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.
HOW MOUNTAINS COME AND GO
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MOUNTAINS constitute the grandest relief features of the earth, and some of the most profound lessons of earth changes may be learned by studying them. To the layman who views great mountains in all their grandeur and massiveness, the expression “everlasting hills” seems appropriate. But the geologist knows that even the loftiest mountains are only temporary features on the face of the earth. Like organisms, they come and go. For example, where the great Rocky Mountains now stand was only a few million years ago (in late Mesozoic time) the bottom of an interior sea. Where the Appalachians now stand there were no mountains late in the Paleozoic era (not less than ten or twelve million years ago), but instead sea water covered the district. Then the Appalachians were formed, lifting their heads much higher than at present, after which they were cut down almost to sea level, and then once more upraised. The Coast Range Mountains of our Pacific Coast have come into existence since the middle of the present (Cenozoic) geologic era. Every mountain, like every organism, has a life history, in some cases simple, and other cases complex. All pass through stages of birth, youth, maturity, old age, and death. Some rear their heads and disappear after a short (geological) existence. Others continue their growth and persist much longer, while still others undergo periods of profound rejuvenation.
Among the various processes by which mountain ranges have been formed, the folding and accompanying general uplift of strata are the most important. In fact, in most of the great mountain ranges of the world the folded structure is conspicuously developed, so much so that they may well be called “folded mountains.” Very commonly, however, mountains of this type have also been subjected to more or less fracturing of the rocks (faulting), and not uncommonly they have also been subjected to igneous activity, including both intrusion and extrusion of molten material. It is among the folded mountains of greater or less degree of complexity that the “greatest exhibitions of geologic phenomena are seen and the lessons which geology as a sciences teaches may be learned. If one desires to know the history of a region, one turns naturally to its mountain ranges, for here may be found the upturned and dissected strata, a study of whose kinds, thickness, and fossils throws light upon past events, while their foldings and dislocations show the nature and results of those great dynamic agencies which, from time to time, have operated upon the outer portion of the earth, and given to it the broad distinctive features which characterize it to-day.” (L. V. Pirsson.) Among the great mountains we may also see wonderful exhibitions of the results of weathering and erosion, especially the work of rivers and glaciers.