. The injection of these magmas into the strata continued for a much longer time, and at present the eroded remains of the sedimentary and volcanic rocks can be seen to be transected by the intrusive acid and basic igneous rocks. Yet even after their consolidation the Lower Palaeozoic volcanic rocks of Wales exerted important influence. Their position and distribution affected the trends of subsequent folds produced by the Caledonian Orogeny (Rast, 1961). The present-day so-called Caledonian trend of the mountains of Snow- donia has been in fact determined by the volcanic structures of Caradocian age, lines of volcanoes stretched from north-east to south-west parallel to the major fractures which allowed the magma to reach the surface. REFERENCES GROOM, T. and LAKE, P. 1908. The Bala and Llandovery rocks of Glyn Ceiriog, North Wales. Quart. Journ. Geol. Soc. Lond., 64, 546-593. JONES, O. T. 1938. The evolution of a geosyncline. Quart. Journ. Geol. Soc. Lond., 94, Ix ex. 1955. The geological evolution of Wales and the adjacent regions. Quart. Journ. Geol. Soc. Lond., 111, 323-351. RAST, N. 1961. Mid-Ordovician structures in South-Western Snowdonia. Liv. and Manch. Geol. Journ., 2, 645-652. RITTMAN, A. 1960. Vulcane und ihre Tatigkeit, 366. Stuttgart. SHACKLETON, R. M. 1959. The stratigraphy of the Moel Hebog district. Liv. and Manch. Geol. Journ., 2, 216-252. STANTON, W. I. 1960. The Lower Palaeozoic rocks of South-West Murrisk, Ireland. Quart. Journ. Geol. Soc. Lond., 116, 269-296. WILLIAMS, H. 1927. The geology of Snowdon. Quart. Journ. Geol. Soc. Lond., 83, 346-431. 1941. Volcanology in Geology 1888-1938. Geol. Soc. of America.