In addition to the laccolites of the foundation and of the main body, there is a series which jut forth from the northern flank like so many dormer windows. They are comparatively small but are rendered conspicuous by the removal of the soft rocks which originally inclosed them. They are higher in the strata than any other observed laccolites, their position being above the Tununk sandstone and in the Blue Gate shale. The core of the main body of the mountain is probably inclosed in the Tununk shale, and the laccolites under the low arches (at the north at least) are entirely below the Cretaceous. The higher series stand on top of the low arches, and are just outside of the sheets which inclose the central body. The largest of them constitutes Sentinel Butte, and stands guard over Penellen Pass. Sapped by the yielding of its soft foundation it is rapidly wasting, and on three sides its faces are precipitous. The huge blocks which cleave from it as they are undermined strew the surrounding slopes for a great distance. Its depth of 400 feet is made up of two layers, of which the lower is the deeper, and between which there is a slight lithologic difference. The upper surface of the butte is smooth and plane with an inclination to the south. It is probably a portion of the original surface of the laccolite. Thus we have in Mount Pennell a great central body consisting of a single laccolite or of a number closely massed together; an inferior group of three or more, evidenced by low broad arches; and a superior group of not less than four, all of which are partially destroyed. MOUNT ELLEN. The crest of Mount Ellen is as lofty as that of Mount Pennell and it is more extended. It stretches for two miles from north to south and is buttressed by many spurs. The sculpture of the crest is alpine, and the structure is consequently obscured. There are dikes of trachyte and perhaps the remnants of laccolites; there are Cretaceous sandstones greatly indurated; and there are Cretaceous shales baked to clinking slate; but they are all carved into smooth, pyramidal forms, and each is half hidden by the _débris_ from the rest so that their order and meaning cannot be seen. The flanks however are full of interest to the geologist. The Ellen cluster of laccolites is a broad one, and all but the central portion is well exposed for study. In the spurs and foot-slopes and marginal buttes no less than sixteen individual laccolites have been discriminated, a number of them most beautifully displayed. They will be enumerated in the order of their position, beginning at the west of Penellen Pass and passing along the western, northern, and eastern flanks to the Scrope Butte, east of the pass. [Illustration: FIG. 38.—Ground Plan of the Ellen Cluster of Laccolites. ]