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HISTORICAL ENCYCLOPEDIA OF ILLINOIS.
ing material, some of the rock being susceptible of a high polish and making a handsome, durable marble. About seventy feet are exposed near Thebes, in Alexander County. All through the Southwest this stone is known as Cape Girardeau marble, from its being extensively quarried at Cape Girardeau, Mo. The Cincinnati group immediately succeeds the Trenton in the ascending scale, and forms the uppermost member of the Lower Silurian system. It usually consists of argillaceous and sandy shales, although, in the . northwest portion of the State, Magnesian limestone is found with the shales. The prevailing colors of the beds are light blue and drab, weathering to a light ashen gray. This group is found well exposed in the vicinity of Thebes, Alexander County, furnishing a durable building stone extensively used for foundation walls. Fossils are found in profusion in all the beds, many fine specimens, in a perfect state of preservation, having been exhumed.
UPPER SILURIAN SYSTEM.-The Niagara group in Northern Illinois consists of brown, gray and buff magnesian limestones, sometimes evenly bedded, as at Joliet and Athens, and sometimes concretionary and brecciated, as at Bridgeport and Port Byron. Near Chicago the cells and pockets of this rock are filled with petroleum, but it has been ascertained that only the thirty upper feet of the rock contain bituminous matter. The quarries in Will and Jersey Counties furnish fine building and flagging stone. The rock is of a light gray color, changing to buff on exposure. In Pike and Calhoun Counties, also, there are out-croppings of this rock and quarries are numerous. It is usually evenly bedded, the strata varying in thickness from two inches to two feet, and breaking evenly. Its aggregate thickness in Western and Northern Illinois ranges from fifty to 150 feet. In Union and Alexander Counties, in the southern part of the State, the Upper Silurian series consists chiefly of thin bedded gray or buff-colored limestone, silicious and cherty, flinty material largely preponderating over the limestone. Fossils are not abundant in this formation, although the quarries at Bridgeport, in Cook County, have
