CONTINENT; SHORE LINES; UNCONFORMITY.
Oropenic movements are mountain making, involving intense crumpling and dislocation of the rock masses, the result 'of powerful lateral thrust which has caused the rocks to arch and buckle and even to break and slide over each other for long distances. The cause of such lateral compression is still one of the great unsolved problems of geology. The in tensity and magnitude of these forces may be 'judged when it is known that in many areas the rocks have shoved over and past one an other distances of several miles. The pressure and accompanying heat often profoundly alter or metamorphose the rocks, with the develop ment of new minerals, the formation of cleav age, and the destruction of bedding and fossils, until in many regions it is impossible to deter mine whether the rocks were originally igneous or sedimentary. Quartzite, gneiss. schist, slate, and marble are the usual resulting products. See Rocx CLEAVAGE; FOLDS; FAULT; META MORPH ISM ; METAMORPHIC ROCKS; MOUNTAINS; STRUCTURAL GEOLOGY ; etc.
Earthquakes.— The processes outlined above may usually be considered as The result ant of a very great number of successive small but sometimes sudden movements. Along any line of weakness the stress gradually accumu lates until suddenly the rock yields by bending or breaking, causing a tremor known as an earthquake. The stress is relieved and years may intervene before there is sufficient accumu lation of forces to cause another movement in the same place. Considered broadly, earth quakes are geological results rather than causes, but they in turn produce certain minor effects of geologic importance. They are a frequent cause of landslides and avalanches. Under ground water circulation is often disturbed by the cracking of the rock, so that springs and wells go dry and new springs are formed. Along the fracture or faults that caused the shock there is frequent shifting of the walls, one side not uncommonly being raised so that a low cliff or scarp is formed, which may ob struct rivers and form lakes or falls. Per manent changes of level frequently accompany earthquakes. In Alaska in 1899 portions of the coast were uplifted 47 feet. In thickly settled
regions destruction to life and property is a notable result. See EARTHQUAKE.
Volcanism.— Volcanism includes all those phenomena dependent on the movement of mol ten rock (magma) within the earth or on its surface. The cause of volcanism is another great geologic problem which is far from being settled. One school believes that the lava is a still uncooled residue from a once molten earth. Another believes lavas form from the melting of rocks in the earth's crust due to re lief of pressure caused by the arching up of folds, to heat generated by crushing and shear ing of rock masses, or to still other causes. From its deep seated source, this fluid rock works its way toward the surface, sometimes along cracks, sometimes by melting its way through the rocks. That which reaches the surface may pour out quietly or burst forth with explosive violence.
Volcanic products are of three types, gases, liquids and solids. Of the gases, steam and carbon dioxide are by far the most abundant. Sulphur gases, chlorine, hydrochloric acid, and a number of rarer substances are also usually present. These may be derived in part from the sediments traversed but are largely original in the magma. Highly heated fluid rock called lava is the most abundant product. Such rock, when it cools slowly at great depth and retains its water content, is usually coarsely crystalline and forms granite, gabbro, etc. When forced out at the surface, where it cools quickly and loses its moisture, it forms very fine crystals or becomes glassy, in which case we have basalt, trap, rhyolite, obsidian, etc. (See BASALT; GRANITE; IGNeous Rocas; TRAP; etc.). During explosive eruptions much solid material is blown out of the crater. If very fine, this is known as ash; if slightly coarser, lapilli; and these when consolidated form volcanic tuff : Still coarser material is known as volcanic breccia or agglomerate. Lava hurled into the air in a semifluid form and rapildy rotated takes on a circular shape forming volcanic bombs. Such material if highly porous is called pumice. The solid volcanic ejecta are known as pyroclastics.