Extrusive Phenomena.— Material which actually reaches the earth's surface is said to be extrusive. Among the most important ex trusive phenomena are cones, craters, lava flows and ash beds. The cone is the hill built up around the volcanic vent, and where formed wholly of.lava it is low and flat, but a cinder cone of solid ejecta is capable of standing in steeper slopes. Composites of both types are most common. Most large cones show numer ous secondary or parasitic cones built over sub ordinate vents. The crater is the hollow in the cone from which the lava issues. In the great Hawaiian volcanoes the craters are several miles across and constitute veritable lakes of hot liquid rock. After volcanoes become ex tinct, the craters may be occupied by lakes, as in Crater Lake, National Park, Oregon. Where the lava is stiff and viscuous it may flow only a short distance, but with very fluid lavas the flows may be many miles in extent. In the Columbia Plateau of Central Washington one such flow followed another, with thin beds of sand and gravel between, to a total thickness of from two to four thousand feet. The sur faces of lava flows are usually cindery or scoriaceous, and full of blow holes. As the flows move forward they often pick up water worn bowlders of rock, fragments of trees, and other debris. They may later be buried under hundreds of feet of sediments, but the scoriaceous surfaces, steam cavities, and in cluded bowlders serve to prove their surface origin. Volcanic ash is often very fine and may be wind drifted for miles. In Nebraska, many miles from the nearest possible volcanic source. are beds of such ash 'varying from a few inches to several feet in thickness now buried under other rock.
Intrusive Phenomena.— Fluid rock which forces its way into and through the surrounding rocks but is not poured out at the surface is said to he intrusive. It occurs in a variety of forms the most important of which are dikes, sills, laccoliths, and batholiths. Fluid rock fill ing fractures in other rocks constitutes dikes, which may occupy any position whatever, but are usually more nearly vertical than horizontal. Thin sheets of lava insinuated between beds along bedding planes are known as sills. They may be either horizontal or vertical, dependent on the position of the beds between which they are intruded. Bodies similar to sills but thick enough to arch up the overlying beds into domes are called laccoliths. Very large irreg ular-shaped bodies which melt their way across the enclosing rocks at any angle are known as batholiths and may be many miles in extent.
Pipes and plugs'are nearly Circular smell bodies, and stocks and bosses are essentially small batholiths.
Contact Metarnorphiam.—At the contact between igneous rocks.and the rocks into which they are intruded, particularly if the igneous bodies are very large and at very high tempera tures, there is usually profound alteration or metamorphism of the wall rocks. New miner
als are formed, usually silicates, since these are best adapted to conditions of high temperature and great pressure. Garnets are ,commonly de veloped in limestones and aluminum silicates in shales. Quartzites suffer less change. See ANDALUSITE ; GARNET; MAGNETITE; METAMOR PHISM ; etc Volcanism and Ore Deposits.—With few exceptions the great ore deposits of world are connected with igneous activity. In a few cases, as in certain magnetite deposits, and in the case of the Sudbury nickel district, the ores have probably actually solidified from a fluid state with the enclosing igneous rock. In other cases, as at the Clifton-Uorend copper camp in Arizona, bodies of igneous rock have meta morphosed the adjacent limestones into a great mass of garnet rock and copper ore, the ore probably coining from the hot igneous rock. In still other cases the igneous rocks upon cooling have given off hot magmatic waters with ores in solution. These have passed up ward into cracks in the overlying rocks and as they cooled have deposited their metals. This is now believed to be the origin of a very large number of the ore deposits. In still other cases the igneous rock has heated the ordinary ground water; thus giving it a greater power of dissolving mineral matter and render ing it more efficient in the gathering together of ore deposits. See DIKE; ECONOMIC Gal:L OGY; METAMORPHISM; MAGMATIC SEGREGATION; ORE DEPostrs; VOLCANOES; etc.
Sraticrtiam. GEOLOGY (GacrracrOxtcs).
This branch of geology outlines the way in which earth materials are put together. Certain structural features imparted to rocks at the tithe they are formed are called original; others imposed on the rocks by deformation at a subsequent period are called secondary or induced structures.
Original Structures.— Practically all sedi mentary rock shows a tendency toward arrange ment in parallel beds, due primarily to the sorting power of wind or moving water. Just so long as the water of a lake, for example, is strongly agitated only coarse material settles out. This makes one bed. When the water becomes quiet, the fine material settles out, and another layer is formed, differing slightly from the first. On a sea bottom where clay has been deposited, uplift may bring the area closer to shore, and sand beds may follow, or sinking may cause the reverse change. Some layers may be many feet thick, others thin as a sheet of paper. But the final result is that most sediments are layered or stratified. The struc ture is highly important, for it aids in weather ing, helps to control the shapes of cliff§ hills, and various land forms, modifies the circula tion of ground water and aids in quarrying. See STRATIIICATION.