[Illustration: Plate 6.--View in the Yosemite Valley from Near the Western Entrance. The great rock called "El Capitan," on the left rises 3,500 feet above the river, and Bridal Veil Falls on the right is 620 feet high. All the rock is granite, the nearly vertical walls of which have resulted from the action of a great glacier which plowed its way through the valley during the Ice Age; the valley walls have been cut back by the removal of large vertical joint blocks. The flat bottom of the valley has resulted from the filling with sediment of a postglacial lake in the valley. (_Photo by F. N. Kneeland, Northampton, Mass._)] During the great earthquake on the coast of Alaska in 1899 notable changes took place along the shore for some miles, one portion having suddenly risen as much as forty-seven feet, while another portion sank below sea level. [Illustration: Fig. 15.--Map showing the principal earthquake regions of the world.] In 1886 the earthquake centering near Charleston, S. C., was preceded by rumbling and roaring noises and the slight quaking increased to violent shaking which lasted more than a minute. Eight minutes later a rather violent earthquake shock took place, followed during the next ten or twelve hours by less severe shocks. Most buildings in the city were wrecked or more or less badly damaged, and some people were killed. The shocks were so violent that the quaking was actually felt by people over an area of more than 2,000,000 square miles, the disturbance having spread at the rate of about 150 miles per minute. Near Charleston openings and fissures were formed through which sand and muddy water were ejected, but the cause of the disturbance was most likely slipping of the old very hard rocks below the loose deposits of the Coastal Plain. From 1811 to 1813 a series of violent earthquakes developed in the general vicinity of New Madrid, Missouri. In an area of over 2,000 square miles, now called the "sunk country," many portions suddenly sank giving rise to small fault scarps or cliffs, and various lake basins were formed. Development of a fissure caused a local change in the course of the Mississippi River. In 1897, Assam, India, was shaken by an earthquake of unusual magnitude, which lasted 2-1/2 minutes. An area of 150,000 square miles was disastrously shaken, and the shocks were distinctly felt over an area of 750,000 square miles. A number of notable fault scarps developed, the movement on one having been thirty-five feet. CHAPTER VIII VOLCANOES AND IGNEOUS ROCKS