SILURIAN ROCKS (ante). The lower Silurian in North America embrace, I. the primordial, or Cambrian period, which again embraces the Acadian and the Potsdam epochs. II. Canadian period, which embraces the calciferous, the Quebec, and the Chazy epochs. III. Trenton period, embracing the Trenton, Utica, and Hudson ricer epochs. The upper Silurian embraces, I. the Niagara period, with Medina, Clinton, and Niagara epochs. 2. Salina period, containing the Onondaga salt group. 3. Lower period, containing the lower Helderberg limestone which includes In New York the water lime group, the lower pentamerous limestone, the delthrys shaly lime stone, and the upper peutamerous limestone. 4. Oriskany period, containing the Oris kany sandstone.
In the Acadian epoch the rocks are exposed to view in valleys in New Brunswick. They are gray and black shales with some sandstones, and have a thickness (42,000 feet. Iu the Potsdam epoch in Labrador and Newfoundland are strata of limestones, sand stones, and shales from 5,000 to 5,600 ft. thick. In New York, Vermont, mid Canada, rocks, lying upon the Archean, are hard sandstones, sometimes pebbly conglomerates. The sandstones are laminated, gray, drab, yellowish, brown, and red, often good build ing stone, as at Potsdam and Malone. In Clinton and St. Lawrence the conglomerate - is often 300 ft. thick., In western Vermont there is red sandrock and Winooski lime stone. Along the Appalachians the striking characteristic is the great thickness of the formation. In Pennsylvania there are in the primal series of Rogers 2,000 ft. of lower slates upon which rests 90 ft. of sandstone, and above this from 200 to 1000 ft. of upper slates. The pictured rocks on the s. shore of lake Superior, and the pillared rocks at the w. end of the lake are considered to be of Potsdam formation. The primordial rocks bear evi dence only of marine life. The plants are chiefly fueoids. The animals are all inverte brates, as crinoids, brachiopods, pteropods, gasteropods, and cephalopods, worms and crustaceans. including trilobites (q.v.).
The rocks of the Canadian period appear at the surface in northern New York, Canada, northern Michigan, and Wisconsin; also in the Green mountains, the eastern Appalachians, from New Jersey southward, and iu Missouri, Arkansas, and the Rocky mountains. The fossil flora and fauna of the Canadian period much resemble that of the primordial, hut are more particularly rich in graptolites (q.v.). The earlier part of the Trenton period was particularly limestone-making. A broad belt of limestone of this period extends from eastern New York to Wisconsin, Minnesota, mid Missouri, including the Galena limestone. It forms the bluffs at Trenton falls. The general thickness is from i00 to 500 ft., reaching 1000, and in Pennsylvania, 2,000 feet. The
Utica shale comes to the surface in the Mohawk valley. Sea weeds are the only known fossil plants. Among the radiates are crinoids and star fish, corals made their first appearance. Various mollusk a were numerous, and among crustaceans trilobites were abundant. The strata are much upturne,l, some of them having a vertical position, others inclining from 10° to 90°, the greater part between 30° and 60°.
In the upper Silurian, the Niagara period contains Oneida conglomerates, in Oneida co., N. Y., the lower member of the Medina epoch, the upper being Medina sandstone. The conglomerate is 500 ft. thick iu the Shawangunk mountains, where it is called Shawangunk grit. The rocks of the Niagara epoch are very extensive, occupying.a large portion of the interior of the continent, and they are to a great extent limestones, this being also a limestone-making epoch. Near Niagara falls there are 165 ft. of lime stone resting on 80 ft. of shale, and at the falls 85 ft. of limestone resting on 80 ft. of shale. Along the Appalachians the rocks have a thickness of 1500 feet. Niagara limestone occurs on the shores of Wellington and Barrow's straits and on King William's island, and common chain coral has been found in the Arctic reirions. There is lead aul coo per in the Shawangunk grit in Ulster co. N. Y., and these metrls are found in other Silurian rocks. Petroleum occurs in large quantities in Niagara limestone at Chicago, Ill., but is not easily worked. The otil;fossil plants are sea weeds. As to animals the sandstones of the Mediva and Clinton groups contain the remains of many brachiopods and lamellibranchs, but only a few corals and erinoids. The Clinton and Niagara lime stones, however, are rich in corals, crinoids, trilobites, and brachiopods, while there arc but few lamellibranchs or muddy bottom mollusks. The fine sandy and clayey charac ter of the Medina beds shows that at the time of their deposition central New York was an extensive area of low, sandy sea shores, flats and marshes, not exposed to heavy waves, and that a similar condition extended westward to Michigan. The beds of the Niagara epoch bear evidence of having been formed in deep seas, from the thickness of the limestones in the Mississippi basin and elsewhere. After the elevation of the Green mountains, according to Dana, there was a gradual sinking which moved the coast line eastward to the Hudson, so that over New 1 ork and the interior basin there was a vast limestone-making sea. At the same time there was another large sea in the St. Lawrence region. In the course of these oscillations, from the beginning of the Trenton to thil close of the Niagara period there was a deposition of 12,000 ft. of rock along the Alle ghanies.