Page:Popular Science Monthly Volume 39.djvu/689

Rh Adjacent to the east end of the Uinta arch, two similar but small upthrust mountains were formed at the same time, and repeat the same structural type in its essential features, but they have sharply arched crests, and their longer axes run from north to south, at right angles with the major axis of the Uinta range. These are Junction Mountain, about twelve miles long and four miles wide, and Yampa Mountain, seven miles long and about three miles wide. Both are cut by the Yampa River, flowing directly through them in deep canons, instead of passing around, thus showing that these very short upthrusts, like that of the larger range, were gradual, not sudden, in their development. The vertical extent of the upward arching of the strata tp form each of these mountains, counteracted meanwhile in large part by denudation, is believed to have been somewhat more than two miles; and this great elevation of so small areas was yet not too rapid to permit the river to keep pace with it in the downward cutting of its cañons.

—The structural type here designated is exemplified by the Henry Mountains in southern Utah, which have been elaborately studied by Gilbert. These mountains were formed as dome-shaped or bubble-like but gigantic uplifts of previously horizontal Carboniferous, Jura-Trias, Cretaceous, and Tertiary formations, by the volcanic injection of immense lenticular masses of porphyritic trachyte between the strata of the series. The injected lava mass is named by Gilbert a laccolite (cisternstone). Whereas in the first type of mountain structure the formerly horizontal strata were thrown into folds, and in the second were curved upward in great arches, they here were simply lifted quaquaversally, as a geologist would say, in vast domes. Mount Ellsworth, the most southern of the Henry Mountains, was lifted by only one laccolite; Mount Holmes, the next northward, by two; Mounts Hillers and Pennell, next in order to the north, each by one large and several smaller laccolite intrusions; and Mount Ellen, the most northern mountain of this group or range, was puffed up by many, perhaps thirty, of these cistern-like masses of lava. The Henry Mountains extend about thirty-five miles from southsoutheast to north-northwest, with a width of five to ten miles; and their highest summits rise about five thousand feet above the plateau of their base, or eleven thousand feet above the sea.

From these summits the view embraces within distances of fifty to one hundred and twenty miles northward, eastward, and southward, no less than five other mountain groups of this type, namely, the Sierra La Sal, the Abajo, La Lata, Carriso, and Navajo Mountains; and two hundred miles to the east the Elk Mountains of Colorado belong to the same class. The Henry Mountains and these other groups were all probably uplifted near the middle of