SILOAM. 173 SILURIAN SYSTEM. form of the letters lends probability to the view that the tunnel was constructed in the days of Hezekiah. The aim in conducting tlie waters tlirough the tunnel into a pool of tlie Tyro- poeoii valley was to make it more accessible to llie inhabitants of the lower part of Jeru- salem. Consult: Tobler, Die HiloahqucUe und dcr Oelberg (Saint Gall, 1S52) ; Socin, Die HiUtahin- schrift (Freiburg, 1899); Driver, Notes to the Hebrew Text of tiamuel {Oxford, 1890). SILPHIUM (Lat., from Gk. <Tl<t>iov, a sort of umbfllileious plant, the juice of which was used in food and medicine) . A genus of about a dozen tall, coarse, American perennial plants of the order Composite. They have a copious resinous juice, and large corymbose-panicled yellow flower- ing beads. Hiipliium laeiniatum, called rosin- weed, is rough and bristly, grows from 3 to 6 and sometimes 10 feet higli, and has pinnatel.y parted leaves. It grows on the prairies of Michigan, Wisconsin, and southward and westward, and blossoms in .July. It is called compass-plant (q.v. ), from the turning of its lower leaves so that their edges point north and south. Another species, Silphium tercbenthinaccum, the prairie burdock, grows from 4 to 10 feet high, and has many small heads in a panicle at the top. SILURIAN SYSTEM (from Lat. Silures, a peoide of ancient liritain). A division of the Paleozoic group of rocks established by Murchi- son (q.v.) to include the strata between the Archiean and Devonian systems. It was subse- quently restricted to the two formations now known as the Ordovician or Lower Silurian and Upper Silurian. These two extend from the up- per limits of the Cambrian to the base of the Devonian. Silurian rocks are extensively de- veloped in both the United States and Europe. The rocks of the Silurian system in America are divided as follows : 3. Upper Pentamenis stuge. 3. Lower Helderberg series. 2. Slialy Limestone stage. 1. Lower Pentamerus stage. Silurian system. 2. Onondaga series. Salina and water- lime stage. 3. Niagara stage. 1 Niagara series. ] 2. Clinton stage. 1. Medina stage. The rocks are largely limestones, but there are also beds of shales and some sandstones inter- stratified with them.. While there was some dis- turbance at the end of the Ordovician era, at the same time it was not sufficiently extensive in America to change materially or increase the ex- tent of the land surface which existed in the Ordovician times. Silurian rocks are present in great thickness in the Eastern States, especially along the Appalachian region. The lowest forma- tion, or that known as the Oneida, is a conglom- erate which appears in central New York, thin- ning toward the eastern sliore line, but is very thick along the Appalachian ranges as far south as Tennessee. Owing to its great hardness, it forms many prominent ridges, notably the Sha- wangunk Mountains of New York, also the crests of the Kittatinny Mountains, and the ridges at Delaware Water Gap. Overlying the conglomerate is a great deposit known as the Medina sandstone, which was formed in shallow water and shows many rip' pic marks. It extends from Central New York with decreasing thickness toward Ohio, but in eastern Pennsylvania the beds aggregate 1800 feet. Overlying tliis is the (Tinton shale, which is well known from New York down into Georgia, and westward into Wisconsin, in which region it changes into limestone, indicating that the Silurian seas were deeper in that area than they were further ' east. A subsequent deepen- ing of the water over a still greater area is iii<li- cated by the formation of the Niagara limestone, which is well developed in the gorge of Niagara River, and whose resistance to erosion causes the abrupt descent of the Niagara River at the Falls. This formation ranges over a very large territory westward to Wisconsin, and then south- ward through Illinois into Missouri and West Tennessee. Small areas are also found in Iowa, the Black Hills, and Nevada. Following this great limestone deposit there comes a series of shallow water deposits of salt, gypsum, and shale of the Salina stage, which are well de- veloped in New York and Ohio, but thin out in Pennsjdvania. In some localities an argillaceous limestone was deposited during the same period, to which has been given the name of water-lime on account of its value in the manufacture of cement. On top of these beds lie great beds of limestones due to the deepening of the water in which the Silurian sediments were being de- posited. To this great limestone series has been given the name of Lower Helderberg. It is proba- ble that the depression made at this time sub- merged some areas which had been dry land since Ordovician times, as in some cases we find the Lower Helderberg rocks resting directly on Ordovician strata. The Lower Helderberg rocks are alnindant in New York, where they form the bold escarpment of the Helderberg Mountains near Albany, but are also known to extend south- ward through Pennsylvania to Virginia, while additional deposits are known in western Ten- nessee and Maryland. Silurian beds are well developed in Europe, China, Northern Africa, South America, and Australia, as well as in North America. At the termination of the Silurian there was a gradual transition into the Devonian, so that it is often difficult to determine the boundary line between the rocks of the two systems. The plant life of the Silurian, so far as re- vealed by the fossil remains, was scanty. Sea- weeds were abundant, but land plants are rarely found. Among the animals there was a great development of invertebrates. Sponges were present in force, but the graptolites had di- minished. Both the hydroid corals and the true corals were very important, the former be- ing especially important as reef builders. Fa- vosites and Halysites are two well-known fossil corals of the Silurian rocks. There was a marked increase of erinoids and also starfishes, while even the sea-urchins were fairly abundant. The trilobites also continued to flourish, although not as numerous as those of the Ordovician; among the common genera were Calymene, Dal- manites, and Lichas. Some insects have also been found, such as scorpions, and prove that there must have been land vegetation. The brachiopods continued in countless numbers, and the genera were quite different on the whole
