UC BERKELEY MASTER NEGATIVE STORAGE NUMBER 03-67.69 (National version of master negative storage number: CU SN03067.69) MICROFILMED 2003 UNIVERSITY OF CALIFORNIA AT BERKELEY LIBRARY PHOTOGRAPHIC SERVICE REPRODUCTION AVAILABLE THROUGH INTERLIBRARY LOAN OFFICE MAIN LIBRARY UNIVERSITY OF CALIFORNIA BERKELEY, CA 94720-6000 COPYRIGHT The copyright law of the United States (Title 17, United States Code) governs the making of photocopies or other reproductions of copyrighted materials including foreign works under certain conditions. In addition, the United States extends protection to foreign works by means of various international conventions, bilateral agreements, and proclamations. Under certain conditions specified in the law, libraries and archives are authorized to furnish a photocopy or other reproduction. One of these specified conditions is that the photocopy or reproduction is not to be "used for any purpose other than private study, scholarship, or research." 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A correlation study of the Modoc lavas of the Warner Mountains 1923 BIBLIOGRAPHIC RECORD TARGET University of California at Berkeley Library Master negative storage number: 03-67.69 (national version of the master negative storage number: CU SN03067.69) GLADIS NUMBER: 1847775341 AD:990922/FZB LEVEL:b BLT:am DCF:a CSC:d MOD: EL:7 UD:030604 /MAP CP:cau L:eng INT: GPC: B10: PIC: CON: ARCV: PC: PD:1923/ REP: CPI: FSI: I1.C: I1: 040 CUScCU 090 SbDISS.SCHMITTOU.GEOL 1923 100 1 Schmittou, Maurice B. 245 12 A correlation study of the Modoc lavas of the Warner Mountains. 260 $c1923 300 ii, 78 leaves :$bplates, maps ;$c29 cm. 502 Thesis (B.A. in Geology) -University of California, Berkeley, April, 1923. 504 Includes bibligraohical references. 610 20 University of California, Berkeley.$bDept. of Geology and Geophysics$SxDissertations. 0 Dissertations, Academic$xUCB$xGeology$y1921-1930 Microfilmed by University of California Library Photographic Service, Berkeley, CA FILMED AND PROCESSED BY LIBRARY PHOTOGRAPHIC SERVICE, UNIVERSITY OF CALIFORNIA, BERKELEY, 94720 DATE: 7/03 REDUCTION: 10 X PM-1 3%"x4" PHOTOGRAPHIC MICROCOPY TARGET NBS 1010a ANSI/ISO #2 EQUIVALENT 0 Ee [10 = 12 22 == gp l= [2 ka I 1] == 22 mS B= . Bg a ———— It 125 lL lis Thesis for the Degree of Bachelor of Arts. A Gorrelation Study ¢f the MNodoo of the Warmer Mountains. MAURICE B. SCHMITTOU. Mey, 19283. IDiss SCHMITTOU GEL )723 ERECT A CORRELATION STUDY OF THE MODOC LAVAS OF THE WARNER MOUNTAINS By Maurice 3B, Schmittou LIBRARY COPY In fulfillment of the thesis requirement for the degree of Bachelor of Arts, in the College of Letters and Science, Department of Geology, University of California | Submitted April 28, 1923 &.7- COR2ENZS Seope of Report Acknowledgements LOCATICH AWD EXTERT OF THE WARNER MOUNTAINS THE WARNER MOUNTAIN VOICANICS = Lower Agglomerates « - « - Hiddle lavas = = = = = « Soper _dglanerates -- - Structure of the Agglomerates Btotritusion to the west of the Farner Nountains Distribution within the ¥arner Hountains - - Structural features of the Modoc lavas Rhyolitiec Obsidian « = = «= = = = « « . Structure sections « « « « = = « « « Faults BA Me dr ME a WE BE WR We ee a we Wx we Relation between faulting and folding DETAILED SECTIONS = = = « = « « « = « « Purpose of the detailed seetions Jess Valley Section « Field Relations =~ Petrography = = « J I LE A t 4 5 Prt Fri gl PEER BEMEE EEE fr i Surface flow Basal Flow ~ Fandango Valley Section Field Relaticns - Petrography = « = Upper flow « = Augite andesite Blotite andesite F505 44 } $ 3% 8 $4 45 4 4 41 £4 ¢ 4%) $f 4444 ¢ 0 : $8 0 40% 4 } 1 2 42% 1% tf 3 4 44% 3 14 : 49040 40% 3 44 50 409 0-40 04 FE $5 0 401 F443 8 41 4 3% 4 4 8% 8 F234 43 ! 3 312 13 8 823 £2 3 4124 F418 81 £4 4 00 :! 430% (Contents, continued) Kt, Bidwell Section « « « = Field Relations « « « = olitie obsidian Second flow « Third flow - Fourth flow Pifsh flow Sixth flow Jevantn flow Eighth flow Ninth flow « = Comparison of the lt. Biaw with other parts of the ar Osceurrence of the Modoc lavas at the Southern end of Surprise Valley = « - = - 76 62 b2 54 65 58 63 66 69 70 71 72 73 73 Frat ' v4 3 ¢ 24 Ean $58 04001 002 L400 048 ft 0 8 04 8 8 0 i Ld TREE ES 3 0 8 60 28 8 1 0 04 058 0 0 8 0 4 4 i 51 3 t red 3 § 3 § t 3 $1 ne 18 11 ® | ot PAGE 2 YY 8 ' APPENDIX EE ue RA AA Views of the Warner Hountaing « « « « « Structure sections of the Warner Mountains Kierophoto PhS = = « « oo © = «= « = «w = Pantographic reduction of the Alturas TOPO hie Shons - ew ew ewww. INZ2ZRODUC TION Scope of the Report The field and laboratory investigations upon which this report is based were made in order to determine the relation between the lavas which form the top of the war- ner Mountair®with those which ocecur along the eastern and western base of that range, and which on the western side are ‘known as the Modoc lavas. In order to do this, detail- ed sections of the mountain front were made at various lo- calities., A description and comparison of the various rocks found in these sections is included. Since the region is as yet little known geological- ly, a brief discussion of the more salient features of the Warner liountains is given before taking up the main part of the problem. Acknowledgments The field work of this report was carried on by the writer during the summer of 1922, while serving in the capacity of assistant to kr. R.J. Russell, who is at the present time preparing a more complete report on the geol- ogy of the Warner Mountains, The writer wishes to express his thanks and appre- ciation to iir., Russell for making this trip into the War- ner kountains possible; for the many suggestions as to the solution of the various problems that arose in carrying out the field work, and for the use of thin-sections of several of the rocks described in this report. The writer wishes also to thank Dr, Re R, Morse for his many helpful suggestions as to the preparation of this paper and for his assistance in the microscopic petrography. Obligations are also due the various writers whose papers are listed, from which considerable information on prob- lems connected with this report was obtained. LOCATION AND EXTENT OF THE WARNER MOUNTAINS The Warner Range of Hountains, lying in part in north-eastern California and in part in south-eastern Oregon, a few rhiles west of the I20th meridian, extends almost due north and south, and is approximately 100 miles in length, 1 This range from its southern end which is about 70 miles south of the California-Oregon boundary, gradually rises from a broad uneven surface volcanic plateau, reach- ing its maximum altitude of 9990 feet at Eagle Peak. From Eagle Peak northward to Mt, Bidwell, a distance of approximately 47 hiles, the crest line reaches elevations that vary between 7000 and 8000 feet. There are a few peaks which rise above 8000 feet among which are Warren Peak, Cedar liountain and Bald liountain. From Mt. Bidwell north, the range gradually decreases in height until about 25 or 30 miles north of the California-Oregon boundary it loses its identity, merging into a number of low distinet Basin Ranges. The width of the range varies from seven to six- teen miles and is narrowest in its central part. Surprise Valley bounds the southern two-thirds of the range along the eastern side. HNorth-ward from Surprise Valley it is bounded by a partially dissected lava plateau. The north- ern one-half of the range is bounded on the west by a val- ley ten to fifteen miles wide, over half of which is covered 1. The greater part of the range lying within California is shown on Plate XX. “lw by Goose Lake. South of Goose Lake it is bounded by an irregular shaped valley, from five to fifteen miles wide, which is drained by the upper branches of the Pitt River. PHYSIOGRAPHY erred nt & The Warner Mountains are a Basin Range differing very little in their essential physiographic and struetu- ral features from other ranges of this type described by various writers, among which are Gilbert, I Russell, 2 Davis, 3 and Louderback, 4. liany of the criteria set forth by the above writers as evidence of Basin Range Structure are well represented in thie range of mountains, It rises very abruptly along normal faults, from comparatively valley floors on either side. The eastern base line is very even but slightly curved, with the concave side facing the east. The con- tinuity of this base is unbroken by ridges extending out into the valley and there are no indentations other than sharp V-shaped canyons extending into the range, See Plate I. 1. Geog. & Geol, Exploration & Surveys West of the I00th meridian- Geology Vol, III, pp. 21-42; 1875. 2. Geological Reconnaissance in Southern Uregon~ U.5.G.3. IV, Ann, Report - 1884; pp. 431-464. 3+ The Ranges of the Creat Basin. Physiographiec evidence of faulting. Science XIV, pp. 457-459; 1901. 4. 3asin Range Structure of the Humbolt Region, Bul. GeS.A, Vol 15, PP. 289-346; 1904, The mountain face is deeply furrowed by sharp V- shaped canyons which extend at right ahgles to the range front. The main streams are entirely consequent and as yet there is very little tendency toward the development of sub- sequent latterals, The mouth of the canyons are usually quite narrow and gorge-like as shown in Figs, 3 and 4, Plate II. Higher up in the middle part of the trunk streams they widen out with a less youthful cross-section. The upper branches of the trunk streams finger out over a fan shaped area and have a grade of I000 to 2000 feet per mile. The grade of the lower course is much less, being from 250 to 400 feet per mile, The change in grade is usually quite sudden. The streams flow out into Surprise Valley over broad alluvial fans which have a slope of two to three degrees. The ridges between the streams are long and sharp crested. They terminate abruptly at the base of the range with their steep front often cut into sharp triangular facets. The western front of the range, for two-thirds of its length from the north is that of a great fault scarp, the greateB® gr which is very similar to the eastern front. The most important difference is the indentation of the range by Fandango Valley. This valley extends about two- thirds the way thru the range. 1t has a broad and rather even floor one to one and one-half miles wide. But unlike other physiographic lines which are normal to the range it cuts thru at an angle of south forty-five degrees east. It is a fault valley with steep escarpments along both sides, that of the north-east side being shown in Plate XI11l and discussed more fully in the latter part of thie report, The parallelism of lassen Creek to the South and Bidwell Creek to the North of Fandango Valley, suggest a similar relationship. The fault along the western base of the range gradual- ly dies out to the south and from Pine Creek to the south- ern end of the range long even slopes of four to five de- grees extend from the base to the summit overlooking Sur- prise Valley. The streams of $his southern area have in many places cut thru the lavas which make up the top of the range and into agglomerates which underlie the lavas. The protection of the upper walls of the canyons by the lavas often give rise to vertical dliffs and box-like canyons. This is particularly true of the canyon of the north fork of the Pitt River below Jess Valley, where it flows to the west for several mdles thru a gorge two hundred to four hun- dred feet deep. Faulting parallel to the range causes a like diree- tion of some of the streams of this area. The upper part of i111 Creek flows almost due south for about four and one- half miles where it makes a right angle turn to the west. This upper part of the stream follows a north-south fault which also crosses East Creek and causes some of its Branch- to flow parallel to the range for some distance. The ridges between the streams are usually broad and smooth preserving as yet a great deal of the prefault- ing topography. This older surface is still better exem- plified on the summit north of Cedar Peak. Here there are broad, almost level areas or gently rounded topographic forms which stand in great contrast to the steep, rugged eastern and western fronts of the range. This flat charac- ter of the top of the range is shown in Figs, 5 to 8, Plates III & IV. THE WARNER MOUNTAIN VOLCANICS The rocks of the Warner Mountains are entirely vol- canic. They include two series of agglomerates and tuffs and two series of lava flows. The lowermost formation con- sists of agglomerates and tuffs, They are overlain by lavas which, are in turn overlain by another series of agplomer- ates and tuffs., The topmost formation consists of lavas which, as shown in the latter part of this report, are a part of the Modoc lavas, The lower three members of this group of volcanics have not as yet been named and for con- venience will be designated in the following manner; Lower Agglomerates, meaning the lowermost formation in the range; the liiddle lavas which overlie the Lower Agglomerates; and the Upper Agglomerates which constitute the third member of the series. Lower Agglomerates ~ Distribution and Character. The Lower Agglomerates occur along the eastern base of the range from a point two ihiles south of Eagle- ville to about half way between lake City and Goose Creek, A total distance of twenty seven or twenty eight miles. They are from two thousand to twenty five hpndred feet thiek between Cedar Canyon and Granger Creek. They dis- appear very abruptly between Eagle Creek and Emmerson Creek, beneath several blocks of overlying Agglomerates and lavas which have been faulted down over them. North of Cedar Canyon their exposed thickness gradually decreas- es until they disappear beneath the iiiddle lavas at the base of the range between Lake City and Goose Creek, They do not occur on the western side of the range south of Fan- dango Valley. Agglomerates very similar to these occur near the mouth of lew Pine Creek Canyon, one-half mile south of the California, Oregon boundary, but their cor- relation was not definite. The greater part of this series of Agglomerates has a distinctly bedded structure which from a distance ap- pears quite uniform, but as a rule the bedding planes are irregular in detail, This irregularity and lack of any sorting of the fragments is evidence of the subaerial de- position of this material, The thickness of the beds of agglomerates varies from a few inches to a hundred or more feet. They consist of tuff and various sized angular frag- ments which are usually quite vescicular. There are also layers of even grained tuffs of various thickness, and often of a bright re# color. The strike of the beds is N 10° Ww. Along the base of the range they dip about eighteen degrees to the west but towards the top of the serdées the dip de- creases to thirteen or fourteen degrees. Figs. 9 and I0 plate V show the attitude and bedded character of this For- mation, Intercalated in the agglomerate are beds of partial- ly water-rounded boulders imbedded in a matrix of tuff and sand. These beds are sometimes twenty or thirty feet thick and consist of unsorted material from a fraction of an ineh to a foot or more in diameter. The boulders are made up entirely of lava apparently like that of the fragments in the beds of angular material, These layers are not restrict- ed to any particular part of the formation but are more nu- merous toward the base. Lenticular layers of sand and dark colored shale are also numerous, and are commonly three to four feet thick, The greater part of this series is made up of ande- sitic material, Megascopic examination of the fragments shows them to consist of an aphanitic groundmass with phe- nocrysts of small lath shaped plagioclase feldspar and a ferromagnesian mineral which is usually hornblende, more rarely augite. Some of the tuffs contain large crystals of hornblende one-fourth to one-half inch in length, On the north side of lill Creek above Lake City at an eleva- tion of 5,500 feet there is a bed of light colored rhyo- litic tuff about 100 feet thick, It is rather fine grain- ed and contains numerous crystals and fragments of quartz, feldspar, hornblende and biotite, It is overlain by a bed of agglomerate consisting of sharp angular fragments of rhyolitic glass, These are the only distinet variations noted in the composition of the Lower Agglomerates. The erosional forms produced by the agglomerates are very irregular, The tuffs usually weather down to a smooth even surface while the beds of coarser material stand out as cliffs or buttresses as shown in Fig, I0 Plate V,. Another characteristic feature of this series are beds deep- ly stained by ferrous oxide. The staining is very uniform, extending thruout the finer material of the matrix and as a thin coating over the surface of the larger fragments. These stains give many of the beds a distinctly green color. Petrified stumps and logs one to two feet in di- ameter are quite common in the tuffs nd sometime in the coarser beds. Iliany of the stumps are standing erect show- ing that they have not been disturbed since they were bur- ried. Their situation at various horizons in the agglomer- ates and their size indicétes that there were &t times quite long periods between successive eruptions. liiddle lavas This series from all evidence seen conformably over- lie the lower Agglomerates. They occur along the eastern side of the range from Cherry Creek six miles south of Eagleville to about five miles north of Goose Creek. On the western side they are exposed in the lower part of Thoms Creek Canyon three miles west of Cedar Pass and in Rast Creek Canyon two miles above Jess Valley. In Cedar Canyon, they are eight or nine hundred feet thick but in Emmerson Creek Canyon they have a thickness of about five hundred feet. Variations in thickness may be in part due to repetition by normal faulting. This is illustrated at Emmerson Creek where three distinct blocks of these lavas; one above the other, are repeated down the mountain front, The lower half of this geries is characteristically very dense and fine grained, They are dark grey to black in color and are very little affected by weathering, there usually being only a thin grey or brown £11m on exposed sur- faces. They consist of an aphanitie groundmass with many small lath-shaped erystals of feldspar, a pyroxene and oc- casional grains of olivine. In one specimen from above Lake City, olivine is more abundant and very fresh. Some of these rocks, as in Emmerson Creek Canyon, are so fine grain- ed that no phenocrysts at all are visible. From their mega - . 8copic character these lower Tlows of this series are evi- dently olivine basalts, The upper half of the series differs quite marked- ly in composition and texture from the lower part. The ground mass is not so dense and the phenocrysts are usual- ly much larger. The latter consist of lath-shaped feldspar showing albite twinning, a great deal of hornblende and oc~- casionally flakes of biotite. The hornblende is often in large crystals an ineh in length as seen in a thick Flow in the lower part of Thoms Creek Canyon. Rooks containing large phenocrysts of hornblende also occur a few hundred yards southeast of Cedar Pass. kany of these upper flows are very vesicular and amygdaloidal. The cavity fillings are usually ealeite or chalcedony, less commonly sheaf-like aggregates of stilbite. Weathering has also caused considerable alteration of the minerals of the rocks accompanied by deep staining by iron oxide. Upper Agglomerates Overlying the middle Lavas is a second series of andesitic tuffs and agglomerates, They occur along the middle and upper slopes of the eastern front of the range between Swarenger Flat in the southern part and the south- eastern side of lit, Bidwell to the north, where they are ob- scured by blocks of overlying lavas which have been faulted down over them. Their extent along the Western front of the range has not been fully determined. They are well de- veloped between Fandango Valley and Jess Valley and appar- ently extend some distance beyond these points. Their ex- posed thickness varies congiderably in different parts of the range from two or three hundred feet to two thousand feet. This irregularity has been caused by faulting in -l3= which blocks of the overlying lavas have been repeated down the front of the mountain obscuring the agglomerates. South of Pine Creek they are only exposed where the overly- ing lavas have been removed from the tops of the ridges and in the deeper canyons where streams have cut thru the lavas. The bedded structure which is characteristic of the Lower Agglomerates is not so well developed in the greater part of this series. At the southern end of Plum Valley on the north fork of Davis Creek, & representative section is exposed. Here they form prominent outerops upon the face of a steep escarpment which rises seven hundred and fifty feet above the floor of the valley. The face of this escarpment is very irregular due to the ¢liff and spire-like erosional forms, Fig. II Plate VI, This section consists of tuffs which are often bright red in color and massive layers of angular unsorted fragments all of which are andesitic in com- position, Most of the fragments are quite vesciocular and sometimes scoriaceous. They are usually porphyritie with an aphanitiec groundmass. The phenocrysts which can be deter- mined megascopically are plagioclase feldspars and hornblende, more rarely pyvaxene or olivine, Weathering has caused a great deal of staining, giving the rocks a brown or reddish color, Calcite and amorphous silica are abundant filling the vescicles of the rock fragments and occasionally as small seams. Lenses of sand and sandy tuff one to four feet thick occur irregularly thruout the series. They can seldom be traced for more than one hundred feet and are very irregular. Numerous basaltic dikes and sills have been intruded into this formation, They are seldom more than ten feet thick, Practically all of the #ikes stand at high angles usually from eighty to ninety degrees. The Upper Agglomerates in the vieinity of Cedar Pass differ somewhat from those at Davis Creek and other parts of the range. They are characterized by much more hornblende, which occurs as phenoorysts in the rocks and as crystals, or fragments in the tuffs, commonly one-half to three-fourths inch long. Un the east side of the range be- low Bear Mountain and Eagle Peak the upper four or five hun- dred feet of this series is composed chiefly of White tuff, It contains an abundance of hornblende and small euhedral feldspar crystals. Class fragments up to an inch in diame-~ ter are also quite plentiful, This tuff is underlain by ag- glomerates similar to those at Davis Creek, At Jess Valley the agglomerates have the same ap- pearance and composition as those 4t Davis Creek. They are exposed Yo a thickness of fourteen hundred feet on the south side of East Creek Canyon, one and one-half miles above Jess Valley. Structure of The Agglomerates Along the eastern side of the range the agglomer- ates dip to the west. In the southern part the dip 4s five to six degrees, but further north at Goose Creek it is twelve to eighteen degrees. There is a decided decrease in dip from lf the bottom to the top of the series which is also true of the underlying formations exposed along this side of the range. The general strike is HN ICO W, altho there are slight variations which are only local and apparently due to secon- dary faulting. At Plum Valley which is almost due west of Goose Creek the béds dip to the east about fifteen degrees and strike § I0° Ww, giving the range a synclinal structure shown in Fig, I, Plate XI, From Davis Creek to Pine Creek the dip to the east gradually béecomes less with the decrease in throw of the fault along the western base of the range. South of Pine Creek the beds dip to the west at an angle of five degrees conforming to the westward slope of the range. Ag the beds extend out into the valley along the western side they become horizontal, MODOC LAVAS Distribution To The West of The Warner Mountains The Modoc lavas apparently cover a large ares in northeastern California and southeastern Oregon, but their total extent is as yet unknown. They occur along the west- ern base of the Warner Mountains for their entire length, To the west only that part of them covering a belt nine to twenty miles wide and parallel to the range was examined. This is apparently ohly & minor part of the totel area cover- ed by these lavas, - In this area to the West of the range they consist of horizontal flows the total thickness of which is varia- ble but in general greater in the northern part. The great- est thickness observed was along the south-western side of Goose lake. The base of the lavas is not shown in this vi- cinity but their exposed thickness is approximately 450 feet. They form a steep and in some cases almost vertical wall a- long a great part of the western side of the lake. rom the edge of this 0liff the lavas extend to the west several miles with a very level monotonous surface. Hig. 13 Plate Vii, shows the character cf the surface of the lavas, South of Goose lake they thin out quite rapidly to a thin rim fifty to seventy-five feet thick along the western bor- der of the valley. Vherever their base 1s exposed in this area they overlie water-lalg tuffa termed by Br, C., 3tock as the Alturas formation, »ig. 14, Plate VII shows the rim of Hodoe Lava overlying tuffs along the western side of the Pitt diver about twelve miles south of Goose lake. Thruout the central part of the Valley in the vi- einity of Alturas, and between Alturas and Likely to the south, the lavag are very thin often consisting of a single flow tewnty-five or thirty feet thick. In many places over this area they have been removed by erosion leaving only isolated remnants, Iut towards the mountains on the east, they increase rapidly in thickness and number of flows. Between Alturss snd Coose ake the thickness of the lavas 175 increases with a corresponding decrease in exposure of the tuffs which they overlie, until the latter disappear: about half way between these points, Across the valley at the south end of Goose Lake alluvium and gravel brought down by the streams from the mountains is quite thiek but along some of the streams channels in the cefitral part of the valley this material has been cut away exposing the lavas, showing that they are continuous from the western glide of Goose Lake to the base of the range. Along much of the eastern side of Goose Lake north of Davis Creek, they form cliffs 50 to 100 feet high, and extend back from these d4liffs to the base of the mountains, where they a~ but directly against the steep western scarp. Distribution of the Modoc lavas within the Warner Mountains Except where cut thru by streams, the Modoc lavas are found over the entire summit of the range north of Cedar Pages, and gover the western slope in the southern part. Their thickness varies a great deal due in part to erosion and in part to the irregularity of the surface up- on which they flowed, At the summit four miles south of Eagle Peak they are about I200 feet thick but from this point towards the west, their thickness gradually decreases to one to two hundred feet at the base of the range and to a single flow twenty-five to fifty feet thick in the cen- ter of the valley near Likeley. At Bald liountain near the central part of the range they are only fifty feet thick put seven miles north at the head of Goose Creek their thickness is at least one thousand feet. On the eastern gide of Mt. Bidwell they are exposed to a thickness of One thousand feet, but repetition of the upper flows by fault- ing has obscured the dower part of the series. At various parts of the range, faulting has caused the recurrence of the lavas at intermediate elevations along the range front, in some cases giving rise to a con- tinuous exposure from the summit to the base as at Kt. Bidwell, Plate XIV, A complete sedtion of the Modoc lavas {gs shown in Fig. 15, Plate VIII, which is a view of the east front of Warren Peak. The lavas here have a thickness of approximately nine hundred feet. With the exception of a difference in attitude the structural features of the liodoc Lavas within the range, and those to the west of it, are alike and not of great variety. The development of columnar structure is quite common, The size of the columns vary somewhat but they are usually quite large. Along the cliffs of the lower part of Davis Creek Canyon they are five to eight feet in diameter and four, five and six sided. The perfection of the col- umns is often very marked. large columns of the same type are well developed along the cliffs overlooking the west slide of Goose lake eight miles due west of the town of Davis Creek. Flow structures are evident in practic=slly all of these rocks. The most common expression of flow structure seen in the fleld 1s the manner in which the rocks weather out into tabular slabs or plates along more or less paral- lel lines. In the finer grained and glassy types these lines are closely spaced causing the rock to weather out inte thin plates, one~fourth to one inch thick and three inches to a foot wide. in other cases the flow lines are greatly contorted and may not show any relation to weather- ing, Another type of flow structure commonly seen is a large ovalold mags of the roek in which the flow lines are concentric and have evidently been formed by eddies in the molten rock as it flowed over the surface. 4 more or leas parailel arrangment of the minerals is usually evident in the hand specimen but this character is much more clearly observed in thin sections. The entire mass of the thinner flows and the upper part of the thicker one have the usual vesicular character. The vesicles are usually elongated in the direction of flow, and are as a rule unfilled by secondary material, but occasionally there are amygdules of ealeite or chalcedony. The surfages of the flows are sometimes sconaceous or slaggy and in the southern part of the range there are a few which have a distinctly ropy sur- face. Rhyolitic Obsidian At several localities in the range rhyolitic glass forms the uppermost part of this series of lavas. it oo~- curs over a limited area four miles south of Kagle Peak, at the summit above Davis Creek, and at lit, Bidwell in the northern part of the range, From these isolated occurences which always have the same relation to the underlying lavas it seems quite probable that the glass at one time covered the entire area. Bbsidian fragments are abundant over the gurface of the lavas west of Goose Lake and is reported to oceur in place at Timbered iountain, twelve wiles west of the southern end of the Lake, but this lucality was not visited. Along the eastern summit above Goose Creek the obsidian is two hundred and fifty to three hundred feet thick and has an equal thickness at Mt. Bidwell. It often shows a great diversity in physical aspects tho in the thin-sections examined it appears quite uniform in composi- tion. The color may be black, grey, white or brown, which is in some cases uniform thruout, in others mingled in blotches or streaks. These variations often occur within a few feet, while on the other hand it may be quite uni- form in color over a large area, Certain parts of the rock are very glassy, others have a felstitlc or lithoidal appearance. Banding due to the parallel arrangment of mi- erolites is usually well marked, There bands are often highly contorted, Weathering produces a light grey film over the surface of the dlack obsidian but is not so ap- parent on the lighbar colored rock, Hacrosconic pheno- crysts are only occasionally evident in the specimens of this roek, but in thin-sections it is seen %o contain a aH few phenceorysts of quarts feldspar and anopague mineral. Its microseople character will be given more fully later, STRUCTURE In order to show more clearly the fault block structure several cross-sections of the range have been constructed, The three sections shown on Plate X and XI, extend entirely across the range and are drawn to a natural scale of two inches to the mile, The detailed sections, 2lates XII XIII, XIV, have been constructed on & scale of eight inches to the mile in order to show as closely as pos- sible the thickmess of individual flows of lava, and the recurrence of these flows at various elevations due to faulting. The profiles were plotted from the U.5. Forest Service map of the Forest Reserve, walch includes most of that part of the range lying within California, In many cases this map does not include the first few hundesd feet of the lower slopes of the range, so that for that part it was necessary to use the U,3.0«2. dap of the Alturas Guad- rangle, Survey of 1885. Faults Faults are very common but they are always normal, and in most cases parallel to the trend of the range, but there are a few which are transverse or oblique, The dip of the fault planes was measured where ever it was peseible and in all cases were found to be about sixty degrees. The ma jor lines of displacement are along the eastern base of the range and the northern two-thirds of the western base. These two fault lines mark the boundary of the main moun- tain block which is broken into secondary blocks by par- allel faults of less extent,which are usually evidenced by both physiographic and stratigraphic discordances, The section showa on Plate X, shows the monoclinal structure of the southern part of the range. The only fault- ing that has occured along the western side is in the viei- nity of Jess Valley and is shown in detail, Plate XII, These faults extend parallel to the range for about seven miles and have a throw of &t least 700 feet. But on the eastern side there are two much more extensive faults and of much greater displacement. The escarpment of the lower fault forms a gteep wall of ilodoc Lavas around the southern end of Surprise Valley. The top of the down-thrown bloek of this fault, forms the floor of Surprise Valley, which in the past has been occupied by & much larger lake than at present, as shown by the distinet shore lines along the eastern side. The upper fault has a throw of about three Lm thousand feet, exposing along its escarpment the entire series of Upper Agglomerates a;d liodoc lavas. A short dis- tance north of this gection the eastern front of the range is broken into several smaller blocks, as evidenced by the occurence of the iilddle Lavas at various elevations. It is this faulting that causes the abrupt ternination of the lower Agglomerates near Emmerson Creek. Thruout a great deal of the central part of the range,the rapid erosion of the agglomerates which have thelr maximum development in this part, obscures to a great extent the secondary faulting. For this reason the vodoe Lavas are found ‘nieaz ihe eastern base of the range except at the northern and southern end, Gorth of Pine Creek the scarp along the western side of the range, gradually increases in height until near the California-Crezon boundary where the maximum vertical displacement has been about thirty-five hundred feet. This western front is also beoken into secondary blocks which extend parallel to the range. The western side of the section across Bald kountain passes thru Joseph Creek 3asin for some distance 80 that the feulted front of the mountain on tAXt side is not evi- dent 1 : “si n the section, but a mile north org... +o displace~ ment of the liodoe lavas, which abut against the scarp at the i base and also oceur on the top of the up-throw block, is about twelve hundred feet. The secondary fault can seldom be traced for more than four or five miles and are often of much less extent, Their throw ls rarely over seven or elght hundred feet and usually it is much less. The smaller blocks are often tilt- ed at slightly different angles and directions giving rise to minor variations in the altitude of the rocke along the range front. The tops of these blooka were covered by lava are quite flat and usually tilted toward the up-thrown side. The scarps are aa a rule very distinet and little affected by erosion, Relation Between Foldins And Paunlting The folding that has ooourred within the range is apparently closely related to the fanlting., In the area to the west of the mountains the !lodoec lavas are lying horison- tal, but as they pass into the southern part of the range, they become tilted to the west about five decrees, conforme- ing to the general westward slope of the mountain block causéd by faulting along the saatern side, North of Pine Creek where faulting has occurred on botn the eastern and western side of the range, the monoelinal astrnefure gradual- ly sasses into that of a syneline, In ths Bald lountaing section the ayncline is quite asymmetric, the dip on the eastern side heing fourteen to eighteen degrees, while on the west they are about three to five degrees. But towards the north the western limb becomes merely steeply inclined with the increase in throw of the gault along the side un- til at Davis Creek the dip is fifteen to eighteen degrees on both sides. At no place within the range are the beds folded to any greater extent. From the above relations it is evident that the faulting is incidental to the folding. DETAILED SECTIONS Purpose Of The Detailed Sections Altho the general cross-section of the range, (Plates X and X1) show quite clearly the relation of the liodoe lavas whigh occur along the western base of the range to those on the top, the detailed sections, (Plates XII, XIII and XIV) in which individual flows have been correlated, meke this relationship much more evident. The sections to be desoribed in detail include that at Jess Valley and Fan- dango Valley on the western side of the range, and the lit. Bidwell section on the eastern side. Several less detail- ed sections were made in other parts of the range. The sections were run as nearly at right angles to the strike of the rocks as possible, this also being normal to che direction of the range withthe exception of that at Fandango Valley. The thickness of individual flows has been plotted as accurately as possible in these sections. The contacts between the flows can usually be closely located Since along the steep face of thie faulted blocks there has been little accumulation of soil or talus, In discussing these sections the field relations will be given first, followed by a petrographic deseription of the rocks in the section, ug JESS VALLEY SECTION Field Relations As was previously stated, the ilodoc Lavas covering the valley floor between Alturas and Likeley, are quite thin, often consisting of but one or two flows twenty-five to thirty feet thick. But toward the base of the range the thickness of the lava increases 10 two hundred feet or more. The South Fork of the Pitt River heads at Jess Valley, about one-third the sistance between the western base and the sum- mit of the range. From there, it flows almost due west to Likeley, thru & canyon two hundred to five hundred feet deep. Along the walls of this canyon which are very steep and of- ten vertical, the contact between the lavas and the under- lying tuffs is exposed continuously. In following this con- tact between Likeley and Jess Valley, no interruption what- ever ean be seen in the continuation of the lavas into the range. It is to be noted, however, that they change from 8 horizondal position at Likeley to an inclination of five degrees to the west at Jess Valley. At Jess Valley the section is intersected by two faults of which the valley floor is the top of the down- ! thrown block. The valley is approximately six miles long | and two and one-half miles wide across the central part, but towards the northern and southern end, it narrows down to a rather blunt V-shape. It has a very even, almost level floor which abuts very abruptly against steep escarp- ments along the eastern and western side. These two es- carpments are very closely the planes of normal faulting along which the relative depression of the valley block has taken place. The fault along the western edge of the valley is more or less branching, giving rise in some cases to two or three small blocks in step-like order between the top and bottom of the ridge slong that sile. The two main faults converge toward both ends of the villey and apparently do not extend far beyond those points as the lavas are contin- uous around either end, and show no evidence of faulting, The throw of the faults along both 8ided of the valley is at least seven hundred feet, this being in some cases distributed among two or more secondary faults as shown in Plate XII, The thickness of the lavas in this vi- cinity is somewhat variable due to erosion but is from five hundred to six hundred feet along the western side. Where tney have not been repeated by faulting, the Upper Agglomer- ates and Tuffs are exposed along the lower slopes 0f the valley sides. Petrography of Jess Valley Section Examination of the rocks in the field and in thin- sections under the microscope shows but two flows to be present in this section. The uppermost of these occurs on the top of both ridges along the east and west of the valley; it forms the top of a small block about nine hun- dred feet wide half way between the top and the bottom of the ridge and also forms the floor of the valley. Careful examination of thim.sections of this rock from the differ- ent elevations show no difference whatever except perhaps a slight variation in grain depending upon whether the specl- men was taken from near the surface of the flow or from the center. At the two lower elevéions it abuts against the gsoarp of the up-thrown block where it is in contact witha rock of quite different character, both in its physical ap- pearance and compositéon, _ This upper flow is about two hundred feet thick, It has 2 medium grey color on freshly broken surfaces, and a somewhat mottled appearance due to alteration of the fer- ro-meégnesian minerals, It is prophyritic with a fine grain- ed ground mass whieh cannot be resolves into its component parts in the hand specimen, The only megascopic phenocrysts are small feldspar laths and a dark colored mineral which under the microscope is seen to be olivine, The flow is —o vesicular near the surface, but thru the central part steam holes are totally lacking or very small, The cavities are seldom filled with secondary minerals. iicroseopic Character Under the wmisroscope thin-sections of thie rock show it to be distinctly porphyritiec with a fine even grain- ed ground mass of feldspar laths and granular augite and a yory amall amount of intersertal glass, in the form of small sngular wedges between the feldspar laths, The feld- spars 4c not show any marked parallel arrangrent due to flow, but lie in nearly all directions, with the anhedral augite and occasional small grains of olivine filling in be- twoen, giving rise to a texture intermediate between the hyalopilitic texture of the andesites and the intersectal texture commonly seen in basalts. The feldspars of the ground mass are typlecally lath shaped crystals O.1I to 0.30 mm long with sharp euhedral outlines, Albite twining is very common, the smaller laths rarely showing more than foru lamellae but on the larger ones, more may be present, Pericline or Carlsbad twinning is rare on these smaller orystals of the ground-mass, Extinction angles on aymetrically extinguishing albite twins were measures in three different thin-sections of this rock a=4d 2 maxisum of thirty-four degrees was found in each one, corresponding to a medium labradonite of about ABgp AR. Zonal growths are sometimes seen in the larger individuals of the ground mass but they are not common and rarely show more than three bands, They are quite free of inclusions only occasionally containing small needle-like crystals «Zw which have the appearance of apatite. The erystals are quite fresh snd show no align of alteration. The feldspar phencerysts vary considerably in size, the largest in the thin-sections reaching 0.7 mm. but in the hand specimens a few were obgerved that were over I mm. in length. As a rule, they show prismatic sections with good euhedral outlines but sections favorably oriented have a tabular shane with more or less rounding due to corrosion, They show rather coarse albite twinning lamellae and ocoa- sionally albite and Carlsbad twinning combined, Two pheno- arysts were found which showed pericline twinning sadbined with thet of albite. An extinction angle of thirty-six de- sprees was messured indleating thet they are more basic than the feldspar of the ground mass, but thelr positive character limits them to a basle labradorite. Iliany of the phenocrysts show a rether sharply defined sonal structure with three, four or five bands. Cleavage is seldom very evident on these crystals, but they are usually traversed by irregular fractures. Fine grained opague inclusions which are pro- bably magnetite or in part ilmenite, are quite common, Zeedles of apatite and small colorless isotropic inclusions which appear to be glass, are also present. The crystals are usually quite fresh. ply occasionally along cleavage or fracture cracks, is there a somewhat clouded appearance indicating & slight amount of alteration. The Pyroxene constituent is chiefly augite but there is also a very small amount of hypersthene. The augite in thin-gsections is almost colorless with a slight yellowish green tinge, and is not pleochroie. It does not occur as phenocrysts but is abundant in the ground mass as small anhedral grains 0,05 to 0.1 mm. in size. The augite is not entirely xenomorphiec toward the feldspar for occasion- ally the latter is penetrated by or included the small aug- {te grains, It 1s sometimes concentrated into ——. ag- gregates to the exclusion of feldspar, The characteristic pyroxene cleavage is usually evident. Its exténction an-~ gle of forty-five degrees and positive sign, distinguish it from the hypersthene, while from the olivine it ean usually be distinguished by its lower birefringence and lack of euhedral outlines, as well as the inclined extinction. Only & few crystals of hypersthene were found in the thin-cections examined. It ocours as rather euhedral orys- tals but not large enough to be called phenocrysts. ome of it is light yellowish-brown while other sections show ijarker shades in part due to stains. It shows greater or less pleochroism depending upon the amount of staining, The unstained hypersthene is very similar to the augite but the latter shows no pleochroism whatever. The hypers- thene is further distinguished Brom the augite by its par- allel extinction and the loeation of the nagative acute bi- sgectrix at right angle to the elongation and cleavage. Olivine is abundant as phenocrysts and occasion- ally as small rounded grains in the ground mass. The phen- ocrysts are often of macroscopic size, the largest being 20 mm, The larger corystals are more or less rounded but many of those of intermediate size have good euhedral out- lines. It is usually automorphic toward the feldspar laths. ‘rhe olivine usually shows two cleavages parallel to the direction of extinetion besides numerous fractures which traverse the erystal on ¥arious directions. It shows high refraction and bouble refraction and an optic sngle of about ninety degrees. Inclusions are not common, The only kind noticed, other than the feldspar mentioned above, is an opague metallic mineral whieh is probably magnetite. Host of the olivine is quite fresh and unaltered but some of the crystals shows a brown stained border or center, gnd a development of fibers along cleavage and fracture eracks, which may represent an early stage of serpentiniza- tion, ‘Magnetite in well developed cubes and octahedrons is quite abundant, There are also numerous irreg iar rigse. v7 a metallic mineral, which may be magnetite or ilmenite, It occurs scattered thru the groun! mass and is the most abundant inclusion in the other minerals present. Glass makes up a very minor part of the groundmass this rock. It occurs as small isolated angular patches between the feldspar laths, It is colorless in thin-sec- tions but contains abundant fine grained opaque inclusions, The light color of the rock, due to the great predominance of feldspar, and the textural features shown in thin sec- tions, give it much the appearance of an andesite, but on the basis of the abundance of olivine and the basic charac- ter of the feldspar it is evidently an olivine basalt. Basal ¥low The lower flow in this section has a distinetly dif- ferent wegadeopie and mieroscopic appearance from the upper flow. It forms rather steep cliffs along the upper part of the western side of the valley and also near the base. It re-occurs near the top of the ridge along the eastern side. It has a dark grey, almost black color, and in the hand specimen shows no sign of weathering, except a very thin brown film in exposed surfaces. It is distinetly por- phyritic with phenoerysts of feldspar, I to 4 mm. long, which stand out very prominently in a dense aphinitiec ground mass. It 18 much more vesicular than the overlying flow, particularly near the surface where in some places it is almost scoriaceous, liieroscopic character: The rock consists of large pheno- erysts of feldspar, olivine and augite, in a ground mass of glass containing feldspar and augite mierolites, and magnetite in such abundance as to appear opaque, The re- lative proportion of phenocrysts and ground mass is about equal, The excessive amount of magnetite conceals %o a great extent the character of the glass except in an occa- sional small area where it is seen to he colorless and ap- parently free of gpherulitic or perlitie structures. FPeldspar is the most abundant phenoorysts. The largest individuals reach 4 nm, in length and have pris- matic shapes with more or less regular boundaries parallel to their elongation, but with very irregular terminations. The frequent penetration of the crystals by embayments of glass, often give them & rather jagged outline. Those of intermediate size (I mm. to 2 mm.) have more euhedral out- lines. Cleavage parallel to the base (VOI) and side pina- cold (CIC) is nearly always present besides numerous irresu- lar fractures. Albite and pericline SwianiE 30 very cone. spicuous, and Carlsbad twins are not uncommon. Cccasional- ly all three are combined on a single crystal, The albite lamellae are usually quite thick and irregular, wedging out or terminating abruptly within the orystal. The large size of the feldspars make it possible to apply several methods of determination all of which give values corresponding to labradorite~bytownite ABg Alpe ‘Only a few sections were found showing zonal growths nd on these there were only two bands visible. Glass inclusions crowded with magnetite are very abundant, Apatite and olivine grains are also included but they are not as abundant as glass. In addition, there are a few very siall inclusions of a highly refracting and double refracting mineral but which is too small to deter- mine its optical character in thin-section. The feldspar of the groundmass ocours in small lath- shape and quadratic sections. They usually show albite twinning but not pericline or carlsbad. Extinction angles measured on these small crystals correspond approximately to the same composition as the phenocrysts. Both the pheno- erysts of feldspar and that of the ground mass have a very fresh, glassy appearance and show no sign of alteration, Olivine is much more abundant in this rock than the one overlying it. It occurs as large phenocrysts 2 mm, to 2 mm. in size and occasionally as small rounded grains. Many of the crystals have almost perfect crystal outlines showing the pinacoid and dome faces in longitudional sec- tions while those parallel to the base have more or less rounded octagonal outlines. The unaltered mineral is al- most colorless in thin-sections but deep brown borders or centers are common to most of the crystals, Two cleavages at right angles are usually evident besides irregular frac- tures. All of the olivine is apparently automorphic toward the feldspar and is freguently included by the latter, Small oubes of magnetite are occasionally included by the olivine, Most of the olivine is considerably altered usually to aggregate of yellowish or brownish green fibers, which have the appearance of serpentine. The alteration takes place along the border or cleavage and fracture cracks, the fibers standing normal to these lines, and extending irregularly into the erystal. In addition, there is a deep red colored alteration produet of the olivine which has more the appearance of iddingsite, but sections of it favorable to definite optical tests were not found. @t is found in other rocks of this area, and its occurrence which is always similar, will be described more fully later. Augite is the only pyroxene mineral in this rock. It has a pale yellowish-green color and is quite free of alteration, It occurs as rather long slender masses (.2 to 1.0 mm, long between the feldspar phenocrysts, and as small irregular grains in the glassy groundmass. As pheno- ecrysts, it is always xenomorphiec towards the feldspar, its shape evidently having been determined by the latter. It shows well developed prismatic cleavage and an extinction angle of forty-five degrees. It contains abundant magne- tite inclusions which sometimes almost completely obscure it. The lagnetite which is so plentiful in this rock is evidently the cause of its dark, almost black color in hand specimens. It occurs as well shaped cubes and oc- tahedrons, some of which are 0,03 mm, to 0,06 mm, in size, but is more common, as small grains or dust in the ground mass, This rock is distinctly more basic than the one overlying it as evidenced by the character of the feldspar and the inerease in olivine and magnetite. all of its minera-- Jlogisal and textural festures correspond to a normal oli- vine basalt; with a vitrophyriec texture. The contrasting physical and chemical fesntures of these two rocks just described, always occuring in the same gequence, make the recognition of faulting very definite. From the top of the eastern ridge of Jess Valley, the lavas are continuous to the summit of the range at xagle Peak, but show a rapid increase in thickness about twelve hun- dred feet. FANDARGO VALLEY SECTION Field Helatlions Fandango Valley lies nine miles south of the Cali- fornia Oregon boundary, snd outs obliquely into the War- ner lountains on the west, at an angle of 345° X, for a distance of approximately six miles. It is separated from the eastern side of the range by a low divide at Fandango Pass. Plate XI1I is a cross-section of the north-east side and floor of the valley, extending south-west from Fandango Peak. The limited time spent in this vicinity was not sufficient to make a study of the south-west side of ¥andango Valley and for this reason it is nct shown in tne section. But from the relations shown on its north- east side, it is evidently a fault valley caused by nor- mal faults which strike obliquely to the range in a north- west south-east direction, the evidence o f whieh is both physiographic and stratigraphic. The floor of Fandango Valley is about one and one- half miles wide and is relatively level. Its elevation is approximately fifty-three hundred feet, which is about two hundred feet higher than the top of the 4liffs along the eastern dide of Goose lake, two limes west of the mouth of the valley. The faulting along the north-eastern side has produced steep escarpments which rise to a height of twenty- five hundred feet at Fandango Peak. The major fault appar- ently extends along the base of this escarpment and crosses to the eastern side of the range at Fandango Pass. Secon- dary faults parallel to the one at the base, divide this side of the Valley into two ot more blocks, which are from two or three hundred feet to one-half mile in width. The topes of these blocks are often more or less flat and when two or more are present, they show quite distinetly, the step-like appearance of repeated block faulting, But as shown by the profile of the section on Plate XIII, erosion has destroyed the flat topped character of some of the small blocks to such an extent, that faulting is evidenced only by the displacement of the lavas, This however, is not cecom- mon a8 the flats are usually well preserved and the scarps are but moderately rounded at the top. The more rapid ero- sion of the central block in this section is due to the less resistent character of the agglomerates which are ex- posed beneath the lavas. The smaller faults can seldom be traced for more than two or three miles along their strike and apparently converge or branch in a rather irregu- lar manner, giving rise to an uneven distribution of the various blocks,some sections of the mountain front showing three or four, others but one or two, This however may be due in part to erosion, PETROGRAPHY OF THEE FANGANGO VALLEY SECTION In the section between the top of Fandango Peak and the floor of the valley, three flows of the lavas were recognized but at Buek Creek two iiles north-west of this section, two additional flows were found. In the first, eight hundred feet below the top of the mountain, only one flow was recognized and is shown as such in the section, But in this vieinity, the mountain front is covered in part by long talus slopes derived from high cliffs of lava im- mediately below the top of the peak and also in part, by a dense growth of brush which obscures all but a few out~ crops. It is quite possible that this part of the section is made up of more than one flow but of such similar char- acter that they cannot be differentiated except by locat- ing the contacts, which is not possible in this locality. A gomewhat similar condition prevails on the faulted block at the base of the escarpment and but one flow was recog- nized, At the base of the lavas ther is exposed about four hundred feet of tuff and agglomerate made up almost entire- ly of glass. The upper half consists of small pieces of black and grey obsidian up to one-half inch in size, with very little tuffaceous matrix, The fragments are fairly well cemented so that rather conspicuous out crops are fommdd. The lower part consists chiefly of smaller fragments and an abundance of light grey tuffaceous material, It is much less resistent than the upper part and weathers down san even surface. Altho material of this character was not found so extensively developed in the upper agglomerates in other parts of the range, from its position with respect to the overlying lavas, it is evidently a part of the agglomer- ate series, Upper Flow Wherever examined, this rock has practically the same megascoplc appearance, the only difference noted being slight variations in the amount of magnetite and the expent of weathering and staining, It has a medium grey color and for the most part shows little alteration in the hand speci- men except numerous small red or brown spots which under the microscope are seen to be caused by the alteration and stadn- ing of olivine. The rock is porphyritic with megascopiec phenocrysts of feldspar and olivine in a fine granular ground mass, altho individual minerals can seldom be recognized. I is moderately viscular near the surface but less so to- wards the central part of the flow. The vessicles are spher- ical in shape and rarely filled with secondary minerals. Along the south-western side of Fandango Peak, this flow forms vertical e¢liffs one hundred or more,feet high. On the face of these, it shows a well developed rectangular joint- ing which causes it to weather out into large blocks or slabs, forming long talus slopes down the side of the moun- tain, A comparison of thin-sections of the lavas found in this section shows that the rock which forms the top of Fan- dango Peak, occurs again fifteen hundred feet below the top of the mountain and also forms the floor of Fangango Valley. See Plate X11, From here it extends out to the eastern edge of Goose Lake, which is two miles west of the base of the range. DBSetween these two points and over the floor of Fan- dango Valley the lavas are lying horizontal but on the north- eastern side of the valley, they are tilted slightly to the north-west, which is apparently a result of the oblique fault- ing. licroscopic Character: Thin-sections of the upper flow from the various elevations at which it was found, show but light variations which are noted below, It consists of a rather uniform holo- crystalline groundmass of small feldspar laths .,06 to .02 mm, long and rounded olivine grains which are enclosed to a great extent by latge ophitic plates of augite. The phenocrysts are feldspar and olivine but are not abundant and show con- siderable irregularity in size. The feldspar laths lie in all directions and show very little tendency toward parallel orientation due to flow. In short, the rock has a distinet- ly diabasie texture with a marked ophitiec habit of the augite. The feldspars of the groundmass, have the usual lath- shape and show from two to four albite twining lomellae. The erystals are for the most part euhedral, but in some cases a mutual interference between two or more feldspars or occasional embayments of olivine, give them more or less irregular outlines. They rarely show cleavage but are often traversed by irregular fractures. Symmetrically extinguishing albite twins give values up to thirty-four degrees corresponding to 2 medium labradorite. Inclusions are’ bommon, the only kind noticed being small needles of apatite, fine opaque grains which are probably magnetite or possibly ilmenite, and occasional minute isotropic bodies which appear to be glass. In all of the section the feldspars have a very fresh unaltered appearance, The phencerysts of feldspar, for most part, have euhedral elongated shapes but are occasionally rounded. They show both albite and Carlsbad twinning. Their extinction angles show them to be labradorite and but slightly if any, more basic than the feldepar of the groundmass., They oc- casionally show zonal growth in two or three bands and con- tain inclusions of magnetite and spatite but not to as great an extent as many of the rocks of this area. Olivine, is very abundant and shows great variation in size from phenocorysts, the larger of which are 1.0 mm, to small rounded grains in the groundmass, .056 to 0.I0 mm. in diameter, The larger erystals rarely show enhedral outlines and are more or less rounded. ¥hen fresh it is colorless but it is more commonly stained various shades of bmown and red. It sometimes shows two fairly distinet cleavages paral- lel to the pinnacoidal faces and is always traversed by nu- merous fractures. The extent of alteration of olivine is the greatest varistion shown in the different thin-sections of this rock. In the thin-sections from the top of Fandango Peak most of the olivine is quite fresh and but slightly stained yellow or light brown, Specimens taken from the sur- face of the flow at the bottom of the valley however, show it to be deeply stained red and brown and nearly always more or less altered along the borders or fracture cracks of fibrous serpentine. The stains which develop from the olivene rarely extend beyond the limits of crystal and are the cause of its red spotted appearance in the hand specimen, The less altered character of the olivine in the specimen from the top of the mountain, is probably due to the fact thal Fook there, has been but recently exposed by the rapid mechanical weathering of the rocks along the cliffs which rapidly ex- poses new surfaces whereas, over the level floor of the val- ley, the rocks at the surface of the flow have been exposed a much longer time. Augite, is second to feldspar in amount, It has a faint yellowish grey color but is not pleochroic. It occurs as large irregular plates often 2.0 to 3.0 mm, in size. I% is sometimes so abundant as to form an almost solid matrix over the greater part of the thin-section in which are poecilitically enclosed the feldspar laths, olivine, and magnetite. It gives an extinction angle of fifty degrees and shows well developed cleavage, It is usually quite fresh but in sections from the bottom of the valley, it sometimes shows a light amount of staining. Small needles of apatite are also occasionally included by the augite. liagnetite, is rather abundant in all of the thin- sections of this rock examinéd, but those from the bottom of the section show somewhat greater amounts than the one from the top of Fandango Peak, Slight variations in the relative proportions of the minerals in different thin sections of the same flow, are quite common and magnetite appears to be the most variable of all, Part of the excess magnetite may have been derived from the alteration of olivine since the sections which show the greater amount of magnetite also show the greatest amount of alteration, Auzite Andisite The second flow from the top of the section is about one hundred and twenty-five feet thick, It is a very vesi- cular porphyritic lava with a dense aphanitic groundmass, -45~ and a few macroscopic phenocrysts of feldspar, the largest of which are about 2.0 mm. long. The vesicles are usually more or less elongated and somewhat irregular in shape, They are partially filled or coated with chalcedony or opal which has a dall green color. The groundmass of the rock has a greyish brown color and is not stained to a very great extent. The extremely vesicular character of the rock ob- F211 Clas scures all flow structure, other than the elongation of the steam cavities, As shown in the section, it occurs at two different elevations on the mountain front, The verticle separation of the two parts is about two hundred feet and represents closely the throw of the fault causing this re- petition, Mieroseopic Character ‘In thin-sections under the microscope the rock has a hyalopititic groundmass, rich in lath-ghaped feldspar mi- erotites , small grains and prisms of augite and lesser a- mounts of magnetite with wekk developed cubicle and octa- hedral forms, in a residium of yellowish grey glass. The erystals of the groundmass show marked flow structure by their parallel arrangement and the manner in which they curve around the phenocrysts. The latter consists of pla- groclose feldspar and augite but are not abundant. The Microscopie feldspar of the groundmass are slend- er laths averaging 0,0I to 0.03 mm. in length and as a rule, are quite euhedral and without irregular terminations. Albite twinning is common with the larger crystals but rare- ly with more than three lamellae, but many of the feldspar mierolites are so small that twinning is not evident. The extinetion angle measures from their direction of elongation, varies from zero to six degrees which corresponds to oli- goeclase. They occasionally contain very minute colorless isotropic inclusions which may be glass Or gas bubbles. The phenocrysts of feldspar show stout rectangular and elongated sections, the largest of which are 2 mm. They vary considerably however, down to small erystals 0,30 mm, in size. Albite andpericline twinning are common and ocea- sionally that of Carlsbad. Fhenocrysys are not abundant in the thin-section and those present do not give very defi- nite optical tests. Two however, showed symmetrically extin- guishing albite twins with an angle of thirty and thirty-two degrees respectively, which would correspond to a labrador- ite. They are occasionally zoned but none were observed with more than three bands. Most of the feldspar phenocrysts are practically free of inelusions only rarely containing apa- tite, magmetite or glass. However, one crystal was found that was densely crowded thruout the central part with fine grained magnetite inclusions arranged apparently in some definite crystaloraphic manner, so as to give somewhat the appearence of graphic intergrowth. This crystal is made up of two zones of which the outer one is quite narrow and free from inclusions. The entire corystal shows a considerable degree of rounding due to resorption. None of the feld- spars show any appreciable alteration, The Augite occurs both as phenoerysts and in the ground mass. It is pale yellowish green to colorless and non~-pleoschroiec. The phenocrysts have euhedral and sub-hedral shapes and are often partially rounded, They vary in size from 0,30 mm. to 0.70 mm, and are not abundant. They show well developed cleavage and an extinetion of forty-five de- grees, while basal sections have octagonal outlines and the characteristic pyroxene cleavage with symmetrical extinction, Some of the augite crystals are twinned and ocoasionally show three or four bands, Inclusions are not common and are either glasa or small rods of apatite. The augite nucrolites of the groundmass are abundant, They occur in small slender prismatic shapes .03 mm, to O0,Imm, - long and also as irregular grains. They never show crystal boundaries but have good cleavage parellel to théftr elonga- tion. A maximum extinction angle of forty degrees was re- corded on some of the larger pleces., They are colorless and outside of the difference in size have much the same appear- ance as the augite phenocrysts, All of the augite in this rock is quite fresh and unstained by alteration, Hagnetite is less abundant than any of the above mentioned minerals and also in smaller amounts than most of the basalts of this area, It occurs in small uniform sized cubes and octahedrons distributed evenly thruout the roek and seldom as irregular masses, as in many of the basalts. Apatite is the only other accessory mineral and is of 1it- tle importance as 1t is only represented by an occasional inclusion. This is another more distinetly contrasting feature between this rock and the basalts, a8 in the lat- ter apatite is usually rather abundant. The most important difference however, between this flow and the rocks which are classed as basalts and which place it as an andesite, is the abundance of oligoclase feldspar and the absence of olivine. This with its charac- teristic andesitic texture as contrasted with the dia- pasic texture of the basalts, are its most distinguishing characters. Biotite Andesite The third flow in the section, is about one hundred and seventy-five feet thick. It is a light grey parphyrit- ic lava, with a dense aphanitic groundmass containing a moderate number of megascopic phenocrysts of feldspar 1.0 to 2,0 mm, long. It is distinctly banded by even and close- ly spaced flow lines which it weathers out into thin plates. The rock has a very fresh unaltered appearance and is not stained except for a thin brown film over exposed surfaces. idigroseopic Character Under the microscope the thin-section of this rock shows & very fine frained pilotaxitic groundmass, consist- -49- ing almost entirely of minute lath shaped feldspar micro- lites, and fine opaque grains and dust which are probably magnetite; phenoerysts of feldspar, which are not abundant and which vagy considerably in dize; and a small amount of biotite. Glass or spherulites were not observed in the groundmass but it is only under highest power of the miero- scope that the smallest crystallites can be detected, The feldspar phenocrysts, range in size from mega- scopic crystals 2 mm, in length, to smaller ones 0.2 to 0.3mm. which are only visible under the microscope. In addition there are numerous miecrolites of quite uniform 3ize, and somewhat larger than most of the crystals of the groundmass but which could hardly be classed as phenocrysts. The larger feldspars have a tabular habit parallel to the glide pinaciod and occasionally have sharp enhedral outlines, but more commonly they are partially rounded. Many of the crystals show a complete development of the basal, dome, and prism faces and show quite characteristic shapes in cross-section, Cleavage is well developed parallel to the base and side pinacoid in addition to numerous irregular fractures, The extinction measures fron the trace of the basal cleavage on sections cut parallel to the side pina- coid give a value of six degrees which with an index of refraction higher than canada balsam and positive sign, correspond to andesine., They show albite, Carlsbad, and pericline twinning but the latter is not always evident, The larger crystals are frequently zoned with two to five bands with a progressive increase in the extinction angle towards the center. All of the feldspar has a fresh un- altered appearance and contains but few inclusions of apa- tite and magnetite or ocoasional emba¥unents pf the ground- mass. The small feldspar mierolites intermediate between the phenocrysts and those of the groundmass have sharp rect+ angular or quadratic shapes, often with a slender spine-like projection from each corner. They resemble tO dome exitent the orthophyric development of orthoclase in some trachytes, but since some of them show albite twinning they are at,least in part plagioclase. Cleavage is not evident but the ex- tinetion measures from the sides of the crystals on a few, gave value from three to five degrees, but on the majority of them it is parallel. Those not showing albite twinning or inclined extinction, cannot be distinguished from ortho- clase, but from the few values obtained, they are at least in part and perhaps all, oliboclase or andesine, From these larger microlites there is a more or less gradation in size to the very minute feldspars of the groundmass which are too small for optical tests. Biotite, showing a deep brown color and rather strong absorbtion is also scattered sparingly thru the rock. All of it has undergone considerable resorbton producing ir- regular or rounded outlines with an abundance of fine grain- ed magnetite along these borders, Some of the biotite has apparently been completely re- placed by the magnetite, as evinced by small areas dense- ly erowded with the latter, which have quite definite boun- daries. Intermediate stages where only a small amount of biotite remains are also shown, Une plece of a green pleochroic mineral which is believed to be hornblende was also found in the thin section, It has a slender prismatic shape and well-developed cleav- age. Only a part of the original erystal is present, the remainder having been torn out in preparing the slide. The fragment is rounded on its unbroken end and is surrounded by a black opaque border of fine granular material which is evidently magnetite. The border is quite shapely defined on either side but there is a small amount of the same ma- terial included within the crystal, Since there is only one piece of the mineral in the thin seetion and it so orient- ed as to give rather indefinite tests, it could not be pés- tively identified as hornblende. There are two holes in the slide of similar shape, to that partiecally occupied by the above mineral, and which are bordered by fine granular magnetite, suggesting a greater abundance of the mineral than is shown in the thin section. lagnetite occurs as occasional erystals of medium | 8ize but is more abundant as fine grains or dust in the groundmass. It is not always possible to determine whether -52- it is primary or secondary after the ferromagnesian minerals by magmatic alteration, It is however, less abundant than in the olivine basalts, Apatite is present both as inclu- sions in the feldspar and as occasional erystals in the groundnass. The Jatter are larger than the inclusions and have long slender prismatic shapes with short pyramidal ter- minations. It has a greyish color and shows slight obsorp- tion parallel to the principle zone, It has the usual low birefringence, and nezative elongation, From the feldspar which could be determined in this rock; the occurrence of biotite, and probably homnm blende in the place of pyroxene, and the finer grained char- acter of the groundmass it is evidently less basic than the rock overlying it and would probably be more properly class- ed as an acid andesite or possibly a latite depending upon the character of the indeterminate feldspar of the ground- mass, NT, BIDWELL SECTION Field Relations In the two fcresoing detailed sections it has been attempted to establish 2 connection between the Modoc lavas along the western base of the Warmer Mountains and the lavas thé form the top of the range. The purpose of the following detailed section of the east side of Mt, Bidwell, is to show a like relation between the summit lavas and those which occur along the northern part of the eastern base. iit, Bidwell is on the eastern side of the range about three and one-half miles south of the California Cre- gon Boundery. It has an elevation of eight thousand two hun- dred and eighty feet, and rises twenty-five hundred feet above the eastern base. The top of the mountain is almost flat and stands form fifty to two-~-hundred feet above the sur- rounding summit level. The section shown on plate XIV extends from the top of Mt, Bidwell, to a point about one and one-half miles east of the base of the range. The faults shown in this section altho more numerous, do not differ essentially from those discussed in the previous section. They are all nor- mal faults which strike parallel to the trend of the range and have their down-throw on the eastern side. Due to the fact that the agzlomerates are not exposed in this vicinity, erosion has been much less rapid than in other parts of the range, 80 that the physiographic expression of the faulting has been but little affected, The major fault occurs immedi- ately below the top «f kt, Bidwell and has a throw of at least one thousand feet, The top of the downthrow block of this fault, forms a distinet flat, eight hundred to one thousand feet wide, which extend along the mountain front for several miles, while the escarpment rising fron the west- ern 8ide of this flat, presents very steep slopes and many “54 vertical cliffs often one to two hundred feet high. Between this fault and the base of the mountain there are five simi- jar flats in step-like order each abutting against a steep scarp along its eastern side, The most conclusive evidence of faulting however, {gs the repetition of the lavas down the moungain front, As shown on the plate XIV the top of lit, Bidwell consists of about two hundred and fifty feet of grey and white obsidian, This same rock occurs on each successive block between the summit and base of the mountain, The two flows immediately below the obsidian, are also exposed on the first three blocks but on the lower two only the glass is exposed, These three rocks differ considerably in their megascopic appearsnce and are quite easily recognized in the hand specimen affording very definite evidence of faulting. In addition to this how- ever, comparisons of thin-sections of these rocks from the various elevations were made and in every case the specimen obtained from the lower elevations, were identical with those taken from the first three below the top of the mountain, PETROGRAPHY CF TSE MT. BIDWELL SECTION The escarpment along the upper part of the east- ern front of Lit, Bidwell, expose, a section of the ilodoe lavas one thousand feet thick, consisting of nine flows, vary- ing in thickness from twenty to two hundred and fifty feet. Due to the great uniformity, both in mieroscopic appearance and in thin-sections, of a great number of these flows, a detailed description of each would involve unnecessary re- petition. In order to alleviate this, only those rocks which have been repeated by faulting or which show imper- tant variations, will be described in detail. In order to designate each flow they will be feferred to by number in order from top to bottom. I. Rhyolitie Obsidian Hegasopical Character: The rhyolitic obsidian at iit, Bidwell, is apparently a part of the same flow that occurs one mile north of Fandango Peak, at the simmitabove Davis Creek and various other localities in the range. It is about two hundred and fifty feet thick on Mt, Bidwell, but thins guite rapidly toward the north, disappearing al- together in a distance of one and & half miles. Lit fosms the top of each of the small fault blocks down the moun- tain front but was not found on the ridges bordering Eight liile Creek, east of the base of the range, except as abun- dant fragments scattered over the surface of the basalt, which in other parts of the section underlies the obsidian, All of the obsidian found in place in the vieini- ty, has a white or grey color. The black and brown glassy varieties found at Davis Creek, were observed here only as loose fragments over the surface at the top of the mountain, Considerable variation however, is to be noted in this light colored obsidian, both in its megascopic and microscopic appearance. The greater part of the rock is light grey, 1lith- oidal and quite free of macroscopic phenocrysts, It is distinetly bended and parts into thin plates along these lines. It is quite vessicular, the vesicle usually being drawn out into slender tubes, Illany of these cavities have been filled with chalcedony or opal, or in other cases are lined with minute quarts crystals, forming drusy surfaces. There is usually a brown stained border around the filled vessicles which with their elongated shape and to the banded appearance of the rock. Cther parts of the obsidian have a very glassy luster and do not show vesicular or band- ed structures. This facles has 2 somewhat mottled appear- ance, in most part white, with grey and bluish streaks or patches, It has a very conchoidal frscture and weathers out into sharply angular fragments. Altho it hss a much more glassy appearance than the lithoidal variety, it con- teins occasional macroscopic phenocrysts of guatrsz, Microscopical Character Thin-sections from different parts of this rock exibit a rather wide varistion in the extent of erystalli- zation, some showing very few phenocrysts of spherulitic growths, others have abundant phenocrysts of quartz and feldspar in a eryptoeryatalline groundmsss with only a small portion of unindividualized glass, It is the lithoidal banded variety that ia the least crystalline. A mioro- spherulitic structure is quite common in the more erystal- line variety, the minute spherulites occurring insuch large numbers and so closely packed as to constitute the chief bulk of the groundmass, Axiolites 0.5 mm. to 0,7 zm. &n length, are also cceasionally seen. These consist of elon gated spherulites where the ninute feldspar fibers extend out from a central axes instead of the more common radiate structure, Trichites 2nd other allied structures are some- times seen but are never sbundant. Phenogrysts-Quarts is rather abundant in some parts of this rock. It oocurs usually as small rounded grains, rarely with enhedral outlines. Some of the larger crystals show irregular corroded borders with frequent em- baynents of glass, It is colorless but contains numerous minute brown and gray inclusions, most of which sppear to be glass, Some of the quarts orystals exhibit stain shadows, but the greater pars gives quite definite optical orienta- tions and ean be distinguished readily from the feldspar by its uniaxial and positive character, and higher inter- ference tints. Feldspar is present in widely varying amounts in all of the thin-seetions examined. In some, 1% consti- tutes the bulk of the phenoeryste, as well as = large part of the ground mass in the form of spherulites and minute microlites, while in others it is present only as occasional -58=~ isolated phenocrysts, in a glassy groundmass, It is rare- ly more than 0.2 mm. in size, and shows &nhedral to sub- hedral guadratiec and tabular sections, It ocoasionally shows Carsbad twinning but not albite., Its index of re- fraction is lower tham that of canada balsam. Sections in which definite determinations could be made, were not found but from the above properties and its parallel or very slightly inclined extinction, 1% is probably orthoolase or sanadine, Small slender hair like erystallites of an opaque mineral are also present in some sections. They give a more or less eutaxitic appearance to the roek in which they are moat abundant, by their parallel orienta- tion and the manner in which they ourve around the pheno- orysts and vesicles. Small cubes of magnetite also are present in limited number. 2nd. Flow The flow immediately below the obsidian at the top of kt, Bidwell is twenty feet thick, It has a medium grey color on fresh surface ani weathering has caused only 8 moderate amount of staining on exposed parte, It is moderately vesicular thruout the vesicles s&s s rule, being quite small and elongated. The rock weathers out inte blocks of various size which are more of less rectangular in shape, with smooth surfaces. These blocks apparently weather out along lines ss jointing is not evident. This rock is exposed beneath the obsidian on the first three fault blocks below the top of Mt. Bidwell. 1t occurs again at th: base of the range and was followed along Eight Mile Creek for about three miles to the east. The flow has 2 greater thickness at the lower elevations wh Nre™ and is exposed to a depth of ome hundred feet in the can- yon of Eight Kile Creek. A careful comparison of speci- mens of this flow from near the top of the mountain and from points one to two miles east of the base of the range show no difference whatever, either in their mehasecopiec appear- ance, or in thin sections under the mieroscope. This, with other definite evidence of faulting mentioned above, seem quite sufficient to show that the lavas at the summit and eastern base of Nt, Bidwell are the same, Microscopie Character In thin-sections from near the central part of the flow, the rock is fine grained holoecrystalline and shows a distinct separation of the minerals into two generations. The phenocrysts do not make up over fifteen percent of the rock but stand out distinetly from the minerals of the ground- mass, The groundmass has & granulitic diabasic texture con- sisting of small feldspar laths, showing an indistinet par- allel orientation with fine granular, rarely enhbdral augite, filling the interstices. Intersertal glass is not evident in any of the thin sections examined, The feldspar of the groundmass occurs as lath- shaped crystals for the most part with well developed, {diomorphic outlines. They very considerably in size, their average length being 0.15 mm. to 0.30 mm, but a great many are smaller. Albite twinning te common $0 ail of them, show- ing two to four damellae on the smaller laths but on the larger ones more may be present, Occasionally they show combined Gar- 1sbad and albite twinning but this is more common with the feldspar phenocrysts. Extinetion angles measured from sym- metrically extinguishing albite twins, were recorded up to thirty degrees corresponding to a labradorite with a composi- tion of about A By; A N55 They are usually free of zonal structures but opaque inclusions of magnetite and small color- less globules are quite abundant, The phenocrysts of feldspar are from 0.60 to I.0mm, in length and have eunhedral outlines, altho sometimes partially rounded, due to resorbtion. They all show albite twinning and frequently Carlsbad and albite twinning combin- ed. They give a positive sign and extinotion angles corres- ponding to a basic labradorite. Zonary banding is not un- common but rarely with more than three or four layers. Small rod like crystals of apatite and fine grained magnetite, are very common as intjusions, and occasionally there are color- less isotropic indlusions which are probably glass, Olivine is very abundant occurring both as pheno- crysts and as a constituent of the groundmass with an indis- tinct grasation in size. The larger erystals as a rule have quite enhedral outlines but the smaller grains are more or less rounded or irregular in shape. The phenocrysts are from 0.70 to 1.0 mm. in size. Gradationsbetween this and minute grains, 0.5 mm, in sige which occur in the ground- mess, are present, Vhen fresh, the olivine is colorless, but it is usually stained and to a great extent replaced by secondary material, If is always traversed by irregular cracks, besides a more or less distinet cleavage whihh is parallel to the direction of extinetion. These features, with its high refraction and double refraction and wide op~ tic angle, distinguish it from the other minerals present. Inclusions in the unaltered olivine were not observed. The relations between the large crystals and the feldspar, shows the former to have crystallized first but the smaller greins may have formed at the same time as the feldspar, The alteration product of the olivine i{# Ia- dingsite. 1% usually has a deep reddish brown color but is occasionally light brown or somewhat yellowish, The darker colors are probably due to iron stains as described by Iawsodi,”, and Iddings?* It ocours as a pseudomorphs after olivine, or more commonly as an aggregate of Tlakes or fibers, the bor- ders of which are quite sharp and are &vidently those of the original minerals. The larger pieces show = marked platy cleavage. Seetions parallel to the cleavage show the émer- gence of the negative acute bisectrix, and a wide optic angle, > Bull. Dept. Geol, Univ, Cal. Vol I,pp36. Monograph XX U,.S5.6.5. (1893) pp 386-394. ’ GB Pleochroism is also evident on the thinest sections but this is often obscured by the deep red stains, Absorbtion is strongest for those rays vibrating in the plane parallel to the cleavage. The alteration sometimes begins at the border of the crystal and progresses rather evenly toward the cen- ter, snd does not show any particular relation to cleavage or fracture cracks. In other cases it begins at the center of the crystal snd progreseecs in a similar manner foward the borers. lt way develop into a fibrous or flaky aggregate, or form a single crystal. The fibers are usually yellowish or brownish green at first, but become dark red or brown as the alteration proceeds, The pyroxene constituent is augite, which occurs as small subhedral and xenomorphiec grains 0,05 mm. to 0.I5 mm. in size. It has a very pale greenish color and is non-pleo- chroie. It is sometimes stained light brown which seems to accompany a slight amount of alteration, The largest grains show an extinction angle of forty degrees, and the character- istic pyroxene cleavage. It only occurs in the groundmass, with about equal propertions of olivine which marks a varia- tion from the greater part of this series of lavas in which augite predominates over the olivine, It is with rare ex- ceptions xenomorphic toward the feldspar, and is evidently the last product of erystalliszation, Magnetite im well shaped cubes and octahedrons, is abundant. It occurs in the ground mass and is also in- -63= cluded by all the other minerals present. It is the most con- spieucus inelusion of the feldspar, often in the form of microscopic grains or just, giving them 8 cloudy sppear- ance or posetines concentrated around the edges of the orye- tn1 forming dark borders. There are also opaque irregular mnases gome of which msy be ilmenite, The distinctly por- phyritie character of this rock and the great predominence of feldspar over the ferromngnesian minerals with a more or less parallel arrangment of the former, give it somewhat the appearance of a basic andesite, but the abundance of olivine is evidence of its more bagic character. It is an Olivine basalt with a micro-disbasic texture, 2rd. Flow The third flow from the tup of the section is about one hundred and fifty feet thick, It has 8 dark grey color on freshly broken surfaces but 1s goated with red and brown stains whereever it 1s exposed, It is quite vesicu- lar near the sufrace of the flow but very dence and con- pact thruout the central part, Flow lines oftan closely apaged are more distinet than in the overlying rock. Teathering along these lines dauses ihe rock to break into rectangular blocks, All éf the rocks in this section are considerably fractured due to the faulting. This rock is exposed on the first three blocks below the top of the moun-~ tain but due to the {nereased thickness of the overiying flow it is not exposed along sight lille Creek east of it. Bidwell iieroseopie Character Specimens of this flow were taken from near the surface and from the central part but they do not show any noticeable variations. Sxcept for fewer phenocrystis of olivine and a greater amount of augite thin-sections of this roek have. nmuch the same appearance as the one overlying it. It is holeoerystalline porphyritie with a fine microdisbasic groundmass and no intersertal glass. The phenoerysys consist principally of feldspar, but there are occasional moderately large crystals of olivine, The latter mineral however, shows such an’ even gradation in size to small grains in fhe groundmass that two separate pericds of erystallization are not evident, The faldspars of the groundmass are small lath ghaped crystals 0,15 to 0.256 mm. long, usually showing fourm to six albite twinning lamellae which is oceasional- ly combined with Carlsbad twinning. Symmetrically ex- tinguishing albite twins show a maximum extinction aidjle of thirty-one degrees corresponding to about the same com- position as the overlying flow, The phenoerysts of feldspar are from 0.50 to 0.70 mm. in size add often show tabular or sometimes square sections, with euhedral boundaries. Albite ani Carlsbad twinning are common and occasionally they show pericline lamellae, They give extinction angles corres- ponding to labradorite bytomnite. The erystals are fre- quently zoned end on some of these, the interior bands are almost round due to partial resorption before the for- mation of the outer layers. !agnetite is cccasionally in- cluded within them bul in larger grains than in the pheno- erysts of the overlying rock. Alteration along cleavage and fracture cracks give rise to a grey cloudy material re- senbling kaolin and brighty polarizing graing which are probably mica, Clivine is less abundant than in the overly- ing roek. If usually occurs as small rounded grains but occasionally shows emhedral forms. It varies considerably in size but is rarely over 0.60 mm. Inclusions of magne- tite were the only ones observed in the olivine and they are not common, The alteration product of the olivine in this roek is chiefly serpentine in the form of aggregates of yellowish and brownish green fibers or masses, It de- velops along the borders, or fractures of the olivine, The fivers stand normal to these lines and extend irregularly into the erystal., liuech of the olivine has been completely replaced by the serpentine which forms a mass of variously oriented fibers." The deep red stains accompanying the al- teration of iddingsite were also noticed on two of the larger olivine phenocrysts in one thin section. The Augite is much more abundant than in the overlying rock and considerably in excess of the olivine in this third flow. It occurs only in the groundmass as xenomorphic grains or angular wedges between the small feld- spar lathe. When cleavage is not evident, the smaller grains cannot always be distinguished from the olivine. Blaish green chlorite 1s apparently associated with the augite as an alteration produet but this relationship is not always evident. 1t occurs as flaky aggregate and has completely replaced much of the original mineral, Magnetite in varying amounts is an ever pres- ent accessory in all of the rocks of this area, It varies considerably in amount in different flows and may show quite noticeable variation in different thin-sections of the same flow. This latter variation is shown in this third flow; the thin-section of the rock from the second fault block showing much more magnetite than the one from near the top of the mountain, 4th. Flow The fourth flow in the section is about fifty feet thick but due to the similarity of t.is and the over- lying roek, the contact between the two was not very close- ly located and is shown in its approximate position in the section on plate XIV by & dotted line. It has a dark grey color on fresh smrfaces but like nearly all of the rocks of this section is coated with red and brown stains which, Irom a distance gives them a very uniform appearance. For the most part, it is very dense and contains but few vesi- cles. This rock and all of these underlying, are only ex- posed in the section immediately below the top of kt. Bidwell. Hicrosecopiec Character In thin-sections under the microscope this rock is readily distinguished from the one above it by a marked ophitic habit of the augite, where 28 in both of the rocks just described, the augite is small xenomorphie grains in the groundmess. This rock is holoerystalline with a fine grained groundmass and a few comparatively large phencerysys of feldspar. The small feldspar laths show no fluxoinal ar- rangment but lie at ail angles and are to a great extent poecilitically enclosed by the augite. The feldspar of the groundmess are from 0.15 mm. to 0.2 mm. long ond have the same development as in the over- lying rocks, They give a maximum extinction angle of thirty- three degrees corresponding to a medium labradorite. The phenocrysts of feldspar are from 0.6 to 1.0 mm, in length, and usually have more or less irregular or corroded outlines. ‘ney are much less abundant than in the rocks previously de~- scribed, only three or four being present in a thin-section. Sections favorable for specific determination were not found, They are usually zoned but rarely with more than three bands, Inclusions of magnetite and apatite are common in both the phenocrysts and in the swaller laths, All of the feldspar is quite fresh and unaltered. The Olivine occurs us rounded grains and rarely ea phenocrysts. In the thin-sections exauined, it is not as abundant as in the overlying rocks and is more extensive- ly nxltered to fibrous serpentine, snd in great part complete- ly replisced by the iatter,. fhe Augite ic much more sbundant than the olivine, It occurs as irregular ophitio plates whioh vary greatly in sige, the average however, is 2,0 em. to 3.0 om, It has & faint yellowish grey color and is non-pleochroic, and gives extinetion angles as high as forty-eight degrees. Inclusions within the augite are abundant, the most common of which are feldspar and magnetite, byt olivine and slender rods of spatite are also usually present, The embayments of these earlier formed minerals, give the augite a very irregu- lar sinuous outline. Bright bluish green ohlorite is associated with the sugite around which it offen forms an irregular border and in many cases has completely replaced the smaller pleces. It occurs both as fibrous and flaky aggregates but ths two are usually separate. A common vocurrence where the altera- tion has been compiete ia & central mass of the flaky mate- tial, which is usually mcre of a brownish green color sur- rounded by a narrow border of bright bluish fibrous chlorite; the fibers standing normal to the sides, These masses of chlorite include and wrap around the feldspar in the same manner as the unaltered augite. A very small amount of reddish brown biotite occurs a8 &n accessory mineral, It is usually irregular in outline, but one cleavage flake was found, showing perfect hexagonal shape. It has an optic angle of almost zero de- grees and is strongly pleochroic. It is apparently of pri- mary origin, There is also a large amount of magnetite the crystals of which are often quite large and have good cubi- cal or octahedral shapes. :spatite in long slender needles is more abundant than in most of the rocks of this section. A comparison of the Hodoec lava in other parts of this area, show that the ophitic type of diabasic texture is much more common than the granulitic or normal type. All of the re- maining rocks in this section have the ophitic texture well developed nAd outside of wings variations in grain and flu- clonal arrangment of the minerals, present a very uniform appearance in thin-sections. They are all olivine basalts, consisting of labradorite feldspar, all of which gives ex- tinetion angles of thirty-one to thirty-three degrees, au- gite, and olivine, with magnetite and apatite as abundant scccasories. The thickness and appearance of the flows, and theirslight variations will be briefly alluded to in the fol- lowing paragraphs. The fifth flow is forty feet thick. It is more or less stained by iron thruout and has s reddish brown color on exposed surfaces. It is very vesicular thruout and shows quite distinet parallel banding due to flow, The _ AT ATT TER er Ty a_i" LY ” hh TI -70- groundmass is fine grained with relatively great variation in the size of the crystals. Phenocrysts of feldspar are not abundant but the few present are from 1.0 mm. to 2,0 mm, long. The olivine is guite extensively altered to fibrous serpentine and an olive green micaceous mineral, whieh is probably antigorite. 14 shows rather strong pleochroism on sections inclined or perpendicular to the cleavage flakes, with the strongest absorption in the plane of the cleavage. Sections parallel to the cleavage are only faintly pleo~ chroic, and show the emergence of the negative acute bisec- trix with a very small optie angle. The alteration begins either around the porder of the olivine crystal or at the center, and spreads uniformly over it. The platy mineral is frequently associated with fibrous yellowish brown Or green serpentine, Swelling takes place with the develop- ment of the latter, for where feldspar phencverysts lie ad- jacent to it they are traversed by fissures which radiate out from the altered ares, The vesicles of this rock have been filled to gome extent by secondary minerals. These have a faint green color and occur in radiate or acicular structures with the fibers extending out from the walls of the cavity, towards the center. The sixth flow is between thirty-five and forty feet thick. It is more deeply stained and altered than the fifth snd has a light red color thruout. It is very vesicular with great variation in the size of the cavities, some of which are one half to three-fourth ineh in diameter. They are usually lined with light blue opal, less commonly, they have drusey surfaces formed by small crystals of clear quarts. sxcept for the greater amount of alteration if the augite ndd olivine, accompanied by a great deal of irom staining, the rock is very much like the flow above. The seventh flow is eighty feet thick. It has the usual deep red stains on the surface but on freshly broken pieces it is very dark gray and unstained, The groundimes is mueh finer grained than in the overlying rocks, consisting of small elosely packed feldspar laths with very marked flow structure. The ophitic plates of augite are also broken and the fragments strung out in the direction of Tiew. The pheno- orysts of feldspar are abundant mid relatively large, vary- ing from 1.0 mm, to 2.0 mm. There is also a great deal of olivine whieh occurs as euhedral or 8lightly rounded pheno- crysts 0.70 mm. to I.5 mm. in size, Most of it is unalbered but occasional orystals show the development of fibrous ser- pentine along the clsavage or fracture cracks. This flow is underlain by a layer of red tuff about five feet thick. It weathers down even with the sur- face of the ground and is only noticeable by the red color vhich it imparts to the soil. It is for the most part very fine grained but contains numerous orystals of plagio- classe, which are considerably altered and have a dull luster. Pseudomorphs: of limonite after magnetite or a ferro-magnes- ian mineral are also rather common, A microscopic examina~ tion of this material was not made. The eight flow is two hundred and fifty feet thick. It is dark grey porphyritic lava, with a microorystal- line groundmass containing many macroscopic phenocrysts of feldspar the largest of which are 6 or 7 mm. long; olivine showing a distinct conchoidal fracture, and large crystals of augite containing inclusions of feldspar. There is an increase in the size of the phenoerysts and orystals of the groundmass between the top and central part of the flow. It is quite vesicular near the surface but the steam holes be- come fewer toward the center. The rock is also greatly frac- tured due to faulting. Secondary calcite occurs filling these fractures and the vesicles. Under the microscope the thin-sections of this rock show a slightly greéter amount of augite than in the overlying rocks but with the same ophitiec habit. The ground- mass im somewhat coarser grained but there is a great varia- tion in the size of the feldspar laths., Thin-sections from the upper part of the flow show a very small amount of in- tersertal glass which oceurs in very siall wedges, between the feldspar, but it is not evident in sections from the cen- tral part. Clivine is very abundant, and partially altered to serpentine, but as a whole the rock is quite fresh. The lowest flow exposed in the section, has a dark grey color and is less stained than the overlying rock, but in thin-sections shows no difference fron it exeept that the phenoorysts, tho rather abundant are not as large. It is quite vesicular and near the surface, the cavities have been filled to a considerable extent by calcite. Comparison of the lit, Bidwell Section with Other Parts of the Area. The marked uniformity in composition and texture of the greater part of the licdoc lavas is well illustrated in the Mt, Bidwell Section. It is also shown by an ex- amination of thin-sections of the lavas from other parts of the area, which can at present be but briefly refered to here. As mentioned in the earlier part of this report, the horizontal flows form rather high cliffs bordering the western shore of Goose lake. At Fletcher's Ranch near the southern end of the lake, the section exposed is somewhat over four hundred feet thick and consists of at least ten flows, with the base of the lavas not in view. In their megascopic appearance they are medium to dark grey in color, with red- dish and brown stains on exposed surfaces. They are with few excei. tions vesieular, several to high degrees. They vary considerably in grain, some being quite doloritic in appear- ance, others almost sphanitic. line thin-sections representing the same number of flows were examined, Of these, eight show the same ophi- tic texture as the lower six flows at it, Bidwell, and the upper flow at Pandango Peak, The other, which is from the seventh flow from the top of the section, has more the normal diabasic texture with the augite filling the inter- stices of the feldspars, but not wraping about or enclosing them. This texture 1s but imperfectly developed however, as much of the augite appears to be earlier than the feldspar. Une quite noticeable difference from the most of the flows at kt. Bidwell, is that those near Goose Lake contain few or no distinet phenocerysts of feldspar and are more uniform in grain except for olivine which often occurs as quite large orystals, a8 well as in intermediate sizes and gall grains, Glass was found in but two of the thim-sections apni that in almost negligible amounts, In composition they do not show any noticeable variation except that augite oconsionally oceurs as more or less euhedral crystals as well &s xenomorphiec grains in the groundmass, This was not observed in any of the thin-gec~ tions of the rocks inthe vicinity of the detailed sections. Ulivine is variable in amount but is present in all of the rocks examined. It frequently ocours as quite large crystals Occasionally with almost perfect euhedral outlines, but it is veually more or less rounded. Some of it shows the usual alteration to serpentine or red and brown staining, which gives many of the rocks = very noticeable fine red spotted appearance in he hand specimen. In some of the rocks the alteration appears to be to 1ddingsite rather than to fi- brous serpentine, but either of these types mey be accompanied by considerable iron staining, kagnetite and apatite ocour in about the same amcunt as in the rocks of the Mt. Bidwell section, the former always rather abundant, In short, no essential difference whatever is shown in the basalts of these two sections. The obsidian however, does not occur in the section at Goose lake except as abundant fragments over the surface of the upper flow. Thin-gections from two flows at the top of the ridge, between Joseph Creek Basin and Thomas Creek near Cedar Pass, were also examined. These do not show any no- tieable difference in composition from the rocks =t Fletcher's Ranch or it, Bidwell, tho there is a difference in texture, They are nct ophitic or distinotly porphyritie, but contain an appreciable amount of clear colorless glasa as angular patches béftween the feldspars giving them an intersertal texture. The augite occurs as irregular or rounded crystals and is frequently twinned. Unly a few pieces were found that partially enclosed the feldspar erystals and is apparently for most part, earlier than the latter mineral. The olivine ls abundant and frequently as large crystals. In one thin- seotion, it i= partiallyy altered to yellowish or brownish green serpentine in the other, to deep red iddingsite, which occurs similarly to that in the second flow at Nt. Bidwell. It frequently forms broad uniform borders around the olivine orystal with a rather sharp boundary between them. In other cases the alteration begins at the center of the crystal and extends uniformly toward the outer margin. The entire border or center of iddingsite extinguishes simultane- ously, and sometimes differs in its position of extinction, from that of the olivine erystal with which it is associated, as much as tem degrees, It also shows & well developed platy cleavage and slight absorption parallel to those lines. Occurrence of the kiodoc Lavas at the Southern End of Surprise Valley The lavas are not exposed in place on the west- ern side of the floor of Surprise Valley, as it is filled to a considerable depth with the deposits of a lake which former- ly oceupied this entire basin, and also by alluvial fans,which extend out into the valley from the base of the range. They also do not occur on the lower slopes of the eastern side of the range between the town of Bidwell and a point about Four miles south of Eagleville. From this point south however, they form a steep wall slong the western side of the valley, which rises abruptly from its almost level floor to a height of four hundred to eight hundred feet, as shown in Plate IX, They also extend around the southern end of the valley and spparently continue into a range which is parallel to and about fifteen miles esst, of the Warner liountains. Detail- ed sections were not made in this area, so that further con- sideration of the extent of the iiodoc Lavas cannot be given. SUMMARY In the study of the rocks included in this re- port, three types were recognized, namely, olivine basalts, endesites, and rhyolite. Of these the basalts, which are re- werkably uniform in composition and show bgt few variations in texture, constitute over ninety percent. The endesites are apparently not common to the entire group, as they were only found a! Pandango Peak, and may represent a produit of local differentiation, From its present distribution, the rhyolitic obsidian at one time extended uniformly over the entire area and represents the most recent of the voleanic products. - This group of lavas ocours over the area border- ing the Warner lountains as horisontal flows, with their greatest observed thickness of four hundred feet or more, along the western side of Goose lake. Within the Warner Mountains they have 28 maximum thickness of about twelve hun- dred feet and are tilted or slightly folded as a result of the forces which caused the uplift or this range. That the lavas which occur at the base of the Warner kHountains and those which form the top are the same, is evidenced woth by field observations and by a comparison of them in thin-sections under the microscope. On the west- ern side of the southern end of the range, they can be fol- lowed continuously, along the Pitt River gorge to points well within the range, changing gradually from horizontal positions at the western end of the section to an ineli- nation of five degrees at the eastern end. In the north- ern and central part of the range, faulting has ogcurred on both the eastern and western side. Here the horizontal flows at the base abut abruptly against the scarps of these faults. A careful comparison of thin-sections of the rocks at the top and the bottom of the western scarp at Fandango Peak, shows them to be the same. A similar study of the roeks in the vieinity of kt, Bidwell, shows a like relation between the summit lavas and those at the eastern base of the Warner Mountains, In adiition the marked uniformity in eomposition and texture of the greater part of this group of voleanie extrusives over the entire area covered, affords strong evidence of its unity. Figure [| — View of the eastern hase of rhe Warner Mountains lookrirrg norlrh frormm Lake Crly. Figure 2. — View of rhe eas’err base orf rhe rarge sourh of Deep Creek. Plare 17 Figure 3. — The gorge of Lagle Creek where rhe s’rearn leaves the mountains and flows nfo Suprise Valley Figure #. — The mouth of North Deep Creek Caryor rhree miles south of Cedarville Plate IT. 5, 55 EL S50 5 ca —— Figure 5 -A view of The westerr Slope in the southern part of the range looking southwest? from near the summit foward Jess valley. Figure 6.— A view of rhe summit looking northeast from a porn! about Iwo miles north of MI-Bicwell Plore IV Figure 7.— The top of rhe Warner Mountains as seer From: near Mr Bidwell looking rors. Flrgure 8.— A view of rhe surmmit in the central par’ of rhe range rear rhe head of rhe south fork of Davis Creek. Figure 9. —A view looking south from [ake Cily showing rhe bedded character and vvesterly dip of the Lower Agglormerafes. Figure 10.— Cliffs of the Lower Agglormerales in the lower part! of Cedar Coryor. The srroolh slopes rr) The foreground are formed by ruffs, Plate ZT Figure //.— The westerr escarprmen’ of The Warrer Moumtains of Plus Valley showing a sectior of Ihe Upper Agglormerares. | Figure /2.-A view /ooking south from Flurr valley along a rit? al the base of rhe escarprment shown ir Figure // Plare YT view [ooking wes! across rhe Modoc Love Fields wes? of Goose [ake Figure 14. — The rim of the Modoc Lavas overlying tuff along ‘he west side of rhe Fitt River: FPlale TI. Figure /5.— The east front of Warren Feak looking south from Squaw Feak. The cliffs are Modoc Lavas with the Upper Agglomerales af 7he base. Figure [6.— The south and eas! side of Logle Feak showing the western dip slope and the faull scarp orn rhe east The scarp or rhe south is tha! of a Irarnsverse raul’ Fla/e IX. Figure /7—A view of the floor and western side of Jess Valley al rhe head of the Fitt Liver gorge. figure 18 — Secondary fault blocks, capped by Modoc Lavas, near Cottonwood Creek on rhe wesrlerr front of rhe Warner Mountains. Hel wCeRel Pall w0 =P (ul Rh «PH 3 The {ollowing mierophotographs represent the prineiple textural and mineralogi-~ eal features of the rock studied. All but one are of olivine basalt, PIATE XV Figure 1 Thin-gsection {rom the surface flow at Jess Valley. Xi 29, crossed nicols. The rock represented is Porphyitic with a poderately fine groundmass containing a small amount of intersertel glass. The phenocrysts are olivine, and labradorite feldspar which is mot shown in the figure. The ground- megs consists of labradorite lathes; fine granular augite rounded grains of olivine snd a few very small patches of intersertsl glass. For a complete description see Pp EB-ZE. Figure 2 Thin-section from the basal flow at Jess Valley. X 82, erogsed nicols. The roek has a vitrophyie to intersertal texture with twenty-five to fifty percent glass. The feldspars sre labradorite bytownite and show both perieline and albite twinning. The other con- stituents shown are olivine, which is usually more or less altered to serpentine; sugite and magnetite. The augite occurs as slender xenomorphie pieces between the feldspars snd is toc & great extent obscured by the extremely abundant magnetite. The darkest areas are glass densely crowded with fine grained magnetite. See pp, 855-37. te ound- ® «@ ord a2 = « ches a8ee valley. texture nd show con-~ re or .