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Show 50 GEOLOGY OF THE HIGH PLATEAUS. rock have been intruded between the Carboniferous and Mesozoic strata, hoisting the upper beds into great domes. Mr. G. K. Gilbert has studied in great detail the Henry Mountains of southeastern Utah, which present this singular phenomenon in perfection. This group of mountains consists of five individual masses, two of which are of great magnitude, and all of them have been domed up by lava rising from the depths and accumulating in reservoirs several thousand feet below the surface. Each of the mountains has a considerable number of these reservoirs and the two larger masses have many of them. The lava intruded itself at various horizons and congealed, leaving lenticular masses, which are now laid bare and admirably dissected by erosion. There are no indications that any notable quantity of the lava ever outflowed. To these intrusive masses Mr. Gilbert has given the name of " laccolites." These are by no means isolated instances of this extraordinary origin of mountains. The Sierra Abajo on the east wall of the Colorado and a small neighboring range called El Late present the same phenomenon. The Navajo Mountain at the mouth of the San Juan Eiver is similarly constructed.* Several of the Colorado ranges, according to Dr. Peale, owe their structure in part to "laccolitic" intrusion. But mountains on the whole are rare occurrences in the Plateau Province. The uplifts there are almost wholly of the tabular form. Yet, when we come to examine their structure, we find that those plateaus which are due to displacement have a construction strikingly similar to the broad platform-ranges of Colorado and to the Uintas. They are found along the western belt of the Plateau Province in the Kaibabs and in still more perfect development in the High Plateaus. Here the uplifts have been blocked out by the usual faults and monoclinal flexures. Most of them have a single fault upon the western side, inclining at a very small angle towards the east. The western limit is the lifted side of the fault; the eastern limit is the thrown side of the next fault. All traces of the anticlinal have vanished and the structure is of the simplest possible order. In a few of these uplifts we have a block between two faults or monoclinals of opposite throws. Such is the Kaibab Plateau itself. Bat the great predominance of the faults which face the west *The Navajo Mountain is a solitary dome-like mass of grand dimensions upon the very brink of the Glen Canon. The caiion slices off a segment of its base, and the spectacle of rock-work, looking at it from the end of the Kaiparowits Plateau across the gulf, is overpovveringly grand. |