Onyx, variety of cryptocrystalline quartz, differing from agate only in that the bands of which it is composed are parallel and regular. Its appearance is most striking when the bands are of s...

Chalcedony [from Chalcedon], form of quartz the crystals of which are so minute that its crystalline structure cannot be seen except with the aid of a microscope. Chalcedony has a waxy luster ...

Scottsbluff, city (1990 pop. 13,711), Scotts Bluff co., W Nebr., on the North Platte River near the Wyo. line; inc. 1900. It is the market, distribution, and processing point of an irrigated f...

Quartz, one of the commonest of all rock-forming minerals and one of the most important constituents of the earth's crust. Chemically, it is silicon dioxide, SiO2. It occurs in crystals of the...

Petrified Forest National Park, 93,533 acres (37,881 hectares), E Ariz.; est. as a national monument 1906, designated a national park 1962. A part of the Painted Desert, it contains the larges...

Month, in chronology, the conventional period of a lunation, i.e., passage of the moon through all its phases. It is usually computed at approximately 29 or 30 days. For the computation of the...

Silica or silicon dioxide, chemical compound, SiO2. It is insoluble in water, slightly soluble in alkalies, and soluble in dilute hydrofluoric acid. Pure silica is colorless to white. It occur...

Taj Mahal, mausoleum, Agra, Uttar Pradesh state, N India, on the Yamuna River. It is considered one of the most beautiful buildings in the world and the finest example of the late style of Ind...

National Parks and Monuments1BFBattlefieldBPBattlefield ParkBSBattlefield SiteHPHistorical ParkHSHistoric SiteHTHistoric TrailISInternational Historic SiteLSLakeshoreMMMemorialMOMonumentMPMili...