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not now clearly exposed.

On the next spur to the eastward the dike sandstone
seems to form a comparatively narrow bund, possibly one
hundred feet wide, across its steep north end. North
of it are the Red Beds, and south of it all is granite.
Beyond this spur is the main west branch of Red Rock
Caiion. Along the west side of this gulch is another very



132 BULLETIN OF THE ESSEX INSTITUTE.

plain transverse fault, the Red Beds on the east side being
jogged to the south or up the gulch fully one thousand
feet and terminating just north of two mining tunnels.
On meeting the transverse fault the sandstone dike appears
to turn and border it on the west side, a very plain indi-
cation that these transverse faults are contemporaneous
with the Ute fault.

The mesa east of this gulch is probably underlain by
Red Beds right up to the steep granite slope, with only
sljorht indications of dike sandstone alons^ the fault line.
In fact, the slide of disintegrated granite hides all contact
phenomena for the next half mile, or to where the main
or quarry ridge of red sandstone meets the granite slope
(20) with a nearly vertical dip (E. 85°). The next and
last red ridge, about six hundred feet farther east, is
swerved to the east as it nears the granite, approaching
the granite at the last very obliquely. Between Red Rock
Gallon and Bear Creek no satisfactory evidence of sand-
stone dikes has been observed ; but the Ute fault appears
to cross Bear Creek without deflection or offset, and the
Jurassic and Cretaceous beds meet it in the same manner
as the last of the Red Beds, each formation in turn expe-
riencing a sharp flexure parallel with the fault and conse-
quent eastward deflection of its outcrop. East of the
Cretaceous (Dakota) hogback, forming the gateway ot
Bear Creek, the Cretaceous beds form first a sharp and
unsymmetrical syncline followed by a gentle anticline.

SANDSTONE DIKES IN THE VICINITY OF CHEYENNE CREEK.

From Bear Creek southeast for one and a half miles
I was unable, for lack of time, to follow the fault line ;
but my observations were resumed at South Cheyenne
Creek, working first northwest and then southeast.



SANDSTONE DIKES OF UTE PASS. 133

Exactly at the mouth of South Cheyenne Canon, where
the carriage road crosses the creek and the burro trail from
the terminus of the electric railroad joins it (21), a large
and entirely typical sandstone dike outcrops on both sides
of the creek. On the west side it is sixty, if not seventy-
five feet wide and both walls can be located. The north
wall is clearly exposed by the roadside and shows sand-
stone penetrating and enclosing the coarse red granite ;
while on the south side the sandstone is involved with
a dark green, fine grained, crushed igneous rock. Crossing
the end of the mountain toward North Cheyenne Creek,
I found a good outcrop in a prospect hole seventy-five to
one hundred feet above the creek. This excavation does
not expose either wall ; but it does show an inclosed mass
of the compact greenish igneous rock. It is dike-like in
form, eighteen to twenty-four inches thick, and hades
southwest 45^^ ; but it is also clearly traversed by dikelets
of the sandstone one-fourth inch to two inches in diame-
ter. Hence the altered trap must be older than the sand-
stone dike. The dike sandstone is quite friable in part,
and some of it rather coarse, even containing pebbles of
quartz and granite. Appearances indicate a wide dike
here, and it is readily traced down the slope to North
Cheyenne Creek and up the west slope of the valley.
Inunediately north of the dike is a large but ol)scure out-
crop of a soft or friable white sandstone which weathers
bufi", yellow, red and purple (mainly yellow). It agrees
very well with the Dakota sandstone of Bear Creek, while
it is entirely difterent from the adjacent dike sandstone.

The great shear plane so conspicuously exposed on the
west side of South Cheyenne Canon (22) demands atten-
tion here. It rises from the creek level near the toll gate,
passes around the end of the mountain at a height of
about two hundred feet, and appears to be traceable east

ESSEX INST. BULLETIN, VOL. XXVII 13



134 BULLETIN OF THE ESSEX INSTITUTE.

and west across both branches of Cheyenne Creek, being
indicated in the topography and also in the contrast pre-
sented by the granite — dark red and brown to gray and
variable above, and a lighter, brighter red and coarser
texture below. The really striking exposure is on the
northwest side of South Cheyenne Canon, where it appears
as a perfectly straight, sloping and open crack or fissure,
with a southwest hade of 60° to 70° (from vertical).
The shear faces are plane, smooth or even polished, and
slickensided in the line of hade (southwest). The fissure
is six to fifteen inches wide and occupied by some broken
granite, but mainly by what appears to be the dike sand-
stone, though some of it might be comminuted granite.
Examination with a lens leaves no doubt that the filling
is chiefly the dike sandstone. This prominent shear plane
is clearly a feature or part of the Ute fault ; and was
probably once connected with the sandstone dike already
described which crosses the ridge between the two creeks
only a short distance below the fissure.

On the northwest bank of North Cheyenne Creek the
sandstone dike crops with a breadth of certainly one
hundred feet. On the slope above is a very large and
prominent outcrop, showing a width of fully two hundred
and fifty feet (23). Neither wall is clearly exposed, but
the boundary on the southwest or granite side can be
readily traced by the float and also by the prospect holes.
The lower hole is in the sandstone only a foot or two
from the granite and shows an irregular streak of granite
six to twelve inches wide running vertically through the
sandstone and parallel with the wall. In the same hole
some of the sandstone appears to be glauconitic. Toward
the middle of the dike are some masses and layers of
conglomerate with water-worn quartz pebbles up to au
inch in diameter, exactly like what may be observed iu



SANDSTONE DIKES OF UTE PASS. 135

different parts of the Manitou l)iisiii at the base of the
Potsdam, and much of the sandstone is rather coarse.

Farther northwest the dike passes beneath a broad,
steep slope of talus from the granite cliffs above. It can
be traced, however, in an occasional prospect hole and
in float. Abont half a mile or so from the creek is a quite
conspicuous outcrop of white Cretaceous (Niobrara)
limestone in beds nine to fifteen inches thick with thinner
shaly partings. It has been quarried somewhat and
shows a dip to the southwest 45°. Overlying it is
a brown and highly fossiliferous limestone with a bitumin-
ous odor when freshly broken. Between this limestone
and the granite cliff is about two hundred feet concealed
by talus, but with indications that the limestone is bor-
dered by the sandstone dike with a breadth of one hun-
dred feet more or less, the Benton shales and Dakota
sandstone being concealed by the fault at this point. In
the next one-fourth mile, going west, the dike seems to
die out completely, no float showing in the slide ; but the
white limestone is exposed in several prospect holes and
ledges almost to the head of this valley, which extends
nearly a mile northwest from the creek to the summit of
the mesa. The dip is constantly southwest, 45° approxi-
mately. Probably these Cretaceous strata, like the Foun-
tain beds farther west, have been overturned by the
oblique upward thrust of the granite.

On tracing the great sandstone dike southeast from
South Cheyenne Creek it is found well exposed on the
first spur with a breadth of one hundred to possibly two
hundred feet. Neither contact is exposed, but the granite
contact can be located within a foot or two at several
points. On the north are extensive but obscure expo-
sures of the gray fossiliferous limestone and the compact
white limestone. The bedding is not clearly exposed,



136 BULLETIN OF THE ESSEX INSTITUTE.

but the gray limestone is south of the white, as before.
The sandstone dike can be traced by float across the lower
slope of the next spur toward the mountain road. Be-
yond the Cheyenne Mountain road my observations have
not extended, and 1 have no information as to whether or
not the Ute fault, with or without the accompanying sand-
stone dikes, can be traced farther in that direction. The
topographic indications are certainly very favorable to
their occurrence, at least as far as Deadman's Caiion ; but
probably the extensive mesa and slide deposits make
satisfactory outcrops few and far between.

The Ute fault series of sandstone dikes has been proved
for a distance of over twenty miles, and an extreme
length of thirty miles or more is certainly by no means
impossible. Professor Stone's observation, cited by Cross,
demonstrates the existence of another extended system of
dikes in the Pike's Peak region. This occurs in the valley
of Turkey Creek, south of Cheyenne Mountain, and is
quite certainly not connected with the Ute fault. Ac-
cording to Stone, the Turkey Creek system has been
located for about twelve miles, and through his courtesy
I am able to cite a third system, since he writes me that
a short sandstone dike occurs about half a mile east of
Nipple Mountain, near Wilbur Station, southeast of Crip-
ple Creek. These facts indicate the probable discovery
of other systems, as the region is more thoroughly studied.

ORIGIN OF THE SANDSTONE DIKES.

Mr. Cross has briefly discussed this topic, without
arriving at a definite conclusion. He recognizes that
these sandstone dikes are radically distinct in character
and origin from those described by Diller in California,
and asserts that the known facts do not indicate the source



SANDSTONE DIKES OF UTE PASS. 137

of the sand ; that the facts do show that the fissures of
this dike complex were filled b}' fine quicksand injected
from a source containing a large amount of homogeneous
material ; that such a system of fissures, large or small,
with their many intersections, could not remain open to
be filled by any slow process ; that the uniformity and
purity of the material filling fissures, varying from mere
films on cleavage planes of orthoclase grains in the granite
to dikes several hundred yards in width, could not have
resulted from infiltration ; and, finally, that none of the
sedimentary formations of the region can be regarded as
prol^able sources of the material.

My study enables me to accept all of these gejieraliza-
tions, except the last one. The main purpose of the pre-
ceding detailed descriptions of the dikes which have come
under my special notice is to set forth the facts which
the true theory of the dikes must explain. The most im-
portant of these are : first, their very evident close rela-
tionship to an important zone of displacement; second,
the homogeneity of the materials and the general absence
of stratification in the dikes; third, the great maximum
and average widths of the dikes.

The relations of the dikes to the great Ute fault are
indisputable. Not only is the fault at most points closely
accompanied by one or more dikes ; but nowhere have
I been able to find any trace of the dikes more than a few
hundred feet (500 to 1000 feet) distant from the princi-
pal line of displacement. Of course it can not be pos-
itively asserted now, in the absence of sedimentary
deposits in those districts, that the dikes of the Turkey
Creek and Nipple Mountain areas accompany lines or
zones of displacement ; bnt it is certainly a fair presump-
tion that they do, in view of the fact that they are,
apparently, in every other respect, identical in character



138 BULLETIN OF THE ESSEX INSTITUTE.

with the Ute fault series. That these fissures, unlike the
relatively narrow ones described by Diller in California,
have not been filled from below becomes perfectly obvi-
ous when we reflect that the inclosing rock formation is
a deep-seated phitonic. The homogeneity and purity of
the sandstone, and especially the absence of feldspathic
or argillaceous material, make it impossible to regard the
dike rock as a fault breccia or as due in any way to the
comminution of the wall rock. Ruling out this theory,
and infiltration, we are forced to the conclusion that the
fissures have been filled from above. But of this theory
two principal forms naturally suggest themselves. First,
the fissures antedate the deposition of the sand, existing
as cracks in the sea-bottom which were filled by the slow
process of sedimentation. Second, the cracks post-date
the deposition of the sand, but antedate its lithifaction to
form a firm sandstone ; and the unconsolidated sand sub-
sided and flowed down into and filled the fissures. As
Cross has pointed out, the necessary slowness of the
process is a valid if not a fatal ol)jecti()n to the first view ;
and it also fails to account for the very general absence
of stratification in the dikes and of more or less water-
worn fragments of the wall-rock. Furthermore, if these
traps for sediments opened on the marginal portion of
the sea-floor, coarse material washed into them would be
protected from further attrition, and the observed fineness
and homogeneity of the dikes could not exist.

By this process of elimination we are forced to the
consideration of the view that the fissures were formed
after the granite had been covered by the sedimentary
deposits and before their complete consolidation, the un-
consolidated portions naturally contributing to the filling
of the fissures and the formation of the dikes. In the
opinion of the writer this view is not, a priori, improb-



SANJDSTONE DIKES OF UTE PASS. 139

able. On the contrary, it postulates conditions which
must be realized now and then — a normal type of geo-
logical accident. The next step, therefore, is to test its
applicability to the actual conditions of the present prob-
lems. There are two questions especially Avhich the
acceptance of this explanation would require to be
answered in the afQrmative. First, are there, among the
sediments of the Manitou and Manitou Park basins, any
that, aside from structural features like stratification,
which would of course be obliterated during the filling of
the fissures, present a reasonably close agreement in
character (composition and texture) with the sandstone
of the dikes? Second, may we reasonably assume that
these sediments were, in part at least, unconsolidated, or
imperfectly consolidated, at the time when the fissures
were formed ? The only sandstone formations that need
be considered in this connection are the Potsdam, Car-
boniferous, Triassic and Dakota. The Laramie and
Monument Creek beds are so far removed in every sense
— lithologically, stratigraphically and topographically —
as to place them quite beyond the possible purview of the
problem. Of the four sandstone horizons first named,
the last three bear no special resemblance to the material
of the sandstone dikes. Cross insists upon this lack of
agreement for the Carboniferous or Fountain beds, which
are ruled out especially by their prevailing coarseness
and arkose character. It is almost as difficult to find
in the bright red Triassic sandstones an equivalent of
the reddish-brown and gray sandstones of the dikes ;
and similarly with the white or butt' Dakota sandstone,
weathering to bright yellow, orange and red tints. Fur-
thermore, the derivation of the dike rock from any of
these higher sandstone horizons would imply the exist-
ence of sandstone dikes in the lower sedimentary forma-



140 BULLETIN OF THE ESSEX INSTITUTE.

tious ; but that the dikes are strictly limited to the granite
no student of this area will question. The Potsdam beds,
on the other hand, resting as they do directly upon the
granite, are in an extremely ffivorable position, strati-
graphic and otherwise, for filling fissures in the granite.

In the foregoing descriptions of the dikes I have several
times stated that the rock resembles the Potsdam sand-
stone, and this resemblance is frequently indicated in my
note-book. In fact, I became convinced before the field
work was finished that the two formations are lithologi-
cally identical, except that the dike rock is usually
unstratified and rarely glauconitic. The glauconite, how-
ever, characterizes only a small part of the Potsdam
strata, and is often wanting altogether. This agreement
holds for both the Manitou and Manitou Park basins.
From the sandstone dikes north of Green Mountain Falls
I crossed the Manitou Park Basin to the highly inclined
Silurian and Potsdam outcrops along its eastern edge, and
found that a part of the latter formation is practically
indistinguishable from the dike rock. Cross describes
the sandstone of the dikes as of fine and uniform texture ;
but this is also the prevailing character of the Potsdam
beds, and my observations show that the dikes, as a
whole, embrace much coarse sandstone and some con-
glomerate, and that their range in texture is fully equal
to that of the Potsdam. Furthermore, the structural
contrast does not hold universally, for at several points,
as I have noted, and including also a part of the great
dike north of Green Mountain Falls, the dike sand-
stone is plainly stratified. The fticts that no sandstone
dikes have been observed in the Potsdam beds, and that,
so far as known, the dike rock is iiever calcareous and
rarely glauconitic, suggests its derivation from the lower
rather than the calcareous and glauconitic upper layers of



SANDSTONE DIKES OF UTE PASS. 141

the Potsdam. The basal member of the Potsdam, it will
be remembered, is a white to gray sandstone more or less
interstratified or blended with the prevailing reddish
brown variety. It is not only calcareous or argillaceous,
but it contains insufficient iron oxide for its thorough
cementation, and has very clearly never been exposed to
volcanic influences. We can, therefore, readily conceive
that it remained unconsolidated for a Ions: time after the
lithifaction of the overlying beds. The dike rock is abso-
lutely indifferent to the changes in the character of the
neighboring sedimentary formations, showing no appre-
ciable variation as, in succession, from Manitou southeast
to Cheyenne Canon, the Potsdam, Silurian, Carbonifer-
ous, Triassic and Dakota beds abut against or border the
great fault.

The close association of the dikes, throughout the en-
tire belt, with the great displacement, and their unvary-
ing lithological similarity to the Potsdam sandstone, have
suggested to me that the dikes probably date from the
formation of the Ute fault ; that the fault probably dates
from the time when the Potsdam beds, which are still at
the base in part of a more or less friable character, were
imperfectly consolidated and covered the entire region ;
that the fault, as is likely to be the case with a great dis-
placement, was not simple, but that a moderate breadth
of the granite and overlying formations was traversed by
a series of parallel fissures ; and that the dikes resulted
from the sinking of the Potsdam sandstone and sand into
the fault-fissures. Such local subsidences of the friable
sandstone would naturally be attended by a more or less
complete obliteration of the bedding. That the structure
of the Potsdam beds has been locally effiiced under shear-
ing and compressive or plicating movements can be seen
at a point on the east side of Ute Pass a few rods below

KSSEX INST. BULLETIN, VOL. XXVn 13*



142 BULLETIN OF THE ESSEX INSTITUTE.

Rainbow Fall. On the south side of a small lateral gulch
a sharp, inverted flexure of the basal Potsdam beds can
be seen (Fig. 4). Throughout the flexure the sandstone
is beautifully slickensided in various directions precisely
as in the sandstone dikes. The extremity of the sharp
lower curve is not clearly exposed, but on both this and
the main curve above it the bedding is much obscured or
completely effkced, and the sandstone closely resembles
the dike rock.

Many of the sandstone dikes are one hundred feet or



- Red.



Hg.^-






4.+X .4-+, +-.-U-L^ +






+ + ■*", - ' ••••■.••••..■ •>: / "- "



/ —



-^^^'



mor in oreadth, and the largest, as described by Cross,
five hundred to one thousand feet ; and certainly no single
featdre of the dikes is more sio^nificant than the 2XQsX
breadth of individual examples. Although presenting,
apparently, an insuperable obstacle to all the other sug-
gested explanations of the sandstone dikes, it offers no
difficulty whatever to the theory proposed here, for we
have only to make the extremely prol)able supposition
that sheets of granite of varying width and bordered by



SANDSTONE DIKES OF UTE PASS. 143

complementary faults have settled down relatively to the
bordering masses, bearing with them their loads of Pots-
dam sediment. The very moderate thickness of the
Potsdam beds (40 to 50 feet) would seem to set a corre-
spondingly narrow limit to the depth of these wide dikes ;
and yet some of them actually outcrop in such strong
relief as to prove a depth of several hundred feet at least.
Escape from this dilemma is aftbrded, however, by the
reasonable supposition that the sandstone was sufficiently
unconsolidated to flow under the great pressure to which
it was exposed ; and also by the facts noted in the great
dike on Sutherland Creek, where the sandstone, still
distinctly bedded, has been strongly folded and tilted to
a vertical or overturned position (Fig. 3). Compression
between converging walls of granite might, obviously,
increase the vertical thickness of the sandstone to almost
any extent. An actual flowing of the sand into the chasms
opened beneath it is plainly indicated in the case of all
the narrower dikes and dikelets and their intricate
branches.

Of course it is a logical though by no means a neces-
sary deduction from this theory that some of the sand-
stone dikes should break the granite-Potsdam contact,
disturbing: or obliteratinij the bedding of the sandstone.
No undoubted instances of this have been observed in a
somewhat thorough examination of this contact throughout
the Manitou area, Imt it is not impossible that they exist.

Concerning the geolosrical age of the sandstone dikes
no positive statements are warranted by the facts now at
our conmiand, although the explanation of the dikes here
proposed aflbrds us a clue, since they must be coeval with
the Ute fault. This displacement was certainly not com-
pleted until Post-Cretaceous times, but it may well have
begun at a nmch earlier period, since, as Cross has stated,



144 BULLETIN OF THE ESSEX INSTITUTE.

orographic movements have affected this region many
times since the early Paleozoic epoch. It is certainly im-
probable that the Potsdam sandstone was, in any Post-
Cretaceous epoch, so imperfectly consolidated as the
formation of the minor sandstone dikes in the manner
here proposed, at so late a date, would require. I sug-
gest therefore that they probably date from some compara-
tively early movement along this line. In this connection
it may be noted that the numerous slickensides in the
dikes show movement after the lithifaction of the sand,
and therefore long subsequent to the first formation of
the dikes.

It is a necessary corollary of the view developed here
that the sandstone dikes of Turkey Creek, Nipple Moun-
tain, and possibly other localities in this region, indicate
formerly overlying Potsdam strata, and thus throw light
upon the former distribution of that formation. They are
narrow linear Grdhen formed along sheeted zones, or more
literally trenches in which portions of the Potsdam sedi-
ments have been buried below the present plane of erosion,
and thus preserved. As erosion cuts more deeply, all the
sandstone dikes will disappear, as they practically have
done already between Cascade and Green Mountain Falls,
or where the Potsdam beds are wantino-.

The sheeting of the granite which this theory demands
is by no means a purely theoretical feature. On the con-
trary, this type of jointing may be regarded as more or
less characteristic of the granite of Ute Pass, as may be
so well seen at Rainbow Fall. And two striking illustra-
tions of sheeted zones in a distant part of the Pike's Peak
Massif accompany the recent monograph by Penrose on
the Mining Geology of Cripple Creek. ^



•^^ lotli Ann. Rep. U. S. Geol. Survey, part 2, plates ill and iv.



SANDSTONE DIKES OF UTE PASS, 145

Two other modes of occurrence of sandstone are, in


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