BULLETIN OF THE UNIVERSITY OF WISCONSIN

SCIENCE SERIES, VOL, 1, No. 2, pp. 35-56, PLS. 1-3.

ON THE QUARTZ KERATOPHYRE AND ASSOCI-
ATED ROCKS OF THE NORTH RANGE OF
THE BARABOO BLUFFS.

BY

SAM

PUBLISHED BY AUTHORITY OF LAW AND WITH THE APPROVAL OF

THE REGENTS OF THE UNIVERSITY

MADISON, WIS.

PUBLISHED BY THE UNIVERSITY
JANUARY, 1895

PRICE 25 CEHTS

<£0tn«tttiee of ^publication

CHARLES KENDALL ADAMS, PRESIDENT OF THE UNIVERSITY

EDITORS

GEORGE L. HENDRICKSON, Philology and Literature

WILLIAM H. HOBBS (Chairman), Science

DUGALD C. JACKSON, Engineering

FREDERICK J. TURNER, Economics, Political Science, and History

Democrat Printinf Company, SUta Printer

BULLETIN OF THE UNIVERSITY OF WISCONSIN
SCIENCE: SERIES, VOL. 1, No. 2, PP. 35-56, PLS. 1-3, JANUARY, 1895

ON THE QUARTZ KERATOPHYRE AND ASSOCIATED

ROCKS OF THE NORTH RANGE OF

THE BARABOO BLUFFS.1

BY SAMUEL WEIDMAN.2

Limestone.

Sandstone and conglomerate.

CONTEXTS.

I. Introduction.

II. Field Geology.

«,**« vv*-.^*^ .— v— «v,v.

Quartzite.

Quartz keratophyre eruption: its areal extent,
contact, absence of bedding.

Types of the quartz keratophyre: quartz kerato-
phyre, sericite schist, volcanic breccia.

Conclusions from field study.

III. Microscopic Geology.

Feldspar phenocrysts, quartz phenocrysts, ac-
cessory minerals.

Groundmass: fluxion, poikilitic, and spherulitic
structures.

General conclusions.

In the south central part of Wisconsin, extending across
the middle portions of the county of Sauk and for a short
distance into that of Columbia, there arises above the surface
of the surrounding area two long ranges of hills or ridges.
These ranges trend across the country in an east and west

1 A thesis submitted for the degree of Bachelor of Science in the Geology Group of the
General Science Course, University of Wisconsin, June, 1894. Read before the Geological
Society of America at the Baltimore meeting, December, 1894.

2 This paper is the result of work undertaken and executed under the direction of Pro-
fessors W. H. Hobbs and C. R. Van Hise. To the former I desire to express my thanks
for superintending the work in the laboratory, for preparing the photo-micrographs, and
for other valuable help, and to the latter for giving me aid and suggestions in the field
study. I also desire to express my obligation to Mr. C. F. Austin who kindly furnished
the chemical analysis for this paper.

36 BULLETIN OF THE UNIVERSITY OF WISCONSIN

direction for a distance of nearly thirty miles, and are
known as the north and south ranges of the Baraboo
Bluffs. In altitude they vary from a mere rise above
the surrounding country to a height of five and even six
hundred feet.

The north range, which is not so prominent as the south
range, is joined to the latter at the eastern and western ex-
tremities, forming thus a canoe-shaped chain of bluffs, en-
closing within a depressed area three or four miles broad
at its widest part. Near the western end, where the
Baraboo River enters the area, and also near the eastern
end, where it emerges again, the north range is broken
down by deep gorges known respectively as the Upper and
the Lower Narrows.

The rock material of the Baraboo Bluffs is mainly of
bedded quartzite, over whose upturned edges lie horizontal
beds of sandstone and conglomerate, capping and flanking
the ranges. Besides these sedimentary rocks there occurs
in the vicinity of the Lower Narrows a considerable area of
eruptive material.

The geology of the sedimentary rocks 1 has already been
quite well worked out. The quartzite, which belongs to
the Upper Huronian formation, is usually hard and
massive, but in a few places there occur beds and zones of
quartzite schists and slates. The dip of the quartzites is
always to the north, and varies from 15° at Devil's Lake
in the south range, to 60° at the Upper Narrows and
even 90° at the Lower Narrows in the north range. This
ever increasing dip from the south toward the north in-
dicates, as shown by the Wisconsin geologists,2 that the
two ranges are the remnants of the north half of a great
anticlinal fold. The sandstone and conglomerate, as well
as a few small areas of limestone, -all of which are of Up-
per Cambrian age, lie in horizontal beds capping the ranges.

i Correlation Papers — Archean and Algonkian, by C. R. Van Hise. Bulletin 86, U. S.
Geological Survey; pp. 105-107, 111, 140, 148. t
t. The Baraboo Quartzite Ranges, by R. D. Irving. Geol. of Wis., Vol. II; pp. 506, 507.

WEIDMAN — QUARTZ KERATOPHYRE OF BARABOO BLUFFS 37

The eruptive rock, which was described as quartz por-
phyry by the state geologists, is a quartz keratophyre,
and occurs in contact with the quartzite along the north
side of the north range in the vicinity of the Lower Nar-
rows. This rock was not discovered until 1874 or 1875 by
Irving and his co-workers upon the Wisconsin Geological
Survey, and all results of work done upon this formation
are embodied in the state geological reports.

Irving * in 1877 described the porphyry as occurring on
the west side of the Baraboo River at the Lower Narrows.
He considered the porphyry to be very distinctly bedded,
with an east and west strike and a dip of 58° to 60° to the
north. He also found that near the quartzite it changes
to a distinct schist, which he thought to be allied to the
greasy quartz schists at Devil's Lake. Farther west at the
northern limit of the porphyry, he found the much
fractured area. A schistose structure, which is apparent
in places, was taken as bedding and as evidence of the
clastic origin of the* rock. A specimen was analyzed and
found to contain 71.24 per cent, of silica and a notably
large quantity of soda as compared with potash.

Chamberlin,2 in ,1882, mentioned the massive quartz por-
phyries which overlie the Baraboo quartzites and referred
to their origin as yet in doubt. According to him they
might be either a metamorphosed silt-like sedimentary
rock; or they might be of eruptive origin, in which case
they must have constituted immense overflows of molten
rock closely allied in chemical composition to rhyolite.

Irving,3 in 1886, in referring to the Baraboo quartzites,
stated that " the bedding structure of the quartzite of these
ranges and of its associated schists and felsitic porphy-
ries, which are taken to have been great eruptive flows, I
studied with a great deal of care a number of years since
*." It is evident from this that he was inclined to

i The Baraboo Quartzite Ranges, by R. D. Irving. Geol. of Wis., Vol. II; pp. 513-515.
8 General Geology of Wisconsin, by T. C. Chamberlin. Geol. of Wis., Vol. I; p. 87.
8 On the Classification of* the early Cambrian and pre-Cambrian Formations, by R. D.
Irving. 7th Ann. Rept. U. S. Geol. Surv., 1885-6; p. 407.

38 BULLETIN OF THE UNIVERSITY OF WISCONSIN

change his opinion as to the origin of the quartz por-
phyry. He, however, made no re -examination of the
area, and it must be supposed that he was led to change
his views on account of his knowledge of the extensive
eruptive areas in the Lake Superior region.

II. FIELD GEOLOGY.

The accompanying map, Plate 1, shows the relations and
extent of the different rock formations which occur in the
vicinity of the Lower Narrows. Four distinct types of
rock are seen to outcrop, representing the quartz kerato-
phyre, Upper Huronian quartzite, Potsdam sandstone and
conglomerate, and the Lower Magnesian limestone. The
cross sections of Fig. 1 cross the range, and in a meas-
ure show the structural relations and vertical extent of
these formations. For the purpose of convenience the
reverse order will be taken in describing these formations.

LIMESTONE.

On the south side of the range, and near its summit, in
the northwest £ of Sec. 25, is a ledge of dolomitic lime-
stone overlying the sandstone. It is non-arenaceous, and
contains many small cavities lined with calcite. In determin-
ing its stratigraphical position, one is met with difficulties
owing to the peculiar fluctuations in level of the horizontal
formations in the Baraboo region, but from the Dikello-
cephalus fauna which it carries, and from its lithological
characters, it has been considered ' as being at least not
below the Lower Magnesian.

SANDSTONE AND CONGLOMERATE.

Overlying both the older formations of quartzite and
quartz keratophyre there occur capping and flanking the

i Geol. of Wis., Vol. II; pp. 594, 595.

WEIDMAN — QUARTZ KERATOPHYRE OF BARABOO BLUFFS 39

range horizontal beds of sandstone and conglomerate of the
Upper Cambrian epoch. The conglomerate is composed of
pebbles and boulders, varying in size from a fraction of an
inch to more than a foot in diameter, imbedded in a matrix
of quartz grains, stained more or less with ferruginous
material. The conglomerate is found in great abundance
in the ravines.

The sandstone which carries the Dikellocephalus fauna is
interbedded with and overlies the conglomerate. It is com-
posed of rounded quartz crystals in a matrix of silica, con-
taining considerable oxide of iron. The sandstone caps
the ridges in thin beds, but along the sides of the range, as
in the area of cross- section V of Fig. 1 (see also Plate 1),
it reaches a thickness of forty-five or fifty feet.

QUARTZITE.

The quartzite is a hard compact rock, and has a reddish
purple hue, with a tendency to granular texture. The
bedding is not everywhere distinct, but in places it is
quite plain. At the contact of the quartz keratophyre
along the ridge the bedding dip conforms to the dip of the
overlying keratophyre schists, but farther south, at the
top of the ridge, the dip is increased to 90°, and some-
times apparently dips to the south. On the south side of
the range, however, it again dips 75° or 80° to the north.
Everywhere the quartzite is jointed, and in many places
the rock is seamed with reticulating veins of pure quartz,
in which, at times, are to be seen small particles of spec-
ular iron ore.

QUARTZ KERATOPHYRE.

Areal Extent. — From the map it is seen that quartz kera-
tophyre extends along the north face of the range for a dis-
tance of over three and one-half miles. Its most eastern
limit is in the northeast corner of the southeast i of Sec.
23, and its most western is found in the northeast i of
the northwest J of Sec. 20, thus extending along the range

40 BULLETIN OF THE UNIVERSITY OF WISCONSIN

in a slightly north of west direction. In Sections 16 and 21
the thickest portion of the eruptive mass is found, for
its contact with the quartzite at this place is distinctly
shown near the northern boundary of the southeast J of
Sec. 21, from whence it extends north forming a broad ridge
cut by ravines, with its northern limit near the middle of
the southeast £ of the southeast i of Sec. 16.

It is apparent that Irving did not find the keratophyre
extending as far east or as far west as a closer study of the
area reveals, for it is described as only reaching from the
Narrows to the south side of Sec. 16.

The Contact. — The contact between the eruptive rock and
the quartzite is well defined, except where a bed of soil
overlies it. At the eastern end of the area in Sees. 22 and
23, where the keratophyre occurs as a narrow strip, the
quartzite is of a higher altitude than the keratophyre, and
stands out as a vertical cliff from ten to thirty feet high.
In the region from the center of Sec. 21 eastward, the con-
tact is easily made out, and it is seen that its direction
conforms very closely with the strike of the underlying
quartzite, which is slightly south of east. From the cen-
ter of Sec. 21 across the greater part of the northwest i
of this section, the contact was not seen, but it was clearly
made out at the boundary of Sees. 20 and 21, near the
southwest corner of the northwest i of the northwest
J of Sec. 21. Prom this place eastward the actual con-
tact was not seen, but many schistose blocks of keratophyre
were found near the outcropping quartzite, and as the
keratophyre was found in situ in the northwest i of the
northwest i of Sec. 20, the contact is thought to be very
near the line indicated upon the map. The eruptive rock
at the contact occurs as schist, but the quartzite forms
massive beds, and is fresh and unaltered. Dikes and veins
from the eruptive rock were closely searched for the whole
length of the ridge at the contact, with the thought that
the eruptive mass might be intrusive and not extrusive,
but no such phenomena were found to occur.

TJKIVEB.SITT

WEIDMAN — QUARTZ KERATOPHYRE OF BARABOO Bl

~ >ir.

ll

o >_
J§ P3

is

§.8
p

42 BULLETIN OF THE UNIVERSITY OP WISCONSIN

The contact between the quartz keratophyre and the
Cambrian sandstone and conglomerate is well shown in the
northwest J- of Sec. 21. At this place a narrow strip of
the conglomerate projecting from the large sandstone area
to the west, lies in horizontal beds upon the dipping edges
of the sericite schist (Fig. 1, Sec. IV). These schists dip
to the north at an angle of 75° or 80°, essentially the same
as those farther south at the contact with the quartzite.
Farther west from this tongue of conglomerate, in a ravine
which marks the eastern boundary of the massive sand-
stone area, the contact is again well shown, the horizontal
sandstone lying upon the upturned edges of the sericite
schist.

Absence of Bedding. — This rock does not, like the quartz-
ite, exhibit stratification, but on the contrary all traces of
bedding are entirely wanting. It does, however, at the
contact form a zone of schists from 150 to 200 feet in
width, and also in a considerable area in the north-
west i of Sec. 21, schists are found. The cleavage
planes of these schists always dip to the north at an angle
of 75° or 80°. Irving l in describing this area, speaks of
the schists, and whenever he does so he invariably men-
tions the clearly defined bedding which the rocks show.
Although he was one of the first geologists who later
learned to recognize the importance of distinguishing
schistosity from stratification, it is evident that he com-
mitted the common error of taking the one for the other
in this region.

Types of Rock. — The eruptive rock as it appears in the
field exhibits marked local variations, due both to condi-
tions in the eruptions of the magma and to dynamic action
since the general outflow. These differences allow a classi-
fication of the rock into three distinct types, which will be
described separately. These types are normal quartz kerat-
ophyre, sericite schist, and volcanic breccia.

i Geol. of Wis., Vol. II; pp. 513-515.

WEIDMAN — QUARTZ KERATOPHYRE OF BARABOO BLUFFS 43

Quartz keratopliyre. — The quartz keratophyre, which forms
the normal rock type of the whole eruptive area, as it oc-
curs in the narrow strip at the eastern extension of the area
is not much fissured, but is jointed and cut by reticulat-
ing veins of quartz. It forms rounded ledges and grades
into the sericite schist zone, which lies to the south of it.
The rock1 is very fresh, and breaks with a conchoidal frac-
ture. It is red in color, containing in about equal num-
bers many beautiful red and white feldspar phenocrysts,
some of which are three-eighths inches in diameter. These
are imbedded in a brownish red matrix. The rock on the
eastern side of the Lower Narrows contains fewer pheno-
crysts than that on the western side. Both phenocrysts
and groundmass become red on weathering.

Farther west, at the large northward projecting ridge
which occurs in the northeast i of Sec. 21 (Fig. 1, Sec.
Ill), the keratophyre 2 is found to be unlike that farther
east, in that it is much fractured, and the feldspar crystals
are rarely apparent in the hand specimen. Reticulating
quartz veins from a fraction of an inch to three or four
inches in thickness are quite numerous. The fractures
which cut the rock run in all directions, so that it breaks
and weathers in small fragments, bounded on all sides by
plane surfaces. On the weathered surface the rock is a
reddish brown, but within it is of a darker hue.

Within the eruptive area are many detached blocks of
quartz keratophyre. Some of these blocks are quite differ-
ent from the quartz keratophyre found in place. A de-
tached block 3 which was found upon the summit of the
range in the northwest J of Sec. 21, unlike the quartz
keratophyre found in situ, is black in color, and on close
examination it shows fluxion structure. Several blocks of
similar black colored quartz keratophyre were found near

i Specimens 3089 and 3090. The specimen and thin section numbers referred to in this
paper are those of the University of Wisconsin Collection.
8 Specimens 3075 and 3092.
s Specimen 3080.

// f ^^ y*

44 BULLETIN OF THE UNIVERSITY OP WISCONSIN

the exposure of volcanic breccia at Mr. Hewitt's place in
the northeast i of the northwest i of Sec. 21.

Specimens from some of these blocks have a large num-
ber of small phenocrysts while others show but few.
These phenocrysts are a light colored feldspar and usually
plagioclase. It is possible that this black quartz kerato-
phyre has its origin in the porphyry areas to the northeast,
but it may be that a closer examination of the area would re-
veal it in place.

Sericite Schist. — Wherever the quartz keratophyre comes
in contact with the quartzite, there occurs a zone of sericite
schist from 150 to 200 feet wide. These schists are a dy-
namic alteration of the quartz keratophyre and they grad-
ually pass into the latter, and hence are not, as Irving l
supposed, closely related to the magnesian schists which
occur at Devil's Lake. The schists vary in color from the
reddish brown of the quartz keratophyre to a grayish white.
In texture they are quite friable and are easily cleaved.
They carry a considerable number of feldspar crystals,
some of which have weathered out leaving cavities that
are usually lined with iron oxide. In some specimens iron
pyrite is seen. Some of the grayish white schists contain
many greenish brown areas of cherty-like material, which
the microscopic examination shows to be secondary quartz,
the rock being a pseudo-breccia.

Besides the schists at the contact, there occurs within
the area of quartz keratophyre in the north half of the
northwest J of Sec. 21, a large area of schist (Fig.l, Sec.
IV), which is capped by sandstone and conglomerate, and
which, like the schist at the contact, has a dip of 75° or
80° to the north. In all respects it is like the schist at
the contact, except that feldspar crystals in the hand speci-
men are rare.

Volcanic Breccia. — There occur within the eruptive area
several good examples of volcanic breccia. Within the

iGeol. of Wis., Vol. II; page 513.

WEIDMAN — QUARTZ KERATOPHYRE OF BARABOO BLUFFS 45

contact zone of schists in the southwest J of the northeast i
of Sec. 22, there is an outcrop of volcanic breccia l frag-
ments in which usually have the typical flow structure
about them. A similar exposure 2 occurs at the contact
near the center of Sec. 22. The included fragments in the
rock of both the above localities are of a hard pink rock, and
unlike the schist in which they are imbedded they are rel-
atively unaltered. In the field these fragments were taken
to be a pink quartzite, but the microscopic slides show them
to be an acid porphyry.

The best exposures of breccia,3 however, were found on
the north side of the road opposite Mr. Hewitt's house, in
the northeast i of the northwest J of Section 21. This
outcrop exhibits a schistosity, the dip of the cleavage
planes being 75° to the north and the strike a little north
of east. The fragments forming this breccia are much
larger than those found in the other schists and are usually
angular and elongated in the plane of schistosity. They
vary in size from an inch to a foot in diameter, and were at
once taken to be of eruptive origin.

The fragments of this breccia represent several rock
varieties. One is brownish pink in color and is composed
almost wholly of secondary spherulites which are from
two-eighths inch to three-eighths inch across. On the
surface of the exposure these secondary spherulites have
weathered out, leaving cavities lined with quartz, which
makes them appear much like amygdaloidal cavities. An
other variety is a brownish gray schistose rock which
contains many feldspar phenocrysts, some of which have
weathered out leaving cavities filled with black ferruginous
material. Other fragments are composed of a dull lead
colored rock, slightly schistose and having no phenocrysts.

A large number of loose blocks * was found in the road

1 Specimens 3084 and 3085.

2 Specimen 3081.

8 Specimens 309 i, 3094.
4 Specimen 3095.

46 BULLETIN OF THE UNIVERSITY OF WISCONSIN

near the house of Mr. Montgomery, in the southeast i of
the southeast J of Section 17. This rock on its weath-
ered surface shows very clearly the flow structure about the
included fragments. Unlike the fragment-bearing rock in
situ this is not markedly schistose, is not much altered,
and the included fragments are few. The weathered sur-
faces seem to give evidence of a few large and widely sep-
arated spherulites, but none of these were included in the
sections which were prepared. An outcrop of fresh brec-
cia l in the northwest £ of the northwest i of Sec. 21,
shows numerous phenocrysts of feldspar intermingled with
small rock fragments.

Conclusions Drawn from Field Study. — The work in the
field shows the areal extent of the eruptive rock to be some-
what greater than was formerly supposed. A marked
difference is found in the rock as it outcrops in various
places. The contact closely conforms in direction for the
greater part of the distance, with the strike of the under-
lying quartzite beds and the dip of the cleavage planes
of the contact schists agrees with the dip of the quarlzite.
Volcanic breccia occurs in considerable quantity in various
localities. All the phenomena observed during the field
study are evidence that the porphyry in its several phases
represents a volcanic outflow which took place over beds
of quartzite. During the elevation which followed, the
overlying eruptive rock was in part metamorphosed into
schist.

III. MICROSCOPIC GEOLOGY.

The structure of the rock, as it is presented by the
microscopic sections in the laboratory confirms the evidence
gathered in the field for the extrusive character of the
quartz keratophyre. The porphyritic structure is developed
in all the types of rock, being well shown in the volcanic

i Specimen 3079.

WEIDMAN QUARTZ KERATOPHYRE OF BARABOO BLUFFS 47

breccia and sericite schist as well as in the unaltered
quartz keratophyre. The porphyritic minerals are chiefly
the feldspars, quartz occurring very rarely in phenocrysts.
The few that are found are intensely corroded, and the in-
dications are that they were once much more numerous,
the rock being at this earlier period mineralogically as it
is now chemically a quartz keratophyre.

Two analyses of the rock have been made, one by Prof.
W. W. Daniells for the State Geological Survey, the other
by Mr. C. P. Austin during the preparation of this paper.
Below are given the analyses of the Baraboo rock in com-
parison with quartz keratophyres from other localities.

I. II. III. IV. V.

71.9
13.4

)

t.6

)

.2

1.0

.2

12.6
.5

Si02

70.97

71.24

72.42

73.00

AI203

13.84

12.20

13.04

15.61

Fe203

3.21

1.71

.68

—

FeO

.78

5.44

2.49

1.95

CaO

1.26

.98

.66

.79

MgO

.20

.13

.58

—

K20

1.57

1.86

4.97

.88

Na8O

6.27

4.29

3.44

4.95

H2 O

.74

.81

1.21

1.06

CO2

.79

—

—

—

P2 05

.08

—

—

—

TiO3

.25

—

.40

—

MnO

—

.97

.09

—

BaO

—

—

.15

—

Li20

—

—

tr.

—

S03

—

—

—

.76

Total

99.96

99.63

100. 33

99.00

Sp. Gr.

—

—

2.62

2.63

48 BULLETIN OF THE UNIVERSITY OP WISCONSIN

I. Quartz keratophyre from Mtihlenthal zw. Elbingerode and Rtibeland,

Harz. (Jacobs ) l

II. Quartz keratophyre from Baraboo, Wis (Darnells.) 2

III. .Quartz keratophjre from Pigeon Pt., Minn. (Bayley.) 3

IV. Quartz keratophyre from Baraboo, Wis. (Austin )

V. Quartz keratophjre from Mt. Elizabeth, Australia. (Howitt.)4

It is seen that the Baraboo rock compares very well in
composition with the quartz keratophyre of other regions.
The large amount of soda as compared with potash in
this rock, as shown by the analyses, indicates the presence
of a feldspar whose composition corresponds with a soda-
orthoclase. The excess of soda over lime in the Baraboo
rock also indicates that the plagioclase present contains a
large per cent, of the Ab. molecule. The specific gravity
of the orthoclase and plagioclase as determined by the
Thoulet solution was found to be about 2.63, both varieties
being apparently the same. This high specific gravity of
the feldspars is explained by the lar^e amount of soda in
their composition, and in part, aliso, by the particles of
oxide of iron which they contain.

A macroscopic description of the several types of rock
as they appear in the different parts of the eruptive area
has already been given. As the microscope shows the
structure of these types to be essentially the same both as
regards the minerals of the first generation and the ground-
mass, the phenocrysts and accessory minerals will first be
described, and then the various structures of the ground-
mass.

Feldspar Phenocrysts. — Feldspar constitutes almost the
only porphyritic mineral. Soda-orthoclase and plagio-
clase are about equally abundant. Sometimes both va-
rieties are equally distributed in the same slide, and at
other times a single variety only is present. Microcline

1 Quoted by Zirkel, Lehrb. d. Petrog., 2te Aufl., Vol. II; p. 334.

2 Irving, Geol. of Wis., Vol. II; p. 515.

s Bayley, Am. Jour. Sci , (3) Vol. XXXVII; p 61.

* Howitt, Roy. Soc. Victoria, 1883, p. 25. Quoted by Rosenbusch, Min. u. petrog. Mitth.
Vol. XI; p. 177.

WEIDMAN — QUARTZ KERATOPHYRE OF BARABOO BLUFFS 49

is found to some extent as both large and small pheno-
crysts.

The phenocrysts vary in size from two- tenths millimeter
to four millimeters across. The crystals are more or less
altered to sericite, sometimes completely, but at other
times only slightly. The alteration takes place very often
at the boundary of the feldspar, and forms zones of sericite
surrounding the crystal. At other times the fine sericite
needles are promiscuously scattered throughout the phen-
ocryst. Very often the feldspar is replaced by secondary
quartz. At times ' it is only partly replaced, but very
often the angular shape which the secondary quartz as-
sumes indicates that there has been a complete replace-
ment of the feldspar.

Numerous beautiful examples were observed of bent
and broken crystals due to the motion of the magma after
the crystallization of the phenocrysts (Plate 2, Figs. 1
and 2). Broken phenocrysts are not uncommon phe-
nomena of effusive rocks. As the viscous streams of lava,
including the crystals of feldspar, flowed over the surface,
the motion within the mass bent and broke the pheno-
crysts. Sometimes they are merely cracked or broken,
but very often the broken parts are separated from each
other (Plate 2, Fig. 2), allowing a thin stream of ground-
mass to flow between the dismembered parts. It very
often happens that the crystals of feldspar, both the
broken and unbroken ones, have their corners rounded by
corrosion. The embayments in the feldspar shown by
some of the slides - show plainly this resorption.

Quartz phenocrysts. — This usually abundant mineral as a
primary constituent of the acid rocks, occurs very rarely
as phenocrysts in the Baraboo quartz keratophyre. In a
total of thirty-two slides examined from this region only
four were found to contain porphyritic quartz, and these
showed only a few crystals, all of which were of small

i Section 3084.1

a Sections 3079 and 3089.

50 BULLETIN OF THE UNIVERSITY OP WISCONSIN

size. All, however, show by their rounded appearance and
embayments, the resorption effects of a corroding magma,
and thus they indicate that they were once of larger size
and probably more numerous. Some1 of the quartz-pheno-
crysts, like those of feldspar, show the effect of a moving
magma by being cracked and broken.

The small number of quartz phenocrysts present would
apparently indicate that this rock is a keratophyre rather
than a quartz keratophyre. Mineralogically it is a kerato-
phyre; but the analyses show it to compare very closely
to quartz keratophyre in the amount of silica contained,
and the few corroded quartz crystals seem to indicate that
the rock was originally a typical quartz keratophyre, though
the silica per cent, has been somewhat increased by the
secondary quartz present.

Accessory Minerals. — Ilmenite is an abundant accessory
constituent occurring in small and large crystals. It is
commonly either partially or completely altered to leu-
coxene. The alteration to leucoxene along the gliding
planes of the ilmenite is well shown in one section.2 Some-
times the ilmenite has gone over to well crystallized sphene,
which is commonly mingled with more or less of the im-
pure variety. In some cases (Plate 3, Fig. 1) the crystals
of ilmenite are partly replaced by secondary quartz, the
latter occupying the space of the former, as in the replace-
ment of the feldspar crystals.

Biotite occurs in considerable abundance in a few of the
sections. It is crystallized into small blades which are
scattered promiscuously throughout the groundmass and in
longer blades 3 arranged in sheaves, or in aggregates which
seem to be about small cavities in the rock.

Zircon occurs to a small extent, sometimes being bounded
by its usual crystal planes. It is frequently associated with
the ilmenite.

i Section 3073.
3 Section 3080.
3 Sections 3079 and 3080.

WEIDMAN — QUARTZ KERATOPHYRE OF BARABOO BLUFFS 51

Magnetite occurs in considerable quantity as shown by
the action of the magnet on the powdered rock, and the
microscopic sections show it disseminated in small particles
throughout the groundmass. Besides the magnetite there
is considerable amorphous iron oxide or ferrite present,
which appears as a red substance about cavities l and in
streaks through the groundmass.

The Groundmass. — The groundmass of the quartz kerato-
phyre is holo-crystalline and composed of quartz and feld-
spar, stained more or less with oxide of iron. It is crys-
tallized in at least three structures which are common to
volcanic rocks, viz. ; the fluxion structure, the poikilitic
structure, and the spherulitic structure.

Fluxion structure. — In all the sections examined under
the microscope the rock shows clearly sinuous lines of flow
in the groundmass. These lines of flow curve and wind
about phenocrysts, and give them the appearance of eyes.
They also curve about the fragments and between the
broken parts of fragments and phenocrysts. The frag-
ments about which flowage is apparent in the hand speci-
men have also a flow structure of their own and in one
case 2 a fragment within a fragment showed the typical
flow lines under the microscope. One section 3 (Plate 3, Fig.
2), shows quite well the flow structure in a volcanic breccia,
in which the lines of flow in the fragments are at right angles
to those in the surrounding mass. In the sericite schists
the fluxion structure is clearly defined, which as well as
the field relations proves them to be the metamorphosed
equivalents of the quartz keratophyre.

Poikilitic Structure. — This structure, which has recently
been described by Haworth,4 Williams,5 and others, is quite

1 Section 30T6.
'Section 3084.1
•Section 3096.

4 A Contribution to the Archean Geology of Missouri, by Erasmus Haworth. Am
Geol., Vol. I; p. 368.

5 On the use of the terms Poikilitic and Micropoikilitic in Petrography, by (i. H. Wil-
liams. Jour, of Geol., Vol. I; pp. 176-179.

52 BULLETIN OF THE UNIVERSITY OF WISCONSIN

common in acid volcanic rocks, and occurs to some extent
in these rocks. This structure is not apparent in ordi-
nary light, but in polarized light on revolving the stage,
small areas in the groundmass appear alternately dark
and light in patches, giving a mottled appearance. These
areas are composed of irregular grains of quartz in close
contact with feldspar. They do not have sharp extinction
individually, but gradually pass into one another as the
stage is revolved. Sections1 taken from the fractured rock
show this structure better than the others.

Splierulitic Structure. — The spherulitic structure is well
developed in many of the sections examined. Some of the
spherulites are a primary crystallization and are composed
of radial fibres of feldspar associated with quartz, and
occur in small circular and semicircular areas,2 and in larger
fan-shaped forms3 (Plate 2, Fig. 2), like those described
by Iddings4 from the Yellowstone Park rhyolites. They
are scarcely perceptible in ordinary light, but in polarized
light the usual distinct dark cross is seen. Besides
those showing the radiating structure there also occur
some spherulites which have a granular texture, and are
of secondary origin. Similar secondary spherulites have
recen ly been described 5 from the acid volcanic rocks of
South Mountain. In the Baraboo quartz keratophyre these
secondary spherulites as they appear in the thin section
are much larger than those of primary origin. In the hand
specimen, too, the only sphe^ ilites found are the secon-
dary ones. One of the specimens6 shows many such spher-
ulites one-fourth inch across, with lines of flowage passing
around them. Under the microscope these spherulites7 ap-

i Section 307^ .
« Section 3074.
•Section 3080.

* Obsidian Cliff, Yellowstone National Park, by J. P. Iddings. 5th Ann. Rep. U. S.
Geological Survey; pp. 276-279.

6 The Structures, Origin, and Nomenclature of the Acid Volcanic Rocks of South Moun-
tain, by F. Bascom. Jour, of Geol., Vol. I; pp. 817-819.

8 Specimen 3094.

7 Stctions 3085, 3094a and 3094b.

WEIDMAN — QUARTZ KEEATOPHYRE OF BARABOO BLUFFS 53

pear to be composed of grains of quartz of smaller size than
that which replaces the feldspar, and which is otherwise
aggregated in the groundmass. Within the secondary
spherulite, too, the quartz seems to be associated with
sericite and iron oxide, whereas the secondary quartz other-
wise aggregated is free from these minerals. One section1
which contains secondary quartz partly surrounding one of
the secondary spherulites shows very plainly both in or-
dinary and polarized light the difference between the two
structures.

The aggregations and general appearance of the second-
ary quartz in the groundmass is of considerable interest in
these rocks. The secondary quartz of the groundmass
most generally forms interlocking areas of grains of var-
ious sizes, and it is possible that much of it may be chal
cedony. Large reticulating veins of quartz up to two inches
in width have been described as occurring in large numbers
in the eruptive rock in the field. The thin sections like-
wise show many minute veins of secondary quartz ramify-
ing throughout the rock. The quartz often occurs in an-
gular forms, when it very probably is a replacement of
ilmenite and feldspar. The spherical shape which the sec-
ondary quartz assumes is a very noticeable and interest-
ing feature (Plate 3, Fig. 1). These spheres vary in size
from one-tenth of a millimeter to one and five-tenths mil-
limeters across, and usually have an elliptical outline, with
the boundary sometimes slightly crenulated. The quartz
in these spheres differs from that in the secondary spher-
ulites in being of coarser grain, and in being also unasso-
ciated with sericite and iron oxide. Some of the sec-
tions,2 show very large areas of secondary quartz. This
secondary quartz is also apparent in the hand specimen,
where it appears in areas about an inch in diameter resem-
bling greenish brown chert, and the rock is considered a

i Section 3085.
8 Section 3077.

54 BULLETIN OF THE UNIVERSITY OF WISCONSIN

pseudo- breccia. These secondary quartz areas, as seen
under the microscope, appear to be composed of much finer
grains of quartz than that which is aggregated in spheres
and angular shapes. The groundmass outside of these
areas shows the fluxion structure, is not mingled with the
fine-grained secondary quartz, and there is no character-
istic arr angement of the minerals at the boundary between
the two.

CONCLUSION.

The eruptive rock of the Baraboo region is thus shown
by chemical analysis to be a quartz keratophyre, whereas
its mineralogical composition alone would class it rather as
a keratophyre. Typical volcanic structures of the ground-
mass are clearly shown. The broken phenocrysts and the
marked fluxion structure indicate the usual motion of a
lava flow. In the field are found outcrops of volcanic brec-
cia, some of the fragments of which are likewise volcanic
breccia. The quartz keratophyre, like the bedded quartz-
ite, lies unconformably below the horizontal formations of
the Upper Cambrian. The dip of the cleavage planes of
the schists conforms to the dip of the underlying quartz-
ite beds, which is at a high angle to the north, and the
general direction of the contact agrees with the strike of
the quarbzite beds and the strike of the schists of eruptive
origin. The changes in the general direction of the con-
tact in the western portion of the area indicates that the
quartzite beds were eroded previous to the deposition of the
porphyry. It may be that the quartz keratophyre eruption
took place when the underlying quartzite beds were hori-
zontal, in which case the eruptive material must have had
the enormous thickness of 3,300 feet, as shown by its areal
extent from north to south. Or it may be that one or more
elevations had taken place in the quartzite previous to the
eruptive flow, in which case its actual thickness cannot be
estimated. Since no quartzite lies to the north of the

WEIDMAN — QUARTZ KERATOPHYRE OF BARABOO BLUFFS 55

eruptive area, the quartz keratophyre is probably the top-
most member of the Huronian rocks in this region, and was
upturned by the orographic movement which tilted the
quartzite beds into their present position.

EXPLANATION OF PLATES.
PLATE 1.

Geological Map of a Portion of the North Range of the Bara-
boo Bluffs. The scale is one and one-half inches to the
mile.

PLATE 2.
Photographs of Thin Sections.

Fig. 1.

Volcanic Breccia. Section 3079. Polarized light, x 25. This
figure shows a large number of fractured soda-orthoclase
phenocrysts. Near the lower side is a fragment of for-
eign rock. The figure also shows crystals of ilmenite
partly altered to leucoxene. The groundmass of this slide
shows the fluxion structure, though less plainly than many
of the others.

Fig. 2.

Black Quartz Keratophyre. Section 3080. Polarized light,
X 25. This section was prepared from a block and shows
a large broken phenocryst of soda-orthoclase. In the lower
left-hand corner are several small crystals of soda-ortho-
clase, belonging to a younger generation than the large
broken one. Quite near the large phenocryst in the
upper part of the figure are a few radial fibres of biotite.
The groundmass shows clearly the lines of flow passing
between the parts of the phenocryst. The groundmass

l!*3';

56 BULLETIN OF THE UNIVERSITY OF WISCONSIN

also contains spherulites, a large fan- shaped one being
quite well defined in the upper right-hand corner of the
figure.

PLATE 3.

Photographs of Thin Sections.
Fig. 1.

Fragment of Volcanic Breccia. Section 3085. Polarized
light, x 25. In the upper part of the figure is seen a
crystal of ilmenite, partly replaced by quartz. Through
the groundmass, which shows fluxion structure, are ellip-
tical areas of secondary quartz.

Fig. 2.

Volcanic Breccia. Section 3096.1. Ordinary light, X 25.
This section was prepared from a block and shows in the
figure a fractured rock fragment, which has flow lines at
right angles to the flow structure of the groundmass. The
space between the broken parts of the fragment is filled
with secondary quartz. The groundmass of the rock and
that of its imbedded fragment are essentially the same.

Bull. Univ. Wis., Set.

Vol. 1, PI. 2.

Fig. 1.

Fig. 2.

Bull. Univ. Wit., Sci. Ser.,

Vol. 1, PI. 3.

Pig. 1.

Fig. .2.

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