Scale, 1 inch = 4,000 feet. 1. Tununk Sandstone; 2. Henry’s Fork Conglomerate; 3. Gray Cliff Sandstone. The full black lines represent trachyte sheets, and the broad black area the Hillers Laccolite. ] [Illustration: FIG. 27.—Mount Hillers, from the south. ] _The Hillers laccolite_ is the largest in the Henry Mountains. Its depth is about 7,000 feet, and its diameters are four miles and three and three-quarter miles. Its volume is about ten cubic miles. The upper half constitutes the mountain, the lower half the mountain’s deep-laid foundation. Of the portion which is above ground, so to speak, and exposed to atmospheric degradation, less than one-half has been stripped of its cover of overarching strata. The remainder is still mantled and shielded by sedimentary beds and by many interleaved sheets of trachyte. The portion which has been uncovered is not left in its original shape, but is sculptured into alpine forms and scored so deeply that not less than 1,000 feet of its mass are shown in section. All about the eroded (south) face of the mountain the base is revetted by walls of Vermilion and Gray Cliff sandstone, strengthened by trachyte sheets. At the extreme south these stand nearly vertical (80°), and their inclination diminishes gradually in each direction, until at the east and west bases of the mountain it is not more than 60°. On the north side there are no revet-crags, and the inclination is comparatively slight. It would appear that the laccolite was asymmetric, and was so much steeper-sided on the south that that side suffered most rapid degradation. [Illustration: FIG. 24.—Ground Plan of the Henry Mountains. The curved lines show the limits of the principal displacements; the triangles, the positions of the main peaks. N, Mount Ellen. P, Mount Pennell. H, Mount Hillers. M, Mount Holmes. E, Mount Ellsworth. ] By reference to the section (Figure 25) it will be seen that the sedimentary strata of the north flank stretch quite to the summit of the mountain. The same beds which form the revet-crags on the southern base constitute also some of the highest peaks. Since these rest directly upon the laccolite, it is assumed that the next lower beds of the stratigraphic series form its floor; and the base of the laccolite is drawn in the ideal section on the level which the Shinarump Group holds where it is unaffected by the displacements of the mountain. It is noteworthy that wherever the sedimentaries appear upon the mountain top they are highly metamorphic. But in the revet-crags there is very little alteration. Massive sandstone, divided by sheets and dikes several hundred feet in thickness, is discolored and indurated at the contact surface only, and ten feet away betrays no change.