Chapter IV: The Laccolite (1)

The principal facts in regard to the laccolites of the Henry Mountains having been set forth in the preceding chapter, an attempt will now be made to deduce from them the natural history of the Laccolite.

There is a question which the critical geologist will be likely to propound, and which should be answered at the outset. “What evidence”, he may demand, “is there that the origin of the laccolite was subsequent to the formation of the inclosing strata rather than contemporaneous with it? May it not have been buried instead of intruded? May not the successive sheets and masses of trachyte have been spread or heaped by eruption upon the bottoms of Mesozoic seas, and successively covered by the accumulating sediments?” The answer is not difficult.

1st. No fragment of the trachyte has been discovered in the associated strata. The constitution of the several members of the Mesozoic system in the Henry Mountain region does not differ from the general constitution of the same members elsewhere. This evidence is of a negative character, but if there were no other it would be sufficient. For there is indubitable proof that at the end of the Shinarump period the Henry Mountain region was lifted above the ocean, and if any of the nuclei of the mountains had then existed they could not have escaped erosion by shore waves, and must have modified the contemporaneous deposits.

2d. The trachyte is in no case vesicular, and in no case fragmental. If it had been extruded on dry land or in shallow water, where the pressure upon it was not sufficient to prevent the dilatation of its gases, it would have been more or less inflated, after the manner of recent lavas. If it had issued at the bottom of an ocean, the rapidity of cooling would have cracked the surface of the flow while the interior was yet molten and in motion, and breccias of trachyte _débris_ would have resulted. The absence of inflation and of brecciation are of course of the nature of negative evidence, but they derive weight from the fact that a great number of distinct bodies of trachyte have been examined in the course of the investigation.

3d. The inclination of the arched strata proves that they have been disturbed. If the laccolites were formed in each case before the sediments which cover them, the strata must have been deposited with substantially the dips which they now possess. This is incredible. The steepest declivity of earth-slopes upon the land is 34° from the horizontal, and they have not been found to equal this under water. Prof. J. D. Whitney noted 23° as the original slope of a deposit on the Pacific Coast, and regarded it as an extreme case. The writer has made many measurements of the inclination of oblique lamination in massive sandstones, and found the maximum to be 24°. But the strata which cover the laccolites dip in many places 45° to 60°, and in the revetments of the south base of Mount Hillers they attain 80°.

4th. It occasionally happens that a sheet, which for a certain distance has continued between two strata, breaks through one of them and strikes across the bedding to some new horizon, resuming its course between other strata. Every such sheet is unquestionably _subsequent_ to the bedding.

5th. The strata which overlie as well as those which underlie laccolites and sheets, are metamorphosed in the vicinity of the trachyte, and the greatest alteration is found in the strata which are in direct contact with it. The alteration of superior strata has the same character as the alteration of inferior. This could never be the case if the trap masses were contemporaneous with the sediments; the strata on which they were imposed would be subjected to the heat of the lava, but the superior strata would accumulate after the heat had been dissipated. In the Henry Mountains a large number of observations were made of the phenomena at and near the contacts of sedimentary and igneous rocks, and in every instance some alteration was found.

In fine, all the phenomena of the mountains are phenomena of intrusion. There is no evidence whatever of extrusion. It is not indeed inconceivable that during the period in which the subterranean chambers were opened and filled, a portion of the lava found its way to the top of the earth’s crust and there built mountains of eruption; but if such ever existed, they have been obliterated.

The Henry Mountains are similar among themselves in constitution. They all exhibit dome-like uplifts; they all contain intrusive rocks; and their intrusive rocks are all of one lithologic type. They are moreover quite by themselves; the surrounding country is dissimilar in structure, and there is no gradation nor mingling of character. Thus similar and thus isolated it is natural to regard the mountains as closely related in origin, to refer their trachytes to a common source, and to look for homology in all their parts. It was the search for such homology which led to the hypothesis that the laccolite is the dominant element of their structure. It is now time to examine this hypothesis, the truth of which has been assumed in the preceding pages, and see how far it accords with the facts of observation.

The facts to be correlated are the following:

1st. There are seven laccolites which lie so far above the local plane of erosion that they are specially exposed to denuding agents. They have no enveloping strata, and their only associated sheets lie in the strata under them. Their original forms have been impaired or destroyed by erosion. They are the Scrope, the Jukes, the Sentinel and its three companions, and the A laccolites.

2d. There are two laccolites so nearly bared that their forms are unmistakable, but which are still partially covered by arching strata, and which have associated sheets and dikes. They are the Marvine and the Steward laccolites.

3d. There are five supposed laccolites situated where the erosion planes are inclined, which run under the slopes and are covered at one side or end, and at the other project so far above them as to have lost something by erosion. These are accompanied by overarching strata and by sheets and dikes. They are the Peale, the Howell, the Shoulder, the D, and the E.

4th. There are seven or eight supposed laccolites, of which only a small part is in each case visible, but which are outlined in form by domes of overarching strata. Their bases are not exposed. Associated with them are dikes and sheets. They are the Hillers, the Pennell, the Geikie, the Newberry, the Bowl Creek, the C, the H, and perhaps the F.

5th. There are five domes of strata accompanied by dikes and (with one exception) by sheets, but showing no laccolite. They are the Ellsworth, the Greater Holmes, the Crescent, the Jerry, and the B arches.

6th. There are nine or more domes of strata with no visible accompaniment of trachyte. They are the Lesser Holmes, the Pulpit, the G, the Dana, and the Maze arches, and those of the foundation of Mount Pennell and of the west base of Mount Ellen.

Upon the hypothesis that all these phenomena are examples of the laccolitic structure, they have the following explanation: Each individual case comprised originally a laccolite, covered by a great depth of uplifted and arching strata, and accompanied by dikes and sheets which penetrated the strata to a limited distance. Lying at different depths from the surface, they have borne and still bear different relations to the progressive degradation of the country, and have been developed by erosion in different degrees. In nine of the instances cited the arch of strata has been truncated, but at so high a level that no dikes nor sheets were unearthed. In five instances the plane of truncation was so low that dikes and sheets were brought to light, but not the main body of trachyte. The truncation was in most of these cases less perfect because of the resistance to erosion by the hard dikes and sheets and by the strata which their heat had hardened. In eight other instances the erosion has left prominent the dome of hardened strata with its sheets and dikes, but has somewhere broken through it so as to reveal a massive core of trachyte. In five instances one side of the dome of strata has been washed away, exposing the core of trachyte to its base and showing undisturbed strata beneath it. In two instances the soft matrix has been so far washed away from the laccolite as to expose its form fully; and seven laccolites have not only lost all cover but have themselves been partially demolished.

Certainly, the hypothesis accords with all the facts that have been observed and unites them into a consistent whole. It explains fourteen dome-like arches of sedimentary rock which are imperfectly exposed, by classing them with seven other arches of the same region which have been opened in section _to the base_ and found to contain laccolites; and it strengthens the case by pointing to a connected series of intermediate phenomena. Until some strata-dome of the Henry Mountains, or of a closely allied mountain, shall be found to display some different internal structure, it will be safe to regard the whole phenomena of the group as laccolitic.

_Form of laccolites._—As a rule laccolites are compact in form. The base, which in eleven localities was seen in section, was found flat, except where it copied the curvature of some inferior arch. Wherever the ground plan could be observed it was found to be a short oval, the ratio of the two diameters not exceeding that of three to two. Where the profile could be observed it was usually found to be a simple curve, convex upward, but in a few cases and especially in that of the Marvine laccolite the upper surface undulates. The height is never more than one-third of the width, but is frequently much less, and the average ratio of all the measurements I am able to combine is one to seven.

The ground plan approximates a circle, and the type form is probably a solid of revolution—such as the half of an oblate spheroid.

_Internal Structure._—Of the laccolites which are best exhibited in section, there are a number which appear to be built up of distinct layers. The Peale exhibits three layers with uneven partings of shale. The Sentinel shows two without visible interval. The Howell shows two. The Pennell has a banded appearance but was not closely examined. The Marvine shows at a distance a faint banding, which near by eludes the eye. No division nor horizontal structure was seen in the Hillers, Jukes, Scrope, or Steward laccolites, but observation was not sufficiently thorough to satisfy me of its absence. It is probable that all the larger laccolites are composite, having been built up by the accession of a number of distinct intrusions.

There is little or no prismatic structure in the trachytes. It is sometimes simulated by a vertical cleavage induced in sheets and laccolites which are undergoing disintegration by sapping, but I did not observe the peculiar prismatic cleavage which is produced by rapid cooling in dikes, sheets, and _coulées_ of basalt. The two structures are not often discriminated, but they are really quite distinct, and their peculiar characters are easily recognized. The cleavage planes produced by cooling are as a rule perpendicular to the cooling surface, and the systems of prisms which are based on the opposite walls of a dike or sheet, do not correspond with each other, and do not run across, but meet midway in a confused manner. The cleavage planes which are produced by the shearing force when a massive bed of trap or other rock is undermined or sapped and yields under its own weight, extend from base to top, and are perpendicular to the plane of the horizon instead of the plane of the bed.[2]

Footnote 2:

See page 172 of the “Exploration of the Colorado River”, by J. W.
Powell.

_Vertical Distribution._—The range of altitudes at which laccolites have been formed is not less than 4,500 feet, and neither the upper nor the lower limit is known.

The highest that are known—those which were intruded at the highest geological horizon—are near the base of the Blue Gate shale (Middle Cretaceous), but it is quite possible that higher ones have been obliterated by erosion. The lowest that is known was intruded in the Shinarump shale, but it is known of the invisible Ellsworth laccolite that upper Carboniferous strata lie above it, so that its horizon of intrusion must be still lower. The plexus of dikes and sheets on the Ellsworth arch indicates that the laccolite is not deeply buried; but in the series of arches there are nine which show no trachyte, and we have no data from which to infer their depth.

How the laccolites are distributed within these limits is more readily comprehended by the aid of a diagram. In Figure 50 the triangles mark the horizons of determined laccolites, and the crosses the horizons which the invisible laccolites cannot exceed. For example, the base of the Scrope laccolite is visible and is seen to lie on the Tununk shale 400 feet above the Henry’s Fork conglomerate; to represent it a triangle is placed at about the middle of the space representing the Tununk shale, and in the column devoted to the Ellen cluster. The upper surface of the Geikie laccolite is visible and upon it rest, first a few feet of Flaming Gorge shale, and then the Henry’s Fork conglomerate; to represent it a triangle is placed near the top of the Flaming Gorge space. The crown of the Pulpit arch has been so far eroded that half of the Vermilion Cliff sandstone is shown in section, but no laccolite is revealed. It is evident that the Pulpit laccolite is lower than the middle of that sandstone; and to represent it a cross is placed below the middle of the Vermilion Cliff space and in the column devoted to the Hillers cluster.

FIG. 50.—Diagram of the Vertical Distribution of the Laccolites of the
Henry Mountains.
]

The first feature which this graphic assemblage yields to the eye is that there are at least two zones of laccolites. The upper ranges from the lower part of the Blue Gate Group, through the Tununk and Henry’s Fork, to the upper part of the Flaming Gorge Group. The lower has not yet been fully developed by erosion, but its proximity is indicated. The Hillers laccolite is uncovered at top, and the dikes of the Ellsworth, the Greater Holmes, and the Crescent have been reached. All of the invisible laccolites indicated in the Vermilion Cliff and Shinarump spaces must be referred to this zone; and there is reason to suspect that all but one of the invisible laccolites whose indication falls in the Tununk and Flaming Gorge spaces belong also to the lower zone. However this may be, it is not probable that the determination of the depths of the invisible laccolites would vitiate the conclusion that there is an upper zone of laccolitic frequency which is separated from a lower zone by an interspace of laccolitic infrequency.

Another feature illustrated by the diagram is that all the determined laccolites are inclosed by soft beds. They have been intruded into the shales, but not the sandstones. They cluster about the Henry’s Fork conglomerate, but none of them divide it. This selection of matrix is confined however to the laccolites and is not exercised by sheets and dikes. Trachyte sheets were seen within the Henry’s Fork, the Gray Cliff, the Vermilion Cliff, and the Aubrey sandstones.

A third feature of the diagram is the restriction of the upper zone of laccolites to the northward clusters; Mounts Ellsworth and Holmes contain laccolites of the lower zone only. This fact of distribution is correlated with a fact of denudation—namely, that in the general degradation of the country, the region about the southern mountains has lost two thousand feet more than the northern. One fact is probably the cause of the other. The absence of laccolites of the upper zone at the south may have permitted the greater degradation; or the greater degradation may have caused the destruction of several of the upper laccolites. The fact that the difference in degradation can be independently accounted for is favorable to the latter supposition. The Colorado River which is the main artery of drainage for the whole region, flows close to the bases of the southern mountains, and the rapid declivity from the mountain summits to the river has given and still gives exceptionally great power to the agents of erosion. No other cause is needed to explain the difference of degradation, and the absence of laccolites of the upper zone is explicable without assuming that they were never present.

The negative objection to the idea that the southern mountains originally possessed laccolites of the upper zone being thus disposed of, it is worth while to inquire whether there is any evidence in its favor. If superior laccolites existed, they would be sure to leave behind them a record of the conduits through which their lava was injected. A dike or a chimney must always connect a laccolite with the source of its material; and the removal of the laccolite necessarily exposes a cross-section of its stem. The discovery of such a dike can be regarded, not indeed as a proof of the former existence of a superior laccolite, but as demonstrating its possibility. The summits and flanks of Mounts Holmes and Ellsworth bear many dikes, which have been regarded as subsidiary features of the laccolites beneath them, but it is quite possible that any one of them formerly led to another laccolite above. The upper laccolite may have been first formed and then have been lifted, dike and all, when the lower was intruded; or it may have been last formed, and been fed through a fissure which traversed the lower after its congelation. The only dike which was discovered in the vicinity of these mountains, without being upon them, stands midway between them. It is the only observed dike of the Henry Mountains (excepting always the alpine district of Mount Ellen), which is not so closely associated with some laccolite as to seem an accessory feature, and its exceptional position has led to the suspicion that it belonged to an overlying laccolite.

Upon such uncertain evidence no positive conclusion can be based, and it is vain to build laccolites in the air. The most that can be said is that the southern mountains need not be distinguished from the northern, because at the present stage of degradation they contain laccolites of the lower zone only.

_The Material of the Laccolites._

The intrusive rocks of the Henry Mountains were sampled with care. Specimens were selected which had undergone little decomposition and which represented all the prominent lithologic varieties. They were chosen from the trachytes of both zones and of each of the mountain masses; and they represent dikes and sheets, as well as laccolites. From about thirty specimens thin slices were cut for microscopic examination.

Captain C. E. Dutton, of Omaha, Nebraska, was so kind as to undertake the study of the collection, and the letter which embodies his conclusions is given below. In accordance with his diagnosis I shall call the intrusive rocks _porphyritic trachyte_; and I am glad to have the weight of his authority in support of my belief that all the rocks of the series are of one type, their resemblances far outweighing their differences.

NOTE.—Persons desiring to examine the Henry Mountain trachytes under
the microscope can obtain mounted thin sections from Mr. Alexis A.
Julien, School of Mines, Columbia College, New York.

REPORT ON THE LITHOLOGIC CHARACTERS OF THE HENRY MOUNTAIN INTRUSIVES.

BY CAPTAIN C. E. DUTTON.

“I have examined with great interest and attention the Henry Mountain rocks you sent me, and proceed to acquaint you with such results as my limited facilities have permitted me to derive from the examination. It is a very well defined series, having some marked characters which distinguish it from the nearest allied group with which I am acquainted. This is all the more interesting, because I am inclined to think that these peculiarities may have a definable association with or relation to the manner in which the intrusive rocks occur in those “laccolites”, as you term them.

“The hand specimens show in most cases large and unusually perfect crystals of orthoclase imbedded in a very compact uniform paste through which hornblende is also disseminated rather more abundantly than is usually the case where the dominant felspar is monoclinic. Micaceous crystals appear to be wholly wanting and this is a notable circumstance, since the trachytes of the Plateau country, to which these rocks are most nearly allied, are seldom without one or more of them. The only other mineral which is of frequent occurrence in the specimens is magnetite (or possibly titanic iron), which is diffused in the usual form of minute granules in many of them, but is scarce in several of them. In general there is a great scarcity of mineral species and any others than those mentioned are of the greatest rarity.

“The dominant felspar is orthoclase, but a portion of it is triclinic, and I presume this portion is albite, with an occasional occurrence of oligoclase. The groundmass in which the crystals are included is in most cases decidedly compact and without distinguishable crystals, but shows between the crossed Nicols closely aggregated luminous points, which with a ¼-inch objective are resolved indistinctly into felspar. In some cases the crystals of the groundmass are quite apparent with an inch objective, and their species determinable. But the greater portion of the groundmass is quite amorphous, and does not polarize light at all. The proportion of crystalline to amorphous matter in the paste is highly variable—in some cases it is quite bright between crossed Nicols, in others far less so, and in none is it entirely dark. Those specimens which have the finer and more amorphous groundmass have the larger and more perfect crystals of felspar—an association of properties which is not wholly without qualification, but still sufficiently decided.

“Turning to the included felspars, their mode of occurrence is quite an uncommon one, I believe, so far as the eruptive rocks of the Rocky Mountain region are concerned, and give rise to some hesitation before assigning them definitely to the trachytic group. In a great many cases the felspathic crystals are well developed, and so large and so nearly perfect that their aspect is decidedly porphyritic. This is especially the case with the dikes of Mounts Ellsworth and Holmes (Nos. 56, 61, 68, and 69). The orthoclase is invariably of the white “milky” variety, with the exception of a single specimen from a Mount Ellsworth dike (No. 57), where it is present as sanidin. (This is an exceptional rock in all respects, and will be spoken of hereafter.) Nearly all of them appear to have been subject to alteration by chemical action since their formation as is indicated by their diminished power to polarize light. Whether this is due to atmospheric weathering or to changes _en masse_ it is of course impossible to distinguish with certainty, though I incline to the latter view since it is manifested as decidedly in specimens which show no external indications of weathering as in those which do show them. It is not uncommon to find crystals which have almost entirely ceased to polarize. The zonal arrangement is very common in the crystals, and some of the zones contain numberless minute fluid cavities in the largest crystals. The foreign substances included in the crystals present no novelty, being the ordinary films of hornblende, minute needles of felspar, granules of magnetite, and those dust-like points of brownish yellow color which are the proper inclusives of the groundmass.

“The orthoclase occasionally presents the adular variety, but this never becomes a marked feature. I have observed the same in many of the trachytes of the High Plateaus and of the Great Basin. Another phenomenon is the occurrence of crystals which are quite typically monoclinic at one end (orthoclase), and at the other end have the arrangement of plagioclase. This is well known elsewhere, and described by Zirkel. (Mik. Beschaff der Mineralien u. Gesteine).

“In classifying these rocks therefore, we may observe that they present a blending of the characteristics which are common to trachyte and felsitic porphyry. Those who regard porphyry as a distinct class of eruptive rocks would have no hesitation in calling Nos. 18, 56, 61, 68, and 69 undebatable felsitic porphyries, and to the same series might with propriety be added No. 33. With equal confidence Nos. 16, 20, 35, and 43 may be called unqualified trachytes. The other rocks are intermediate in character between these two extremes, and the whole may be regarded as a series in which the individuals form a graduated scale.

“You will recall the fact that many lithologists object to the terms porphyry or porphyritic being used to designate a distinct class or group of rocks, holding that they merely characterize a single feature which is more or less frequently presented by all igneous rocks and having no necessary relation to any of them, and that this is no more adequate to such an important distinction than the color or relative degree of fineness or coarseness of texture. Although this latter view seems to me to underrate the distinctive value of the porphyritic character in general, I incline very decidedly to the belief that it is true as applied to these Henry Mountain rocks. Here at least the porphyritic character has but little significance. In some varieties the crystals are larger and more perfect and the groundmass more homogeneous; in others the crystals are smaller and imperfect, and the groundmass more coarse and irregular; while still others are ‘betwixt and between’. The most careful scrutiny fails to show any fundamental differences in the groundmass or in the included minerals. Hence I think these rocks would be accurately designated _as a group_ by calling them porphyritic trachytes. Such varieties as Nos. 16, 20, and 35 may by themselves be called simply trachyte, the porphyritic character being insufficiently distinct in them to warrant any qualification of the name.

“There is one specimen (No. 57, from dike in Shinarump shale, Mount Ellsworth) which constitutes an exception to the foregoing. By inspection of a hand specimen it might hastily pass for a very compact andesite, but the observer will be instantly undeceived by applying the microscope. It consists of imperfect crystals, which must be sanidin, imbedded in a very close groundmass composed of a material which differs from the foregoing trachytes in being wholly amorphous. Between crossed Nicols the paste transmits no light whatever, though between the parallel Nicols it closely resembles the others. The hand specimen is very dark colored (gray), but the slide is sufficiently translucent. The felspar crystals are either fragmental or very imperfectly developed as to their edges and angles, but polarize very sharply. The amount of twinning is very small, but it appears occasionally. The rock is undoubtedly a trachyte, but an unusual one. Its dark color is not due to hornblende nor to magnetite, both of which occur in it very sparingly, especially the former.

“I have already remarked that the only frequent minerals besides felspar are hornblende and magnetite. Apatite occurs and is tolerably plentiful in a few of the specimens, but absent from most of them. The crystals are all small, requiring a low power for the determination of the larger and a high power for the smaller. They present no peculiarities. Of very rare occurrence is nepheline. This mineral is usually associated with the more basic volcanic rocks and seldom penetrates the trachytic group. Quartz is almost equally rare. The absence of any great variety in the mineral species is quite normal, except possibly the total absence of mica, which is usually present and frequently the only associate of felspar in the western trachytes. The Henry Mountain rocks do not so far as I can discover contain a trace of it.

“In answer to your particular inquiries—

“1st. ‘Is the paste vesicular, or is there any evidence in the crystals to indicate the pressure under which they were formed?’ I can only say that the paste is extremely compact and contains no vesicles even in those specimens of which the aspect is most decidedly trachytic. Some of the larger crystals of felspar contain an abundance of pores or vesicles which may have contained liquids, but with a ¼-inch objective they are too small for treatment by the method you refer to. A few large cavities, usually of irregular shape, occur, but I am quite unfamiliar with the practical treatment of this subject and cannot advise you. I do not find any cavities still containing fluids, and I presume that even if they existed they would be difficult if not impossible to gauge on account of the impellucidity of the felspar. I presume quartz is the most favorable mineral for this investigation on account of its transparency and the greater frequency of its large cavities. Quartz however is almost the scarcest of the contents of these rocks.

“2d. ‘Do any mineralogic differences correlate with the superficial or geographic distribution of the rocks?’ and

“3d. ‘Do any mineralogic differences correlate with the vertical distribution of the trachytes?’ As it appears from your account of the distribution of the masses that the upper zone belongs (with one exception) to Mounts Ellen and Pennell and the lower to the other mountains, the answer to one is the answer to both, and this is in the negative. The mineralogical differences are exceedingly small, considering the number of distinct masses, and this covers the inquiry entirely. Regarding the texture or habitus, on the contrary, it appears to me that the true trachytes predominate in the upper zone and the porphyries in the lower, but not without exceptions.

“4th. ‘Do any mineralogic differences correlate with the size of the intrusive masses?’ No mineralogical differences thus correlate, but I find a preponderance of the porphyritic texture in the smaller masses and of the trachytic texture in the larger. It is not without exception, and the preponderance is small.”

_Metamorphism and Contact Phenomena._—Wherever the trachytes came in contact with the sedimentaries the latter were more or less altered. Large bodies of trachyte produced greater changes than small. The laccolites both metamorphosed their walls more completely, and carried their influence to a greater distance than the sheets and dikes. The summits of the laccolites had a greater influence than the edges; a phenomenon to which I shall have occasion to revert. The sandstones were less affected than the shales, at least in such characters as readily catch the eye. Clay shales were indurated so as to clink under the hammer, and Captain Dutton discovered with the microscope that minute crystals of felspar had been developed. Sandstones were usually modified in color, and their iron was segregated so as to give a mottled or speckled appearance to the fracture. They were indurated, but the granular texture was always retained.

The trachyte carries numerous small fragments of sedimentary rock broken apparently from its walls, and these are as thoroughly crystalline as their matrix.

The altered rocks are usually jointed, but nothing approaching to slaty cleavage was seen, nor has there been any crumpling.

The reciprocal influence of the sandstone and shale upon the trachyte was small. Specimens broken from the contact surface of a laccolite and from its interior cannot be distinguished. In the Marvine laccolite however there is a difference between the exterior and interior portions in their ability to withstand erosion.

_Historical._—Before leaving the subject of the structure of the mountains it is proper to place on record certain observations by others which antedated my own but have never been published.

While Professor Powell’s boat party was exploring the cañons of the Colorado, Mr. John F. Steward a geologist and member of the party climbed the cliff near the mouth of the Dirty Devil River and approached the eastern base of the mountains. He reported that the strata had in the mountains a quaquaversal dip, rising upon the flanks from all sides.

The following year Prof A. H. Thompson then as now in charge of the geographic work of Professor Powell’s survey crossed the mountains by the Penellen Pass and ascended some of the principal peaks. He noted the uprising of the strata about the bases and the presence of igneous rocks.

In 1873 Mr. E. E. Howell at that time the geologist of a division of the Wheeler Survey traveled within twelve miles of the western base of the mountains, and observed the uprising of the strata.

My own observations were begun in 1875, at which time a week was spent among the mountains. They proved so attractive a field for investigation that in the following year a period of nearly two mouths was devoted to their study.

_Other Igneous Mountains._—The Henry Mountains are not the only igneous group which the Plateau province comprises. They are scattered here and there throughout its whole extent. From the summits of the Henry Mountains one can see the Sierra La Sal ninety miles to the northeastward, and the Sierra Abajo seventy miles to the eastward. Beyond them and two hundred miles away are the Elk Mountains of Colorado. Fifty miles to the southwestward stands the Navajo Mountain on the brink of the Colorado; and one hundred and twenty miles to the southeast the Sierra La Lata and the Sierra Carriso are outlined against the horizon. Westward it is less than thirty miles to the Aquarius Plateau, the nearest member of the great system of volcanic tables among which the Sevier and Dirty Devil Rivers rise.

Beyond the horizon at the south and southwest and southeast are a series of extinct volcanoes; Mount Taylor and the Marcou Buttes in New Mexico; the Sierra Blanca, the Sierra Mogollon, the San Francisco Group and the Uinkarets in Arizona; and the Panguitch Lake Buttes in Utah.

Of the groups which are visible, all but that of the Aquarius Plateau are allied in character to the Henry Mountains.

The Sierra Abajo was studied in 1859 by Dr. J. S. Newberry, geologist of the Macomb Expedition, who writes: “Within the last few weeks we have been on three sides of this sierra, and have learned its structure quite definitely. It is a mountain group of no great elevation, its highest point rising some 2,000 feet above the Sage-plain, or perhaps 9,000 feet above the sea. It is composed of several distinct ranges, of which the most westerly one is quite detached from the others. All these ranges, of which there are apparently four, have a trend of about 25° east of north, but being arranged somewhat _en echelon_, the most westerly range reaching farthest north, the principal axis of the group has a northwest and southeast direction. The sierra is composed geologically of an erupted nucleus, mainly a gray or bluish-white trachyte, sometimes becoming a porphyry, surrounded by the upheaved, partially eroded, sedimentary rocks. The Lower Cretaceous sandstones and Middle Cretaceous shales are cut and exposed in all the ravines leading down from it, while nearly the entire thickness of the Cretaceous series is shown in spurs which, in some localities, project from its sides; apparently the remnants of a plateau corresponding to, and once connected with, the Mesa Verde. Whether the Paleozoic rocks are anywhere exposed upon the flanks of the Sierra Abajo I cannot certainly say, though we discovered no traces of them. It is, however, probable that they will be found in some of the deeper ravines, where, as in most of these isolated mountains composed mainly of erupted material, they are doubtless but little disturbed, but are buried beneath the ejected matter which has been thrown up through them.

“The relations of the Cretaceous rocks to the igneous nucleus of the Sierra Abajo are very peculiar, for, although we did not make the entire circuit of the mountain mass, and I can, therefore, not speak definitely in regard to the western side, as far as our observations extended we found the sedimentary strata rising on to the trachyte core, as though it had been pushed up through them.” (Geology of the Macomb Expedition, page 100.)

Of another group Dr. Newberry says in the same report (page 93): “Of the composition of the Sierra La Sal we know nothing except what was taught by the drifted materials brought down in the cañons through which the drainage from it flows. Of this transported material we saw but little, but that consisted mainly of trachytes and porphyry, indicating that it is composed of erupted rocks similar to those which form the Sierra Abajo, of which it is in fact almost an exact counterpart. From the cliffs over Ojo Verde we could see the strata composing both the upper and second plateaus, rising from the east, south, and southwest on to the base of the Sierra La Sal, each conspicuous stratum being distinctly traceable in the walls of the cañons and valleys which head in the sierra. It is evident, therefore, that the rocks composing the Colorado Plateau are there locally upheaved, precisely as around the Sierra Abajo * * *.”

These mountain groups have been since visited by the geologists of Dr. Hayden’s survey, Dr. A. C. Peale ascending the Sierra La Sal and Mr. W. H. Holmes the Sierra Abajo. Mr. Holmes has also examined the La Lata and Carriso Mountains and found in them the same upbending of Cretaceous strata and the same association of igneous material.

The Navajo Mountain has been viewed by Mr. Howell and by the writer from a commanding position on the opposite side of the Colorado River, and fragments of its trachyte have been gathered on the river bank by Professor Powell, but no geologist has yet climbed it. Still there can be no question of its general structure. It is a simple dome of Jura-Triassic sandstone, springing abruptly from a plateau of the same material, and veined at the surface by sheets and dikes of trachyte—the counterpart in fine of Mount Ellsworth, only of more imposing proportions.

The La Sal, the Abajo, the La Lata, the Carriso, the Navajo, and the Henry Mountains agree in their essential features. Structurally they have no trends. Their phenomena are grouped about centers and not axes. In all of them the strata are lifted into dome-like arches, and associated with these arches are bodies of trachyte. The trachytes are all of one lithologic type, and are so closely related that a collection of rock specimens representing all the groups would show scarcely more variety than a collection representing the Henry Mountain laccolites. With so many characters in common they can hardly fail to agree in the possession of laccolitic nuclei.[3]

Footnote 3:

While these pages are passing through the press a paper by Dr. A. C.
Peale “On a peculiar type of eruptive mountains in Colorado” (Bulletin
U. S. Geol. Sur., Vol. III, No. 3) comes to hand. He groups together
as of one type not only the Elk, La Sal, Abajo, La Lata, and Carriso
Mountains, but also the Spanish Peaks, Park View Mountain, Mount
Guyot, Silverheels Mountain, the San Miguel Mountains, the La Plata
Mountains, and certain smaller masses in Middle Park and near the
Huerfano River. He says, “Although modified in several instances, the
general plan appears to be the same. The igneous material came up
through fissures in the sedimentaries, sometimes tipping up their
ends, and sometimes passing through without disturbing them. On
reaching the Cretaceous shales, it generally spread out in them, and
pushed into and across them dikes and intrusive sheets of the same
igneous rock. The elevation in some cases appears to be due to actual
upheaval caused by the eruptive force. The mountains as they now exist
are doubtless largely the result of erosion, the hard igneous rock
opposing greater resistance to erosive influences than do the
surrounding soft sedimentary beds.”

The Elk Mountains are at the very margin of the plateau, and geographically might be connected with the Sawatch Range which bounds the plateau province on that side. But structurally they are a group instead of a range, and affiliate with the groups which are insulated by an environment of tables. Thanks to the labors of Mr. Holmes and Dr. Peale their general structure is known. The Eastern Elk Mountains consist of four great bodies of “eruptive granite”, over which are arched not only Mesozoic but Paleozoic strata. Their foundation must be a floor of Archæan metamorphics. Two of them, the Snow Mass and White Rock laccolites, are joined by a continuous line of disturbance, in the description of which by pen and pencil Mr. Holmes has made an important contribution, not only to dynamical geology, but to the methods of geological illustration. The others are more symmetric and are complementary illustrations of the common structure. One, the Sopris, is half truncated by erosion so that the core is exposed at top with an encircling fringe of upturned sedimentaries; and the other, the Treasury, retains a complete arch of Paleozoic strata. The Western Elk Mountains are a cluster of smaller laccolites which are inserted between strata of Cretaceous age. Their traps include porphyritic trachytes undistinguishable from those of the Henry Mountains, and eruptive granites identical with those of the Eastern Elk Mountains; and they exhibit a gradation from one to the other. Indeed the two rocks are nearly related, and their assignment to classes so diverse as trachyte and granite is merely an illustration of the imperfection of our classification of rocks. The description of the Elk Mountains will be found on pages 61 to 71 and 163 to 168 of the Annual Report for 1874 of the “Geological and Geographical Survey of the Territories.”

If we turn now to the distinctively volcanic mountains of the Plateau province—to those which are built by eruption at the surface—we leave at once the porphyritic trachytes. Mount San Francisco, Mount Bill Williams, Mount Sitgreaves, Mount Kendrick, Mount Floyd and the Sierra Blanca (of Arizona) are all composed of basic trachytes, and so are the Aquarius Plateau and the many tables that lie beyond it.

The Mogollon group, the Marcou Buttes, the minor cones about Mount San Francisco, the Uinkarets, and the Panguitch Lake group are basaltic. In each of these instances the igneous rock issued above the surface and there is no evidence by displacement that any portion of it was deposited below.

Mount Taylor may be an exception. In the character of its lava and its general features it resembles Mount San Francisco, but there are disturbed strata on its southern flank, and it is possible the mountain is both extrusive and intrusive. Extrusion and intrusion are probably combined in some small tables lying fifty miles north of the Henry Mountains. They are built of Flaming Gorge shale, preserved from erosion by dikes, sheets, and (probably) outflows of basalt.

Combining all these facts we attain to a simple relation between two types of igneous rock on the one hand, and two types of mountain structure on the other. One type of rock is acidic, including “porphyritic trachyte” and “eruptive granite”, and its occurrence is without exception intrusive. The other type of rock is basic, including basic trachyte and basalt, and its occurrence is almost uniformly extrusive.

It is not possible to combine the two groups of phenomena by saying that in one case the eruptive cones cover laccolites, and in the other the laccolites have been covered by eruptive cones which have disappeared; first, because many of the eruptive cones are too well exposed to admit of the concealment of laccolitic arches beneath them; second, because the two types of lava are essentially different. The acidic type if extruded at the surface would be an ordinary trachyte; the basic type if crystallized under pressure would be classed with the greenstones.

The basis for the generalization is exceedingly broad. I have enumerated only seven groups of laccolitic mountains and ten groups of eruptive; but with few exceptions each group is composed of many individuals, each one of which is entitled to rank as a separate phenomenon. In the Uinkaret Mountains Professor Powell has distinguished no less than one hundred and eighteen eruptive cones, and in the Henry Mountains I have enumerated thirty-six individual laccolites. In one locality basic lava has one hundred and eighteen times risen to the surface by channels more or less distinct, instead of opening chambers for itself below. In the other locality porphyritic trachyte has thirty-six times built laccolites instead of rising to the surface.

If our attention was restricted to these two localities we might as naturally correlate the types of structure with some accidents of locality as with types of lava; but when all the localities are taken into account it is evident that there is no common mark by which either the laccolitic or the volcanic are distinguished.

THE QUESTION OF CAUSE.

We are now ready to consider the question: Why is it that in some cases igneous rocks form volcanoes and in other cases laccolites?

It is not necessary to broach the more difficult problem of the source of volcanic energy. We may assume that molten rock is being forced upward through the upper portion of the earth’s crust, and disregarding its source and its propelling force may restrict our inquiry to the circumstances which determine its stopping place.

Let us further assume, but for a moment only, that the cohesion of the solid rocks of the crust does not impede the upward progress of the fluid rock, nor prevent it from spreading laterally at any level. The lava will then obey strictly the general law of hydrostatics, and assume the station which will give the lowest possible position to the center of gravity of the strata and lava combined.

(1) If the fluid rock is less dense than the solid, it will pass through it to the surface and build a subaërial mountain.

(2) If the upper portion of the solid rock is less dense than the fluid, while the lower portion is more dense, the fluid will not rise to the surface but will pass between the heavy and light solids and lift or float the latter.

(3) If the crust be composed of many horizontal beds of diverse and alternating density, the fluid will select for its resting place a level so conditioned that no superior group of successive beds, including the bed immediately above it, shall have a greater mean specific gravity than its (the fluid’s) own; and that no inferior group of successive beds, including the bed immediately beneath, shall have a less mean specific gravity than its own.

FIG. 51.—Diagram to illustrate the application of the law of
Hydrostatic Equilibrium to the movements of lavas. The shaded bands
represent heavy strata; the open, light.
]

[In the diagram, a series of light and heavy beds are represented in section by open and shaded spaces. A lava stream free to move upward or laterally will intrude itself at some point (_c_) so placed that every combination of superior beds (_a_), which includes the lowest, shall have a less average density; and every combination of inferior strata (_b_), which includes the highest, shall have a greater average density than that of the lava.]

The first case is that of a volcano; the second is that of a laccolite; and the third is the general case, including the others and applying to all volcanoes and laccolites.

Conversely we may say that, given a series of strata of diverse and alternating density, a very light lava will traverse it to the top and be extruded; a heavier will intrude itself at some lower level; and a series of dissimilar lavas may select an equal number of distinct levels.

It is easy to imagine such a balancing of conditions that a slight change in a lava will determine a great change in its level of intrusion.

Having seen the general application of the hydrostatic law, it is time to recall the condition which we laid aside at the start. Cohesion, or rigidity, is never absent and must affect every phase of vulcanism. It certainly opposes the free circulation of lavas, and it cannot but modify their obedience to the hydrostatic law.

But granting this, and believing that a full comprehension of the subject must include this condition, I am at a loss to tell in what way it influences the selection by a lava flood of a subaërial or a subterranean bourne. Whether it will on the whole oppose upward progress more than lateral, or _vice versa_, is not clear. If it resists lateral intrusion the more strongly, it favors the formation of volcanoes; if it resists upward penetration the more strongly, it favors the formation of laccolites; and in either case the working of the hydrostatic law is modified.

But in neither case is the working of the law more than modified. The law is not abrogated, and in obedience to it light lavas still _tend_ to rise higher than heavy however much the rising of all lavas may be hindered or favored.

In brief, since lavas are fluids they are subject to the law of fluid equilibrium, and their behavior is conditioned by the relations of their densities to the densities of the solids which they penetrate; and since the latter solids are rigid and coherent, it is further conditioned by the resistance which is opposed to their penetration. When the resistance to penetration is the same in all directions, the relation of densities determines the stopping place of the rising lava; but when the vertical and lateral resistances are unequal, _their_ relation may be the determining condition.

If we can decide whether the determinative condition in the Plateau region was that of densities or that of penetrability, we shall have solved our problem.

Assuming, first, that the essential condition is that of penetrability, we should expect that some particular stratum or that a few particular strata, being less penetrable than others, would check the rising lavas and accumulate them in a system of laccolites, which would occupy one, or a few definite horizons. Volcanoes would occur in districts from which such impenetrable strata either were originally absent or had been removed before the igneous epoch; and we should expect to find the same variety of material in laccolites and in volcanoes.

Assuming, second, that the essential condition is that of densities, we should expect as before to find certain stratigraphic horizons more favorable than others to the accumulation of laccolites, and we should also expect to find certain lavas usually volcanic and certain others usually laccolitic.

That is to say—since the condition of impenetrability resides in the solid rock only, and the condition of density pertains to both solid and fluid, either condition might determine laccolites at certain stratigraphic horizons, while the latter only could discriminate certain lavas as intrusive and others as extrusive.

The vertical distribution of laccolites is not inconsistent with either assumption. In the Henry Mountains there are two zones of occurrence; in the Eastern Elk Mountains there is a third; and it is probable in the present state of our knowledge that all other laccolites of the Plateaus can be assigned to one or another of these. The fact that the laccolites of the upper zone have a vertical range of two thousand feet is rather favorable to the idea that their stations were determined by relations of density, but is not decisive.

When however we turn to the relation between the constitutions and the behaviors of lavas, we find the entire weight of the evidence in favor of the assumption that conditions of density determine the structure. The coincidence of the laccolitic structure with a certain type of igneous rock is so persistent that we cannot doubt that the rock contained in itself a condition which determined its behavior.

We are then led to conclude that the conditions which determined the results of igneous activity were the relative densities of the intruding lavas and of the invaded strata; and that the fulfillment of the general law of hydrostatics was not materially modified by the rigidity and cohesion of the strata.

Having reached this conclusion it is natural to seek for confirmation by the investigation of the densities of the rocks concerned in the phenomena. As will appear by a table given further on, the density of the Henry Mountain trachyte has been determined to be 2.61.; but the densities of the erupted lavas of the Plateaus are not yet known. There can be no doubt however that the latter are heavier. Von Cotta in his Lithology gives 2.9 to 3.1 as the density of basalt, and 2.6 to 2.9 as the density of the more basic trachytes. And in general, it is well established that where the state of aggregation is the same, basic igneous rocks are always heavier than acidic. But in order that the laccolitic structure should have been determined by density, the acidic rock of the laccolites must have been heavier in its _molten_ condition than the more basic rocks of the neighboring volcanoes; and since in the _crystalline_ condition the acidic is the lighter, it follows that it has gained less density in cooling than the basic.