Not so the Marvine laccolite. Lying at the foot of the mountain where erosion is so conditioned as to discriminate between hard and soft, and surrounded by nothing firmer than the Tununk shale and Tununk sandstone, it has suffered a rapid denudation, in which nearly the whole of its cover has been carried away without seriously impairing its form. It stands forth on a pedestal, devoid of talus, naked and alone. The upper surface undulates in low waves preserving the original form as it was impressed on the molten mass. Over a portion there is a thin coating of sandstone, the layer next to the trachyte being saved from destruction by the induration acquired during the hot contact. From the remainder this also has disappeared, and the contact face of the trachyte is bare. For some reason the exterior portion of the laccolite disintegrates more slowly than the interior. It may be that there was some reaction from the surrounding sedimentaries during the cooling, which modified the crystallization. Or it may be that at a later epoch a reciprocal metamorphism was induced along the contact of the diverse rocks. At all events there is a crust a few feet in thickness which is specially qualified to resist destructive agents, and which by this peculiarity can be distinguished from the trachyte of the interior. All about the northern and western faces, which are steep, this crust has been broken through and the interior excavated; but it is only along the upper edge of the face that the crust is completely destroyed. Along the lower edge it is preserved in remnants sufficiently numerous to fully define the outline of the base. Each remnant is a sort of revet-crag standing nearly vertical and joined by a buttress to the cliff behind it. In the accompanying sketches (Figures 43 and 44) the most of these features find better expression than words can give them. The solemn order of the tombstone-like revetments is not exaggerated, nor is the contrast between the ruggedness of the cliff and the smoothness of the upper crust. At the left in the upper view, and at the right in the lower, there can be seen inclined strata, the remnants of the arch which once covered the whole.

The extreme depth of the laccolite is 1,200 feet, and its diameters are 6,000 and 4,000 feet.

The G arch, the Dana, the Crescent, and the Maze agree in having no visible laccolites. They are mere dome-like uplifts, by which inferior beds are brought to light. In the middle of the Crescent arch, and there only, is a small dike. They differ in their dimensions and in their erosion and topography. The G arch is low and broad, and lifts the Henry’s Fork conglomerate a few hundred feet only. It is covered by that bed except where Bowl Creek and one or two others cross it in shallow cañons.

The Dana arch bears the same relation to Mount Ellen and Jukes Butte that the Pulpit arch bears to Mount Hillers. The streams which flow down have truncated it, and afterward carved a system of cañons below the plane of truncation. The Henry’s Fork conglomerate overlooks it from the base of the Jukes laccolite on one side, and on the other margins it with a monoclinal ridge; and in the interval the Flaming Gorge and Gray Cliff rocks come to the surface.

The Crescent arch is so perfectly truncated by the existing mountain streams that their flood-plains unite above it in a single broad slope. On the side toward the mountain there are a few insular hills of the conglomerate, and on the side toward the plateau the same rock lifts a low monoclinal ridge which circles about the base of the arch in a crescent. Within the crescent there are few outcrops, but it is probable that the Gray Cliff Sandstone is brought to the surface. Near the center a solitary thin dike juts forth, like a buoy set to mark the place where a laccolite is sunk. Judging the magnitude of the laccolite by the proportions of the uplift, it has a diameter of nearly four miles and a depth of 2,500 feet; and these dimensions indicate a volume of three and a half cubic miles.

The Maze arch covers a smaller area than the Crescent, but its height is greater. The drainage from the mountain crosses it on a number of lines, but there is no indication that at any recent stage of the degradation they have produced an even truncation. At present they divide the Gray and Vermilion Sandstones by so intricate a labyrinth of deep cañons that the whole area of the uplift is almost impassable. The monotony of red sandstone, for here all members of the Jura-Trias are brick-red, the variety of dip, complicated to the eye by oblique lamination, the multiplicity of cañons and ridges, conspire to give an impression of chaos from whatever point the tract is viewed; and even from the most commanding stations I was unable to make out completely the arrangement of the drainage lines. Still no faults were discerned, and it is probable that the Maze arch, intricate as it seems, is a simple circular dome. On the north it adjoins the Crescent arch, and the monoclinal ridges of conglomerate which margin the two at the east are confluent. On the south it adjoins rather more closely one of the low arches which have been referred to the Pennell cluster. On the west, or toward the mountain, it is probably met by other arches at its own level; but these if they exist cannot be fully known until the progressing degradation of the country shall have removed certain laccolites which lie above them and slightly overlap the Maze arch.

The four arches just described are of the foundation series of the eastern base, and cover laccolites of unknown depths. Higher in the strata, and at the same time absolutely higher, is a second series, which to a certain extent overlap them. The Bowl Creek encroaches on the G laccolite; the Jukes, on the Dana and on the Bowl Creek; the H laccolite, on the Crescent; the Peale and Scrope, on the Maze.

The Bowl Creek arch is so masked by the overlapping Jukes laccolite, and by the encroachment of certain large dikes, that its general form and proportions are not known; but it is laid bare at one point in a fine natural section. Bowl Creek crosses it near the center, and in the walls of the cañon are exhibited two hundred feet of the laccolite, together with two hundred and fifty feet of superjacent beds. The curve of the strata is unbroken by faults or dikes, and carries them below the level of the creek at each end of the cañon. Next above the trachyte lies a clay shale which has been baked to the hardness of limestone. It is one hundred feet thick, and is more or less altered throughout, as is also the sandstone which overlies it. The extent of the metamorphism indicates that the trachyte mass by which it was produced is not a mere sheet, but is the body of the laccolite itself.

The Jukes laccolite encroaches upon the G, the Dana, and the Bowl Creek arches, and is superior to them all; it may be said to stand upon them. The trachyte has a depth of only one thousand feet, but it lies so high with reference to the general degradation that it is a conspicuous feature of the topography. The edges of the laccolite are all eaten away, and only the central portion survives. All of its faces are precipitous. The cover of shale or sandstone has completely disappeared, and the upper surface seems uneven and worn; but a distant view (Figure 47) shows that its wasting has not progressed so far as to destroy all trace of an original even surface. The eminences of the present surface combine to give to the eye which is aligned with their plane the impression of a straight line. The hill is loftier than the laccolite, for under the one thousand feet of trachyte are five hundred feet of softer rock which constitute its pedestal, and by their yielding undermine the laccolite and perpetuate its cliffs.

The H laccolite is not well exposed. It lies on one edge of the Crescent arch and is covered by the Henry’s Fork conglomerate.

Little is known of the form of the Peale laccolite. One edge is lost in the obscurity of the alpine sculpture, and the other has been removed along with the crest of the Maze arch, on which it rested. But by this removal a section has been opened across the laccolite, revealing its internal structure from top to bottom. It is shown to be composite, attaining its height of 850 feet by the compilation of three distinct beds of trachyte, separated by partings and wedges of shale. The lowest bed is thin and of small extent. Next is the main bed, six hundred feet thick and a mile or more broad; and on top is a bed two hundred feet thick and proportionately narrow. Each of the beds is lenticular in section, and the piling of the less upon the greater produces a quasi-pyramidal form. The interleaved shale bands are metamorphosed to the condition of slate. Under the laccolite are one or two hundred feet of shale, apparently unaltered except at the contact. Then come the Henry’s Fork conglomerate, 350 feet thick, and the Flaming Gorge shale, several times thicker. Within the upper shale is a restricted trachyte sheet and near the top of the lower shale is a broader one. The latter is double throughout, its two layers having been intruded at different times.

The top of the escarpment is at the top of the laccolite, and only a short distance back from the brow are shale and sandstone resting upon the trachyte and conforming to its uneven surface.

The Scrope laccolite resembles the Jukes in everything except that its erosion has progressed so far as to obliterate every trace of the original upper surface. Its position in the strata is about the same, and the erosion of its matrix has left it a conspicuous crag. It rests on the flank of the Maze arch, just as the Jukes laccolite rests on the Dana arch. The remnant of trachyte is less than one thousand feet high, and has been carved into a subconical form in which no hint of the original size and proportions of the trachyte body is conveyed.

Figure 49 is a view of the east flank of Mount Ellen, as seen from Mount Hillers. It groups together many of the details that have been enumerated. The conical hill at the left is the Scrope laccolite. The spur from the mountain which ends just at the right of it is the Peale laccolite. The bold butte which terminates the last spur of the mountain in the distance is the Jukes laccolite. Across the base of the latter one can trace the outcrop of a hard bed. This is the Henry’s Fork conglomerate, and the upward curve which it shows belongs (probably) to the Crescent arch. Nearer than the Jukes Butte and in the same direction is a low hill, marking the dike within the Crescent. Just to the left of it is an insular outcrop of the conglomerate, and nearer by is another outcrop in the form of a cliff, which is continuous to the base of the Peale laccolite. Between the Peale and Scrope laccolites the conglomerate is hidden by an embayment of the cliff, and it reappears in the base of the Scrope Butte. In all these outcrops the escarpments of conglomerate face toward the east and at the same time toward the Maze and Crescent arches. On the opposite side of the arches the same conglomerate outcrops in a monoclinal ridge with its escarpment facing to the west. The ridge can be traced in the sketch from a point in the foreground under the Jukes Butte nearly to the eastern base of the butte, the course at first being almost directly toward the butte and then curving far to the right and forming the Crescent. Between the Scrope and Peale laccolites on one side and the monoclinal ridge on the other, lie the Maze and the Maze arch. Beyond the Maze arch and limited at the right by the Crescent, is the Crescent arch. The Scrope, the Peale, and the Jukes are visible laccolites; the Maze and Crescent arches cover invisible laccolites.

The determinate laccolites and arches which compose the lower slopes of Mount Ellen, while they are of prime importance for purposes of investigation, must not be considered superior in number and magnitude to those of the central region. That region is known to abound in trachyte masses, and it far exceeds the marginal district in the extent and degree of its metamorphism. There is every reason to believe that the mountain crest marks the zone of greatest igneous activity, and that the foot-hills are as truly subsidiary from a geological point of view as they are from a topographic. There is no evidence of a great central laccolite, such as the Hillers and Pennell clusters possess, and I am disposed to regard the mountain as a great congeries of trachyte masses of moderate size, separated and in the main covered by shales and sandstones. The sedimentary strata of the summits are all of the Cretaceous series, but they are too greatly altered to permit the discrimination of the Masuk, Gate, and Tununk groups. The Henry’s Fork conglomerate, which might have been recognized even in a metamorphic condition, was not seen.

STEREOGRAM OF THE HENRY MOUNTAINS.

For the double purpose of mapping and studying the mountains a model in relief was constructed, in which great pains was taken to give all the principal features their proper altitudes and proportions. This model was photographed, and has been reproduced by the heliotype process in Plate III, at the end of the volume. After it had been completed, a second model was constructed by adding to the surface of the first. Wherever the Blue Gate sandstone appeared in the original model no addition was made. Where the Tununk sandstone appeared there was added an amount equivalent to the combined thickness of the Blue Gate sandstone and the Blue Gate shale; and in general, enough was added in every part to bring the surface up to the summit of the Blue Gate sandstone. In this way a restoration was made of the form of that sandstone previous to its erosion. It is not to be understood that the mountains ever possessed this form; for when the surface of the Blue Gate sandstone was unbroken by erosion it was unbroken only because it was covered by other strata, which while they shielded it were themselves eroded. If the sandstone had been indestructible, this is the form which would have been developed by the washing away of all the overlying beds; and in this form are embodied the arches and domes which were impressed upon the sandstone by the upswelling of the laccolites. The model became a stereogram of the displacements of the Henry Mountains, and a photograph from the stereogram appears in Plate IV.

It will be observed that over the central district of Mount Ellen, where by reason of the peculiar sculpture of the rock its structure was concealed, the restoration of the sandstone was not carried; it seemed better to represent our lack of knowledge by a blank than to bridge over the interval by the aid of the imagination.

If the reader will study the plate, he will find that it expresses a great body of the phenomena which have been described in this chapter. The simplicity of the Ellsworth and Holmes arches is contrasted with the complexity of the others; the greatness of the Hillers and Pennell domes with the smallness of those which lie upon their flanks. One point that is especially striking is the relation between the upper and lower domes of the Ellen cluster. The upper, which give rise to all the conspicuous features of the mountain flanks, are comparatively small, while the lower, which might almost be overlooked in a rapid examination of the mountain, are comparatively large and constitute the great mass of the uplift. The smaller laccolites, because they are the upper, have been denuded of their covers, and in virtue of their hardness stand forth salient. The larger, because they are the lower, have not been laid bare, and the comparatively feeble resistance which their covers have opposed to erosion has impressed their forms but slightly on the topography.