Welsh Journals

ORDOVICIAN VOLCANOES OF NORTH AND CENTRAL WALES PART II. VOLCANIC ROCKS AND THEIR ENVIRONMENT N. Rast University of Liverpool THE GEOLOGICAL SETTING OF THE VOLCANIC ROCKS Traditionally the study of the Lower Palaeozoic rocks in Wales involved the deduction of the conditions of their formation with a view of sketching a complete history of these rocks. In this respect sedimentary rocks were used almost exclusively, and since most of the Lower Palaeozoic rocks of Wales are marine, unconsciously an idea arose that the story of these rocks was the story of marine deposition. In two comprehensive addresses to the Geological Society of London (1938 and 1955), Professor O. T. Jones has summarized his version of these ideas. He points out that the Lower Palaeozoic rocks of Central and North Wales, consisting of dark shales and coarser bands of siltstone and impure sandstone (known as "grey- wacke "), contain a very sparse fauna of fossils amongst which only the free-floating graptolites are at all abundant. Thus these rocks must represent off-shore sediments deposited in the central part of a basin of sedimentation, where the total thickness of Cambrian, Ordovician and Silurian sediments reaches over 40,000 ft. Towards the Welsh borders and Anglesey, this thickness decreases and numerous breaks in sedimentation appear, and at the same time the types of sediments and fossils change. In Shrop- shire, the Cambrian sediments are represented by a thin condensed sequence of fairly pure sandstone and phosphatic limestone, while the Ordovician and Silurian strata consist of sandstones, mudstones and limestones rich in brachiopods, trilobites and corals. Such sediments with their fossils are interpreted as near-shore and shallow-water in origin. In Anglesey the sedimentary rocks do not provide such a spectacular evidence of near-shore conditions. Nevertheless, even there the complete absence of the Cambrian deposits and a relatively thin and incomplete sequence of Ordovician and Silurian strata suggest near-shore conditions. Thus it is envisaged that throughout the Lower Palaeozoic times a sedimen- tary trough occupied Central and North Wales, and the Ordovician volcanic rocks are placed about the central part of this trough. This setting of the volcanic rocks in the past prompted numerous suggestions to the effect that the Ordovician volcanoes were submarine. In fact, in Snowdonia, H. Williams (1927) suggested a great depth of water to account for the considerable lateral spread of the so-called rhyolitic lavas", since it is known