CLASSIFICATION OF MOUNTAIN RANGES. 671 



Adjacent to the east end of the Uinta arch, two similar but 

 small upthrust mountains were formed at the same time, and re- 

 peat 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 canons. 



3. Domed Mountains. 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 lenticu- 

 lar masses of porphyritic trachyte between the strata of the series. 

 The injected lava mass is named by Gilbert a laccolite (cistern- 

 stone). Whereas in the first type of mountain structure the for- 

 merly 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 south- 

 southeast 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 south- 

 ward, no less than five other mountain groups of this type, namely, 

 the Sierra La Sal, the Abajo, La Lata, Carriso, and Navajo Mount- 

 ains ; 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 



