Mount Pennell is not so easily studied as the lower mountains at the south. Its summits are timbered and are carved into alpine forms which do not portray the structure but rather mask it. Upon its flanks however the topography and the structure are in so close sympathy that the latter is easily read; and by their study some of the general features of the mountain have been made out. From the east and south and west the strata can be seen to rise toward it. The uprising strata on the west are the Tununk sandstone and the shales above and below it. The sandstone forms no revetments, but accords so closely in its dips with the slopes of the mountain that it is the surface rock over a broad area, outcropping wherever there is rock exposure. At the head of the foot-slope trachyte sheets are associated with it, some overlying and others underlying it; and at one point a gorge reveals a laccolite not far below it—a laccolite that may or may not be the great nucleus of the mountain. On the west and south flanks the uprising strata are the Blue Gate and Tununk sandstones, with their shales. The Gate sandstone has been worn away nearly to the foot of the slope, and forms a monoclinal ridge circling about the base. The ridge is interrupted by a number of waterways, and it sends salients well up upon the flank, but it is too continuous to be regarded as a mere line of revetments. The Tununk sandstone is not to be seen without search, being covered by heavy trachyte sheets. Trachyte sheets also underlie it, and the whole are carved into a conspicuous series of revet-crags. [Illustration: FIG. 36.—Mount Pennell, from the north. ] The association of overarching strata with sheets of trachyte leaves no doubt in my mind that the core of Mount Pennell is laccolitic, but whether it is simple or compound is not so clear. The collation of all the observed dips shows that if there be but one laccolite it has not the simplicity of form which usually characterizes them. There are low arches of the strata at the southern, northern, and northeastern bases, which reveal no trachyte and give the impression that there may be a foundation of low-lying laccolites upon which the main trachyte mass or masses of the mountain are based. One of these low arches, that at the northeast, is shown in the foreground of Figure 36. The strata which portray it are of the Henry’s Fork conglomerate, and the laccolite which they cover is of necessity distinct from that which is revealed on the east flank of the mountain. [Illustration: FIG. 37.—Sentinel Butte. ]