_The Howell laccolite_ lies apart from the cluster and is well exposed. It differs from all that have been enumerated in its extreme thinness. With a breadth of more than two thousand feet, it has a depth of only fifty. Seen from the east, it might readily be mistaken for a _coulée_, for on that side it is the thin, hard, black cap of a table carved out of soft, sandy shale (Flaming Gorge Group) by circumdenudation. But followed westward, the table is found gradually to lose its height by the rising of the adjacent land, and at last the lava-bed runs into the slope and disappears beneath the upper layers of the same sandy shale on which it rests. How far it extends under ground can only be conjectured. How far it originally stretched in the opposite direction cannot be known because it is broken away. The original edge is concealed at one end and has been undermined and destroyed at the other, so that the only place where it can be seen is the point at which it emerges from the shale. At this point, where erosion has bared but has not yet attacked the lava, the form and character of the edge are exhibited. In place of the tapering wedge which usually terminates intrusive sheets, there is a blunt, rounded margin, and the lava scarcely diminishes in depth in approaching it. The underlying strata, locally hardened to sandstones, lie level; the overlying curve downward to join them, and between the curved strata is interleaved a curved lava-sheet. In all these characters the intrusive body is affiliated with the typical laccolites, and distinguished from the typical sheets. [Illustration: FIG. 32.—The Howell Laccolite, as seen from the north. ] [Illustration: FIG. 33.—The Edge of the Howell Laccolite. ] [Illustration: FIG. 34.—Mount Pennell, from the west. ] _The laccolite which is marked “D”_ on the diagrams of the Hillers cluster is identical in nearly all its characters with the Howell; it is thin and broad. One margin is wasting as its foundation is sapped; the other is hidden from view. Its depth does not diminish toward the edge. Moreover, a few dikes issue from its margin, or from beneath its margin. It was intruded within the same formation as the Howell (the Flaming Gorge shale), but at a horizon several hundred feet higher; and, what is specially noteworthy, the rock of which it is composed is identical in _facies_ with that of the Howell laccolite, and notably different from all others which were observed in the Hillers cluster.