BULLETIN OF THE GEOLOGICAL. SOCIETY OF AMERICA VOL. 48. PP. 961-972. PUS. 77-78.1 FI6. JUNE 30.193S

BASALTIC ROCKS IN THE UMPQUA FORMATION *f BY FRANCIS G. WELLS AND AARON C. WATERS (Accepted by the Committee on Publications, 1934) CONTENTS Page Introduction...... 961 Summary of geology...... 963 Effusive rocks...... 963 Amygdaloidal and ellipsoidal basalts...... 963 Distribution...... 963 Petrography...... 963 Structure and extrusive origin of the basalts...... 964 Palagonite tuff and breccia...... 966 Intrusive rocks...... 967 Olivine basalt dikes...... 967 Gabbro and norite...... 968 Norite sills in the Umpqua formation...... 968 Distribution and character...... 968 Petrography...... 968 Volcanic necks and dikes cutting the Calapooya formation...... 968 Distribution and character...... 968 Petrography...... 969 Hypersthene basalt dikes...... 969 Chemical differences between Eocene and Miocene (?) igneous rocks...... 971

INTRODUCTION In 1930, during a study of the geology of an area in west-central Oregon within which occur the quicksilver deposits of Blackbutte, Elk- head, Bonanza, and Nonpareil, a series of extrusive and intrusive basal­ tic igneous rocks were encountered. Part of this area lies within the Roseburg quadrangle, mapped by Diller in 1898. Diller called all these rocks “diabase.” 1 It is the purpose of this paper to show that the basaltic rocks include amygdaloidal and ellipsoidal basalt flows, olivine basalt dikes, norite sills and necks, and hypersthene-augite * Manuscript received by the Secretary of the Society, September 10, 1934. t Published by permission of the Director, U. S. Geological Survey. 1J. S. Biller: Description of the Roseburg quadrangle [Oreg.], U. S. Geol. Surv., Geol. Atlas, Roseburg folio, no. 49 (1898) p. 3. (961)

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/46/6/961/3430507/BUL46_6-0961.pdf by guest on 26 September 2021 962 F. G. WELLS, A. C. WATERS----BASALTIC HOCKS IN UMPQUA FORMATION

EXPLANATION BEDDED ROCKS

<§¡1!i Alluvium E3 1( ESI ,p (Calapooya formation ^ UNCONFORMITY „[ Tev

F igure 1.— Geologic sketch map of Blackbutte-Elkhead-Nonpareil area, Oregon

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/46/6/961/3430507/BUL46_6-0961.pdf by guest on 26 September 2021 INTRODUCTION 963 basalt dikes and that they represent more than one period of igneous activity. SUMMARY OF GEOLOGY A full discussion of the geology of this area is found in a recent bulle­ tin of the United States Geological Survey,2 it can be briefly sum­ marized as follows: The southwestern part of the area (Fig. 1) is underlain by the Umpqua formation, a series of interbedded sandstones and shales of Eocene age, that have been compressed into gentle folds with a north­ west trend. Intercalated in and contemporaneous with the sandstones are lenses of conglomerate, beds of palagonite tuff, and flows of amygda- loidal and ellipsoidal basalt. The Umpqua formation has also been intruded by norite sills and basalt dikes. The Calapooya formation, a series of volcanic conglomerates, andesitic tuffs, breccias, and flows, of Eocene (?) age, that dips at a low angle toward the east, rests un- conformably on the Umpqua formation in the eastern part of the area. The Calapooya formation is cut by basalt dikes and norite necks. EFFUSIVE ROCKS AMYGDALOIDAL AND ELLIPSOIDAL BASALT FLOWS Distribution.—Basalt flows are exposed in irregular, usually elongate outcrops, the long axes of which trend parallel to the strike of the Umpqua beds. These outcrops are most abundant in the southern part of the area, in Township 25 South, Range 4 West, and in a large elliptical area in Township 23 South, Ranges 4 and 5 West. Petrography.—Megascopically, specimens of the basalt are dark greenish gray to black and are usually aphanitic—though the amygda- loidal varieties are commonly porphyritic, the phenocrysts of plagio- clase being aggregated together in more or less conspicuous clots. A few small vesicles filled with zeolite minerals are present in most of the specimens. The amygdaloidal varieties are characterized by abundant white amygdules a centimeter or more in maximum diameter. The cavernous areas between the ellipsoids as well as the closely spaced radial joints have greatly facilitated weathering, and as a result the glass selvages are commonly stained. Under the microscope the lava is seen to be of normal basaltic char­ acter. In general the texture of the ellipsoidal flows is intersertal; of the amygdaloidal flows, intergranular. Augite and labradorite (An56) *F. G. Wells and A. C. Waters: Quicksilver deposits of sottthwestem Oregon, TJ. S. Geol. Surv., Bull. 850 (1934).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/46/6/961/3430507/BUL46_6-0961.pdf by guest on 26 September 2021 964 F. G. WELLS, A. C. WATERS— BASALTIC ROCKS IN TJMPQUA FORMATION are the most abundant minerals and are present in approximately equal amounts, the augite with much opaque dark-brown glass occupying the spaces between the randomly arranged minute laths of labradorite. Augite and labradorite occur in two generations, and in each the pheno- crysts are aggregated together in glomeroporphyritic clots. Pseudo- morphs of a yellowish-brown alteration product after olivine are spar­ ingly present. Magnetite is a common accessory, and minute crystals of apatite are also found. In some specimens the groundmass has been partly altered to serpentine and a reddish-brown product. The amygdaloidal cavities commonly present an outer covering of chlorite and a central filling of fibrous chalcedony with negative elongation, or they may be filled with calcite and zeolites, of which thomsonite (PI. 77, fig. 1)—easily recognized by its low relief, positive elongation, and parallel extinctions—is the most abundant. Although glass is very abundant in the groundmass of the ellipsoidal basalts, the dark-brown opaque glass that forms the mesostasis be­ tween the adjacent minerals differs strikingly from the transparent glass formed by chilling in contact with water at the surface of the ellipsoids. This is well shown by a thin section of glass breccia exposed in a small quarry near the northeastern extension of the lava mass on Banks Creek. Under the microscope this rock is seen to be composed of large shards of perfectly clear light yellowish-brown glass in which are embedded a few tabular crystals of feldspar and grains of augite (PI. 78, fig. 1). The glass is perfectly isotropic. It is traversed by numerous cracks, along which it has been hydrated to a yellowish col- loidal-appearing substance showing very faint aggregate polarization and a relief less than that of canada balsam. This specimen prob­ ably represents an incipient stage in the alteration of the glass to palagonite. In the same thin section, fragments of exceptionally glassy lava may also be seen, but the glassy base of the lava is of the opaque variety. A chemical analysis of the flow rock (Table 1, col. 1) shows it to be a typical basalt, corresponding closely to the average of plateau basalts given by Daly (Table 1, col. 3). Structure and extrusive origin of the basalts.—Though most of the flows are massive, showing no structure, they may grade up­ ward into amygdaloidal or ellipsoidal material, or, more commonly, the whole flow may contain such material. In extreme examples, filled amygdules may constitute as much as 25 percent of the mass, and the ellipsoidal flows may consist of large individual pillows embedded in

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/46/6/961/3430507/BUL46_6-0961.pdf by guest on 26 September 2021 BULL. GEOL. SOC. AM. VOL. 46, 1935, PL. 77

F i g u r e 1. BASALT FROM GASSY CREEK F i g u r e 2. DIABASE DIKE Showing a large amygdule filled with thomsonite. Exposed in the northwestern part of the Nonpareil- (Ordinary light, X 18) Note the low degree of crys- Bonanza area. Light gray (plagioclase), dark gray tallinity of the basalt as compared with the basalt (augite), black (magnetite). (Ordinary Light, X 18) dike illustrated in figure 2.

F i g u r e 3. NORITE SILL FROM CAZAD MOUNTAIN Showing magnetite enclosing hypersthene, augite, and labradorite. (Ordinary light, X 18)

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Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/46/6/961/3430507/BUL46_6-0961.pdf by guest on 26 September 2021 EFFUSIVE BOCKS 965 palagonite tuff. The pillows average two feet in diameter but range from one to eight feet. They conform in general to a fairly uniform globular or ellipsoidal outline, though many of them have rounded ex­ tensions like the irregularities of a knotty potato. Individual ellip­ soids manifest a characteristic radial columnar jointing, which sepa­ rates the globular mass of lava into pyramidal blocks with bases rarely more than two inches across. The flows are arranged in definite layers from three to sixty feet thick. The top and, to a lesser degree, the bottom in the amygdaloidal varieties is clearly marked by the greater abundance of amygdules in the highly scoriaceous surface. At many localities, notably on the hillside northwest of Adams Creek and in the Elkhead mine, the con­ tact between the amygdaloidal lavas and the overlying sedimentary rocks of the Umpqua formation can be seen. The contact is clearly erosional: the overlying sediments show no evidence of baking and commonly contain pebbles of the amygdaloid. The structure of the lava flows is difficult to ascertain, but in the few localities where dips and strikes can be accurately determined the structure appears to conform in a general way to that of the Umpqua formation. These structures indicate that the basalts are flows intercalated in the Umpqua formation. Additional facts that support this inter­ pretation are: 1. The conglomerates of the Umpqua formation, instead of being intruded by the basalt (“diabase”), are filled with pebbles of it. Basal­ tic material locally forms more than 90 percent of these conglomerates. 2. The local occurrence of unmetamorphosed conglomeratic material as a filling between ellipsoidal masses affords definite proof that the basalt is not intrusive. 3. Amygdaloidal and vesicular texture can be observed in the basalt at almost every locality. 4. The basalt shows remarkably low crystallinity and always con­ tains a considerable mesostasis of glass in the groundmass. The tex­ ture is intersertal, nowhere diabasic. This is believed to be due to drastic chilling resulting from extrusion into water. 5. Individual ellipsoids are commonly margined by a chilled glassy surface, produced by contact with the water into which they were ex­ truded. 6. At certain localities the ellipsoidal lavas are intimately associated with a variety of basaltic glass (sideromelane) and its associated altera­ tion product, palagonite—basaltic glass, which has been shown by re­

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/46/6/961/3430507/BUL46_6-0961.pdf by guest on 26 September 2021 966 F. G. WELLS, A. C. WATERS— BASALTIC BOCKS IN UMPQUA FORMATION cent writers8 to originate through the drastic quenching of basaltic magma either by water or by glacial ice. It seems doubtful if any considerable part of the large body mapped as diabase in the Roseburg quadrangle is actually intrusive. Diller4 considered the finding of fragments of shale enclosed in the igneous rock near Roseburg to constitute proof of intrusive origin. Fuller5 has shown, however, that this relationship may be brought about in the normal course of subaqueous extrusion. In the roadcut along the Pa­ cific Highway, just south of Roseburg, ellipsoidal lavas occur at several horizons in the Umpqua shales, which show no baking at the upper con­ tacts. From these shales, F. E. Turner has collected foraminifera which are being studied by Turner and S. A. Berthiaume.® PALAGONITE TUFF AND BRECCIA Intimately associated with the ellipsoidal lavas is fragmental mate­ rial, composed of altered basaltic glass (palagonite) that ranges in texture from exceedingly fine-grained tuff to breccia in which the individual fragments are two or three inches in length. At some localities, as for example, near the southeastern comer of section 35, Township 24 South, Range 4 West, this fragmental material fills the cavernous spaces between adjacent pillows, and individual exposures may show ellipsoids and pyroclastic material intermingled in all proportions. At other localities, as in the southern part of section 16, Township 25 South, Range 4 West, the fragmental material occurs as a definite stratum that is not in contact with lava, but none of it has been found at an appreciable stratigraphic distance from the ellipsoidal lava, and the local intimate intermingling of the two, as well as the purely basaltic composition of the fragmental pyroclastic material, definitely establishes their contemporaneous origin. This fragmental material was separately mapped by Diller7 as the Wilbur tuff lentils. The finer-grained tuffs resemble rather closely a hard mudstone or argillite. On fresh fracture they are a dark green or steel-blue, but they weather readily to dull black and then resemble a fine-grained carbona­ 8 M. A. Peacock: The petrology of Iceland. Pt. 1. The basic tuffs, Roy. Soc. Edinburgh, Tr., vol. 55 (1926) p. 51-76; The geology of Videy, southwest Iceland—o record of igneous action in glacial times, Roy. Soc. Edinburgh, Tr., vol. 54 (1926) p. 441-465. M. A. Peacock and R. E. Fuller: Chlorophaeite, sideromelane, and palagonite from the Columbia River Plateau, Am, Min., vol. 13 (1928) p. 365-383. R. E. Fuller: The aqueous chilling of basaltic lava on the Colttmbia Rivet Plateau, Am. Jour. Sci., 5th ser.t vol. 21 (1931) p. 281-300. * J. 8. Diller: ibid. 5R. E. Fuller: The aqueous chilling of basaltic lava on the Columbia River Plateau, Am. Jour. Sci., 5th ser., vol. 21 (1931) p. 297. • Personal communication from F. E. Turner. 7 J. S. Diller: ibid.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/46/6/961/3430507/BUL46_6-0961.pdf by guest on 26 September 2021 EFFUSIVE ROCKS 967 ceous sediment. The coarser breccias, as is well illustrated by the ex­ posures on the small rounded hill in Salt Lick Valley, are locally tran­ sitional into the conglomerates of the Umpqua formation. The breccias vary greatly in color, but medium greenish-gray or light-brown material is the most common. The very fine-grained varieties are almost black. Although under the microscope these pyroclastic rocks show structure characteristic of palagonite breccia, original clear palagonite is not abundant, having been for the most part transformed into an almost irresolvable mass of obscure, very feebly bi-refracting substances, some of which appear to be of chloritic nature. Occasionally, however, frag­ ments can be seen which are nearly isotropic and which show the typical colloform structure (PI. 78, fig. 2) and low index of refraction of palago­ nite. Material identifiable as palagonite is seen to be completely tran­ sitional into the more common aggregates of weakly bi-refracting sub­ stances. In addition to the palagonite and its decomposition products, fragments of unaltered basalt and isolated fragments of the constituent minerals of the basalt are commonly present. In many specimens, car­ bonates are abundant; opal and chalcedony are common, and in some specimens, zeolites also occur. Embedded in specimens of palagonite tuff collected from Banks Creek, Diller8 found the remains of calcareous micro-organisms suggest­ ing Globigerina, and minute bodies doubtfully identified as the tests of siliceous organisms. INTRUSIVE ROCKS OLIVINE BASALT DIKES Three large dikes and a few smaller dikes of olivine basalt cut the Umpqua formation in the northwest corner of the Nonpareil-Bonanza area. These dikes are very dark gray, rather coarsely crystalline, and very fresh. Under the microscope the rock is seen to have a typical ophitic texture and to consist of nearly equal amounts of augite and labradorite in grains of varying size (PI. 77, fig. 2). Olivine was for­ merly common but has been changed largely to a light-brown, almost isotropic alteration product. Its remaining fresh grains have optical properties (negative, 2V close to 90°) that indicate a high iron content. Magnetite and apatite are accessory. A little chlorite is present. These dikes correspond closely in chemical composition (Table 1, col. 2) to the basalt flows intercalated in the Umpqua formation and probably represent feeders to these flows. * Ibid.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/46/6/961/3430507/BUL46_6-0961.pdf by guest on 26 September 2021 968 F. G. WELLS, A. C. WATERS— BASALTIC BOCKS IN UMPQUA FORMATION GABBRO AND NORITE At many places within the Blackbutte-Elkhead-Nonpareil area the rocks of the Umpqua and Calapooya formations have been intruded by small sills, dikes, and volcanic necks of gabbroic and noritic composi­ tion. These are relatively resistant to erosion and form peaks such as Cazad Mountain, Harness Mountain, Bald Mountain, and Steens Butte. NORITE SILLS IN THE UMPQUA FORMATION Distribution and character.—Sills of intrusive rock crop out south and east of the Elkhead mine. The upper portion of Cazad Mountain is composed of a norite sill, about 400 feet thick, which dips eastward with the enclosing Umpqua formation. Along the crest of the mountain the Umpqua beds near the norite contact have been hardened, and a pecu­ liar mottling has been developed in certain beds of sandstone. The sill intruding the Umpqua formation east of the Elkhead mine, in sections 26 and 25, Township 23 South, Range 4 West, has also produced note­ worthy alteration of the enclosing sediments. Petrography.—The norite on Cazad Mountain has a granular texture and a medium-gray color not very different from that of the dark-col­ ored diorites. Light-colored minerals predominate slightly. The microscope shows that the rock consists of labradorite (An59), augite, hypersthene, and magnetite, named in order of decreasing abun­ dance. Hypersthene and augite are present in nearly equal amounts and together make up nearly half of the rock. The feldspar and the hypersthene have typically euhedral outlines. Some of the augite shows crystal boundaries, but most of it is interstitial to the other minerals. Magnetite, which occurs as large continuous anhedral plates, appears to have completed its crystallization late, for it encloses all the other minerals and sends minute fingerlike projections into cracks within the other minerals or into openings between them (PI. 77, fig. 3). Near the middle part of this sill there are segregated bodies of a very coarse-grained, lighter-colored rock composed essentially of labradorite, augite, and magnetite with varying amounts of hypersthene. They can, therefore, be called “augite gabbro.” The observed relations indicate that these bodies were produced through differentiation subsequent to the injection of the igneous magma. VOLCANIC NECKS AND DIKES CUTTING THE CALAPOOYi FORMATION Distribution and character.—The higher peaks surrounded by the Calapooya formation consist of intrusive rock, forming bodies with the oval or elliptical outlines characteristic of volcanic necks or small stocks.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/46/6/961/3430507/BUL46_6-0961.pdf by guest on 26 September 2021 INTRUSIVE BOCKS 969 Two such masses, on Bald Mountain and on Harness Mountain, occur within the Blackbutte-Elkhead area, and a third prominent mass forms Steens Butte, which lies just east of the area mapped. Steens Butte probably illustrates the nature of these intrusive bodies better than the others. It is a roughly cylindrical plug of dark-gray microgranular diabasic rock, about a mile in diameter, that has forced its way through the upper, or dominantly igneous, facies of the Calapooya formation. It has well-developed columnar jointing. The joints are vertical in the center of the plug, but toward the margins they curve outward to a horizontal position approximately perpendicular to the surrounding wall. Curving columns of this character are developed to a lesser de­ gree in the plugs of Harness Mountain and Bald Mountain. The Har­ ness Mountain plug is elliptical, with its longer axis about two and one- half times the shorter. The Bald Mountain plug is roughly circular. Petrography.—The intrusive body of Steens Butte has a fine-grained subhedral gabbroid texture and in mineral composition shows relation­ ship to the Cazad Mountain sill. Its constituents, named in order of decreasing abundance, are labradorite (AnB6), augite, hypersthene, biotite, quartz, orthoclase, magnetite, and apatite. Biotite is scarce and is invariably associated with hypersthene. Quartz and orthoclase occur together in fine-grained aggregates. The texture of the rock is that of a fine-grained gabbro, with a noticeably strong tendency toward idio- morphism among the constituent minerals. Though mineralogically the rock resembles the norite of Cazad Mountain, the norm calculated from the chemical analysis has a high percentage of quartz and alkali feldspar, so the rock may be designated a “quartz-augite diorite.” This rock shows marked chemical similarity to a small augite diorite plug (Table 1, col. 6) in the Bohemia district, about 15 miles to the east. A specimen collected within the border of the Bald Mountain intru­ sion consists of labradorite, augite, and minor amounts of hypersthene and well-formed magnetite embedded in a glassy groundmass crowded with tiny microlites. The texture is, like that of an intersertal basalt, exceptionally rich in feldspar. As the feldspar is labradorite, the rock may be called a “hypersthene basalt.” HYPERSTHENE BASALT DIKES A series of dikes differing slightly in texture and component minerals, but all of basaltic composition, intrude the Umpqua and Calapooya formations in the Blackbutte-Elkhead-Norpareil area. A basalt dike exposed on the 900-foot level in the Blackbutte mine is typical. Under

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/46/6/961/3430507/BUL46_6-0961.pdf by guest on 26 September 2021 970 P. G. WELLS, A. C. WATERS— BASALTIC ROCKS IN UMPQUA FORMATION the microscope it shows aggregates of augite and hypersthene embedded in a felt of roughly parallel Iabradorite laths. Well-formed magnetite is an abundant accessory. The rock is surprisingly fresh in appearance, as contrasted with the adjacent highly altered andesite, and this sug­ gests that it may have been intruded subsequent to the deposition of

T able 1.—Analyses of typical igneous rocks from the Blackbutte- Elkhead-Nonpareil area

1 2 3 4 5 6 7

Si02...... 47.51 47.90 48.80 50.23 53.63 52.67 53.27 12.81 13.79 13.98 15.85 17.95 17.36 17.08 FeOa...... 6.28 5.13 3.59 2.95 3.24 3.37 2.93 FeO...... 6.86 8.26 9.78 7.22 4.84 5.14 6.06 MgO...... 6.58 6.18 6.70 6.84 4.94 5.06 5.12 CaO...... 10.84 10.69 9.38 10.16 7.88 8.80 9.60 Na^O...... 2.09 1.73 2.59 1.58 2.25 3.06 2.28 KsO ...... 31 .74 .69 .97 1.16 .73 .72 H20 ...... 1.91 1.13 1.80 1.67 1.12 2.15 1.52 H20 - ...... 2.33 .90 .51 1.10 .18 .15 C02...... 05 .14 None None .09 .08 TiOj...... 1.75 2.13 2.19 1.32 .91 1.13 1.04 P20*...... 76 .82 .33 .49 .72 .29 .20 MnO...... 19 .16 .17 .12 .11 .17 .15 .02 .02 .02 .01

1. Basalt flow, Douglas County, Oregon, sec., 24, T. 25 S., R. 4 W. J. J. Fahey, analyst. 2. Diabase dike, Douglas County, Oregon, sec. 29, T. 24 S., R. 4 W. J. J. Fahey, analyst. 3. Average plateau basalt.9 4. Norite, Cazad Mountain, Douglas County, Oregon, sec. 44, T. 23 S., R. 4 W. J. J. Fahey, analyst. 5. Quartz diorite stock, Steens Butte, Lane County, Oregon, sec. 24, T. 23 S., R. 3 W. J. J. Fahey, analyst. 6. Basalt, The Dalles, Oregon.10 H. S. Washington, analyst. 7. Labradorite andesite, near Grizzly Saddle, Bohemia district of Lane County, Oregon. George Steiger, analyst.11 the ores. A slight amount of alteration is perceptible in the dike, how­ ever. Most of the hypersthene crystals have been changed to serpentine pseudomorphs, and the feldspars show some cloudiness, but the augite is very fresh, and the rock as a whole shows no marked traces of the profound decomposition that has affected the andesite.12 9 R. A. Daly: Igneous rocks and the depths of the earth (1933) p. 17. 10 Eugene Callaghan: Some features of the volcanic sequence in the Cascade Range in Oregon, Am. Geophys. Union, Tr. (1933) p. 247, table 2, no. 1. “ Eugene Callaghan: op. tit., p. 246, table 1, no. 4. 12 F. G. Wells and A. C. Waters: op. cit., p. 29-31.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/46/6/961/3430507/BUL46_6-0961.pdf by guest on 26 September 2021 CHEMICAL DIFFERENCES, EOCENE-MIOCENE (?) IGNEOUS ROCKS 971 CHEMICAL DIFFERENCES BETWEEN EOCENE AND MIOCENE(?) IGNEOUS ROCKS It seems from the analyses available that the intrusive dikes and extrusive flows of Eocene age differ markedly from the intrusive rocks of Miocene (?) age. The Eocene rocks (analyses 1, 2) are typical basalts characterized by a low percentage of silica and alumina and Table 2.—Norms *

1 2 3 4 5 6 7

Q...... 6.30 6.24 5.34 11.22 6.72 8.82 or...... 1.67 4.45 6.12 7.23 3.89 3.89 ab...... 17.82 14.15 13.62 18.86 25.68 19.39 an...... 24.46 27.80 33.08 34.75 31.97 34.47 c ...... 20 10.09 8.12 5.80 3.94 5.34 en...... 16.50 15.50 17.00 12.40 12.70 12.80 fs...... 4.75 7.79 8.84 5.02 5.15 7.26 mt...... 9.05 7.42 4.18 4.64 4.87 4.18 il...... 3.34 3.95 2.43 1.67 2.13 1.98 ap...... 2.02 2.02 1.34 1.68 .67 .34 CO...... 20

* Norms recalculated by Jewell J. Glass according to methods of T. F. W. Barth [Pro-posed change in calculation of norm4 of rocks, Min. pet. Mitt., vol. 42 (1931) p. 1-7] and H. S. Wash­ ington {The me of "Ferrosilite” aa a name for the normative molecule FeSiOz, Min. pet. Mitt., vol. 43 (1932) p. 63-6«.

high ferrous and ferric oxide; the Miocene (?) intrusives (analyses 5, 7) have a much higher silica and alumina content and are corre­ spondingly lower in ferrous and ferric oxide. Nevertheless, both types are characterized by high lime—a feature that, according to Calla­ ghan,18 is common to all the rocks of the Cascade Range—and the analyses show sufficient similarity to indicate a possible consanguinity. The differences in chemical composition are shown mineralogically by the presence of olivine in the diabase and of hypersthene in the younger intrusives. The difference noted for the intrusive rocks also holds for the flows, the Eocene flows being olivine basalts chemically almost iden­ tical with the diabase dikes, and the andesite flows in the Bohemia dis­ 18Eugene Callaghan: Some features of the volcanic sequence in the Cascade Range in Oregon, Am. Geophys. Union, Tr., 14th Ann. Meeting, April 1933 (1933) p. 243-249.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/46/6/961/3430507/BUL46_6-0961.pdf by guest on 26 September 2021 972 F. G. WELLS, A. C. WATERS— BASALTIC ROCKS IN UMPQUA FORMATION trict corresponding in chemical composition to the Miocene (?) intru­ sives. The analysis (no. 4) of the norite sill occurring within the Eocene sediments cannot be definitely allocated to one or the other group. The contents of silica, alumina, and ferric oxide are in accord with the analyses of the Miocene intrusives, but the ferrous oxide, lime, mag­ nesia, and alkalies show close correlation with the analyses of Eocene basaltic rocks. Mineralogically the sills resemble some of the Miocene (?) intrusives, and on this basis they are tentatively assigned a Miocene (?) age.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/46/6/961/3430507/BUL46_6-0961.pdf by guest on 26 September 2021 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/46/6/961/3430507/BUL46_6-0961.pdf by guest on 26 September 2021