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Primordial :: Primordial (a.) Of or pertaining to the lowest beds of the Silurian age, corresponding to the Acadian and Potsdam periods in American geology. It is called also Cambrian, and by many geologists is separated from the Silurian..

Cambrian :: Cambrian (a.) Of or pertaining to the lowest subdivision of the rocks of the Silurian or Molluscan age; -- sometimes described as inferior to the Silurian. It is named from its development in Cambria or Wales. See the Diagram under Geology.

Trenton Period :: Trenton period () A subdivision in the lower Silurian system of America; -- so named from Trenton Falls, in New York. The rocks are mostly limestones, and the period is divided into the Trenton, Utica, and Cincinnati epochs. See the Chart of Geology..

Asaphus :: Asaphus (n.) A genus of trilobites found in the Lower Silurian formation. See Illust. in Append.

Silurian :: Silurian (n.) The Silurian age.

Utica :: Utica (a.) Of, pertaining to, or designating, a subdivision of the Trenton Period of the Lower Silurian, characterized in the State of New York by beds of shale..

Chaetetes :: Chaetetes (n.) A genus of fossil corals, common in the lower Silurian limestones..

Wenlock Group :: Wenlock group () The middle subdivision of the Upper Silurian in Great Britain; -- so named from the typical locality in Shropshire.

Atrypa :: Atrypa (n.) A extinct genus of Branchiopoda, very common in Silurian limestones..

Ludlow Group :: Ludlow group () A subdivision of the British Upper Silurian lying below the Old Red Sandstone; -- so named from the Ludlow, in Western England. See the Chart of Geology..

Tilestone :: Tilestone (n.) A kind of laminated shale or sandstone belonging to some of the layers of the Upper Silurian.

Cliff Limestone :: Cliff limestone () A series of limestone strata found in Ohio and farther west, presenting bluffs along the rivers and valleys, formerly supposed to be of one formation, but now known to be partly Silurian and partly Devonian..

Salina Period :: Salina period () The period in which the American Upper Silurian system, containing the brine-producing rocks of central New York, was formed. See the Chart of Geology..

Halysites :: Halysites (n.) A genus of Silurian fossil corals; the chain corals. See Chain coral, under Chain..

Pentamerus :: Pentamerus (n.) A genus of extinct Paleozoic brachiopods, often very abundant in the Upper Silurian..

Heliolite :: Heliolite (n.) A fossil coral of the genus Heliolites, having twelve-rayed cells. It is found in the Silurian rocks..

Delthyris :: Delthyris (n.) A name formerly given to certain Silurian brachiopod shells of the genus Spirifer.

Niagara Period :: Niagara period () A subdivision or the American Upper Silurian system, embracing the Medina, Clinton, and Niagara epoch. The rocks of the Niagara epoch, mostly limestones, are extensively distributed, and at Niagara Falls consist of about eighty feet of shale supporting a greater thickness of limestone, which is gradually undermined by the removal of the shale. See Chart of Geology..

Graptolite :: Graptolite (n.) One of numerous species of slender and delicate fossils, of the genus Graptolites and allied genera, found in the Silurian rocks. They belong to an extinct group (Graptolithina) supposed to be hydroids..

Chazy Epoch :: Chazy epoch () An epoch at the close of the Canadian period of the American Lower Silurian system; -- so named from a township in Clinton Co., New York. See the Diagram under Geology..

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