Brief descriptions of a few typical carefully studied earthquakes during recent years will serve to make the main features of earthquakes still clearer to the reader. The violent Japanese earthquake of 1891 was caused by the sinking of a block of earth forty miles long from two to thirty feet below that on the other side of a fault fracture. There was also considerable horizontal shifting, and cracks developed in the adjacent region. A distinct fault scarp, fifteen to twenty feet high, developed, and in some cases extended right across cultivated fields. [Illustration: Fig. 13.--Map of the United States, showing the large areas over which three of the greatest of our earthquakes were actually felt by people. These earthquakes were recorded in many parts of the world by delicate instruments: New Madrid, 1811; Charleston, 1886; San Francisco, 1906.] [Illustration: Fig. 14.--Sketch map showing the trace of the great fault fracture along which a renewed sudden movement of as much as twenty feet took place to cause the San Francisco earthquake of 1906. (After U. S. Geological Survey.)] The San Francisco earthquake of 1906 was produced by renewed movement along the great fault which extends lengthwise through the Coast Range Mountains for several hundred miles. It is literally correct to say that, for 250 miles along this great earth fracture, one part of the Coast Range instantaneously slipped from two to twenty-two feet past the other. More or less of the movement extended at least several thousand feet down into the earth. In this case both sides slipped and the movement was very largely horizontal rather than vertical. The land on the east side of the fault moved south and that on the west side moved north, the amount diminishing away from the fault on each side so that some miles out the actual crustal movement was only a few inches. When one thinks of the tremendous volumes of earth material involved in this shifting of the earth's crust, is it any wonder that such destructive earthquake waves were produced? Many buildings were wrecked, several hundred people were killed, the disastrous San Francisco fire resulted, water mains were broken, and fences and roads crossed by the fault were dislocated as much as fifteen to twenty feet. [Illustration: Plate 5.--Swift Current Valley in Glacier National Park, Montana. This was once a deep V-shaped canyon carved out (eroded) by stream action. Then a great valley glacier slowly plowed its way through it during the Ice Age and, by ice erosion, the present nearly straight U-shaped canyon has resulted. (_Photo by Campbell, U. S. Geological Survey._)]