Cambrian System

Cambrian System, the name applied by Sedgwick about 1834 to the great series of slaty rocks and limestones in North Wales, then believed to be older than the Silurian of Murchison, Sedgwick afterwards made three divisions, Lower, Middle, and Upper Cambrian, the Lower being the Cambrian of Murchison, the Middle being the Primordial Silurian of Murchison, or Upper Cambrian of more recent authors, and the Upper being the Lower Silurian of Murchison or Ordovician of Lapworth. Recent tabulation of the species of fossils shows that there are three distinct faunas below the Old Red Sandstone, so that the names Cambrian, Ordovician, and Silurian may well be limited by them. As thus restricted the Cambrian rocks of North Wales consist of purple, reddish-grey, and green slates, grits, sandstones, and conglomerates, estimated at 25,000 feet thick and mostly unfossiliferous, though yielding altogether nearly 200 species belonging to 60 genera. They seem universally to rest, as at Bangor, unconformably upon older [Archaean] rocks (fragments of which occur as pebbles in the conglomerates) and are very uniform in character, slates, greywackes, quartzites, and conglomerates, over the whole world. Often ripple-marked, sun-cracked and false-bedded, they have been formed in shallow water, possibly in inland basins. They are often cleaved, highly inclined, or folded. The fossils, considering that they are the earliest undoubted traces of animal life, are singularly varied, comprising sponges, cystideans, polyzoans, brachiopods, heteropods, pteropods, pelecypods, cephalopods, annelids, and ostracods, though trilobites are by far the most numerous. From the prevalence of the two genera of this group Paradoxides and Olenus, the Cambrian has been divided into two divisions, the Lower or Paradoxidian, and the Upper or Olenidian. The Lower Cambrian consists of the Harlech and Longmynd groups of Wales and Shropshire, with the Barmouth sandstones and Llanberis and Penrhyn slates, and the Menevian beds, named from the Roman name of St. David's. The Upper Cambrian comprises the Lingula Flags, so named from the brachiopod Lingula Davisii, in which the gold and copper ores of North Wales occur, and the Tremadoc slates. Igneous rocks are associated with them. Upper Cambrian rocks appear in the Malvern Hills. There is no marked unconformity in the Cambrian system, but a slight one above it. In North America Cambrian rocks are divided into Acadian and Potsdam, and they are well represented in Brittany, Normandy, the Ardennes, Sweden, and Bohemia, but appear to thin out eastward.