Silurian System

Silurian System, the name originally applied by Sir Roderick Murchison in 1835 to those rocks below the Old Red Sandstone that occupy the former territories of the Silures on the South Wales border. He afterwards extended the name downwards to all rocks below the Old Red Sandstone that contain trilobites, thus including the equivalents of the rocks described by Sedgwick under the name of Cambrian in North Wales. To obviate this conflict of nomenclature, when it had been shown by Mr. Etheridge that between the Archaean and the Old Red Sandstone there are three distinct faunas, the name Ordovician was proposed by Professor Lapworth for the Lower Silurian of Murchison or Upper Cambrian of Sedgwick. As now defined, the Silurian system is a series of sandstones and shales, with three bands of limestone, having a total thickness of from 5,500 to 7,000 feet, occupying in Britain a large area on the Welsh border, and in the Lake district, and found in deep borings to the north of London. Its subdivisions are as follows:-

Ludlow Series, with Kirkby Moor Flags and Bannisdale Slates. { Ledbury Shales - 300 ft.; Downton Sandstones - 100 ft.; Upper Ludlow Shale with bone-bed - 900 ft.; Aymestry Limestone - 30-40 ft.; Lower Ludlow Shale - 900 }

Wenlock Series, with Denbigh and Coniston Grits. { Wenlock or Dudley Limestone - 100-300 ft.; Wenlock Shale - 640-1,400 ft.; Woolhope or Bair Limestone - 40 ft.; Tarannon Shale - 1,000-1,500 ft. }

Upper Llandovery, or May Hill Series. - 1,500 ft.

Besides May Hill, in Gloucestershire, the Lickey Hill quartzite, in Worcestershire, belongs to the Upper Llandovery sandstone series. The limestones of the Wenlock series, which though thin are crowded with fossils, are burnt into quicklime. The bone-bed in the Upper Ludlow, though less than a foot thick, is traceable over 1,000 square miles to the south of Ludlow. There is an unconformity at the base of the series, and though near Ludlow it passes conformably up into the Old Red, in North Wales it bas been tilted, crumpled, faulted, and cleaved before being covered by that formation. Land plants are represented in Silurian rocks; a fish has been found in the Lower Ludlow, and others occur in the bone-bed; Palaechinus, a sea urchin, occurs in the Upper Llandovery; and Pseudocrinites, a cystidean, in the Wenlock Limestone. This light grey limestone is full of corals, crinoids, trilobites, and brachiopods, and also contains the enrypterids, Eurypterus and Pterygotus. The chief corals are Omphyma, Favosites, and Halysites; the chief trilobites Calymene, Phacops, Homalonotus, and Illaenus; the chief brachiopods Orthis, Rhynchonella Strophomena, Atrypa, and Pentamerus; and the chief cephalopod Orthoceras.