"Notwithstanding the apparent irresistible nature of these layers of rock, they slowly yield to the enormous lateral pressure of contraction and gradually huge folds are pushed up in long mountain ranges. Usually this process goes on so slowly and gradually that the yielding of the rock masses takes place without noticeable jar, but occasionally a sudden slip occurs under the gigantic forces, and an earthquake is the result. This slip is usually only a few inches, but when two continents fall together for only a few inches enormous energy is developed. "Such slips usually occur along the line of an old fissure previously formed, and the depth below the surface of the earth varies from one to twelve miles. Thus places situated near these old internal fissures are more likely to experience earthquakes than those farther away. It is a well known geological fact that the Pacific coast in California contains several of these fissures and earthquakes are more common there. The entire western part of the United States has been slowly rising for many centuries, and the shifting of soil due to erosion and transportation doubtless contributes to produce these seismic disturbances. "Earthquakes are more common than most persons think. Modern instruments for detecting slight tremors within the earth's crust show that there is scarcely an hour in the day free from these shocks. In mountain regions, and especially in the highest and youngest mountains, erosion is most rapid, and on the sea bottom, along the margin of the continents sedimentation is greatest. In these regions, therefore subterranean temperature and pressure changes are most rapid and earthquakes most frequent. "A study of earthquakes develop these general facts. The origin is seldom more than twelve miles below the surface; the size of the shaken region bears a certain relation to the depth of the origin or focus, the smaller shaken region indicating a relatively shallow origin; the energy of the shock is approximately indicated by the area of the shaken region; the origin is seldom a point, but generally a line many miles in length; the subterranean stress is not relieved by a single movement, but rather by a quick succession of movements causing a series of jars. "The transmission of an earthquake shock through the earth takes place with wonderful rapidity. The elastic wave varies in velocity from 800 to 1,000 feet per second in sand or clay to three miles per second in solid granite. "Sometimes these vibrations are of such a character as to be imparted to the air, and their transmission through the air outstrips the transmission through the earth and the ear detects the low rumbling sounds before the shock is felt. "If the origin of the shock is under the sea near the coast any upheaval of the bottom of the ocean that frequently accompanies an earthquake, gives rise to a great tidal wave that frequently inundates the neighboring coast with much damage.