158 THE EARTH AS MAN'S ABODE type of instrument employs a small mirror mounted on a balanced weight, which causes a beam of light to move on a moving piece of photographic paper. The problem of the high cost of developing the photographic paper has recently been eliminated by the use of a photoelectric cell,' which sets up a fluctuating electric current as the beam of light flickers across it. This current, amplified by a vacuum-tube hookup, drives a pen which records the wave on a plain white paper. The essential part of the seismograph is like a pendulum, the bob of which tends to remain stationary in space, while everything attached to it moves. It is impossible to attach the pendulum in such a way that it docs not move to a certain extent with the earth, but this motion may be corrected so as to obtain the true motion of the earth relative to it. It has been found that the vibrations produced by earthquakes pass through the center of the earth. The rate at w^hich vibrations pass through different media is quite w^ell know^n; and from this information it is concluded that, if the center of the earth is a molten liquid, these vibrations would travel much less rapidly than they actually do travel. Data obtained with the seismograph and other lines of evidence suggest that the earth has an outer layer of soil and rock about 37 miles thick. Beneath this layer there is thought to be a layer of compounds of iron, magnesium, and silicon about 950 miles thick. Below this there is another similar layer about 875 miles thick, in w^hich the iron content increases with the depth. The center of the earth is thought to consist of a core of iron and nickel, which accounts for its magnetism. The other elements are found throughout these layers in relatively insignificant quantities. If these ideas are correct, it seems quite possible that the earth may have been in a molten condition at one time, these substances being separated in layers in accordance with their densities. Earthquakes are recorded by seismographic stations throughout the world. The deformation of the earth under the influence of stresses which develop within it frequently takes the form of sudden fractures. Displacements at the surface are produced by these fractures, which start vibrations that spread throughout the earth. Inasmuch as any earthquake is made up of three linear displacements along directions at right angles to each other, a fully equipped observing station requires three seismographs, set to read the north-south, east-west, and vertical components of the earthquake. A rough estimate of the position of the earthquake origin can be obtained by stations thus equipped, without the use of records from other stations. Although the primary causes of earthquakes lie within the earth, it ' This topic will be studied in a later unit.
DIASTROPHISM AND VULCANISM 159 is quite possible that the actual time of occurrence is determined by such external factors as the change in pressure due to annual and daily changes in atmospheric pressure. Earthquakes occur more frequently during the night than the day and more frequently in winter than in summer. Years of many sunspots show a greater number of earthquakes than usual. Microseisms, or very small oscillations, are thought to be produced by storms; they are much more frequent in winter than in summer. Mountain Ranges Have Been Formed by Slow Uplifts of the Earth's Surface. The forces which have caused earthquakes have been producing very slow changes in the earth's surface. Some portions of .the earth's surface are being elevated gradually. The Italian island of Palmarola has risen more than 200 feet since 1822. The northern part of the Scandinavian Peninsula has been rising for several thousand years. Shore lines have been raised 1000 feet above the sea in northern Sweden. An elevation is taking place in the region of the Great Lakes along the Canadian border, where the present elevations rise 600 or 700 feet above former levels. On the other hand, the eastern coast of the United States is sinking, as is also the northern coast of France. The Bermuda Islands have sunk to the extent that a former area of 576 square miles has been reduced to 20 square miles. Some mountains are wholly volcanic in origin, while others, like the Catskills, were formed by erosion of high plateaus. The Black Hills of South Dakota were formed by the erosion of a huge dome upthrust 9000 feet by a lava intrusion. The great mountain ranges of the world, like the Alps, the Himalayas, the Rockies, and the Sierra Nevadas, were formed by faulting and folding. These mountain ranges represent rock layers produced by sedimentation in shallow ocean beds to a depth of 25,000 to 30,000 feet, which have been forced upward in the form of mighty folds. Inasmuch as the continental shelves are not over 600 feet deep, a layer of sedimentary rock 30,000 feet thick could be formed only by the gradual sinking of these shallow basins as they became filled with sediment. At the same time the neighboring land must have been rising gradually to provide sufficient material for the tremendous amount of erosion necessary to form such huge amounts of sediment. When the Appalachian range was folded, the circumference of the earth was decreased about 100 miles or more, and it was decreased perhaps twice as much when the Alps were formed.