Silurian System
in geology, a term made public by Sir Roderick Murchi- SILVER 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 universally adopted. Murchison divided his Silurian 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 Laurentian system, since established, is unequivocally older than the Silurian. Under the Upper Silurian were ranged in descending order: (1) the tilestone of Brecon and Caermarthen; (2) the Ludlow ironstone and shale, and (3) the Wenlock or Dudley limestone. Under the Lower Silurian: (1) the Caradoc sandstone; (2) the Llandeilo flags and schists. Now the Table of Strata stands thus:
I. Upper Silurian: 1. Ludlow formation; 2. Wenlock formation; 3. Llandovery formation 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 between the Upper Silurian and the Devonian. During the early part of the Silurian the land was sinking; during the deposition of the Llandeilo the sea was moderately deep. Algæ, corals, brachiopods, trilobites, and other crustacea, and, in the upper strata, fishes, are the characteristic 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 formation to be Upper Llandovery and Wenlock, and the Salina, the Lower Helderberg, and the Oriskany to be formations of Ludlow age.