THE STRUCTURE OF THE HENRY MOUNTAINS.
The mountains stand within the province of the great flexures, but are
independent of them. Fifteen miles to the westward runs the Waterpocket
flexure. Thirty miles to the north is the San Rafael fold. At the east
the strata rise toward a great uplift, of which the full form is
unknown. But where the Henry Mountains stand the rocks are unaffected by
these disturbances. They have a uniform dip of about 45′ to the
northwest, and form a perfect datum-plane from which to measure the
magnitude of the displacements which have given rise to the mountains.
The mountains are composed of a large number of parts which are in a
certain degree individual and homologous. By the generalization of the
characters of those parts a conception has been obtained of a _type
structure_ to which the entire series of phenomena has been referred.
In laying the material before the reader, the following plan will be
followed:
First. The type of structure will be briefly set forth.
Second. The phenomena by which the type is at once demonstrated and
illustrated will be described in detail.
Third. The type of structure will be discussed.
If the structure of the mountains be as novel to the reader as it was to
the writer, and if it be as strongly opposed to his preconception of the
manner in which igneous mountains are constituted, he may well question
the conclusions in regard to it while they are unsustained by proof. I
can only beg him to suspend his judgment until the whole case shall have
been presented. On some accounts it would have been well to follow in
writing the order of investigation, and develop the general plan of
structure as it was developed in the field, by the addition here of one
element and there of another; or at least to assemble the facts before
announcing my deductions. But such a course would be at the expense of
an important element of convenience and brevity. As will appear in the
sequel, the preliminary explanation of the type structure furnishes a
complement of categories and terms by the aid of which the description
of the details of observation, essentially tedious, is greatly
abbreviated.
[Illustration:
FIG. 7.—Ideal Cross-section of a Mountain of Eruption.
]
[Illustration:
FIG. 8.—Ideal Cross-section of a Laccolite, showing the typical form
and the arching of the overlying strata.
]
It is usual for igneous rocks to ascend to the surface of the earth, and
there issue forth and build up mountains or hills by successive
eruptions. The molten matter starting from some region of unknown depth
passes through all superincumbent rock-beds, and piles itself up on the
uppermost bed. The lava of the Henry Mountains behaved differently.
Instead of rising through all the beds of the earth’s crust, it stopped
at a lower horizon, insinuated itself between two strata, and opened for
itself a chamber by lifting all the superior beds. In this chamber it
congealed, forming a massive body of trap. For this body the name
_laccolite_ (λάκκος, _cistern_, and λίθος, _stone_) will be used. Figure
7 and Figure 8 are ideal sections of a mountain of eruption and of a
laccolite.
The laccolite is the chief element of the type of structure exemplified
in the Henry Mountains.
It is evident that the intrusion of a laccolite will produce upon the
surface as great a hill as the extrusion of the same quantity of matter,
the mass which is carried above the original surface being precisely
equivalent to that which is displaced by the laccolite; and it is
further evident that where the superior rock is horizontally stratified
every stratum above the laccolite will be uplifted, and, unless it is
fractured, will be upbent, and will portray, more or less faithfully, by
its curvature, the form of the body it covers.
Associated with the laccolites of the Henry Mountains are _sheets_ and
_dikes_.
The term _sheet_ will be applied in this report to broad, thin,
stratified bodies of trap, which have been intruded along the partings
between sedimentary strata, and conform with the inclosing strata in
dip. _Dikes_ differ from sheets in that they intersect the sedimentary
strata at greater or less angles, occupying fissures produced by the
rupture of the strata.
The logical distinction between dike and sheet is complete, but in
nature it not unfrequently happens that the same body of trap is a sheet
in one place and a dike in another. Between the sheet and the laccolite
there is a complete gradation. The laccolite is a greatly thickened
sheet, and the sheet is a broad, thin, attenuated laccolite.
[Illustration:
FIG. 9.—Ideal Cross-section of a Laccolite, with accompanying Sheets
and Dikes.
]
In the district under consideration the laccolite is usually, perhaps
always, accompanied by dikes and sheets (see Figure 9). There are sheets
beneath laccolites and sheets above them. The superior sheets have never
been observed to extend beyond the curved portion of the superior
strata. Dikes rise from the upper surfaces of the laccolites. They are
largest and most numerous about the center, but, like the superior
sheets, they often extend nearly to the limit of the flexure of the
uplifted strata. The larger often radiate from the center outward, but
there is no constancy of arrangement. Where they are numerous they
reticulate.
In the accompanying diagrams dikes are represented beneath as well as
above the laccolites. These are purely hypothetical, since they have not
been seen. In a general way, the molten rock must have come from below,
but the channel by which it rose has in no instance been determined by
observation.
The horizontal distribution of the laccolites is as irregular as the
arrangement of volcanic vents. They lie in clusters, and each cluster is
marked by a mountain. In Mount Ellen there are perhaps thirty
laccolites. In Mount Holmes there are two; and in Mount Ellsworth one.
Mount Pennell and Mount Hillers each have one large and several smaller
ones.
[Illustration:
FIG. 10.—Ideal Cross-section of Grouped Laccolites.
]
Their vertical distribution likewise is irregular. Some have intruded
themselves between Cretaceous strata, others between Jura-Triassic, and
others between Carboniferous. From the highest to the lowest the range
is not less than 4,000 feet. Those which are above not unfrequently
overlap those which lie below, as represented in the ideal section,
Figure 10.
The erosion of the mountains has given the utmost variety of exposure to
the laccolites. In one place are seen only arching strata; in another,
arching strata crossed by a few dikes; in another, arching strata filled
with a network of dikes and sheets. Elsewhere a portion of the laccolite
itself is bared, or one side is removed so as to exhibit a natural
section. Here the sedimentary cover has all been removed, and the
laccolite stands free, with its original form; there the hard trachyte
itself has been attacked by the elements and its form is changed.
Somewhere, perhaps, the laccolite has been destroyed and only a dike
remains to mark the fissure through which it was injected.