miles, one lying north, and the other, the main range, lying south of
the City of Baraboo. The geology of this district is admirably given by
the late Professor Irving.[8] Not only is the general geology clearly
described, but remarkably accurate descriptions are given of the
character of the quartzite, and the phenomena shown by it, considering
the fact that the report was written nearly twenty years since. The
unconformity existing between the quartzite and the Cambrian was later
more fully described.[9] The induration of the Baraboo quartzite has
been explained as due to the enlargement of the original quartz grains;
and to the deposition of independent interstitial quartz.[10] The
present note is based upon recent observations on the East Bluff at
Devil’s Lake and on the exposures at the Upper Narrows of the Baraboo
River.
The section across the ranges, as given by Irving, is shown by Fig. 1.
The two ranges together, as thus represented, are less than the north
half of a great anticline, the south side of the south range being near
its crown. This structure involves a very great thickness of quartzite,
and was offered with reservation by Professor Irving. He says: “The
hypothesis is not altogether satisfactory. The entire disappearance of
the other side of the great arch, as well as the peculiar ways in which
the ranges come together at their extremities are difficult to explain
by it. It may be said in this connection that the dip observations
toward the west are not so satisfactory or numerous as they might be.”
The question naturally arises whether or not the great width of the
ranges in the central part of the area may not be partly explained by
monoclinal faulting, and thus reduce the supposed thickness of the beds.
[Illustration: FIG. 1.--Ideal Sketch, showing structure and amount of
erosion of the Baraboo Ranges.
After Irving.
Scale natural, 12,000 feet to the inch.]
The layers of quartzite are ordinarily very heavy, but the changing
character of the original sediment is such as to make it easy to
follow the layers. Some beds were composed of fine grains of quartz,
mingled with clayey material, others of coarse grains with little
clayey material, and others of pebbles so large as to pass into an
unmistakable conglomerate. The pebbles of the conglomerate are mainly
white quartz and red jasper. It is thus easy to discriminate the
bedding of the series from the heavy jointing which occurs, cutting
the bedding in various directions, and from a secondary cleavage and
foliation which occurs in certain localities.
From the general work of many geologists on dynamic action in folding,
it is to be expected that the amount of movement necessary for
accommodation between beds, and consequently the dynamic metamorphism
resulting from shearing, would be less near the crown of the anticline
than on the leg of the fold. That is, dynamic metamorphism ought
not to be so extensive in the south range as in the north range. The
facts described by Irving,[11] and those noted by me, fully agree with
this anticipation. The central parts of the heavy, little inclined
beds of the south range are largely indurated by simple enlargement.
The pressure has not been sufficient to obliterate the cores, but has
apparently granulated the exterior of some of the larger fragments, as
in hand specimens the exteriors of the large blue quartz grains are
white. Very generally the grains show slight wavy extinction. A few of
them are distinctly cracked. The crevices thus formed and those in the
interstices have been filled in large part by infiltrated silica, but
their positions are plainly indicated by difference in extinction, by
bubbles, by iron oxide, or by secondary mica which has taken advantage
of the minute crevices.
However, as described by Irving, between the heavy beds of quartzites
are often layers, cut by a diagonal cleavage which dies out in passing
into the thick beds. The layers showing cleavage sometimes pass into
those showing the beginning of foliation, the rock then nearing a
schist. In the centers of the schist zones, the schistosity approaches
parallelism with the bedding, and in passing outward curves from this
direction until it crosses the bedding at an angle, at the same time
becoming less marked and grading into ordinary cleavage, which dies out
in the quartzite. Upon the opposite side the transition is of the same
character, but the curve is in the opposite direction.
Irving apparently regarded these shear zones as originally beds of a
different character from the adjacent quartzite, and his conclusion is
fully borne out by the thin sections. The microscope shows that the
grains of quartz are of small size, and separated to a greater or a
less extent by interstitial clayey material. Because of this partial
separation of the grains of quartz, they have not been granulated to
the extent that one would expect from the schistosity of the rock, most
of the original cores being plainly visible. They, however, often show
wavy extinction and even cracks, but not to a greater degree than the
grains in the massive quartzite; for in the latter the full stress of
the pressure has been borne by the grains in full touch, not separated
by a plastic matrix, as are the grains of quartz in the argillaceous
layers. In the matrix of the schist are numerous small flakes of
muscovite, arranged with their longer axes in a common direction, much
finely crystalline quartz, and a good deal of iron oxide.
It is concluded that the clayey character of the beds, and,
consequently, the greater ease of movement within them, has located the
slipping-planes and shear-zones, necessary in order to accommodate the
beds to their new positions. On the south range, near Devil’s Lake,
these shear-zones are generally not more than six or eight inches
wide. They may be well seen just back of the Cliff House, and on the
Northwestern Railway, about one-half mile south of this house. All of
these shear-zones are parallel with the bedding, and illustrate the
possibility, so far as I know first mentioned by H. L. Smyth, that
a crystalline schist, with schistosity parallel to bedding, may be
produced by shearing along the bedding-planes.
On the railroad track, near the locality where these shear-zones may
be seen, is also an almost vertical shear-zone, two to four feet wide.
It therefore cuts almost directly across the beds of quartzite, which
here incline to the south about twelve or thirteen degrees. Throughout
this band, the quartzite is broken into angular trapezoidal fragments,
the longer directions of which are vertical, and which may be picked
out with the hammer. In certain parts of the zone well-defined gruss
or friction clay produced by the grinding of the fragments against one
another, has been produced. This is clearly a plane of faulting. How
much the throw of this fault is it is not easy to say, as the heavy
beds of quartzite are so similar that it is impossible to certainly
identify them. At this place there is, however, a change in the
character of the quartzite, layers of light color being overlain by
other beds, which are more heavily stained with iron oxide. This same
succession is seen on both sides of the fault, and if beds of like
character correspond, the amount of the throw is twenty to thirty feet,
and the south side has dropped relative to the north side. In other
words, the faulting is in the right direction to reduce the theoretical
thickness of the sediments as given by Irving. The district has not
been closely examined for other faults, but the existence of one fault,
even of a minor character, suggests that a careful study of the whole
area with reference to faulting should be made, in order to determine
what deductions may possibly be made from Irving’s estimate of the
probable thickness of the quartzite.
At the upper narrows of the Baraboo, near Ablemans, we are on the north
leg of the anticline. The dip is throughout from seventy to ninety to
the north, and in some places the layers are slightly overturned. The
slipping along the bedding has here been much greater. While in this
area there are heavy beds of quartzite which have not suffered great
interior movement, other beds have been sheared throughout, being
transformed macroscopically into a quartz-schist, but the foliation
is strongly developed. In other places, as described by Irving,[12]
where the rock is a purer quartzite, for a distance of 200 feet or more
across the strike, the rocks have been shattered through and through,
and re-cemented by vein quartz.
For the most part the rock is merely fractured, the quartz fragments
roughly fitting one another, but there are all gradations from this
phase to a belt about ten feet wide of true friction conglomerate, the
fragments having been ground against one another until they have become
well-rounded (a Reibungs breccia). Between the boulders of this zone
is a matrix, composed mainly of smaller quartzite fragments. The whole
has been re-cemented, so that now the mass is completely vitreous. This
belt of friction conglomerate at first might not be discriminated from
the Potsdam conglomerate, immediately adjacent, but a closer study
shows how radically different they are. In one the cementing material
is vein-quartz; in the other the sandstone has been feebly cemented by
quartz enlargement.
A movement later than the one which produced the cemented fractured
rocks and breccia has broken broad zones of the massive beds of
quartzite into lozenge-shaped blocks, the longer axes of which are
parallel to the bedding and movement. These later-formed blocks have
not been re-cemented by secondary quartz, and the cracks are taken
advantage of in quarrying, the fragments being easily picked apart.
Thus the rock has been affected by at least two dynamic movements,
separated by a considerable interval of time.
The shear-zones, often several feet in width, particularly affect the
more finely-laminated layers, which are lean in quartz, while the
relief in the more massive layers has resulted in complex fracturing.
In the first phase of production of the schist, the irregular fractures
pass into rather regular fractures, cutting the beds nearly at right
angles. As the action becomes more intense in the more argillaceous
beds, the angle of fracture, or cleavage, as it may now fairly be
called, becomes more acute, and in the most intense phase this cleaved
rock passes into a well-developed schist, the foliation of which is
parallel to the bedding. The phenomena of shearing are here therefore
very similar to those at Devil’s Lake, except that the process has gone
farther.
When studied in thin section, the massive beds of quartzite show more
decided effects of dynamic action than at Devil’s Lake. However, the
major portions of the grains of quartz have distinct cores which are
often beautifully enlarged. In some cases nearly every grain has thus
grown, perfectly indurating the rock. But, also, nearly every grain of
quartz has a wavy extinction, and many of them have been fractured,
as mentioned of a few of the quartz grains of the quartzites of the
south range. In one case the pressure has been so great as to produce
rather numerous roughly parallel lines of fracture. It is thus seen
that the dynamic effects are not confined to the schist zones, but
are also prominent within the heavy beds of quartzite. This was to
be expected; for while the major part of the accommodation necessary
to bend the rock mass as a whole took place along the shear zones,
the accommodation required to bend each of the rigid heavy beds of
quartzite must have taken place within each layer. To the consequent
intense pressure and the rubbing of the grains over one another, are
wholly attributed their wavy extinction and fractures.
In the schists of the shear zones, as at the south range, the thin
sections show that the original quartz grains were small; interstitial
material was present, and mica has developed more largely than in the
quartzite. However, in the most crystalline phases, the fragmental
cores of the quartz grains and their frequent enlargements are plainly
seen. Thus the shearing has not been sufficient to produce a completely
crystalline schist, although this would not be macroscopically
discovered, unless it were suspected because the rock is not thinly
foliated.
As the dip of the quartzite is so steep at this locality, it is
difficult to say how far the shifting of the beds over one another
lessens the apparent thickness. The shear zones as well as the friction
conglomerates appear to be parallel to the bedding. If they are exactly
so, this shearing action would necessitate an estimate of the original
thickness greater than now shown, since the shear zones probably have
less width at the present time than the beds from which they were
originally produced.
Cutting the bedding are heavy joints inclined to the north at an angle
of 20° to 30°. If slipping had occurred along these in the right
direction, this might cause a small thickness of beds to have a great
apparent thickness. However, the schists above described weather out
on the face of the cliffs, and are therefore marked by recessions in
the walls. If slipping parallel to the jointing had occurred since the
schists were formed, these depressions ought not to match on opposite
sides of the joints; but, on the contrary, they continue unbroken from
foot to top, and probably the joints were formed simultaneously with or
later than the belts of schist. Consequently, at the upper narrows of
the Baraboo no evidence was found of faulting which could reduce the
estimated thickness of the quartzite as given by Irving.
As Irving clearly saw, bearing strongly in favor of the theory of a
great fold, is the increasing steeper dip of the layers in passing
north. The phenomena of movement and metamorphism corresponding so
exactly to those required by a simple fold, the question may be asked
if these are not evidence of some weight in favor of the general
correctness of Irving’s conclusion as to the structure. Had monoclinal
faulting extensively occurred, it would not have been necessary to have
had so great a readjustment of the beds as has been shown to occur by
the schists, cleavage, and the exceedingly intricate macro-fracturing
and micro-fracturing of the rock beds and their constituent particles.
In addition to the phenomena described by Irving, in summary, the
Baraboo quartzite ranges show results of dynamic metamorphism as
follows: A fine example of the Reibungs Breccia may be seen. A fault
zone of limited throw exists. All phases are exhibited, between a
massive quartzite, showing macroscopically little evidence of interior
movement through a rock exhibiting in turn fracture and cleavage, to
a rock which macroscopically is apparently a crystalline schist. The
foliation of the schists is parallel to the original stratification,
being consequent upon the movements of the beds over one another,
readjustments occurring mainly in the softer layers. In thin sections
the schists still give clear evidence of their fragmental origin,
but also show the mechanical effects of interior movement. These
same effects are apparent within the heavy beds of quartzite, some
readjustment of the particles to their new positions being here also
necessary. There is no evidence that the semi-crystalline character
of the schist and quartzite are due to high heat. Nowhere are the
particles fused. So far as they are destroyed it is by fracture, and
the rock is again healed by cementation.
The rock, in its most altered condition being a semi-crystalline
schist, and in other parts showing less change, can be connected with
its original state. Had the folding been more intense, it is reasonable
to suppose that the entire rock would have been transformed into a
completely crystalline quartz-schist, showing no evidence of clastic
origin, and possibly the foliation throughout would have corresponded
to the original bedding.
C. R. VAN HISE.
FOOTNOTES
[8] The Baraboo Quartzite Ranges, by R. D. Irving. In Vol. II,
Geol. of Wis., pp. 504–519.
[9] The Classification of the Early Cambrian and pre-Cambrian
Formations, R. D. Irving. In 7th Annual Rep., U. S. G. S., pp.
403–408.
[10] Enlargement of Quartz Fragments and Genesis of Quartzites, by
R. D. Irving and C. R. Van Hise. In Bull. 8, U. S. G. S., pp.
33, 34.
[11] The Baraboo Quartzite Ranges, by R. D. Irving. In Vol. II,
Geol. of Wis., pp. 510, 516.
[12] The Baraboo Quartzite Ranges, by R. D. Irving. In Vol. II.,
Geol. of Wis., p. 516.
THE CHEMICAL RELATION OF IRON AND MANGANESE IN SEDIMENTARY ROCKS.
Iron and manganese are frequent constituents of sedimentary rocks,
in some places occurring finely disseminated through sandstones and
shales, or forming a part of limestones, in other places forming
the mass of the deposit in which they occur. They are both derived
primarily from similar, and often from the same sources, and are in
many respects alike in their chemical behavior in nature. For these
reasons it is to be expected that they would frequently, if not
generally, be deposited in intimate association. Such is found to be
the case, and iron and manganese are often closely associated in the
same deposits. Very often, however, iron and manganese deposits occur
close together, but distinctly separated, while sometimes extensive
deposits of iron, and less commonly of manganese, occur with little or
almost no association with each other.
It is the object of the present paper to discuss the agencies which
are instrumental in causing these substances to be deposited sometimes
together and at other times separately. The subject is of interest as
showing how slight differences in the chemical behavior of their salts
may cause the almost complete separation of metals once intimately
associated.
THE CONNECTION OF IRON AND MANGANESE IN NATURE.
A few words concerning the relation of manganese to iron in nature
will perhaps make the following discussion clearer. One of the most
common modes of occurrence of manganese is with iron, though extensive
deposits containing manganese more or less free from iron often
occur. When associated with iron, manganese occurs with it in various
ways. Sometimes the two are intimately mixed, so that they have the
appearance of a homogeneous mass, resembling iron ore when iron is
in the preponderance and manganese ore when manganese predominates.
In such cases there appears to be no tendency to combine in one fixed
proportion, though, as iron is a much more abundant substance than
manganese, the mixture most commonly contains an excess of iron, and
exists in the form of a manganiferous iron ore. The manganese, when
not intimately mixed with the iron, may occur in it in pockets or as
scattered nodules and concretions. Such occurrences as those described
are frequent in the Lake Superior iron region, the Appalachian Valley
of the eastern states, in Nova Scotia, Arkansas, Colorado, New Mexico
and innumerable other places. In Virginia very common occurrences are
alternating layers of iron and manganese ore. The iron in such cases is
generally in the larger quantities and the more continuous deposits;
while the manganese is often represented by thin lenticular layers or
by bands of nodules.
From such cases, where iron predominates, there are all gradations
in admixture, up to the rarer cases where manganese predominates.
Frequently a given geologic horizon is characterized by both iron and
manganese, though in one case it may contain only iron, in another
only manganese, and in still another iron and manganese mixed in
various proportions. A remarkable case of this is seen in the iron
and manganese horizons immediately above, or a short distance above,
the Paleozoic quartzite, on the east side of the Appalachian Valley,
especially in the Valley of Virginia.[13] Here deposits of iron
ore, of manganese ore, and of both ores mixed, are found at various
points along the same geologic horizons. Similar alternations also
occur in the Lower Silurian novaculites of the Ouachita Mountains of
Arkansas,[14] in Cebolla Valley, in Gunnison county, Colorado,[15] and
in many other places. In many cases certain horizons are characterized
over large areas by iron alone, and but little manganese, as is well
seen in the Clinton formation and in the Tertiary iron-ore horizons of
Arkansas and Texas; while, on the other hand, some areas of certain
horizons contain considerable quantities of manganese and very little
iron, as is seen in parts of the Marine limestone in New Brunswick and
Nova Scotia, and also in parts of the metamorphosed Cretaceous shales
of California.
THE SOURCE OF IRON AND MANGANESE IN SEDIMENTARY ROCKS.
The iron and manganese contained in sedimentary strata may be
considered as derived primarily from the decay of pre-existing rocks.
Some of the later sedimentary rocks may have derived a part or all of
their iron from older sedimentary rocks, which, in turn, had derived
their iron and manganese from still older rocks. In this way the iron
and manganese in a given geologic horizon may have formed a part of
various older horizons before they reached their present resting place,
but, in every case, their primary source can be traced back to the
original materials from which sedimentary rocks were first formed.
In certain cases the sea water has supplied a certain amount of iron
and manganese to sedimentary rocks, but in such cases the sea water
acts only as a carrier of these materials from the land areas or from
submarine sources to the strata then forming.
THE TRANSPORTATION OF IRON AND MANGANESE IN NATURE.
The process that goes on in this interchange of iron and manganese from
older to younger rocks is as follows:
(1) The conversion, by surface agencies, of the minerals containing
iron and manganese into forms that can be taken into solution by
surface waters.
(2) The solution of the iron and manganese in surface waters,
acidulated with organic and sometimes inorganic acids, and their
transportation in this form from the areas of older rocks to areas over
which younger rocks are being deposited.
(3) Finally, the precipitation in one or more of several ways of the
iron and manganese contained in solution.
The iron and manganese thus chemically precipitated may be deposited
either with mechanical sediments, such as sand, clay etc., or without
them. If the deposition of mechanical sediments is largely in excess
of the precipitation of iron and manganese, the final products will be
beds of ferruginous shale, sandstone, etc., common in many geologic
horizons. If the precipitation of iron and manganese is in excess of
the deposition of mechanical sediments, the resulting products are
deposits of more or less pure iron and manganese ore. Between these
two extremes there are all gradations in the admixture of the iron and
manganese with mechanical sediments.
Frequently the iron and manganese which were originally finely
disseminated through shale, sandstone, etc., are subsequently
concentrated into bodies of comparatively pure ore, and very commonly
this concentration takes place by a process of re-solution of the iron
and manganese and re-deposition by replacement with limestone, or,
more rarely, with some other material. The limestone or other material
which thus acts as a precipitant is often in the same series of strata
from which the iron and manganese were removed, and thus these two
substances, which were once in a finely disseminated condition, may be
converted into deposits of comparatively pure ore and yet remain in the
same general series of strata in which they were originally deposited.
A remarkable case of this is seen in the iron deposits of the Penokee
series in Michigan and Wisconsin,[16] to be mentioned again on page
370. It has also been suggested by H. D. Rogers[17] that certain
siderite deposits in the Coal Measures were formed by the conversion
of finely disseminated sesquioxide of iron into carbonate of iron by
organic matter, and the subsequent segregation of the carbonate as now
found in layers and nodules.