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Riley, Donald A study of the heavy mineral assemblages of certain tertiary formations in the vicinity of Walnut Creek, California 1926 BIBLIOGRAPHIC RECORD TARGET University of California at Berkeley Library Master negative storage number: 03-67.67 (national version of the master negative storage number: CU SN03067.67) GLADIS NUMBER: 1847771173 FORMAT : BK AD:990922/FZB LEVEL:b BLT:am DCF:a CSC:d MOD: FL:"7 UD:030604 /MAP CP:Ccau L:eng INT: QPC: B10: FIC: CON: ARCV: PC:s PD:1926/ REP: CPI: FSI: TiC: rT: 040 CUscCU 090 SbDISS.RILEY.GEOL 1926 100 1 Riley, Donald 245 12 A study of the heavy mineral assemblages of certain tertiary formations in the vicinity of Walnut Creek, California. 260 $Scl926. 300 [vi], 23 leaves :$bcol. ill., map; $c29cm. 502 thesis (M.A. in Geology)--University of California, Berkeley, April 1926 610 20 University of California, Berkeley.$bDept. of Geology and GeophysicsS$SxDissertations. 690 0 Dissertations, Academic$xUCBS$xGeology$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 pet PM-1 3%"x4” PHOTOGRAPHIC MICROCOPY TARGET NBS 1010a ANSI/ISO #2 EQUIVALENT lle ll %. i ES I= hie pe DISS RILEY GéEolL 192. EAT A STUDY OF THE HEAVY MINERAL ASSEMBLAGES OF CERTAIN TERTIARY FORMATIONS IN THE VICINITY of WALNUT CREEK, CALIFORNIA LIBRARY COPY PRESENTED BY: Prrntet Biley APRIL 27, 1926 CONTENTS Page xX. Introduction, cocsesssssssscesacsasascascscscnccsscanl PUTPOB@«cssssssessssssssasssassssascsenssscncncel Acknowledgment ++ sseosssscersesssvcsconcoccncoanal 1X. LooBULPl is cesssenssrosnsntesnssasesssangnsnnvsnnnel 1X. PhySiography «+eeeesescscsscrsssssscrcrcosncarensl ROLi@E ss cossssvtssvossssnsavsrvnnsnsonsnsavennsal Relation to surrounding BT@B cesses sessssssnsnsssel I¥. GEOLORY ++ +sesssssssssassnsnsnsnsnsscsnsssncansananl Tertiary Seri@ge esse ssecessscssccasnccscsrecaseel MiOGEN® +++ sssssssssssssssssssssssscssnncvansesl Sebrante Sendstone of the Monterey Group: +2 BOGONG ++ sss t88 0s 0sssssssssssnssssscssssssssetd Tejen Pormationsescssessscoescscscscsssnssecd Y. Lecatien of Sampling AT@B ccsosesssasssesnancsessnscd Sebrante BaNdStONes «+s eccessssssssanccsssssccsssd Sample NO, cossoesssosscesssesscsscsssscssccsssni PGT oes sssessnssrssassssnsrnsensssnnsensd PI sv cesssssssssssasssnsernesransasareed PRILE cossresssssssnssssssessesasnssssneeb I Sample N@scscoecervcsssscssessocsnnacoccel PhiT eoovossossessosssssscsacscrsnscnscesd PEITBecvsosovscssvsanssscas ssesencceccsbd PSII encnennisinnnesssinnessernsnnseb VI, Potrogra8phTcsveesrssreliiteiihaseencseenecenceseesa SAMPle HOscevscossnnessssasssnsssnsessnnsnssneeeh PB evecenvensvsrsenevasnssanencanennsrsnvsnens? PBIlescvrennsncsnnsonsnsssnensesnssennnesneveed PBII ener vvesvensrnnsssnsvnsnnsannnrsenssnnesell PLilenensenenscsnvsnisnnbobohiddoscensaressnsesls PLITL sannsnsnnsnecer nnnvovennsnsnsnsesonnnvesld L311] anvssescsssnescnnvessanvesssneesneseenvalbs Vil, SUNMMBTY eecocvesnssrocaccescosssssnssscensasssessesslB Tobulated SUMMBIYssseeteeeseessseerestssersssesalB Analysis of TabulationNBescsecsecssccsscnssnesesl? VIII. ConcluBiofB.aveveeseovnsosesnvssearanssvsannerenels IX: BibliograpPliF eens es ss te csccossssserescscscenssnnnnse2d X. MBE esvsasvsnsnssssossssnssnessssncerscsnvssnnssanes Al. CPrO8S-S8aLi0Re cs cer csssssvnsrvsranenessesveveesese Page I. INTRODUCTION Purpose This paper was written as a partial fulfillment of the requirements for the degree of Bachelor of Science in the College of Mining, University of California. Acknowledgment I wish te express my sincere appreciation of the assis- sance rendered me by Professor Carlton D. Hulin for his help- ful suggestions and guidance in this work. II. LOCATION The area from which semples were taken for this report comprises a strip about three miles and length and one half mile in breadth. sbeut one snd one helf miles northeast of the town of Walnut Creek, Contra Cesta County, Califernis, and about seven miles directly west of the North Peak of Mt. Disblo. The section under consideration is represented on the topographic and geclogic maps of the United States Geological Survey Folie #193 (San Francisce Folio), Cencord Quadrangle. The field work in this report was completed in five days in the months of Nevember and December, 1925. The leboratory work was done during thespring of 1926. Ree PT SE pp ei III. PHYSIOCGRAPHY Relief The area here considered consists of pert of a low ridge running from northwest to southeast. The elevation ranges frem 100 feet to 600 feet above sea level. At the northwest end, which terminates the ridge, the surface slopes gently up from the almost flat valley beyond .Toward the southwest the sides of the ridge grow steeper and the general topography more rugged. Relation te Surrounding Area To the northwest, nerth, and northeast this ridge is bounded by the Ygnacie Velley. Te the southwest, and run- ning parallel to it, is Shell ridge. The ridge continues on to the southeast to the foet of Mt. Diesblo. IV. GEQLOGY The stratigrephic, areal and structural geology of this region is teken from the United States Geological Survey Fo 1io #193 end wes checked locally by observations in the field. Tertiary Series Miocene Sobrante Sandstene The Sobrante Sendstene is the lowermost bed of the Mon- terey group. It outcrops in this area in a belt about 500 feet wide with strike paralleling the ridge. It dips at an angle of between 80° and 90° te the northeast. As this strike and dip is fairly constant thruout the area a long narrow belt ig formed which comprises the southwestern slope of the ridge. The contact with the Tejen forms the crest of the ridge. The formetion is chiefly sandstone with layers of shale and lenses of gravel and cenglemerate. Outcrops are cemmen and fossils seem fairly abundant. | The prevailing sandstone varies from medium fine te coarse in texture. It is peerly cemented and easily broken even when fresh. When fresh the send is gray in coler with a greenish tinge in places. On weathered surfaces the celor is decidedly lighter. Incorporated in these sandstones are numerous cencretionary bodies, generally small in size, vary- ing from one to eight or ten inches acress. These concre- tions seem to be of the same mineralogical composition as the sendstene but are much better combutad and of a darker color. They ere irregular in shape and do not break freely from the surreunding reck. The shale occurs in narrow beds and lenses within the sandstones. It is dark bluish gray in celor. The conglomerate consists of pebbles and beulders ranging in size from one inch to a foot or mere in diameter. The ma- trix is sandstone. They are fairly well rounded. The deposit is evidently shallow water marine, deposited near the shore line. Eocene Tejon Formation The Tejon Formation apparently rests on the Sobrante in this area as its dip is else the northeast. The strike of the bed is parallel te the line of the ridge and the dip ap- perently from 80° te 90°. The nertheast side of the ridge is mede up of this formation. The formation veries from one quarter to one half mile in width, dipping under the alluvium of the Temescal formation to the northeast. Geod outcrers are rare in this area and no fossils were seen. Phe formation in this erea seems to be made up entirely of stmdstones. Ne shale or conglomerate were observed. The sandstone is a light buff color, medium to fine grained, in some pleces well cemented with calcereous cement. In this case it is rather hard to break with a hammer. On weathered surfaces it becomes easily breakable with the fingers. In some outcrops the sandstone is red frem the decomposition of iron minerals. This is evidently & marine deposit at mederate depth. The grains are uniform and well sorted. V. LOCATION OF SAMPLING AREAS In sampling this ares an attempt was made to obtain es representative samples as possible at definite intervals along the strike in both the Eoeene and Miocene beds. This method could not be absolutely adhered to because of lack of suita- ble outcrops in the Tejon. Thus in the Tejon samples could not be taken by trenching, but were taken from whatever ex- posure could be found. However, these may taken as fairly representative as the sandstone seems to be very uniform in lithology end mineralogiml composition in this area. Sebrante Sandstones Sample TsI This sample was taken along & road cut about three quar- ters of a mileffrem the northwest end of the area. Here the read cuts both the Sobrante and Tejon at approximately right angles to the strike. 4 good section of the Sobrante is ex- posed for about one hundred feet but no suitable Tejon could be found to semple. A sample was taken by trenching aleng the exposure. It was mixed well and quartered, opposite quarters being taken for the field sample. Sample TsII This sample was taken at approximately a mile along the contact to the seutheast of ssimmple TsI on the southwest flank of the ridge. Here the outcrop is not prominent but there are good exposures near the contact and near the base of the ridge. Samples were taken from these end mixed and a part extracted for the field sample. Sample TsIII Sample TsIII was taken frem an area about a mile along the contact from Sample TsII. This area lies ai right angles to the strike on the flank of the ridge. There is no sure outcrop near the contact but dewn the side good exposures are found. Tejon Semple TtjI This semple was taken from a small area of exposed sand near the crest of the ridge sbout one-half mile northwest of Sample Tsl. The material obtained was almost white and was not well cemented. Sample TtJII Ths sample was not collected by the writer but was given by Professor Hulin, and was worked out in conjunction with the course Geology 110B at the University of California. It wes collected from the Tejon frem a small outcropping nearly op- posite,on the nermel te the strike, from the spot from which Sample TsIIl was taken. This sample before treating was red in color. Sample Tt+jJIII This sample was taken from an outcrop near the contact with the Sobrante, approximetely eppesite frem Sample TsIII. Here the outcrop stands out prominently at the crest of the ridge so that a good wample was obtained. The sandstone here ig light colered and well cemented with calcereeus cement. VI. PETROGRAPHY The heavy minerals were extracted from the samples by means of liquid gravity separations. For this purpose bromo- form, heaving a specific gravity of 2 2.88, was used. Thus mineral sssemblages centeining minerals of specific gravity of 2.88 or greater were obtained. These assemblages were stud- ied under the petrographic micrescepe by 0il immersion methods. These assemblages were studied in detail in an effort te determine if the same formations could be correlated from point to peint, to see if the Eocene formation could be distinguished fom the Miocene, and alse te obtain all evidence possible con- cerning methed of depesitien, conditions existing at the time of deposition, and source of the material. In designating the relative amount of a mineral in an as- semblage the follewing symbols will be used: F.-Flood V.Ab--Very Abundant Ab--Abundant C==Commen Oc--Occasional R--Rare In cased particularly representative mineral a percentage ratio to the entire assemblage may be given, Sample Tsl A moderate crop of heavy minerals was obtained comprising sbout 3% of the amount run. The dark minerals predominate so that the assemblage apveared abmost black to the naked eye. Minerals Magnetite (Ab) This mineral occurs in opaque grains ir- regular in shape. Alse there are some subangular te nearly rounded. The irregular grains usually show hackly structure around the edges. It occurs also as inclusions in hornblende and hypersthene. The face grains are relatively large in size. Hypersthene (V.Ab.) Occurs in subengular grains from shapeless to prismatic in form, showing good cleavage. Geod pleochroism is exhibited, green to yellew brown. It is char- acterized by large numbers of small inclusions, probably il- quenite or magnetite. Some greins show alteration products. The mineral is apparently of rather high iren content. Cem- prises sbout 356% of entire separate. Hornblende (Ab.) Occurs as subamgular grains which are seldom prismatic and only pleochreiec. The mineral contains many opaque inclusions and shows tendency toward alteration. IsSdark green in cdor. Ilmenite or Chromite (8) These are rather large opaque subangular grains giving e purplish luster by reflected light. Brown Biotite (Oc.) Occurs as slaty, brewn, nonpleo- chreic fragments. Alse seme leeched biotite. Aegirite-sugite (¢.) Irregular forms 2ight green in col- or. They are nonpleochreic and are seen in angular, subangu- lar, and shapeless forms. Many show crown point terminations. Basaltic Hemmblende (Oc) Occurs es angular to subangular greins, mrely prismatic. Pleochroic cark brown to almost black. The grains are comparatively large with few inclusiens. Hematite (Ye.) Occurs as angular fairly equant opaque greins giving a red luster by reflected light. They eppear te be altered fiom hornblende. Seme grains appear te be about half hemnblende. Epidote (0c.) These occur as small equant colorless grains having & high refractive index. They sre anguler and contah ne inclusions. Titenite (R.) These are irregulsr,angular, yellowish green grains having shigh refractive index. zircon (R.) This mineral is rare in this assemblage, oc- curing in scattered rounded and crystalline forms giving char- acteristic shapes. Spinel (R.) Rounded isotropic grains, colorless to light red with surface much abraided. Garnet (R.) This mineral is very rare in this assemblage and occurs as rounded colorless isotropic grains. Glaucephene (R.) One grein found. It was pleochroic, violet to blue and much room. . Actimolite (R.) Peuné as elongated colorless fragment with geod cleavage. It contained rounded opaque inclusions. Sample TsII A heavy crop was obtained from this sample, being about 5% of the smeunt of materisl run. The separate was almost black te the naked eye. Minerals Magnetite (Ab.) Occurs as irregular opaque grains show- ing hackly structure. Also occurs in great numbers as inclu- gions in hornblende end hypesthene accompenied by decomposi- tion products. Hypersthene (V.Ab.) Subangular grains from shapeless to prismatic and showing geod cleavage. Pleochreic-green to yellow brewn. Characterized by large number of small opaque inclusions and seems to be of high iron content. The grains are relatively large and very frequent so that the hypersthene content is exceedingly high, amounting te about 50% ¢ the en- tire sediment. Chromite (Ab.)--Occurs as subamguler grains. Opaque with purplish luster by reflected light. Hornblende (C.) Occurs as dark green nenpleochreic sub- angular grains containing opaque inclusiens. The edges tend to be rounded. Aegirite-Augite (Oc.) Irregular te shapeless grains ex- hibiting crown peints. Neonpleochreic, light greem in color. Epidete (Oec.) Fairly frequent greins having a high re- fractive index. They are irregular in shape and usually some- what pleechroic--colorless te green. Bassltic Hornblende (R) This mineral occurs in irregule subangular grains which are pleochroic from brown te very derk brown. It contesins few inclusiens. Brown Biotite (R) These are rare and occur as platy forms without outlines. Medium brown in celer. Titanite (R) Occurs as very infrequent irr8gular grains yellewish green in celer, having a very high refractive in- dex. Garnet (R) Very rare. When found is subangular and ceol- orless with perhaps a slightly pink tinge. Hematite (R) Scattered opacue irregular grains showing red in reflected light. Semple TsIII This sample yielded a fairly large crop, approximately 2% of the amount run. Centeins & high percentage of dark minerals. Minerals Magnetite (C) Occurs almost entirely as large and small inelusiens in hornblende and hypersthene. Sometimes seems as if half a grain is cempesed of hornblende and half of meg- netite. Hypersthene (V.Ab.) Occurs as subangulaer prismatic grains showing geod cleavage. Pleochroism is marked--brewn te green. Grains contain large numbers of small opaque in- clusions and seem to be of light iren content. Grains are very frequent and seem te form about 35% to 40% of the sepa- rate. Hornblende (Ab.) Occums as dark green slightly pleechroic grains showing meny inclusions. ddges tend te be rounded. Chremite (C) Occurs as opaque subangular grains. Has purplish luster by reflected light. Aegirite-Augirite (C)Occur as irregular angular frag- ments having frequent crown point terminatiens. They are nenpleeochreic and light green in celer. Epidete (0c.) Occurs as smell greins. They are slight ly pleochreic--colorless to light green and are angular in form. Grains occur fairly frequently. Brown Biotite (Oc.) Occurs as platy brown and leeched fragments without definite outline. Titanite (R) Occurs very infrequently as irregular green ish yellew grains with high refractive index. Zircon (R) Infrequént rounded grains showing a very high refractive index. The light separates upon examination with a bineculer mi- crescope. Seem te be principally composed of angular guartz, rounded feldspar, end some muscovite. In general these samples frem the Sobrante show enly med- erate effects of chemical decompesition as evidenced by the comparatively high amounts of minerals that would be quickly destroyed if this were true. These minerals all tend te be subangular and se de not show extended effects of mechanical decomposition. Sample TtJjI The erep in this sample was very peor. Less than one per cent of the material run. The assemblage to the eye was light gray with few dark particles. Minerals Magnetite (C) Opaque irregular grains sometimes showing heckly structure areund edges. The fragments are relatively large. Alse some evidences as inclusion in ether minerals which have changed te limenite. Chromite (Ab.) Occurs as subangular te rounded grains. They are opaque and slightly vielet in reflected light. There is much more of this material than of magnetite. Garnet (Ab.) These are beth pink snd colorless. They are well rounded and the surface well abraidéd. Epidote (C) Oceurs as subsngular grains showing traces of cleavage. Have a high refractive index. Someiimes is weakly pleochroic--celerless te greenish. Zircon (C) Frequent small angular grains. They are color- 1ess and show & very high refractive index. Frequently they show geod characteristic crystalline outline. Pitanite (0e.) Occasional greenish yellow subangulsr to rounded grains are found having a high refractive index. Hornblende (Oc.) Oceurs as subangular grains which are thiek end subby; almost opaque. The edges are much rounded. Besaltic.Hornblende (0c.) Occasional plecchreic cleav- age fregments usually almost equant in shepewwith rounded edges. Pleochroic--brown te black. Hypersthene (R) One or twe grains were seen, much rounded, light green and enly slightly pleochroic. Ikely has a lew iren content. Rutile (R) Infrequent grains having a regular angular shape with very high refractive index. %The color was brown- ish red. Kyanite (R) Irregular subasngular grains. Celor--color- less to bluish. The grains show geod cleawyage and & high re- fractive index. Crossite (R) One frsgment only was observed. It was strorgdy pleeochreic, yellow te derk blue, and showed good cleavage. Sample TtjII This sample gave a poor orpp, the separétion amounting to enly a fraction of a percent of original sample. It was highly colored. Minerals Magnetite (Oc.) Ocours as irregular opaque grains having ’ hackly euthine around edges. Magnetite is only subordinate in this sample. Chromite (&b.) Occurs as opaque subsngular grains which are well rounded on cerners. They show the effect of much wear and apparently form about 30% of the separate. Garnet (Ab.) Occurs as isotropic reunded grains. They are colorless. There are alse some grains showing good cry- stal outline with corners tending te be rounded. Epidete (C) Is found as subanguler te rounded £rag- ments having a high refractive index and which are slightly pleochroic--colerless to yellowish green. These are equally small in size. zireen (Oe.) Is found in subanguler celerless fragments having a very high refractive index, and exhibiting charac- teristic pointed shapes. Hornblende (Oe) Occurs as angular fratgments dark green in eeler, slightly pleechroic, showing decomposition effects. Besaltic Hornblende (R) Is rare. Shows excessive deco m- position--subsngular with rounded edges. Is pleochroic, brown te dark brown. | Hypersthene (RK) Occasional fragment found. They are angu- ler with rounded cerners, and pleochroic--1ight green to red- dish yellow. Zeisite (R) Occurs in celerless fragments with ragged edges. The refractive index is medium high. Contains many in- clusions. Sample TtjIII This sample yeilded a poor crop of heavy minerals. The assemblage was light colored showing a fairly low percentage of dark minerals. Minerals Magnetite (0c) Occurs occasionally as ragged opaque greins. Chromite (Ab.) Occurs as subangular te rounded opaque greins. Shews evidence of abrasion. It apparently com- prises about 30% of the entire assemblage. Garnet (Ab.) Occurs as subangular irregular isotropic grains showing tendency to rounding on dorners. Alse there are some grains that shaw distinct crystalline outlines. This mineral is ceorless. Epidote (C) Occurs in subangular, small, shapeless frag- ments, colorless te light green, having =a high refractive in- déx, and shows some cleavage. 72ircen (Oe) Is found frequently as small celorless angu- lar grains, some having distinet cyrstal outline. Hernblende (Oe) Occurs in stubby, thiek, almost equant grains. It is celered very dark green with only slight ple- echreism. The corners are well rounded. Basaltic Hornblende (R) Infrequent grains. They are prismatic in shape with small rounded cerners. Pleochroic, brown te black. . Pitanite (R) Oeccasienal small,angular,greenish grains having a high refractive index. Rutile (R) Occurs very infrequently as angular red brown fragments having & high refractive index. When examined under the Hnecular microscope the light separates of these samples seem te be compesed almost entirely of anguler quartz grains with a small ameunt of much altered feldspar. In general these samples frem the Tejon show the ef- fects of long centinued and intensive chemical actien. This is shown by the fact that the great proportion of the minerals in the sediment are those mest resistant te chemical decom- position. Tabulated ANALYSIS OF TABULATIONS An analysis of the foregoing tabulations gives rise te certain observations. Er TR 1. The mineral assemblages from the Sobrante are very 9 00 9 different frem these of the Tejen. Yrbaugh LB 2. The heavy mineral assembleges from the same forma- Q | i o o Spm a Rn tiens are quite similar. 3. There are certain minersls that are quite charaecteris- tic of the formation in which they appear in the limited area 00 : 2] v0 °Ggqv ¥ ¥ 99 "SIG }[VU0BWI0—00 {uoWmOD—) {JuvpuNqY—qV juvpunqe Siaa—qv A studied. l. The main distinguishing features between the Eocene and Miocene are not the absolute difference of minerals con- goray 9%03 tained, althe some of these are quite charccteristic, but the differences in appearance and porpertional eccurence in the as- semblages. The minerals contained in the “obfante assemblages are, — in general, in larger grains than these of the Tejon. They Botne i are not se well reunded as these of the Tejon. They centasn Staurolite rants 8 many inclusiens. They shew effects of decompesition due te Tourmaites |§ weathering such as the presence of hematite and limonite. q L Zeem | These features are practically sbsent in the Tejon. Chromity | |Hematitel 0H qv . °0 0 ¥ ay o0 qv, 20 qv 0 The difference in proportion of certain minerals contained im perhaps the mest striking distinguishing feature between q | s u the twe horizons. Thus the Sobrante has by far the larger proportion of ut I0T00 minerals that would be broken down by any prelonged chemical 03 WET peu decomposition, such as hypersthene, sigirite-sugirite, snd fez USTT IUBTT fear UIXIBQ euoqs -puss | LITHOLOG! 81800 much hornblende. The very resistant minerals such as garnet, 07 SUI JI0T0D 9IN1X09 wunypeu ® tID1Spuss zircon, titanite, spinel, etc. are decidedly subordinate and rare in mest cases. On the other hand the Tejon assemblages show, for the mest pert, only these minerals which are resistant to chemi- esl action, the greater portion of the assemblage consistin g of chromite, garnet, epidete, etoc,, with hornblende subordi- nate, hypersthene very rare, and adgirite-sugite entirely ab- sent . The relative prepertien of heavy minerals in a given sam- ple is in itself a distinguishing feature. 2. A easusl study of the tabulation ef minerals contained in the various assemblages obtained frem samples of either group will show thet each group in itself is very similar. Thus in the Sebrante the minerals contained in each assemblage are, for the mest part, the same. Alse they occur in nearly the same relative prepertien and have practicaily the same appearance. Hypersthene is the most characteristic. Aegirite- dugite is persistent and charseteristic. About the same &- mounts of magnetite and chromite were found in each sample. Biotiéte is persistant. The rarer minerals are mere scattered but many of these occur in all the samples. In the Tejon assemblages similarities are even mere marked. Minerals are remarkably similar in kind, propertien, size, and general appearance. Garnet is perhaps the mest char- acteristic. Epidote oacurs in all assemblsges in nearly the seme propertien and appaearamce, 83 does chromite and also zircon. 3. The mest characteristic mineral of the Sobrante, es here studied, is hypersthene. It eccurs in large amounts end has the same features throut the area studied. While it ecurs in the Tejen, it is found only as rare fragments that have no value. Anether characteristic mineral eccuring in this horizon is segirite-sugite. In this area the sbservatien of this mineral in any proportion greater than "rare" would almost conslusively preve the sample te be Miocene. Garnet is the mest characteristic minerel of the Tejon in this area. It is persistant and forms about 20% of the gssemblege. It often shows a six-sided crystal outline. This might be proved characteristic upen further study of the area. Rutile, while rare im the Tejon, was net found in the Sobrante area and was of seme value in differentiating the beds. In considering the cenditiens of depesition and the source of this meterial, the minerals themselves, sorting, the mechenical decempesitien, and the chemical decomposition must be considered. The Sobrante minerals are peorly sertedi They do not show extensive effects of mechanicel action and only slight effects of chemical decempesitien. This would seem to indicate that they were transperted over & relatively short distance and thet the dlimate was moderate or arid. The presence of such minerals as sugirite-augite, epi- dote, titanite, and veme glaucephane seems to indicate that the depesit has its source in some natemerphic formation. This is probably the Franciscian. The Tejon minerals are well sorted and rounded, and show extensive effects of mechanical and chemical decompeosi- tion. This would indicate a comination of long transpertation and © humid, warm climate. The presence of minerals such as garnet, epidote, etc. indicate that the material was derived from & metemerphic area, probably from the coast ranges. VIII.CONSLUSIONS As a result of the foregoing studies the following ten- tative conclusiens, subject te further study, were reached by the writer: 1. That the Sebrante can be distinguished from the Te- jen in this region by means of the mineral content. 2. That these beds can be cemrelated over short dis- tances by means of their mineral content. 2. That the Tejon was deposited under warm humid cli- metic conditions, and the SBebrante was devosited under geel- er or more arid conditions. 4. That these sediments were derived either from meta- merphie recks or reworked from sediments so derived, or frem beth, probably the Franciscian fermatien. IX. BIBLIOGRAPHY Hh. Lewson, A. C., San Frencisce Felie, U. S. Geel. Surv. Ne. 193, 1914. 2, Milner, H. B., Sedimentary Petrography. 3. Lawsen, E. 8., The Micrescopic Determination of the Neno- peque Minerals. 4. Holmes, A. L., Petrographic Methods and Calculations. $ ? 5 \ @ < hd Q “ - | & : X J SOBRANTE (Somple Ts I) 7x) SSEMBIAGE TEVON (Sample TY CHARACTERISTIC A Seca level EI pi i Missone NORTHEAST — SOUTHWEST 3 abramn 8 Tao Fosere CRe3S-SECTIUNS APPROXIATE SAIrPLE AREAS SCALES VERT 17:50" HORIZ.{1 1M. END OF TITLE