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I

ICE—A solid form of water. Water frozen, or reduced to the solid state by cold. Ice is a transparent, nearly colorless, crystalline, and brittle substance. Water in freezing expands about one eleventh of its volume, the specific gravity of ice being 0.9166, that of water at 4°C (39.2°F) being 1.0. Pure water freezes at 0°C (32°F), and ice melts at the same temperature.

ICE AGE—(1) A cold period marked by episodes of extensive glaciation alternating with episodes of relative warmth. 2. (Ice Age) The most recent glacial period, which occurred during the Pleistocene epoch.

ICE APRON—A wedge-shaped structure for protecting a bridge pier from floating ice.

ICE BARRIER—The outer margin of the antarctic ice sheet.

ICEBERG—A massive piece of ice that breaks off and floats away from a Glacier. Icebergs occur as huge blocks, or in peaked forms of great variety and beauty. About one ninth of the bulk of an iceberg projects above sea level.

ICEBLINK—(1) A white or yellow streak in the sky near the horizon, caused by the reflection of light from an area of ice. Also call Blink. (2) A coastal ice cliff.

ICEBOUND—Locked in or covered over by ice.

ICECAP, or Ice Cap—An extensive dome-shaped or plate-like perennial cover of ice and snow that spreads out from a center and covers a large area, especially land. A very large ice cap is an Ice Sheet, or continental Glacier, as that of the antarctic continent.

ICEFALL—(1) The part of a glacier resembling a frozen waterfall that flows down a steep slope. (2) An avalanche of ice.

ICEFIELD—A large, level expanse of floating ice that is more than eight kilometers (five miles) in its greatest dimension.

ICEFOG—A fog of ice particles. Also referred to as Pogonip.

ICEFOOT—A belt of ledge of ice that forms along the shoreline in Arctic regions.

ICE-FREE—(1) Free of ice and open to travel or navigation, as an ice-free channel in a river. (2) Marked by a lack of obstructive ice.

ICE-MINUS—Of or relating to a strain of genetically altered bacteria that are applied to crop plants to inhibit the formation of frost.

ICE NEEDLE—A thin ice crystal floating high in the atmosphere in certain conditions of clear, cold weather.

ICE NUCLEUS—Any particle that serves as a nucleus in the formation of ice crystals in the atmosphere.

ICE-OUT—The thawing of ice on the surface of a body of water, such as a lake.

ICE PACK—(1) A large area of floating pieces of ice driven together more or less closely. (2) A folded sac filled with crushed ice and applied to sore or swollen parts of the body to reduce pain and inflammation. Also referred to as an Ice Bag.

ICE POINT—The temperature, equal to 1.0°C (33.8°F), at which pure water and ice are in equilibrium in a mixture at 1 atmosphere of pressure.

ICESCAPE—A wide view or vista of a region of ice and snow.

ICE SHEET—A very large Ice Cap, also called continental glacier, as that of the antarctic continent.

ICE STORM—A storm in which snow or rain freezes on contact, forming a coat of ice on the surfaces it touches.

ICE WATER—Chilled or iced water, especially served as a beverage.

ICHTHYOLOGY—The study of fishes.

ICHTHYOSAURUS (ICHTHYOSAUR) [Nevada]—The chief genus of Ichthyosauria, also known by a prior name, Proteosaurus. An ancient, extinct marine reptile whose name means "fish-lizard" and who ruled the world's oceans during the Mesozoic era some 200 million years ago. Fossil remains of the Ichthyosaur have been found on every continent except Africa. Possessing a fish-like body, porpoise-like snout, short neck, dorsal and caudal fins with limbs flattened into paddles, the Ichthyosaur ruled the seas for some 135 million years as the pre-eminent marine predator. The Ichthyosaur attained a length of approximately 60-70 feet. The Ichthyosaurus shonisaurus popularis was the name given to a species discovered in Nevada in 1928. Some 40 of these reptiles became stranded in mud flats from a receding equatorial sea which once covered the state. The longest specimen found at this site, located at an elevation of 7,000 feet in the Shoshone Mountain Range near the town of Berlin in northwestern Nye County, Nevada, was 55 feet long and represented the only complete fossilized skeleton of the species ever found in the United States. In 1977 the Nevada State Legislature named the Ichthyosaurus Shonisaurus popularis as Nevada's official state fossil.

ICICLE—A tapering spike of ice formed by the freezing of dripping or falling water.

IDENTIFICATION—(Statistics) A term used to describe the ability to determine an econometric model's structural parameters, i.e., the coefficients of the exogenous (or independent) variables. An econometric model is said to be exactly identified if the data support a unique set of parameters for the independent variables. A model is said to be Under-identified if there is no way of estimating all the structural parameters and Over-identified if more than one value is obtainable for some parameters.

IGNEOUS ROCK—(Geology) A rock formed by the solidification of molten materials (magma). The rock is extrusive (or volcanic) if it solidifies on the surface and intrusive (or plutonic) if it solidifies beneath the surface.

ILLINOIAN—(Geology) Of or relating to one of the glacial stages of the Pleistocene epoch which occurred in North America, which consisted of the Nebraskan (first stage), Kansan (second stage), Illinoian (third stage), and Wisconsin (fourth stage).

ILLUVIAL—Describing soil material, usually minerals and colloidal particles, that is removed from the upper soil horizon to a lower soil horizon. Illuvial deposits can form a Hardpan.

ILLUVIATION—The deposition in an underlying soil layer of colloids, soluble salts, and mineral particles leached out of an overlying soil layer.

IMBIBITION—(Chemistry) Absorption of fluid by a solid or colloid that results in swelling.

IMHOFF CONE—A clear, cone-shaped container used to measure the volume of settleable solids in a specific volume of water.

IMHOFF TANK—An anaerobic sewage treatment tank in which solids are withdrawn from the bottom of the tank.

IMMERGE—To submerge or disappear in or as if in a liquid.

IMMERSE—To plunge, drop, or dip into or as if into a liquid, especially so as to cover completely.

IMMERSIBLE—Capable of being completely immersed in water without suffering damage.

IMMISCIBLE—Applied to liquids which are insoluble in each other. The chemical property where two or more liquids or phases do not readily dissolve in one another, such as oil and water.

IMPACT ZONE—The spot on a wave where the water is just about to collapse and explode, the spot of greatest danger to and opportunity for a surfer.

IMPERIAL VALLEY [California]—A valley, southeast California, bounded by the Salton Sea on the north, the Chocolate Mountains on the east, and the desert ranges of the Santa Rosa and Vallecito mountains on the west. The valley, crossed by the border between the United States and Mexico, is part of a larger valley that extends south into Mexico; t