SILURIAN SYSTEM, a term variously used in geology; as originally defined by Murchison in 1835 it included the rocks de veloped on the borders of England and Wales, a region formerly inhabited by the Silures ; now used in Britain in a restricted sense to denote rocks lying between the Ordovician system below and the Old Red Sandstone or Devonian system above. There is no complete agreement as to the exact horizon at which the lower and upper limits of the system should be drawn. The upper limit of the Ashgillian has yet to be defined accurately in those areas where sedimentation proceeded continuously from Ordovician to Silurian time ; and opinion is somewhat divided as to the upward limitation of the system, on the one hand on account of the physi cal conditions attending deposition where the Old Red Sandstone facies succeeds, and on the other from the difficulty of putting a definite boundary where there is a gradual passage, owing to con tinuous sedimentation under the open sea conditions that usher in the Devonian facies.
As is the case with the rocks of the ORDOVICIAN system the strata belonging to the SILURIAN system may be divided into several contemporaneous facies or types of deposit controlled by different physical conditions ; four main facies can be recognized: Again, as in the Ordovician Period, the deposits belonging to the shallow water facies may pass into each other laterally, i.e., in a direction parallel to the original shore line, whilst in a direc tion at right angles to this shore line they pass into the deeper water Graptolite Shales, and as this transition often takes place gradually, the shallower water beds being replaced stage by stage, the contemporaneity of the two is placed beyond doubt.
It may be said to be characterized:— (a) By the rise of the spine-bearing brachiopods (e.g., Atrypa,
Meristina) and the great development of those with a definite internal skeleton (Pentamerus). The Silurian is pre-emi nently the age of brachiopods.
(b) by the abundance of trilobites, though as these have passed their zenith there are but few new genera, and these of no great importance.
(c) by the predominance of the uniserial scandent graptolites (Monograptus). Other noteworthy features are : (d) the rise in importance of the mollusca as a whole, especially the Cephalopods, the earliest Ammonoid making its appear ance.
(e) the incoming late in the period of the first vertebrates (fish).
(f) the increased importance of corals and crinoids, especially as rock-builders.
(g) the abundance towards the close of crustaceans of Eurypterus type.
The shallow water fauna is predominantly a brachiopod-trilobite fauna, characterized by mutations of Pentamerus (including the subgenera Barrandella, Conchidium, Sieberella, etc.), though in some places it is less prominent than Stropheodonta, and in the higher beds Rhynchonella (Camarotaechia) and Chonetes pre dominate. In the cleaner waters, which seem to have occupied wide areas, there seems to have been considerable reef-building, not only by corals but by other classes of organisms such as bryozoa, crinoids and stromatoporoids. Brachiopods also, to gether with other organisms, gave rise to considerable shell-banks of limestone.
The fossils of commonest occurrence are Trilobites Phacops Sensu lato, Calymene, Encrinurus, Acidaspis, Lichas, Cheirurus, Homolonotus, Illaenus, Arethu sina, Proetus, S phaerexochus.
Brachiopods Pentamerus Sensu lato, Stricklandinia, 1Vlerislina, Stroplieodonta, Atrypa Coelospira, Plectambonites, Leptaena, Camarotaechia, W ilsonia, Dayia, Chonetes, Dalmanella, W hitfieldella.
Corals Halysites, Heliolites, Favosites, Acermlaria,amphyma, Palaeocyclus, Goniophyllum, Syringopora.
Cystids Lepadocrinus, Prunocystis, Schizocystis.