The Milan exchange. (Milan, Gibson County, Tenn.) 1874-1978, 1876-08-31
The Structure and Age of the Rocky
Mountains,

Mountains.
| From the Missouri River westward
the whole country gradually rises, at an
average grade of barely ten feet to the
mile, until about the meridian of 1861
degrees w. is reached, and then the
Rocky Mountains rise abruptly from
the plain. Thence to somewhat beyond
10S degrees w. the country is traversed
by numerous mountain ranges, sepa-
rable into two series The first series
comprises two complex axes of eleva-
| ton, the front or eastern and the San
gre de Christie, whose trend is from n.
10 degrees w. to n. Sc degrees w. The
second series is made up of the San
Juan, Los Pinos, La Plata and San
Miguel ranges, which have a trend of
N. 30 degrees w. to N. As degrees w.
Each series shows a parallelism in its
ranges, and the whole system terminates
en- echeLOn southwark, most of the axes
ending in Colorado.
The eastern range, which consists of
several closely packed parallel axes, and
rises sharply from the plain, is COM
posed of metamorphic rocks, badly
tissured by dykes of lava, and not un-
frequently capped by lava over1ows.
The schists are much torn and faulted,
and side throws of mineral veins are
not uncommon. Along the median line
of the axis, exposed here and there by
| deep cuts, compact granite, more or
less syenitic, appears to prevail. The
sedimentary rocks occur as ''hOg backs
along the eastern base, and curve
round the southern examinations of the
several axes.
The second range, Professionally
named the Sangre de Christie by DR..
SteVenson-to whom (Report of En-
gineer Department, Wheeler Expedi-
ton, 1879 we are indebted for these
particularS-is in the main almost par
aliel with the eastern range, but is much
more complex in its structure. Its
width is about twenty-five miles in the
northern part, diminishing to twelve
miles at Sangre de Christie Pass. With
its extension, the Spanish range, it is,
in DIR. Stephenson's opinion, but the
southern portion of a magnificent group
which once covered the whole region
from East River to South Park. 1t re-
mains for future explorations to solve
the many problems which its compli-
Gated structure involves. In the main
portion no rocks have been found of
later date than the carboniferous.
The third great axis is the San Juan,
for the most part buried under a great
mass of volcanic rocks, which almost
conceal those Of sedimentary origin.
Wherever exposed, marked uncOn-
formabilIty is seen between the car
boniferous and the everything rocks.
The older formations are inclined at a
very high angle, while the cretaceous
and (doubtfully) the triassic, which are
conformable to each other, have very
small dip.
The next great axis toward the west
is the one termed by DR.. Newberry the
Los PinOs, in part the divide between
the Rio de los PinOs and the Rio Piedra.
The only rocks involved are the car
boniferOus and (probably) the Silurian.
On each side of the range, which is not
more than five or six miles wide, the
cretaceOus ROCKS are seen forming
mesas and dipping only two or three
degrees.
The next axis, the La Plata (Newber-
ry), forms in part the divide between
the Rio de la Plata and Rio de los Ari-
mas. The course of the uplift is almost
northwest, and the dip is very gentle
where the strata have not been locally
disturbed by lava dykes. The only rocks
involved are the paleOzOic, against
which the triassic and the cretaceous
abut at slight angle.
The San Miguel axis is still farther
westward, and, like the La Plata, in
wolves only pal9OzOic rocks, those Of
mesOzOic times forming mesas around
it. Beyond, to the westward, extends
a cretaceous plateau separating the
Rocky Mountains from the Great Basin.
From his admittedly partial explora-
tions, DR.. Stevenson finds it sufficiently
evident that the Rocky Mountains are
not the result of single grand up
heaval, and that the several axes are
not wholly SynchronouS in origin. The
general diminution of disturbance west
ward, as shown by the diminishing
steepness of dip, together with the gen
eral trend of the several axes, shows
that the disturbing force was propagat-
ed from the east or east of northeast.
The relations of the strata of the sew
eral periods make it easy to determine
the era and the comparative energy of
the successive upheavals. The first
was at the close of the carboniferous
period. The Silurian and the carbonif-
erous are every where cSnrofmable,
showing that, during the time of their
deposition, there must have been either
comparative quiet or continued sub
sidence. The line of continuous action
thereafter seems to have been that now
occupied by the eastern range. In this
region there was a subsidence during
the trias, which but slightly, if at all,
affected the interior.
The second epoch of elevation began
toward the close of the triassic, and was
marked by an exceedingly energetic
action along the eastern line, accompa-
MED by grand eruption of igneOus
rocks. The cOnformability of the trias
and the cretaceOus in the San Juan area
shows that the energy of the convul-
siOns diminished westward and south
westward from the main line of disturb
A1ICl bhC SCCOIlu tll IuOAO
was an extensive subsidence, the record
of which appears in the prevalence of
cretaceous deposits over the whole
Rocky Mountain area.
The third epoch of elevation followed
hard upon the cretaeious period. The
action is generally violent, in some
parts terrific, resulting in a perfect
maze of cross faulting. Everywhere
north and east of the Rio Grande the
volcanic disturbance was excessive, a
vast area being buried under a sheet of
lava from two thousand to three
thousand feet thick; and enormous
dykes, stretching from the Sangre de
Christie southeastward far out into the
plain, remain to attest the widespread
effects of the disturbance.
During the tertiary age, another but
much slighter elevation took place,
giving the rocks of that age a dip of
five degrees. Of the four upheavals,
the first and third were much the most
general in their effects. The first was
synchronous with that during which
the Appalachian chain was completed.
I-Scie7atitic American