SILURIAN SYSTEM, in geology, a term made public by Sir Roderick Murchi son in 1839, though he had begun to use it as early as 1835. It implied that, speaking broadly, the rocks so described were well developed in the country of the old Silures. The term has been univer sally adopted. Murchison divided his Si lurian system into Upper and Lower Silurian, contending that the Cambrian system of Sedgwick was not independent, but simply Lower Silurian. Whatever be the case with the Cambrian, the Lau rentian system, since established, is un equivocally older than the Silurian. Under the Upper Silurian were ranged in descending order: (1) the tilestone of Brecon and Caermarthen; (2) the Lud low ironstone and shale, and (3) the Wenlock or Dudley limestone. Under the Lower Silurian: (1) the Caradoc sand stone; (2) the Llandeilo flags and schists. Now the Table of Strata stands thus: I. Upper Silurian: 1. Ludlow formation; 2. Wenlock formation; 3. Llandovery for
mation or group.
II. Lower Silurian: 1. Bala. and Caradoc beds; 2. Llandeilo flags; 3. Arenig or Stiper Stone Group.
There is a great break between the Upper and Lower Silurian, which are uncomformable, and a greater break be tween the Upper Silurian and the Devo nian. During the early part of the Silurian the land was sinking; during the deposition of the Llandeilo the sea was moderately deep. Alm corals, brachio pods, trilobites, and other crustacea, and, in the upper strata, fishes, are the charac teristic fossils. The higher vertebrates had not yet appeared. Vast areas in Russia, etc., are covered by Silurian rocks. In America, the Canadian and the Trenton formations are believed to be Lower Silurian; the Niagara forma tion to be Upper Llandovery and Wen lock, and the Salina, the Lower Helder berg, and the Oriskany to be formations of Ludlow age.