Petrography and Petrology of 

Volcanic Rocks in the 

Mount Jefferson Area 

High Cascade Range, Oregon

GEOLOGICAL SURVEY BULLETIN 1251-G

PETROGRAPHY AND PETROLOGY OF

VOLCANIC ROCKS IN THE

MOUNT JEFFERSON AREA

HIGH CASCADE RANGE, OREGON
Mount Jefferson from the north. Jefferson Park is in the foreground. Photograph

by G. W. Walker.
^etrography and Petrology of 

Tolcanic Rocks in the 

Mount Jefferson Area 

High Cascade Range, Oregon

B > ROBERT C. GREENE

CONTRIBUTIONS TO GENERAL GEOLOGY

GEOLOGICAL SURVEY BULLETIN 1251-G

JNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1968
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 30 cents (paper cover)
CONTENTS

Page

Abstract-. _ _____-______-________________-__---____-_-_-__---__--_- Gl

Introduction---_--_________-____-_______--____--___-_-_---_-_---__ 2

Scope ________________________________________________________ 2

Location and geography._______________________________________ 2

Previous work.._______________________________________________ 2

Accessibility. _________________________________________________ 6

Fieldwork and acknowledgments.-.----------------------------- 6

Rock nomenclature.___________________________________________ 7

Laboratory work and petrographic techniques._________--___-__--_ 7

Fused beads.____--_____________-_-___-____-__-_---------- 8

Plagioclase_ ___--___________________-__--_____-_-_----_-- 9

Olivine________-_________._.______-.____-_________-__-_--- 10

Clinopyroxene.- _ __________________________________________ 10

Orthopyroxene ____________________________________________ 10

Oxy hornblende.___________________________________________ 11

Lithology and stratigraphy.__________.______-_____________-----_--- 11

Introduction._________________________________________________ 11

Field description.________________________________.__---------_ 11

High volcanic plateau.-____________________________________ 11

Composite cones--_---__________----_--___-__----_--------- 13

Cinder cones, intrusive plugs, and intracanyon lava flows.------ 13

Petrographic description__-____________________---_-_____----- 14

Basalt (49^-52 percent SiO2 )__-. _ -- - -- 14

Andesite (52-57*4 percent SiO2 )_---------------------------- 17

Silicic andesite (59-65 percent SiO2 )________________________ 21

Dacite and rhyodacite (66-74 percent SiO2)________-___------- 26

Recent andesite-_-______________________-_-__-___--------- 28

Intrusive rocks.___________________________________________________ 29

Petrology_.._________________________________________________ 32

Analyses _____________________________________________________ 32

Alkali-lime index_______________________________-_--------_ 35

Comparison with other suites_______________________________ 36

Normative minerals________________-_______-_------_----- 40

Phase relationships and crystallization history___________---------- 40

Origin and derivation of the volcanic rocks._____________-.----_--- 43

References cited__________________________________________________ 46
VI CONTENTS

ILLUSTEATIONS

Page 

FRONTISPIECE. View of Mount Jefferson.

FIGURE 1. Index map showing Cascade Range and principal peaks__ G3

2. Generalized geologic and sample locality map of the

Mount Jefferson area.____________________________ 4

3. Photograph showing Three Fingered Jack from the

south_ __________________________________________ 5

4-6. Graphs showing:

4. Comparison of silica content and refractive index 

of fused bead for eight chemically analyzed 

rocks_ ____________________________________ 8

5. Comparison of silica content and refractive index 

of fused bead for several different suites of 

volcanic rocks-_-_____--____--__-____-_____ 9

6. Frequency distribution of silica content in rock

samples from the Mount Jefferson area_______ 12

7-9. Photomicrographs showing:

7. Diktytaxitic-intergranular texture in basalt____ 16

8. Olivine phenocryst with iddingsitized margin in

andesite._________________________________ 16

9. Olivine phenocryst with clear core, iddingsitized

zone, and clear margin in basalt_____________ 17

10. Diagram showing comparison of composition of plagioclase, orthopyroxene, and olivine phenocrysts with 

silica content of the enclosing rock_________________ 22

11. Diagram showing composition of clinopyroxene_________ 23

12. Photomicrograph showing olivine phenocryst in andesite_ 25

13. Photomicrograph showing intersertal texture in andesite.. 25

14. Diagram showing comparison of modal color index and

silica content___________________________________ 26

15-17. Photomicrographs showing:

15. Hypersthene phenocryst in silicic andesite_____ 27

16. Oxyhornblende phenocryst with opaque margin

in silicic andesite___________________________ 27

17. Intersertal-trachytic texture in intrusive andesite_ 32 

18-23. Silica variation diagrams of:

18. K2 O, Na2 O, CaO, and K2 O + Na2O_______--_-_- 35

19. A12 O3, FeO + Fe2 O3, MgO, TiO2 , and MnO______ 36

20. Crater Lake rocks-_-_____-____--__---__-_-_-_ 37

21. K2 O, Na2 O, and CaO for Western Cascade rocks. 38

22. A12 O3, FeO + Fe2 O3, and MgO for Western Cascade

rocks _____________________________________ 39

23. Rocks of the Koolau Volcanic Series, Oahu, Hawaii- 41 

24. Triangular diagram showing composition of normative

feldspar.________________________________________ 42
CONTENTS VII

Page 

FIGURE 25. Triangular diagram showing ratios of normative quartz,

plagioclase, and orthoclase-________________________ G43

26. Diagram showing comparison of iron enrichment and

silica content of experimental liquids._______________ 44

27. Diagram showing comparison of iron enrichment and

silica content of various magma series.______________ 45

TABLES

TABLE 1. A partial classification of volcanic rocks._-____-_-____- G7 

2-7. Modes of 

2. Basalt__________-__________________________- 15

3. Andesite________--_-__---__________________- 18

4. Silicic andesite-_--------_----_-___-_-__-_____ 24

5. Dacite and rhyodacite._______________________ 28

6. Recent andesite-_--------------_-_-___-__-__- 29

7. Intrusive rocks____----------_-----_---_-___- 30

8. Chemical analyses, norms, and modes of Mount Jefferson area rocks___________________________________ 33

CONTRIBUTIONS TO GENERAL GEOLOGY

PETROGRAPHY AND PETROLOGY OF

VOLCANIC ROCKS IN THE MOUNT JEFFERSON AREA

HIGH CASCADE RANGE, OREGON

By ROBERT C. GREENE

ABSTRACT

The Mount Jefferson area lies along the crest of the Cascade Range in Oregon 

and is about 25 miles long by 10 miles wide. It includes the major volcanic peaks 

of Mount Jefferson and Three Fingered Jack.

The area is underlain almost entirely by lava flows and local deposits of cinders 

and breccia; collectively these flows and associated deposits are known as the 

volcanic rocks of the High Cascades. The rocks, which are undeformed, are of 

Pliocene to Recent age. They form a high volcanic plateau that is surmounted 

by the composite cones of Mount Jefferson and Three Fingered Jack and that is 

locally overlain by Recent cinder cones and intracanyon lava flows.

The volcanic rocks are classified primarily by their silica content, determined 

from the refractive index of fused beads. In general, the rocks increase in silica 

content with increasing elevation (decreasing age).

Basalt (49^-52 percent Si02) contains phenocrysts of olivine and (or) bytownite in a groundmass of plagioclase, clinopyroxene, opaque minerals, and glass.

Andesite (52-57 percent Si02) is the most abundant type of rock. Some andesite 

is porphyritic, the phenocrysts being of bytownite, olivine, and (or) calcic augite. 

Aphyric rocks and the groundmass of porphyritic rocks consist of plagioclase, 

clinopyroxene, opaque grains, and glass. Some andesite contains orthopyroxene.

Silicic andesite (59-65 percent Si02) contains phenocrysts of labradorite, and, 

commonly, calcic augite, orthopyroxene, and oxyhornblende. The aphanitic 

groundmass consists of plagioclase, alkali feldspar, silica minerals, pyroxene, 

and glass.

Dacite and rhyodacite (66-74 percent 8102) contain phenocrysts of labradorite 

or andesine and, in some, orthopyroxene, salite, and oxyhornblende in an aphanitic 

groundmass similar to, but more silicic than, that of the silicic andesite.

The Recent intracanyon flows are andesite with 54-55 percent Si02 and phenocrysts of plagioclase, olivine, and clinopyroxene in a groundmass of plagioclase, 

clinopyroxene, glass, and opaque grains.

Silica variation diagrams for eight chemically analyzed specimens yield smooth 

curves. The alkali-lime index of 61 is close to that for other areas in the High 

Cascades. The Mount Jefferson area rocks contain more alumina and less iron 

oxide than those of either the Western Cascades or the Koolau Volcanic Series 

from Oahu, Hawaii. The norms show that the four more silicic rocks are perGl 

285-003 O 68 2
G2 CONTRIBUTIONS TO GENERAL GEOLOGY

aluminous. Triangular feldspar and quartz-orthoclase-plagioclase diagrams show 

the paucity of potassium and illustrate the consanguinity of this suite of rocks.

Comparison of Mount Jefferson area rocks with the experimental data on the 

system SiC^-FeO-FeaOs-MgO shows that they very likely crystallized at nearly 

constant oxygen pressure.

The magma forming the volcanic rocks of this area was erupted from a number 

of vents, probably chiefly from Mount Jefferson and Three Fingered Jack. The 

Pliocene and Pleistocene lavas represent a differentiated sequence, fed by one or 

more magma chambers. The Recent intracanyon flows originated from a new, 

undifferentiated magma.

INTRODUCTION 

SCOPE

This report describes the petrography and petrology of a suite of 

rocks belonging to the volcanic rocks of the High Cascades (Callaghan, 

1933), collected from the Mount Jefferson area in Oregon.

LOCATION AND GEOGRAPHY

The Mount Jefferson area is in Linn, Marion, and Jefferson Counties, Oreg., about 40 miles northwest of Bend and about 55 miles 

east of Salem (figs. 1 and 2). It is part of the Cascade Range, a mountain chain that trends north and south from northern California 

across Oregon and Washington into British Columbia. The area from 

which rock samples were collected, about 25 miles long by 8-12 miles 

wide, lies along the crest of the range. It includes the major volcanic 

peaks of Mount Jefferson (alt 10,497 ft; frontispiece) and Three 

Fingered Jack (alt 7,841 ft; fig. 3).

The Cascade Range in the Mount Jefferson area is a north-southtrending ridge whose crest lies at an elevation of 5,500-6,500 feet. 

The ridge is surmounted by numerous major and minor peaks on its 

summit and slopes. Spurs lead outward from the crestal ridge and 

are separated by deep U-shaped valleys which end at steep headwalls 

against the ridge. Such deep valleys on the west side of the ridge 

have floors as low as 3,000 feet and thus provide as much as 7,500 

feet of relief against Mount Jefferson.

PREVIOUS WORK

The earliest work on the Mount Jefferson area was a report on the 

geology of Mount Jefferson by Hodge (1925), who described the 

general sequence of volcanic rocks and discussed the physiography and 

glacial history.

Thayer (1937) described the petrography and petrology of the rocks 

from an area extending from Detroit (18 miles west of Mount Jefferson 

to Olallie Butte (north of the area shown in fig. 2) and Mount Jefferson. Rocks in this area belong to both the volcanic rocks of the Western 

Cascades and those of the High Cascades and were divided by Thayer
VOLCANIC ROCKS, MOUNT JEFFERSON AREA, OREGON G3

124° BRITISH COLUMBIA

122° 120°

48'

Western Cascade area

Mount McLoughlin 

Klamath Falls

_assen 

Peak

118°

-148°

H 46°

H 44°

42°

50 100 MILES

-[ 40°

FIGURE 1. Location of Cascade Range (shaded), principal volcanoes, part of 

Western Cascades studied by Peck, Griggs, Schlicker, Wells, and Dole (1964), 

and area of this report (stippled). Boundary between Western Cascade Range 

and High Cascade Range in Oregon, from Peck, Griggs, Schlicker, Wells, and 

Dole (1964).
G4 CONTRIBUTIONS TO GENERAL GEOLOGY

MJW-40X 

MJW-42 X

Mount X MJW-23, 26 

Jefferson X MJW-28

MJW-21 X

Pamelia X MJW-105

North

Cinder

Peak

Dacite and 

rhyodacite

Three Fingered 

Jack

Basalt, andesite Intrusive

and rocks 

silicic andesite

Volcanic and

sedimentary

rocks

Contact

FIGURE 2. Generalized geologic and sample locality map of the Mount Jefferson 

area (from Walker and others, 1966, pi. 1, and additional mapping in 1965 by 

R. C. Greene and G. W. Walker). Numbers of chemically analyzed samples 

underlined; X, location of specimen; +, geographic feature.
VOLCANIC ROCKS, MOUNT JEFFERSON AREA, OREGON G5

0) I

ao

£ 

Mo

"-S
G6 CONTRIBUTIONS TO GENERAL GEOLOGY

into several local map units. Sixteen chemical analyses were presented 

by Thayer, five of which (Thayer, 1937, p. 1622-1623, nos. 5-7, 

11, 15) were of High Cascade lavas from within the area of the present 

report. Four of these are basalt (andesite in my terminology) that 

contains 52.96-54.09 percent SiO2 and are comparable to MJW-92 

(52.6 percent SiO2 , table 8), except that the iron is less oxidized. The 

other analysis reported by Thayer is of a pyroxene andesite (silicic 

andesite in my terminology) that contains 60.11 percent SiO2 and is 

comparable to MJW-105 (61.2 percent SiO2 , table 8). Thayer's 

conclusion that the volcanic rocks of the High Cascades near Mount 

Jefferson are poorer in K2O and total iron and richer in A12O3 than 

those of the Western Cascades is substantiated by the analyses given 

in this report.

Thayer later (1939) described additional structural details of the 

lavas in the area covered by the 1937 report and discussed the physiography and glacial history of Mount Jefferson, Jefferson Park, and 

adjacent areas.

Williams (1957) briefly described the Three Fingered Jack volcano 

and has also written several other reports on volcanic rocks of the 

High Cascades (Williams, 1932a, 1932b, 1933, 1935, 1942, and 1944).

The geology of much of the Western Cascade Range in Oregon has 

been described by Peck, Griggs, Schlicker, Wells, and Dole (1964). 

Their report provides descriptions of the stratigraphy and structure of 

the early Tertiary rocks that emerge from beneath the High Cascades 

west of the Mount Jefferson area. The eruptive history is traced 

from the Eocene through the culminating eruptions forming the 

volcanic rocks of the High Cascades.

ACCESSIBILITY

The Mount Jefferson area is accessible by U.S. Highway 20 from 

Bend and State Highway 22 from Salem; these highways join near 

Santiam Pass at the south edge of the area. Logging roads from State 

Highway 22 provide access to the west edge of the area and logging 

and U.S. Forest Service roads along the Metolius River (southeast 

of fig. 2) lead to the east edge. An extensive system of trails penetrates 

the area.

FIELDWORK AND ACKNOWLEDGMENTS

Fieldwork in the Mount Jefferson area was done by the writer and 

G. W. Walker during the summer of 1965 as part of a mineral resource 

evaluation of the Mount Jefferson primitive area. Traverses were made 

on all major trails and up many canyons, ridge crests, and outlying 

peaks. A summary of the geology and mineral resources has been 

published (Walker and others, 1966).
G7

ROCK NOMENCLATURE

The rock classification scheme used in this report is in part modal 

and in part chemical and essentially follows that of Howel Williams 

(Williams and others, 1954). This scheme is a simple one requiring 

the use of only a few names and is especially useful for the rocks in 

the report area, many of which are too fine grained for complete 

modal analysis. Chemical data necessary for classification were 

supplied by eight chemical analyses and by estimates of silica content 

based on the refractive indices of fused beads. The classification used 

is summarized in table 1.

TABLE 1. A partial classification of volcanic rocks

Rhyodacite_________ >10 percent modal or normative quartz; alkali feldspar

M-% total feldspar. 

Dacite_____________ >10 percent modal or normative quartz; alkali feldspar

>3^§ and <CM total feldspar. 

Andesite__________ 52-66 percent Si02 ; color index, <40; alkali feldspar

<JHj total feldspar. 

Basalt______________ Plagioclase more calcic than Abs0 Anso; <10 percent

alkali feldspar; color index, >40.

Silica content, as determined from the refractive indices of fused 

beads, is used in this report as the principal independent variable with 

which to compare the other properties of the rocks. A smooth curve 

is obtained by plotting the silica contents of the eight analyzed 

samples versus the refractive index of their fused beads (fig. 4). Only 

two samples lie off the curve, and these, at short distances.

Huber and Rinehart (1966) discussed the curves of the refractive 

index of fused beads versus silica content for several volcanic rock 

suites. They emphasized the necessity of constructing an individual 

curve for a given suite rather than relying on an average curve derived from samples of varying affinity. The Mount Jefferson curve 

(fig. 5) is plotted with several curves discussed by Huber and Rinehart. It lies close to the curve for the Western Cascade suite near its 

ends, but bulges toward the more alkalic suites at intermediate SiC>2 

content.

LABORATORY WORK AND PETROGRAPHIC TECHNIQUES

About 200 rock samples were collected from the Mount Jefferson 

area. Eighty-one thin sections were prepared, of which 44 were selected 

for modal analysis and detailed study. Fused beads were prepared for 

all sectioned rocks and several samples of cinders. Chemical analyses of 

eight rocks selected on the basis of the fused beads to represent a wide 

range of silica content were prepared by the rapid rock analysis laboratory of the U.S. Geological Survey.
G8 CONTRIBUTIONS TO GENERAL GEOLOGY

74

70

66

62

58

54

50

1.480 1.500 1.520 1.540 1.560 

REFRACTIVE INDEX OF FUSED BEAD

1.580 1.600

FIGURE 4. Comparison of silica content and refractive index of fused bead for 

eight chemically analyzed volcanic rocks from the Mount Jefferson area. 

Extrapolated to 50 percent Si02 .

Modal analyses were made by counting a thousand points in a 

standard thin section. Where groundmass was too fine for a reasonably 

accurate count of its individual minerals, it was counted as "groundmass," and a visual estimate of the mineral proportions was made.

The color index was calculated by dividing the sum of the modal 

percentages of ferromagnesian minerals by the sum of all minerals 

present. Glass and voids were ignored.

To identify groundmass minerals, X-ray diffractometer charts were 

made from whole-rock powders of the silicic andesite, dacite, and 

rhyodacite. Some scans from 26 values of 10°-52° at 2° per minute were 

supplemented by scans of 20°-40° at 1° per minute.

FUSED BEADS

Fused beads were prepared from powdered whole-rock samples with 

a carbon arc. Three or four beads were made for each sample. The re-
VOLCANIC ROCKS, MOUNT JEFFERSON AREA, OREGON G9

80

75

70

65

60

55

50

45

'Western Cascade suite

Mount Jefferson suite

Lake County, Oregon suite

Mammoth Lakes, Calif, suite Hawaiian tholeiitic suite

Hawaiian alkalic suite

40

1.480 1.500 1.520 1.540 1.560 1.580 1.600 

REFRACTIVE INDEX OF FUSED BEAD

1.620 1.640

FIGURE 5. Comparison of silica content and refractive index of fused bead for 

several different suites of volcanic rocks. All curves except the one for the 

Mount Jefferson area taken from Huber and Rinehart (1966).

fractive index of fragments of each bead was determined by oil immersion, by using a set of oils with 0.002 divisions. A sodium light was 

employed, and temperature corrections were applied. Because fusedbead fragments vary in index, commonly within the range of 0.002- 

0.005, it was necessary to record the percentage of the fragments with 

indices higher and lower than any given oil and interpolate accordingly 

between the two closest oils. In this way the median index of the bead 

was obtained to an accuracy of ±0.001 or better. Triplicate determinations gave a total deviation of 0.003 or less in 60 percent of the measurements, of 0.006 or less in 85 percent, and of 0.008 or less in 

95 percent.

PLAGIOCLASE

Plagioclase composition was determined by oil immersion. The ft 

index was measured in sodium light using a 50+200-mesh separate 

and a set of oils with 0.002 divisions.

285-003 0 68 3
G10 CONTRIBUTIONS TO GENERAL GEOLOGY

The An content was determined from the /3 refractive index using 

the curve of J. R. Smith (1958). This curve is derived from plagioclases 

from plutonic rocks which were heated to invert them to the "high" 

structural state. It was chosen in preference to curves derived from 

"low temperature" plu tonic plagioclase (Calkins and Hess, in Kennedy, 1947; Smith, in Hess, 1960) or composite curves for plagioclase 

from a variety of sources (Chayes, 1952). Compositions obtained from 

Smith's /3 index curve may be slightly in error, however, for many 

natural calcic volcanic plagioclases have intermediate structural states 

owing to partial ordering during cooling.

This method should yield refractive indices that are within ±0.001 

of the true value and that correspond to ± 2 percent of the An content. 

The chief limitation of this method is that fragments of the larger 

crystals, especially the core zones of phenocrysts, tend to be selected, 

and they are more calcic than the average for the rock.

OLJVINE

The composition of olivine was determined by measurement of the 

optic axial angle on the universal stage, using the conoscopic method. 

Corrections for refraction were made by using the graph of Emmons 

(1943, pi. 8). The Fa content was found by comparing the 2V values 

with those on the graph of Poldervaart (1950), whose nomenclature is 

also used.

Scatter of the 2Vs was rather large, and the optic-angle measurements are accurate to only ±4°. Such measurements correspond to 

about ± 8 percent of the Fa content and thus produce only very approximate compositions.

CLINOPYROXENE

Clinopyroxene was determined by refractive index and optic axial 

angle measurement. The /3 index was measured in sodium light by 

using a 50+200-mesh separate and oils with 0.004 divisions. 

Accuracy is probably about ±0.002. Optic axial angles were determined with the universal stage and the conoscopic method. Scatter 

of the 2V's was appreciably less than for olivine, and the 2F's are 

accurate to ±2°-3°.

Composition was determined by plotting the optic angles and 0 

refractive indices on the diagram of Hess (1949). The specified accuracy 

of the 2F's gives ±2 percent Wo, that of the /3 refractive indices 

gives ± 1-2 percent Fs. The nomenclature for clinopyroxene is that 

of Poldervaart and Hess (1951).

ORTHOPYROXENE

Orthopyroxene was determined by measurement of the optic axial 

angle. The Fs content was taken from the curve of Kuno (1954).
VOLCANIC ROCKS, MOUNT JEFFERSON AREA, OREGON Gil

The nomenclature is that of Poldervaart and Hess (1951). Scatter of 

2V values was large, so the optic angles are probably accurate to 

only ±4°, a value which corresponds to ±3 percent Fs at less than 

35 percent Fs.

|8 refractive indices were determined for two orthopyroxenes as a 

check on the 2V determinations. When these indices were applied to 

a curve calculated from the a, 7, and 2V values of Kuno (1954), they 

gave the same composition as that given by the 2V's.

OXYHORNBLENDE

The optic axial angles of oxyhornblende were determined with the 

universal stage, using the conoscopic method. Scatter of the 2V's was 

large, as was expected since the grains vary widely in color and 

birefringence within the same specimen. Refractive indices were 

determined by immersion with oils of 0.004 and 0.005 divisions.

LITHOLOGY AND STBATIGKAPHY

INTRODUCTION

The oldest rocks exposed in the Mount Jefferson area are volcanic 

and sedimentary rocks correlative with the volcanic rocks of the 

Western Cascade Range (Peck and others, 1964, p. 7-8). They are of 

Miocene(?) and Pliocene age and crop out only in a few deep canyons 

on the west side of the area. They were briefly described by Walker, 

Greene, and Pattee (1966) and are not further considered in this 

report.

The remainder of the Mount Jefferson area is underlain by lava 

flows with local cinders and breccia collectively known as the volcanic 

rocks of the High Cascades (Callaghan, 1933; Peck and others, 1964, 

p. 7). They are of Pliocene to Recent age and are undeformed. Most 

of these rocks are andesite (52-65 percent Si02, fig. 6), but they 

range from basalt (49j^-52 percent Si02) to dacite and rhyodacite 

(66-73 H percent Si02).

FIELD DESCRIPTION

To facilitate a field description of the volcanic rocks, the Mount 

Jefferson area is best divided into (1) a high volcanic plateau; (2) the 

composite cones of Mount Jefferson and Three Fingered Jack; and 

(3) cinder cones, intrusive plugs, and intracanyon lava flows.

HIGH VOLCANIC PI^ATEAU

The high volcanic plateau consists primarily of flows of andesite 

and basalt. Some have initial dips of 5°-10°, but most are flat lying. 

They range from a few feet to a few tens of feet in thickness. The 

upper and lower parts of many flows are somewhat vesicular and, 

rarely, weakly brecciated. Exposed top surfaces commonly-show ropy
G12 CONTRIBUTIONS TO GENERAL GEOLOGY

48 52 54 56 58 60 62 64 66 68 70 72 74 

SILICA CONTENT FROM FUSED BEAD, IN PERCENT

76 78

FIGURE 6. Frequency distribution of silica content in rock samples from the 

Mount Jefferson area. Dashed line is projected curve (log scale) for a systematic 

series of samples.

texture. Platy jointing is common in the fine-grained flow rocks but 

rare in aphanitic rocks. Such jointing may or may not be parallel to 

the top and bottom of the flow.

The andesite and basalt flow rocks are light to dark gray 1 and 

brownish gray. (See section on "Petrographic description.") Weathered 

surfaces are slightly darker and browner. Dacite is pale red and 

weathers to a grayish red.

1 Color terminology is that of the National Research Council's Rock-Color Chart (Goddard and others,
VOLCANIC ROCKS, MOUNT JEFFERSON AREA, OREGON G13

Interbedded cinders are rare in most of the high volcanic plateau 

and are essentially absent near the base of the High Cascade volcanic 

rocks. Cinders are more common, however, near the bases of Mount 

Jefferson and Three Fingered Jack, where the rocks of the high plateau 

merge with those of the composite cones.

COMPOSITE CONES

Mount Jefferson (frontispiece) is a slightly dissected stratovolcano 

consisting of flows of andesite (dominantly silicic andesite) with interbedded cinders. Both cinders and flows crop out toward the base of 

the cone where it has been deeply eroded by glaciers, but aretes high 

on the flanks and near the summit are almost entirely flows. Initial 

dips of flows and cinder beds are as great as 25° and are generally 

away from the summit. The silicic andesite flows are from several feet 

to a few tens of feet thick and rarely show original surface features. 

These medium- to light-gray rocks show little surface weathering, 

owing to the rapid spalling of the surface. The cinder beds exposed 

toward the base of the cone are similar to those on Three Fingered 

Jack. A small intrusive plug near the summit may represent the filling 

of the principal lava conduit.

Three Fingered Jack is a severely dissected stratovolcano (fig. 3). 

Erosion, primarily glacial, has exposed a precipitous face exhibiting 

alternating lava flows and beds of cinders. Most of the flows are only 

a few feet thick, are scoriaceous near their tops and bases, and have 

rough, commonly ropy, surfaces. The surface color is brownish; where 

weathered it is duller yellowish brown. The cinders are an unsorted 

mixture of fine and coarse material ranging from ash through lapilli 

to blocks and bombs. They are generally agglutinated into a coherent 

rock which may be crudely bedded. The cinder agglutinate is a colorful 

rock, ranging from various shades of red and reddish brown to yellowish 

brown and yellowish orange. Flows and cinders are interlayered 

remarkably evenly in 5- to 20-foot layers (fig. 3) from the summit to 

the base of the mountain. The lower part of the cone is about half 

flows and half cinders, but cinders become dominant toward the top. 

Initial dips are as much as 35°. Numerous dikes transect the stratified 

rocks, and an intrusive plug north of the present summit marks the 

probable site of the conduit.

CINDER CONES, INTRUSIVE PLUGS, AND INTRACANYON LAVA FLOWS

Cinder cones and intrusive plugs are scattered irregularly about 

the Mount Jefferson area, but intracanyon lava flows occur only in 

the central part and on the south margin (fig. 2). These rock units are 

commonly related, though cinder cones or plugs may occur separately.
G14 CONTRIBUTIONS TO GENERAL GEOLOGY

Most of the cinder cones are prominent rounded hills. They are 

composed chiefly of cinders, predominantly grayish to dusky red, but 

in places dark gray and brownish gray. Sorting is poor, and the cinders 

may be loose or agglutinated. Vesicular flow material is locally interbedded.

Some of the cinder cones contain intrusive plugs of vesicular and 

flow-banded rocks which apparently represent the final surge of magma 

into the eruptive vents. Other intrusive plugs show little obvious 

relationship to vents. They form topographic highs and owe their 

greater resistance to erosion to the fact that they have less closely 

spaced jointing than the surrounding flow rocks. They are composed 

of varied types of rock, which differ from their extrusive equivalents 

mainly in their somewhat coarser textures.

Intracanyon lava flows are distinguished by their unmodified bare 

rock surfaces and their perched positions conforming to present 

topography. Each flow emanates from the flank of a cinder cone and 

has an aa, or blocky, surface. A crude pattern of pressure ridges may 

be seen where the flows are gently sloping. The andesite making up 

the flows is vesicular, and surface weathering stains it dark gray to 

grayish black.

PETRO GRAPHIC DESCRIPTION

For purposes of description, the Pliocene and Pleistocene rocks of 

the area are divided into groups based on silica content, which was 

estimated from the refractive index of their fused beads. These 

groups also have stratigraphic significance, for the younger rocks at 

a higher elevation are generally more silicic. The recent intracanyon 

flows andesite with 54-55 percent silica are treated last.

BASALT (49^-52 PERCENT SIOz)

Basalt is found principally at low elevations on the flanks of the 

crestal ridge. It forms a unit, perhaps mappable, which underlies the 

main mass of andesite of the High Cascades, and which overlies the 

volcanic and associated sedimentary rocks of the Western Cascade 

Range. Five samples of basalt were studied in detail (table 2), four 

from the basal basalt unit, and one from higher in the section at 

Sant'am Pass. These rocks are dark to medium gray, rarely mottled 

with olive gray. They are aphanitic to fine grained, commonly diktytaxitic, and aphyric to porphyritic (fig. 7).

The basalt contains phenocrysts of olivine and (or) plagioclase in a 

groundmass of plagioclase, clinopyroxene, opaque grains, and glass. 

Olivine is fresh, iddingsitized, (fig. 8) or partly or wholly altered to 

clays. In sample MJW-34, several unusual patterns of iddingsitization
VOLCANIC ROCKS, MOUNT JEFFERSON AREA, OREGON G15 

TABLE 2. Modes, in volume percent, and other characteristics of basalt

Phenocrysts:

(An)....... ... ...

Olivine.. ............

Groundmass:

(An)............. .

(Wo:En:Fs)-.-.

Magnetite and

Glass ___ .........

Void.................

Total..............

Alteration materials:

Total..............

Total..............

MJW-34

(75)

.9 .

.9

51.9

30.7

(44:32:24) .

2.7

1.2 .

10.1

96.6

2.5 .

2.5

100.0

MJW-86

Modes

3.2 .

(77) .

3.2

32.6

37.6

10.6

4.6

85.4

1.3 .

10.1 .

11.4

100.0

MJW-90

15.6

15.6

50.1

(73)

17.2

(41:29:30) .

3.5 .

4.2

9.4

84.4

0

100.0

MJW-88

10.3

10.3

43.7

(AVt

16.1

27.1

1.6

88.5

1.2 .

1.2

100.0

MJW-11

1.0

(77)

3.6

4.6

52.3

29.4

(44:32:24)

4.0

2.2

7.5

95.4

0

100.0

Other characteristics

Refractive index of fused 

bead. 

Silica content from fused 

bead. 

Texture _ ..---.-. _ ...

Grain size (mm):

41

1.608 

49^ 

Diktytaxitic,

porphyritic, intergranular.

0.6-1.5

0. 03-0. 5

58

1.604 

50

intergranular, 

flow-alined.

0. 5-1. 5

0.05-0.3

42

1.600 

50 

Diktytaxitic, intergranular to 

intersertal.

0.1-2.

38 41

1.592 1.590 

51 51^ 

Intersertal . . . Porphyritic,

diktytaxitic, intergranular to 

intersertal, 

flow-alined.

.............. 0.6-1.

. 05-2. 0. 05-0. 5

are seen (fig. 9). Some grains have iddingsite centers and clear margins; 

others have clear centers, an iddingsite zone, and then a clear margin. 

In these latter grains the iddingsite zone has a fuzzy inner margin 

and sharp outer margin. Obviously, these olivine phenocrysts were 

iddingsitized while the lava was still in part molten; then additional 

olivine was deposited on their surfaces (Sheppard, 1962).

The cores of plagioclase phenocrysts and larger microlites from 

nonporphyritic rocks are bytownite (fig. 10). Clinopyroxene and 

opaque grains fill much of the space between groundmass plagioclase 

microlites; the remainder is occupied by glass, which is tachylitic 

in some specimens. Clinopyroxene, where indentifiable, is calcic 

augite or calcic ferro-augite (fig. 11). The alteration materials are 

clays derived from olivine or glass.
G16 CONTRIBUTIONS TO GENERAL GEOLOGY

FIGURE 7. Intergranular texture with diktytaxitic openings and flow-alined 

plagioclase microlites (P) in basalt. Augite (A) and magnetite (M) are in 

intergranular position, voids (V) are largely surrounded by plagioclase, and an 

olivine phenocryst (Ol) is near the upper margin. Plane light, X 30. Sample 

MJW-11.

FIGURE 8. Olivine phenocryst with iddingsitized margin (I) and preferential 

iddingsitization on 010(?) plane, in andesite. Groundmass is plagioclase (P) 

and clinopyroxene (C). Plane light, X 50. Sample MJW-59.
VOLCANIC ROCKS, MOUNT JEFFERSON AREA, OREGON G17

FIGURE 9. Olivine phenocryst with clear core (C), iddingsitized zone (I), and 

clear margin (M), in basalt. Groundmass is plagioclase (P), augite (A), and 

magnetite (Mag). Plane light, X 240. Sample MJW-34.

ANDESITE (52-57^ PERCENT S1O2)

Andesite (52-57% percent SiO2) is by far the most abundant type of 

rock in the Mount Jefferson area (fig. 6) and is found at all elevations 

from directly above the lower basalt unit to the ridge crest between 

the two main volcanic cones. It is most commonly medium dark gray 

but ranges from dark to medium light gray and brownish gray. Some 

is gray mottled with brownish gray; in general, the more brownish the 

rock, the higher the content of glass. The andesite is aphanitic to fine 

grained and commonly is porphyritic.

The andesite is composed primarily of plagioclase, olivine, clinopyroxene, opaque material, and glass (table 3). Some is porphyritic, 

the principal phenocrysts being equidimensional grains of bytownite 

(fig. 10). Compositions of plagioclase from andesite and other rocks 

plotted in figure 11 are the core portions of larger phenocrysts. In 

general, the core portions are oscillatory zoned, and so the composition 

reported represents the average of the alternating zones. (See Harloff, 

1927; Bowen, 1956, p. 274-275; Hills, 1936; Carr, 1954; Vance, 1962; 

and Vance, 1965, for discussion of oscillatory zoning). Outside the 

oscillatory zoned core, a progressively zoned margin completes the 

phenocryst. The width of the margin is generally one-fifth or less the 

radius of the phenocryst; a sharp contact locally showing resorption 

separates the margin from the core.

285-003 O 68 4
G18 CONTRIBUTIONS TO GENERAL GEOLOGY

Bytownite phenocrysts are accompanied by phenocrysts of olivine 

(fig. 12), and, in a few rocks, of clinopyroxene. Where the composition 

of the clinopyroxene can be determined, it is calcic augite (fig. 11). 

Orthopyroxene phenocrysts (Fs30) are present in one example. The 

porphyritic rocks contain a groundmass of plagioclase laths more 

sodic than the phenocrysts, minute grains of clinopyroxene, opaque 

minerals, and interstitial glass (fig. 12).

Nonporphyritic andesite samples exhibit considerable variety in 

composition and texture. In general they are similar to the porphyritic rocks except that the plagioclase grades continuously in size 

from microphenocrysts to smaller microlites, and equidimensional 

grains are absent. Larger plagioclase grains are calcic labradorite to

TABLE 3. Modes, in volume percent,

Sample. MJW-46 MJW-9 MJG-14

Modes

Phenocrysts:

Plagioclase---........................... . 7.3 ................ 16.3

(An)....................................... (79) ................ (79)

Olivine........................................ 3.2 2.9 4.2

Clinopyroxene--.---_..._... .1 _.........._ ..__.........

WoiEn-.Fs)............................... ................ ................ .................

Orthopyroxene._._.__....___ . ....__...-. ___.-._. .__....

(^)....................................... ................ ................ .................

Total....................................... _______10.6 _______2.9 _______20.6

Groundmass:

Plagioclase......-............... ........... 38.4 60.9 28.0

(An).............................-......... ................ (82) .................

Clinopyroxene................................. 22.6 16.6 22.1

(Wo:En:Fs). ..._..................... ................ ................ -----------------

Orthopyroxene_____.____ . . . _____.__ ________ _______....

(tfs)....................................... ................ ................ .................

Magnetite and ilmenite._________ _ . ________ ________ ____... ...

Glass......................................... 28.6 '23.1 27.3

Void................ __-,_ ................ ___ 7.6 ______«2.1

Total_..................................... 89.4 97.1 ______79.6

Alteration materials:

Iddingsite..__________._______ ________ ________ Trace 

Clays after olivine_____. ________ _____. __ ____. ___ _______ 

Clays after glass.__._.__________ ...______ ________ _____......

Total........................................ ................ ........ -- -.

Total........................................ 100.0 100.0 100.0

Other characteristics

Color Index.......................... 36 27 37

Refractive index of fused bead__.. 1.684 1.681 1.576

Silica content from fused bead..................... 52H 53 53H

Texture.. _______._____ Glomeropor- Intersertal to Porphyritic, Inphyritic, in- subophitic. tersertal to 

tergranular to interto intersertal. granular. 

Grain size (mm):

Phenocrysts. .__............. 0.1-0.8 ......._...... 0.1-1.

Groundmass. ----___........_......__ <0.01-0.1 0.06-0.8 0.02-1.

See footnotes at end of table.
VOLCANIC ROCKS, MOUNT JEFFERSON AREA, OREGON G19

calcic bytownite. As in the porphyritic rocks, the spaces between 

plagioclase laths are principally filled with clinopyroxene, opaque 

minerals, and glass. In some samples, orthopyroxene (ferrous bronzite 

or magnesian hypersthene) is also present. One unusual rock (MJG42) contains bronzite and olivine but no clinopyroxene.

All the andesite samples contain glass in the groundmass; several 

have as much as 30 percent. The color, and probably the composition 

of this glass, varies widely. In some samples the glass is a black 

tachylite that is nearly opaque (fig. 13); in others, it is a transparent 

brown.

Texture is intergranular-intersertal or intersertal-intergranular 

(fig. 13), depending upon whether pyroxenes or glass predominate in

and other characteristics of andesite

MJQ-42 MJW-92 MJQ-6 MJG-11 MJG-36 MJG-6

Modes Continued

11.6

7.2 

(81) 

2.5 0.5

31.3 

(73) 

4.0 

3.0 

(45:32:23)

20.1 

(82) 

6.6 

3.6 

(44:32:24)

3.0

11.6

63.0 

(69)

19.0 

(34) 

4.4 

.8

87.2

9.7

46.9

28.3

311.9

3.2

90.3

0.5

67.6 

(75) 

U9.3

4.0 

(35) 

3.8 

4.0

98.7

38.3

25.9

11.0

.9

22.2 

1.7

61.7

30.3

42.2

20.4

3.1 

4.0

69.7

3.0

46.5 

(81) 

10.3

31.1 

9.1

97.0

1.2

.4 

.4

1.2 ................ .8 ................ ................ .......--..----

100.0 100.0 100.0 100.0 100.0 100.0

Other characteristics Continued

36 

1.574 

54 

Intergranular. .

44 

1.574 

54 

Porphyritic, 

intergranular.

29 

1.573 

54 

Intergranular 

to intersertal.

25 

1.573 

54 

Diktytaxitic, 

porphyritic, 

intersertal to 

intergranular.

35 

1.572 

54 

Porphyritic, 

intergranular.

22 

1.571 

54 

Intersertal, 

trachytic.

-----.... 0.1-1. ................ 0.2-1. 0.1-1. .................

0.05-0.8 0.05-0.1 0.05-1.2 0.02-1. <0.01-.05 0.07-0.7
G20 CONTRIBUTIONS TO GENERAL GEOLOGY

TABLE 3. Modes, in volume percent,

- -.__ _____-_________-______-__ MJG-59 MJG-40 MJG-8

Modes

Phenocrysts:

Plagioclase--_--_______ _______ ...._... . ___ _ ___________ __________ _- .................

(An)....................................... ................ ................ _.._.............

Olivine----_-__--__-__---_--_____-__.________ Trace 2.3 0.4

Clinopyroxene ............................... ................ .--..---.------. .--..-.-.--------

(Wo:En:Fs)............................... ___._........... ................ .................

Orthopyroxene__............................ ................ ................ .....-......-..--

(Fs)....................................... ................ ..........------ .................

Total---.-.-.--.............................. ................ _______2.3 _______0.4

Groundmass:

Plagloclase.................................... 55.6 52.7 58.9

(An)....................................... (77) (67) (86)

Clinopyroxene. .................. . 31.7 25.2 23.6

(Wo:Bn:Fs).....--..-..--............... ................ ................ .................

Orthopyroxene.---.-----.---.-...-..........-. ................ -.... ......-.. Trace

(Fs)-.... ............................... ................ ................ ..... _ 

Magnetite and llmenlte........................ 5.1 4.8 2.8

Glass.......................................... 3.7 10.8 9.1

Void.-............................. ----------- ................ 4.2 5.2

Total...................................... _______96.1 _______97.7 _______99.6

Alteration materials:

Iddingsite....._............................. _____ __ - Trace .................

Clays after olivine _. _-_-_ ___- 3.9 ....-.--...-...- --.---..---------

Clays after glass.------------------------------ ................ ---------------- .-..-----------.-

Total.................................... 3.9 ................ -.----.----------

Total.. _____________ _________________ 100.0 100.0 100.0

Other characteristics

Color index..-........... 42 38 31

Refractive Index of fused bead...-- .....-....-. 1.571 1.568 1.566

Silica content from fused bead-......-.--...-...... 54 55 55

Texture...._ _ __________ ______ __ __. Intergranular.. Intergranular Integranular

to intersertal, to intersertal,

flow-alined. trachytic. 

Grain size (mm):

Phenocrysts..------------ _ ______ .._... _............. .....-...-.-....-

Groundmass.----_..........____......... 0.05-1. 0.02-1. 0.05-1.

1 Contains 17.3 percent auglte and 2.0 percent pigeonite.

2 In part after olivine.

s This figure includes 8.3 percent partially devitrifled tachylitic glass.

the spaces between plagioclase laths. In one sample, larger clinopyroxene grains partly enclose plagioclase subophitically. Alinement 

of plagioclase is weak to absent in most of the rocks, but it is strong 

and forms trachytic texture in some.

In general, the andesite is little altered. Olivine is partially iddingsitized in several samples (fig. 8); in others it is partially altered to 

clays, even though the remaining minerals are unaffected.

Color indices of these rocks range from 20 to 44 (fig. 14); this 

rather wide variation can be largely attributed to the abundance of 

glass. The generally low color index for the composition of rocks 

with high glass content suggests that the glass is relatively mafic.
VOLCANIC ROCKS, MOUNT JEFFERSON AREA, OREGON G21

and other characteristics of andesite Continued

MJW-59 MJG-13 MJW-21 MJW-42 MJG-36 MJW-28

Modes Continued

2.1

2.1

64.0 > 

(67) ......

22.2

3.8

7.0

97.0

0.9 ......

0.9 .....

100.0

24.8 -----

(71) .....

4.5 

4.6 -----

33.9

23.2

22.9

4.1 

11.8 

4.1

66.1

100.0

0.5

0.5

53.4 

(67) ....

8.6

14.1

(27) .....

3.4

16.6

99.5

100.0

8.2 --..

.4

.9 ....

(24)

9.5 ....

65.1

19.4

1.1

2.3 62.6

90.5

100.0

Trace

68.4

1.7 

(40:37:23) .....

13.6 ---.

1.5 

9.7

5.1 -....

100.0

100.0

42.3

(75)

.2 

.6

10.9

(35)

54.0

29.5

11.3

2.5 

62.7

46.0

100.0

Other characteristics Continued

31

1.566 

55

Intergranular, 

trachytic.

0.02-1.

43

1.565 

55

Porphyritic, 

intergranular 

to intersertal.

0.1-1. 

0.02-0.07

33

1.563 

56

Interpranular Porphyritic, 

to intersertal, trachytic. 

trachytic.

L552

25 20 

1.557 1.554 "~~

56K 57 57 

Intergranular Porphyritic, 

to intersertal. intergranular.

0.1-0.5

0.3-1.2 

0. 02-0.3

....._... 0.1-2. 

0.05-1. 0.02-0.1

4 This figure includes 0.3 percent opal lining vesicles, 

s May be in part K-feldspar.

SILICIC ANDESITE (59-65 PERCENT SiO2)

Silicic andesite (59-65 percent SiO2) is found principally on Mount 

Jefferson, but can also be found interbedded with different andesite at 

other localities of high elevation within the report area.

The silicic andesite contains abundant phenocrysts of plagioclase 

and sparse phenocrysts of dark minerals in an aphanitic groundmass 

(table 4). The groundmass is dark to medium gray and rarely mottled 

with pale brown or light gray. Most of the rocks are somewhat vesicular. The core of the plagioclase phenocrysts is generally intermediate 

labradorite. The mafic phenocrysts include calcic augite or salite 

similar to that found in the less silicic andesite (fig. 11). The somewhat 

more abundant orthopyroxene is ferrous bronzite or magnesian
G22 CONTRIBUTIONS TO GENERAL GEOLOGY

ra

UJ

O

|

UJ

o

cc

UJ

0.

30

20

10

n

i i

Hyalosiderite

Chrysolite 

 

Forsterite

i i

i i

 +

 

**. ..

 «{- 

i i

OLIVINE

60

ii 50

UJ

S

H 40

UJ

o

g 30

0.

Ferrohypersthene

Hypersthene

20

Bronzite

ORTHOPYROXENE

100

90

80

70

60

50

40

30

Anorthite

. .* Bytownite

Labradorite

Andesine

48 50 52 54 56 58 60 62 64 

PLAGIOCLASE PERCENTAGE Si0 2

66 68 70 72 74

FIGUBB 10. Comparison of composition of plagioclase, orthopyroxene, and 

olivine phenocrysts with silica content of the enclosing rock for various samples 

from the Mount Jefferson area. +, Recent; , Pliocene and Pleistocene.

hypersthene (fig. 15). The Fs content is higher in rocks of higher SiC>2 

content (fig. 10). Oxyhornblende, also present in some of the samples, 

is rimmed with opaque material (fig. 16). Part of the oxyhornblende
VOLCANIC ROCKS, MOUNT JEFFERSON AREA, OREGON G23

PERCENTAGE Fs

FIGURE 11. Composition of clinopyroxene from rocks in the Mount Jefferson 

area: +, from basalt; , from andesite; |, from silicic andesite; x, fromdacite; 

O> from intrusive andesite; A, from Recent andesite. Nomenclature from Poldervaart and Hess (1951).

in each rock studied is strongly colored, pleochroic, and highly birefringent, and part is nearly colorless and weakly birefringent. Optical 

data for the strongly colored oxyhornblende from MJW-76 is as 

follows: n«=1.674, %=1.718, ny=l.74G, 2F=75(-).

The groundmass in the silicic andesite samples is much too fine 

grained to determine a point count. Estimates of the composition 

commonly give about 50 percent plagioclase with varying amounts of 

pyroxene, glass, alkali feldspar, cristobalite and indeterminate murky 

"dust" along grain boundaries. Sample MJW-40 contains quartz in 

addition to cristobalite.

Plagioclase phenocrysts are commonly flow alined in these rocks; 

in the two most silicic samples, alined plagioclase microlites form a 

distinct trachytic texture (fig. 16). Color indices are generally low 

(fig. 14), reflecting, as in the andesite, the presence of a rather mafic 

glass.
G24 CONTRIBUTIONS TO GENERAL GEOLOGY

e »*s

ooca T m^

to c^t^ c^coS"

. to .(M .Si - -

I* I

11 s 1 l §

«a

S-S

PH

 Rl

s-s

PH

**"* -^

 f CO l

i-H C<5C

. -1 u ° Ja 

b5 .S
VOLCANIC ROCKS, MOUNT JEFFERSON AREA, OREGON G25

FIGURE 12. Olivine phenocryst (01) embayed and partially altered to magnetite (M) in andesite; plagioclase phenocrysts (P). Groundmass is composed 

of plagioclase microlites with intergranular clinopyroxene and minor glass. 

Plane light, X 50. Sample MJW-92.

FIGURE 13. Intersertal texture in andesite. Euhedral plagioclase laths (P), 

clinopyroxene grains (C) clinging to larger plagioclase grains, and tachylitic 

glass (T). Plane light, X 50. Sample MJG-6.
G26 CONTRIBUTIONS TO GENERAL GEOLOGY

60

50

X 40

u 

0

30

20

10

Basalt

Andesite

Dacite and rhyodacite

46 50 54 58 62 66 70 

SILICA CONTENT FROM FUSED BEAD, IN PERCENT

74

FIGURE 14. Comparison of modal color index and silica content of rocks from

the Mount Jefferson area.

DACITE AND RHYODACITE (66-74 PERCENT SlOs)

Dacite and rhyodacite (66-74 percent SiO2) are present in several 

isolated patches on ridgetops in the northern part of the Mount Jefferson area (fig. 2, table 5). They form a mappable unit of minor 

extent. The rocks are rather variable in appearance; the most typical 

(MJG-73 and MJG-80) are light grayish red to pale red and are flow 

banded so that narrow alternate bands are lighter in color and visibly 

porous. The rhyodacite, MJG-64-2, is light olive gray and less porous. 

There is a small amount of obsidian associated with it at this locality. 

Sample MJW-36 (not separately mapped) is a grayish-black rock with 

glassy luster and dense texture. AU these rocks have an aphanitic 

groundmass with sparse, progressively zoned phenocrysts of plagioclase. The composition of the core portions ranges from sodic labradorite to intermediate andesine.

Oxyhornblende phenocrysts are present in MJG-73 and MJG-80, 

and small amounts of magnesian hypersthene and salite, in MJW-36. 

The groundmass consists of alkali feldspar, plagioclase, cristobalite,
VOLCANIC ROCKS, MOUNT JEFFERSON AREA, OREGON G27

FIGURE 15. Hypersthene phenocryst (H) in silicic andesite. Plagioclase (P) and 

magnetite (M) grains in brown glass (G) with minute crystallites. Plane light, 

X 120. Sample MJW-105.

FIGURE 16. Oxyhornblende phenocryst (H) with opaque margin in silicic 

andesite. Groundmass is plagioclase (P) and magnetite (M) grains with interstitial cristobalite and alkali feldspar; trachytic texture. Plane light, X 75. 

Sample MJW-76.
G28 CONTRIBUTIONS TO GENERAL GEOLOGY

TABLE 5. Modes, in volume percent, and other characteristics of dacite and

rhyodacite

Sample..___-___________._ ....... MJW-36 MJG-73 MJG-80 MJG-64-2

Modes

Phenocrysts: 

Plagioclase____ _________ ______

(An) ___._._._...._____

Clinopyroxene _ ______

(Wo:En:Fs)_________.._

(Fs)... _.______._.__

Oxyhornblende + opaque 

alteration......

Total.. ____________ .

Groundmass: 

Mode expressed as total

Void.. _______________________

Visually estimated composition: 

Alkali feldspar and silica 

minerals __ _ ____ _

Opaque or nearly so . __ 

Total_-___-___.. ___________

4.3

(48) 

.8 

(46:30:24) 

Trace 

(38)

.3

5.4

94.6

(28) 

(62) 

(5)

100.0

5.7 

(50)

2.0

7.7

83.3

9.0

(42) 

(17) 

(21) 

(4)

100.0

13.3

(52)

2.0

15.3

76.0

8.7

(37) 

(42)

(4)

100.0

1.2 

(39)

Trace

.4

1.6

98.4

(72) 

(24)

(2)

100.0

Other characteristics

Refractive index of fused bead. _ . 

Silica content from fused bead . _ _ 

Texture___. _________________ ....

Grain size (mm) :

13

1.516 

66 

Porphyritic, 

intersertal, 

trachytic.

. 05-0. 5

8 

1.510 

67.5 

Porphyritic, 

trachytic.

0. 1-2.

2 

1.508 

68 

Porphyritic, 

granular.

0. 1-1.

H

1.492 

73.5 

Porphyritic, 

granular.

0. 1-0. 5

and glass with scattered opaque minerals. The textures of these rocks 

are porphyritic with a granular to trachytic groundmass.

RECENT ANDESITE

The Recent andesite studied comes from several intracanyon flows 

in the east-central part of the Mount Jefferson area. It is medium 

dark gray, porphyritic, and vesicular and is very fresh in appearance.

Abundant phenocrysts of plagioclase, sparse phenocrysts of olivine 

and clinopyroxene, and in some samples, rare phenocrysts of orthopyroxene (table 6) are set in a fine-grained groundmass of plagioclase, 

clinopyroxene, glass, and opaque minerals. In sample MJW-32 the 

opaque minerals are finely divided throughout the abundant black 

tachylitic glass; consequently, the rock has a low color index for its 

SiO2 content. Plagioclase microlites are flow alined in each of the 

Recent andesite samples.
VOLCANIC ROCKS, MOUNT JEFFERSON AREA, OREGON G29

TABLE 6. Modes, in volume percent, and other characteristics of 

Recent andesite

.Sample... __..................... MJW-18 MJW-95 MJW-32

Modes

Phenocrysts:

Plagioclase........-.-...-... 4.5 9.3 6.3

(An)..................... (56) (79) (77)

Olivine__--_____--------_-__- 1.5 .8 .7

Clinopyroxene___..._._ .1 1.6 2.2

(Wo:En:Fs). -_.---_-. .......___ ...._______ (41:38:21)

Orthopyroxene.------------. ................ .8 .6

Total.-----.---..---------. 6.1 ___ 12.5 ____9.8

Groundmass:

Plagioclase-------. __________ 48.3 31.5 27.7

Clinopyroxene________... 32.8 29.8 9.2

Magnetite and ilmenite_ ... 5.8 3.7 _....------_---_.

Glass........................ 4.7 9.8 33.2

Opal .-.......--.-.-... .....___________ ................ .8

Void..._____.___._..____.____ 2.3 12.7 19.3

Total-..-.-.....--------.. 93.9 87.5 90.2

Total--.-.-----------..-- 100.0 100.0 100.0

Other characteristics

Color index..____________________ 43 47 26

Refractive index of fused bead... 1.574 1.569 1.566

Silica content from fused bead... 54 55 55

Texture_________________...___ Porphyritic Porphyritic Porphyritic

granular intersertal intersertal

flow-alined. flow-alined. flow-alined. 

Grain size (mm):

Phenocrysts.---------.----__ »1.0-2.0; 0.1-1. 0.2-1. 0.1-2.

Groundmass. _ ..____.... 0.02-0.10 <0.01-0.2 0.01-0.5

1 Includes two generations of plagioclase phenocrysts.

INTRUSIVE ROCKS

Most of the intrusive rocks occur as plugs, or round to oval bodies 

that probably fill old vents. They are 500-2,500 feet in maximum dimension and are scattered throughout the Mount Jefferson area. All 

these rocks are andesite except MJ G-72, which is a dacite dike with a 

composition similar to the dacite flow of MJW-36 (p. 26). The intrusive andesite samples form a heterogeneous group and are described separately (table 7).

The coarsest grained rock studied from the Mount Jefferson area is 

an intrusive andesite (MJW-97) that contains sodic labradorite, an 

iron-rich ferro-augite (fig. 11, point farthest to right), and hypersthene. Its low color index indicates that the inferred SiO2 content of 

54% percent is low for this rock; apparently a high iron content caused 

the fused bead to give an abnormally high refractive index and, hence, 

the low inferred SiO2 percentage.

A sample from the plug, at North Cinder Peak (MJW-96) is texturally similar to MJW-97 but is less rich in iron. It contains calcic 

labradorite, calcic augite. and intermediate bronzite.
G30 CONTRIBUTIONS TO GENERAL GEOLOGY

1

 a

 e» 

t.

 §

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a

 5

I 

j

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5 St: ^

H OO

S*S

"3 S

§ i

3 S ^^3 3

 a a3 .s c spas

(-5 S >H *« O
Other characteristics

Grain size (mm):

. .. 23 35

. .- . 1.570 1.561

............ 54# 56. __ ____ Granular _____ Intergranular............ __ 0.1-1.5 <0.01-0.5

24 

1.559 

56 

Granular ____ .

0. 1-0. 8

35 

1.557 

56^ 

Diktytaxitic, intergranular to intersertal, 

trachytic.

0. 02-0. 7

30 

1.547 

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Microporphyritic, intersertal, 

trachytic.

0. 2-0. 5 

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71 

Porphyritic, granular.

0. 1-1. 0 

<0. 01-0. 05

1 Contains trace of clays. 2 Contains 2.8 percent altered inclusions now consisting principally of clinopyroxene and opaque minerals.

O oo
G32 CONTRIBUTIONS TO GENERAL GEOLOGY

A sample from the central plug of Three Fingered Jack (MJG-20) 

is an aphyric rock with plagioclase microlites surrounded by minute 

"beady looking" clinopyroxene grains of indeterminate composition. 

The larger plagioclase microlites are calcic bytownite. The rock contains clinopyroxene-rich patches, which are probably altered inclusions.

Samples from two intrusives near Santiam Pass (MJG-41 and 

MJW-10) have a pronounced intersertal-trachytic texture (fig. 17). 

They contain sodic to intermediate bytownite and their principal 

mafic mineral is orthopyroxene.

PETROLOGY

ANALYSES

Eight rock samples from the Mount Jefferson area have been chemically analyzed by the Rapid Rock Analysis Laboratory of the U.S. 

Geological Survey. The analyses, their norms, and the modes of the 

analyzed rocks are presented in Table 8.

The percentages of the major elements composing these eight 

samples are plotted against percent silica in variation diagrams in 

figures 18 and 19. The curves are drawn by inspection. Each element

FIGURE 17. Intersertal-trachytic texture in intrusive andesite. Euhedral plagioclase microphenocrysts (P) and elongate microlites (M) separated by brown glass 

(G). Orthopyroxene (Or) grains partly in intergranular position. Plane light, X 

120. Sample MJW-10.
TABLE 8. Chemical analyses, norms, and modes of M

[Analysts: P. L. D. Elmore, S. D. Botts, G. Chloe, Dennis Taylor, Hezekia

---- __. Basalt

......... MJW-11

Andesite

MJW-92 MJW-21

Chemical analysis

Si02 .. . ...... ---_

A1203 .- - ....

FeO..... _-.... .....

MgO._ -.._-- ..

CaO. -........ ....

Na2O... ...............

KjO... . ..... ..

JJ Q_j_

TiO2

p Q

MnO.........--. ...

CO2 -. ___..._

 ...... 51.6

....-..-- 17.9

......... 1.5

------- 7.1

......... 7.0

......... 8.6

......... 3.2

------- .60

......... .11

-- --. .45

 .... 1.4

......... .42

...--.__. .16

... ... <.05

...- 100

52.6 

17.1 

5.7 

2.6 

6.9 

8.4 

3.2 

.82 

.04 

.40 

1.2 

.40 

.14 

<.05

100

55.7 

17.8 

1.4 

5.8 

4.6 

7.6 

3.5 

1.0 

.15 

.53 

.98 

.35 

.13 

<.05

100

Si

MJG-63

(weight percent)

58.8 

17.0 

2.6 

3.6 

3.9 

6.2 

3.6 

1.3 

.61 

.59 

.87 

.21 

.11 

<.05

99

Norms (weight percent)

CIPW: 

Q.-... .... .....

C-.

ab_. ........... ....

fs._ ................

ap_. ....... .......

Salic............ ....

Femic. ....____ .......

Niggli: 

K ..............

......... 0.1

... -. 3.5

......... 27.1

......... 32.7

..... 3.0

......... 17.4

.......-- 9.8

.... 2.2

......... 2.7

..... . 1.0

......... 99.5

......... 63.4

......... 36.0

......... 11

5.7

4.9 

27.2 

30.0 

3.9 

17.3

5.4 

2.0 

2.3

1.0

99.7

67.8 

31.8

14

6.9

5.9 

29.8 

30.0 

2.3 

11.5 

8.2 

2.0

1.9

.8

99.3

72.6 

26.7

16

13.6

7.7 

30.6 

26.5 

1.3

9.8 

3.3 

3.8

1.7

.5

98.8

78.6 

20.3

19
G34 CONTRIBUTIONS TO GENERAL GEOLOGY

 a J

C3 ^

« §

« M

Rfc 10.

^H CO-*

. .

oocgoo

o o

o

-*S

ss a » xH o o> 

r SH fe^S

! -*>t>. Q,,w m

! nt> oPM 'O r- 

.3 f 

o o
VOLCANIC ROCKS, MOUNT JEFFERSON AREA, OREGON G35

O 5

o- 3

50 54 58 62 66 70 

PERCENTAGE SiO 2

74 78

FIGURE 18. Silica variation diagram showing percentage of Si02 plotted against percentages of 

K2 0, Na2 0, CaO, and K2 0 + Na2O for Mount 

Jefferson area rocks.

varies in a fairly smooth fashion in this suite of rocks. Iron is highly 

oxidized in some of the rocks and less so in others, but total iron 

decreases smoothly as silica increases.

ALKALI-LIME INDEX

The alkali-lime index (Peacock, 1931), or the percent SiO2 at which 

K2O+Na2O equals CaO, is about 61 (fig. 18), a number which is on 

the boundary between Peacock's calc-alkalic and calcic suites. The 

Mount Jefferson area rocks are thus slightly more calcic than the 

Western Cascade suite (Peck and others, 1964), which has an alkalilime index of 60. The indices of other suites of High Cascade rocks are 

62 for Crater Lake, 63.7 for Mount Shasta, 63.9 for the Lassen region, 

and 63.2 for Mount St. Helens (Williams, 1942, p. 153). Thus the
G36 CONTRIBUTIONS TO GENERAL GEOLOGY

50 54 58 62 66 70 

PERCENTAGE SiO2

FIGURE 19. Silica variation diagram showing percentage of SiO2 plotted against percentages of 

Af2O3, FeO + Fe2O3 , MgO, Ti02, and MnO for Mount 

Jefferson area rocks.

volcanic rocks of the High Cascades form a unified group in the lower 

part of the calcic suite.

COMPARISON WITH OTHER SUITES

The silica variation curves for the Mount Jefferson area are compared with points for 17 rocks from Crater Lake (Williams, 1942, 

p. 148, analyses 4-8; p. 150-151, analyses 14-25) in figure 20. The 

two suites, both from the High Cascades, show similar trends, the
VOLCANIC ROCKS, MOUNT JEFFERSON AREA, OREGON G37

 o

AI 2 O3 MgO

 OX 

FeO+Fe2 O3 K2 O

58 62 66 70 

PERCENTAGE SiO 2

FIGURE 20. Silica variation diagram for Crater Lake 

rocks compared with Mount Jefferson area rocks. 

Points represent analyzed Crater Lake rocks (Williams, 1942). Curves are from Mount Jefferson area 

trends in figures 18 and 19.

most notable difference being the higher content of N&zO in Crater 

Lake rocks at 68-72 percent SiO2 .

The variation curves for the Mount Jefferson area rocks are compared in figures 21 and 22 with points for 22 rocks from the Western 

Cascades (Peck and others, 1964, p. 44-45, analyses 1-22). The trends 

for some elements show a close relationship; others show clear differences, despite the greater scatter in the more heterogeneous Western 

Cascade group.
G38 CONTRIBUTIONS TO GENERAL GEOLOGY

The Mount Jefferson rocks contain more A12O3 , and, at less than 

58 percent SiO2, less FeO-j-Fe2O3 than the Western Cascade suite. 

This confirms the results reported by Peck, Griggs, Schlicker, Wells, 

and Dole (1964, p. 46). Other elements show smaller differences: 

K2O is less abundant in Mount Jefferson rocks, though points overlap; 

Na2O and CaO are comparable; MgO is less abundant in Western 

Cascade rocks in the range of 51-60 percent SiO2 . The Western

50 54 58 62 66 

PERCENTAGE SiO2

70 74

FIGURE 21. Silica variation diagram of K^O,

and CaO for Western Cascade rocks compared with 

Mount Jefferson area rocks. Points represent analyzed 

Western Cascade rocks (Peck and others, 1964). 

Curves are from Mount Jefferson area trends in 

figure 18.
VOLCANIC ROCKS, MOUNT JEFFERSON AREA, OREGON G39

4 -

3 -

2 -

1 -

46 50 54 58 62 66 

PERCENTAGE SiO 2

74

FIGURE 22. Silica variation diagram of A12 03 , FeO + Fe2 03 , and MgO for Western 

Cascade rocks compared with Mount Jefferson area rocks. Points represent 

analyzed Western Cascade rocks (Peck and others, 1964). Curves are from 

Mount Jefferson area trends in figure 19.
G40 CONTRIBUTIONS TO GENERAL GEOLOGY

Cascade rocks, therefore, differ significantly from the High Cascade 

rocks in chemistry as well as age.

A further comparison with a more distant tholeiitic suite is given 

in figure 23. The points in this diagram represent two lavas and a 

differentiated intrusive from the Koolau Volcanic Series on the Island 

of Oahu, Hawaii (Wentworth and Winchell, 1947). The Koolau is 

richer in TiO2 and much richer in FeO+Fe2O3 than the Mount Jefferson rocks, but poorer in A12O3 and Na2O. Percentages of K2O, CaO, 

and MgO are similar. Despite the lower Na2O, the alkali-lime index 

for the Koolau is only 62, owing to the marked decrease in CaO in 

the most silicic differentiate.

NORMATIVE MINERALS

Standard CIPW norms and ratios (table 8) for plotting were determined by using a Boroughs B-5500 computer. All the chemically 

analyzed rocks contain normative quartz. There is some similarity 

between normative and modal plagioclase but very little between 

normative and modal pyroxene. The highly oxidized state of the 

iron in several of the rocks creates large amounts of normative magnetite at the expense of ferrosilite, and in one sample the Fe2O3 :FeO 

ratio is so high that rutile appears in the norm. Normative corundum 

appears in the four most silicic rocks. This extra alumina must be 

present in the glass or substituted in the pyroxenes.

The presence of normative wollastonite in the four least silicic rocks 

shows them to be meta-aluminous (mol. percent of A12O3>K2O+ 

Na^O but <K2O+Na2O+CaO) and the presence of normative 

corundum in the four more silicic rocks shows them to be per-aluminous 

(mol. percent of Al2O3>K2O+Na2O+CaO).

Niggli K values (molecular ratio K2O to K2O+Na2O) exceed 20 

only above 67 percent SiO2 . The paucity of K in these rocks is illustrated by the triangular plots (figs. 24 and 25) showing the proportions of the normative feldspar components and the proportions of 

normative ^plagioclase, orthoclase, and quartz. The latter diagram 

also suggests that differentiation of the magma or magmas in the 

Mount Jefferson area approached compositions near the isobaric 

minima in the silica-alkali feldspar-water system (Tuttle and Bowen, 

1958).

PHASE RELATIONSHIPS AND CRYSTALLIZATION HISTORY

Plots of the Mount Jefferson area rocks on an iron enrichment 

versus silica diagram define a curve similar to experimentally derived 

curves for crystallization in the system SiO2-MgO-FeO-Fe2O3 

(Osborn, 1959; figs. 26, 27 of this paper). The Mount Jefferson area 

curve also resembles Osborn's curve for volcanic rocks from the
VOLCANIC ROCKS, MOUNT JEFFERSON AREA, OREGON G41

2 -

1 -

46

AI 2 0 3

EXPLANATION 

Na 2 O

O ©

AI 2 03

© 

FeO + Fe 2 O3

 

CaO

O 

MgO

X 

K 2 O

TiO 2

50 54 58 62 66 

PERCENTAGE SiO 2

70 74 78

FIGURE 23. Silica variation diagram for rocks of the Koolau Volcanic Series, 

Oahu, Hawaii, compared with Mount Jefferson area rocks. Points represent 

analyzed rocks of the Koolau Volcanic Series. Curves are from Mount Jefferson 

area trends in figures 18 and 19.
G42 CONTRIBUTIONS TO GENERAL GEOLOGY

FIGURE 24. Composition of normative feldspar in eight chemically analyzed 

rocks from the Mount Jefferson area. Weight ratios plotted.

Cascade Range, and, more poorly, the curve of Nockold's averages. 

The Mount Jefferson area curve is probably better than the latter 

two for comparison with the experimental curves because it represents 

probable successive differentiates collected in a small area.

Insofar as Osborn's experimental data for a simple system are applicable, the Mount Jefferson area rocks can be interpreted to have 

crystallized under constant or increasing oxygen pressure. The steep 

slope of the Mount Jefferson curve suggests, however, that the pressure may actually have decreased slightly during crystallization. 

Recent work (Presnall, 1966) on the system SiO2-CaO-MgO-FeOFeaOs has confirmed Osborn's conclusions about the influence of 

oxygen pressure on crystallization and has shown them to be applicable to the more complicated system including CaO, which more 

closely resembles basalt magma.

Osborn (1959) proposed that the oxygen necessary to maintain 

constant or rising oxygen pressure in a crystallizing magma is supplied 

primarily by the dissociation of water, and hence, that the magmas 

crystallizing under these conditions are relatively water rich. Therefore,
VOLCANIC ROCKS, MOUNT JEFFERSON AREA, OREGON G43

Q

Ab+An

FIGURE 25. Ratios of normative quartz, plagioclase, and orthoclase in eight 

chemically analyzed rocks from the Mount Jefferson area. Weight ratios 

plotted. Crosses represent isobaric minima in the system NaAlSisOg-KAlSisOgSiO2~H2 O between 500 and 4,000 kg per cm2 pressure (from Tuttle and Bowen, 

1958, p. 75).

the curves also suggest that the parent magma for the Mount Jefferson 

area rocks contained a relatively abundant supply of water.

ORIGIN AND DERIVATION OF THE VOLCANIC BOCKS

The volcanic rocks of the High Cascades were erupted from a northsouth line of vents lying east of those that produced the volcanic rocks 

of the Western Cascades (Peck and others, 1964, p. 52). The line of 

active vents appears to have migrated eastward since the early 

Oligocene, and its activity culminated in the building of the High 

Cascades during the Pliocene and Pleistocene.

In the Mount Jefferson area, eruption was from a number of vents. 

The principal ones were Mount Jefferson and Three Fingered Jack, 

but venting of lavas also occurred at other places.

Mapped areas of cinders and associated flow rocks (fig. 2) certainly 

represent vents, and the intrusive plugs very likely mark vent locations. Some of the older vents, particularly those that erupted the
G44 CONTRIBUTIONS TO GENERAL GEOLOGY

1.0

0.9

0.8

0.7

0.6

132

30 35 40 45 

WEIGHT PERCENTAGE SiO 2

50

FIGURE 26. Comparison of iron enrichment and 

silica content of experimental liquids undergoing 

fractional crystallization under different conditions: 

curve n-24, constant total composition; curve n-d, 

constant O2 pressure; curve n-b increasing 62 pressure (from Osborn, 1959).

lowermost basalt of the High Cascade volcanic rocks, are probably 

buried. The Recent intracanyon flows came from small vents not coincident with Mount Jefferson or Three Fingered Jack, but on the 

same north-south line.

The field positions of the volcanic rocks are compatible with a 

general pattern of eruption of successively more silicic lavas from both 

the main and subsidiary vents. The most silicic rocks are found on 

ridge crests north of Mount Jefferson. These may be remnants of
VOLCANIC ROCKS, MOUNT JEFFERSON AREA, OREGON G45

1.0

0.9

0.8

0.7

0.6

0.5

0.4

Sk

45 50 55 60 65 

WEIGHT PERCENTAGE SiO2

70 75

FIGURE 27. Comparison of iron enrichment and silica content of various magma 

series: Ski-Sk3, Skaergaard intrusion (Wager and Deer, 1939); I-III, averages 

for tholeiitic rocks (Nockolds, 1954); IV-VI, averages for andesite, dacite, and 

rhyodacite (Nockolds, 1954); 2-7, assorted rocks from the Cascade Range 

(Turner and Verhoogen, 1951); , analyzed samples from the Mount Jefferson 

area (in part from Osborn, 1959).
G46 CONTRIBUTIONS TO GENERAL GEOLOGY

formerly more extensive flows that originated from Mount Jefferson, 

or they may have come from local vents.

The graph of frequency of types of rock (fig. 6) is not based on 

systematic sampling, but it suggests (dashed line) an overall distribution that is unimodal and strongly skewed toward the mafic compositions. Thus there are large volumes of lava with 53-56 percent silica 

and successively smaller volumes with higher percentages of silica. 

Small volumes of lava with less than 53 percent silica may be basic 

differentiates.

Probably systematic sampling designed to give a quantitative estimate of volume versus silica content would give a frequency distribution curve of similar shape. Such a curve plus the regular trends 

displayed by the petrologic diagrams (fig. 18, 19, 24, 25) suggest that 

these rocks are either successive differentiates from a common parent 

magma with about 53-54 percent SiO2 or from several magmas of 

similar original composition which have undergone similar differentiation.

The Recent intracanyon flows have compositions similar to the most 

common composition of the older flows (fig. 6). Therefore, they must 

represent a reappearance of undifferentiated parental magma in new 

vents along the same line of crustal weakness occupied by the major 

vents of the High Cascades.

REFERENCES CITED

Bowen, N. L., 1956, The evolution of the igneous rocks, with a new introduction by

J. F. Schairer: New York, Dover Pub., Inc., 332 p, [repr. from 1928 ed.]. 

Callaghan, Eugene, 1933, Some features of the volcanic sequence in the Cascade

Range in Oregon: Am. Geophys. Union Trans., 14th Ann. Mtg., p. 243-249. 

Carr, J. M., 1954, Zoned plagioclases in layered gabbros of the Skaergaard intrusion, east Greenland: Mineralog. Mag. [London], v. 30, no. 225, p. 367-375. 

Chayes, Felix, 1952, Relations between composition and indices of refraction in

natural plagioclase: Am. Jour. Sci., Bowen Volume, pt. 1, p. 85-105. 

Emmons, R. C., 1943, The universal stage (with five axes of rotation): Geol. Soc.

America Mem. 8, 205 p. 

Goddard, E. N., chm., and others, 1948, Rock-color chart: Washington, Natl.

Research Council (repub. by Geol. Soc. America, 1951), 6 p. 

Harloff, C. E. A., 1927, Zonal structure in plagioclase: Leidse Geol. Medd., v. 2, pt.

2, p. 99-114. 

Hess, H. H., 1949, Chemical composition and optical properties of common clinopyroxenes, Pt. 1: Am. Mineralogist, v. 34, nos. 9-10, p. 621-666. 

 1960, Stillwater igneous complex, Montana a quantitative mineralogic

study: Geol. Soc. America Mem. 80, 230 p. 

Hills, E. S., 1936, Reverse and oscillatory zoning in plagioclase feldspars: Geol.

Mag. [London], v. 73, no. 860, p. 49-56. 

Hodge, E. T., 1925, Geology of Mount Jefferson [Oregon]: Mazama, Ann. Ser., v7, no. 2, p. 25-58.
VOLCANIC ROCKS, MOUNT JEFFERSON AREA, OREGON G47

Huber, N. K., and Rinehart, C. D., 1966, Some relationships between the refractive index of fused glass beads and the petrologic affinity of volcanic rock 

suites: Geol. Soc. America Bull., v. 77, no. 1, p. 101-110.

Kennedy, G. C., 1947, Charts for correlation of optical properties with chemical 

composition of some rock-forming minerals: Am. Mineralogist, v. 32, nos. 

9-10, p. 561-573.

Kuno, Hisashi, 1954, Study of orthopyroxene from volcanic rocks: Am. Mineralogist, v. 39, nos. 1-2, p. 30-46.

Nockolds, S. R., 1954, Average chemical composition of some igneous rocks: 

Geol. Soc. America Bull., v. 65, no. 10, p. 1007-1032.

Osborn, E. F., 1959, Role of oxygen pressure in the crystallization and differentiation of basaltic magma: Am. Jour. Sci., v. 257, no. 9, p. 609-647.

Peacock, M. A., 1931, Classification of igneous rock series: Jour. Geology, v. 39, 

no. 1, p. 54-67.

Peck, D. L., Griggs, A. B., Schlicker, H. G., Wells, F. G., and Dole, H. M., 1964, 

Geology of the central and northern parts of the Western Cascade Range in 

Oregon: U.S. Geol. Survey Prof. Paper 449, 56 p.

Poldervaart, Arie, 1950, Correlation of physical properties and chemical composition in the plagioclase, olivine, and orthopyroxene series: Am. Mineralogist, v. 

35, nos. 11-12, p. 1067-1079.

Poldervaart, Arie, and Hess, H. H., 1951, Pyroxenes in the crystallization of 

basaltic magma: Jour. Geology, v. 59, no. 5, p. 472-489.

Presnall, D. C., 1966, The join forsterite-diopside-iron oxide and its bearing on 

the crystallization of basaltic and ultramafic magmas: Am. Jour. Sei., v. 

264, no. 10, p. 753-809.

Sheppard, R. A., 1962, Iddingsitization and recurrent crystallization of olivine 

in basalts from the Simcoe Mountains, Washington: Am. Jour. Sci., v. 260, 

no. 1, p. 67-74.

Smith, J. R., 1958, Optical properties of heated plagioclases: Am. Mineralogist, 

v. 43, nos. 11-12, p. 1179-1194.

Thayer, T. P., 1937, Petrology of later Tertiary and Quaternary rocks of the 

north-central Cascade Mountains in Oregon, with notes on similar rocks in 

western Nevada: Geol. Soc. America Bull., v. 48, no. 11, p. 1611-1651.

 1939, Geology of the Salem Hills and the North Santiam River basin, 

Oregon: Oregon Dept. Geology and Mineral Industries Bull. 15, 40 p.

Turner, F. J., and Verhoogen, Jean, 1951, Igneous and metamorphic petrology 

[1st ed.]: New York, McGraw-Hill Book Co., Inc., 602 p.

Tuttle, O. F., and Bowen, N. L., 1958, Origin of granite in the light of experimental studies in the system NaAlSisOg-KAlSiaOg-S^-^O: Geol. Soc. 

America Mem. 74, 153 p.

Vance, J. A., 1962, Zoning in igneous plagioclase normal and oscillatory zoning: 

Am. Jour. Sci., v. 260, no. 10, p. 746-760.

 1965, Zoning in igneous plagioclase patchy zoning: Jour. Geology, v. 73,

no. 4, p. 636-651. 

Wager, L. R., and Deer, W. A., 1939, The petrology of the Skaergaard intrusion,

Kangerdlugssuaq, East Greenland, Pt. 3 of Geological investigations in

East Greenland: Medd. Gr0nland, v. 105, no. 4, 352 p. 

Walker, G. W., Greene, R. C., and Pattee, E. C., 1966, Mineral resources of the

Mount Jefferson primitive area, Oregon: U.S. Geol. Survey Bull. 1230-D,

p. D1-D32. 

Wentworth, C. K., and Winchell, Horace, 1947, The Koolau basalt series, Oahu,

Hawaii: Geol. Soc. America Bull., v. 58, no. 1, p. 49-77.
G48 CONTRIBUTIONS TO GENERAL GEOLOGY

Williams, Howel, 1932a, Geology of the Lassen Volcanic National Park, California: California Univ. Dept. Geol. Sci. Bull., v. 21, no. 8, p. 195-385.

 1932b, Mount Shasta, a Cascade volcano: Jour. Geology, v. 40, no. 5, p. 

417-429.

 1933, Mount Thielsen, a dissected Cascade volcano: California Univ. Dept. 

Geol. Sci. Bull., v. 23, no. 6, p. 195-213.

 1935, Newberry volcano of central Oregon: Geol. Soc. America Bull., 

v. 46, no. 2, p. 253-304.

 1942, The geology of Crater Lake National Park, Oregon, with a reconnaissance of the Cascade Range southward to Mount Shasta: Carnegie Inst. 

Washington Pub. 540, 162 p.

 1944, Volcanoes of the Three Sisters region, Oregon Cascades: California 

Univ. Dept. Geol. Sci. Bull., v. 27, no. 3, p. 37-83.

1957, A geologic map of the Bend quadrangle, Oregon, and a reconnaissance geologic map of the central portion of the High Cascade Mountains: 

Oregon Dept. Geology and Mineral Industries, in coop, with U.S. Geol. 

Survey, scale 1:125,000.

Williams, Howel, Turner, F. J., and Gilbert, C. M., 1954, Petrography an introduction to the study of rocks in thin sections: San Francisco, W. H. Freeman 

and Co., 406 p.

U. S. GOVERNMENT PRINTING OFFICE : 1968 O - 285-003