Thus there are grouped in this one cluster laccolites of the most varied character, differing in form, in magnitude, in the stratigraphic depth at which they were intruded, in the extent to which they have been uncovered or demolished in the progress of erosion, and also, but very slightly, in their lithologic characters. The greatest is one thousand times as bulky as the least. The length of the most obese is three times its depth; the length of the most attenuated is more than one hundred times its depth. The one highest in the strata lies a thousand feet above the base of the Cretaceous rock series; the lowest is not higher than the summit of the Carboniferous. The latter has not yet been touched by erosion, others have been completely denuded, and some have been partially demolished and removed. MOUNT PENNELL. Mount Pennell and Mount Ellen are distinct mountain masses separated by a low pass, but there is no interval between the clusters of laccolites by which they are constituted. Whether the site of a laccolite shall be marked by a mountain depends in great measure on the relation of the laccolite to the progress of erosion. In the Henry Mountains the laccolites which have not been reached by the denudation scarcely affect the topography. The arched sedimentaries above them are no harder than the same strata in the surrounding plain, and they are brought substantially to the same level. It is to those which the downward progress of erosion has reached and passed that the mountains are due. In virtue of their hardness they survive the general degradation, and conserve with them broad foundations of more perishable material. Mounts Ellen and Pennell mark the positions of the highest of a great cluster of laccolites, and the pass between them marks a part of the cluster where all the laccolites lie low in the strata. [Illustration: FIG. 35.—Cross-section of Mount Pennell. 1, Blue Gate Sandstone. 2, 2, Blue Gate Shale. 3, 3, Tununk Sandstone. The full black lines represent Trachyte. ]