A description of the structure of the mountain must include, first, the arch of the strata; second, the faults which modify the arch; third, the system of trachyte dikes and trachyte sheets; and fourth, the sculpture of the mountain. In its general proportions the arch is at once simple and symmetrical. From all sides the strata rise, slowly at first, but with steadily increasing rate, until the angle of 45° is reached. Then the dip as steadily diminishes to the center, where it is nothing. A model to exhibit the form of the dome would resemble a round-topped hat; only the level rim would join the side by a curve instead of an angle, and the sides would not be perpendicular, but would flare rapidly outward (see Figure 11). The base of the arch is not circular, but is slightly oval, the long diameter being one-third greater than the short. The length of the uplift is a little more than four miles; the width a little more than three miles, and the height about 5,000 feet. The curvature fades away so gradually at its outer limit that it is not easy to tell where it ends, and the horizontal dimensions assigned to the dome are no more than rude approximations. But there is another element which can be given more exactly. The line of maximum dip, which separates the convex upper portion of the dome from the concave periphery, is easily traced out in nature, and runs at the foot of the steep part of the mountain. It surrounds an area two miles in width and two and two-thirds miles in length. [Illustration: FIG. 11A.—View from the west spur of Mount Ellsworth, showing the trachyte dikes of the north spur and revetments of sandstone and trachyte. ] [Illustration: FIG. 11.—Stereogram of Mount Ellsworth; an ideal restoration of the form of the overarching strata. ] The Ellsworth arch is almost but not completely isolated. The Holmes arch, upon the east side, stands so near that the bases of the two impinge and coalesce, and the same thing happens, though less notably, with the Hillers arch at the north. The simplicity of the arch is further impaired by faults—not great faults dividing the whole uplift, but a system of small displacements which are themselves subordinate phenomena of the uplift. They are restricted to the central portion, and never occur so low down as the line of maximum dip. The strata of the upper part of the arch are divided into a number of prismoid blocks which stand at slightly different levels but are not sufficiently deranged to destroy the general form of the arch. The greatest throw is only a few hundred feet. All or nearly all of the fault planes are occupied by dikes of trachyte.