That a fissure several feet or several scores of feet in width should end thus abruptly, demands explanation, and the phenomena immediately concerned offer none. Nevertheless it is easy to make an assumption which if true renders both cases clear. If we assume that the fissure instead of ending at the crosshead is merely offset, and resumes its course beyond, and that the dike contained in it has two bodies connected by a thin sheet (Figure 22), we shall have no difficulty in conceiving the erosion which will produce either of the natural appearances described. The rocks which constitute Mount Holmes are the same as those about its base. The Vermilion Cliff and Gray Cliff Sandstones alone appear in the crests. The underlying Shinarump shales are cut by the erosion at a few points only, and those are near the base. For this reason the Vermilion Sandstone is not undermined about the base, and the circle of revet-crags which surrounds Mount Ellsworth finds no counterpart. There are, indeed, a few revetments of Gray Cliff sandstone, but they are scattered and for the most part inconspicuous. In the general view of Mount Holmes (Figure 16), one of the main dikes crowns the nearest spur, and another the spur leading to the right. At the left are minor dikes, and high up is a trap sheet notched on its lower edge. At the left base of the mountain lies the lesser arch. Figure 23 gives a section exhibited by one of the northward cañons. It shows one of the faults of the upper part of the arch and illustrates the thinning of the sheets as they descend. [Illustration: FIG. 23.—Section shown in a northward cañon of Mount Holmes. _a_, Vestige of Trachyte sheet. _b_ _b_ _b_, Trachyte sheets. _c_, Trachyte dike. 1, Gray Cliff Sandstone. 2 2, Purple Sandstone. 3, Vermilion Cliff Sandstone. 4, Shinarump Shale. ] MOUNT HILLERS. Next in order to the north is Mount Hillers. Let it not be supposed, however, that there is discernible system in the geographic arrangement of the mountains or of the laccolites. A chart of the mountain peaks and a chart of the laccolites would alike prove intractable in the hands of those geologists who draw parallel lines through groups of volcanic vents by way of showing their trend. They are as perfectly heterotactous as they could be made by an artificial arrangement. The diagram (Figure 24) shows the relation of the laccolite groups to each other and to the meridian. The principal mountain summits are indicated by triangles, and the curved lines inclose areas of disturbance. Mount Hillers and its foot-hills are constituted by a group of no less than eight laccolites, and a ninth, the Howell laccolite, is conveniently classed with them, although not contiguous. [Illustration: FIG. 25.—Cross-section of Mount Hillers. FIG. 26.—The same, with ideal representation of the underground structure.