[Illustration: Plate 8.--(_a_) A Ledge of Igneous Rock (Granite) in Northern New York. This illustrates so-called "joints" or natural cracks, commonly separating most hard rock masses into more or less prismatic blocks. (_Photo by the author._)] [Illustration: Plate 8.--(_b_) A Fault Fracture in a Ledge at East Canada Creek in the Mohawk Valley, New York. The Ordovician limestone formation in thin layers on the right has sunk hundreds of feet along vertical fault to the left of middle, bringing it sharply against the older (Cambrian) massive formation on the left. The hole is artificial. (_Photo by Darton, U. S. Geological Survey._)] Geysers are periodically eruptive hot springs found only in a few of the volcanic regions of the world. They are most wonderfully displayed in the Yellowstone National Park, where they send columns of hot water to all heights up to 250 feet at various intervals of time. Almost incredible amounts of hot water are sent into the air every day in the geyser basins of Yellowstone Park. The single geyser "Old Faithful," which erupts at intervals of about seventy minutes, sends a column of water several feet in diameter to heights of from 125 to 150 feet. During each eruption about 1,500,000 gallons of water are sent forth, or every day enough to supply the need of a fairly large city. A very brief explanation of the cause of geyser eruptions may be stated as follows: The very irregular, narrow, geyser tube extends nearly vertically downward into yet uncooled lava. The tube is more or less rapidly filled by underground water. The bottom, or near-bottom, portion of the water gradually becomes heated by the lava until finally the boiling point is reached for that depth. But, because of the pressure of the overlying water column, the boiling point at that depth is considerably greater than for the surface. A little steam develops far down and this causes the whole column of water above it to lift slightly, thus relieving the pressure on the superheated water far down. This relief of pressure allows much of the superheated water far down to flash into steam, which violently forces the column of water out of the geyser tube. CHAPTER X HOW MOUNTAINS COME AND GO