Loading...
HomeMy WebLinkAboutATTACHMENT P0 965 debris n( the rockslides and ruW' "s. Intermittent streams, such as 13eatunt i-Iollu\y, have worn small valleys across the top of the basalt on Table Mountain, but their c(Tcct. is thought to be rclativcl\ small when compared to the mass -wasting at the edges of the flow. The near -planar surface of the lava is still plainly distinguishable. At the cast edge of \o-th Table Mountain,hold cliffs such as those found on the \vcst side da not occur. At most E laces, Sawniill Ravine being a notable cxcrlttion, the soft Ternary strata are thin or absent, and the lata rests essentially upon bedrock. The bedrock is nearly as hard as the lava, so that diiTcrcntial cri,sion \yith'attendant mass -'wasting, while it docs occur here, is of much less relative ini- portancc than on the \vest Side. Tlic upper surface of the "older basalt" on both North and South Table \fountains is, ext:cpt for local Irregularities, nearly planar, and dips slightly, with near uniformity, toward the \vest -southwest. It is clear that the basalt has undergone little or no deformation, but'It. hc.n. gcndy: tilted. As ihdicarcd above, erosion appears .to have proceeded principally at the edges, rather than on the surface, of the basalt. 1-111ts the present' sitrfacc js believed to closely approximate, in broad aspert,'titc original surface Of tl : lava, cxcchi that the surface arca has been greatly reduced. 'phis prescr\ ration is duc in part to the highly resistant ' nature c,f the basalt, but it may. be that the basalt wasonce covered by later deposits which have since been stripped away. For example, it is conceivable that the andcsitic brcccias which no\y mantle.tho: basalt at many localities to the cast (Turner, 1897, iS9g) alight once have extended much farther to the west and covered the "older b .salt" on Table Mountain. The "older hasalt" in must of the irca rests, ­1111slight angular unr-o, onnity, un several of the older Tertiary formations previously described. On the cast side of Forth Table Mountain the lava overlies bed- rock v.•ith.marked angular unconformity. The young- est rocks directly underlying the basalt arc the frng- mcnt2l andesite and volcanic sediments of the ,\-lchrten ( ) Formation. The basalt overlaps the andcsitic rocks to the cast and north, and over most of Forth Table Mountain it rests on Eocene strata. A part of the a\•crlap nligm be due to a slight angular discordancebetween the lrasalt and the Mchrten ( = ), but most of it k probably caused by the marked r•liscunformity between the \Iehrten (=) and the underlying Eocene strata. The thickness of the "uld.er: basalt", \t here it has not bccn affected by later erosion, is fairly uni- form. It is thickest on South Table Mountain, \%-here-1 ahout 250 feet of lava are c-pnscd. he average thick- ness in other places is 175 or 200 feet. This uvcrnl! • uniformity was thickness sho\yn by the basalt indicates tllat clic flow as poured our upon a rather even plain. The 'older basalt" overlies Tertiary fragmental an- desite in the Orovilic arca, and is in turn overlain by OItuv1I.t.F. Qt;Al,ll.4\Gt.li-CRF.F I-V C 61 fragmented andesite to the east (Turner, 1.94, p)\. 493- 494). The agc of the basalt therefore lies sonte\vhcrc vithin the time of andcsitic cruptinr in the Siena. -1 jtc latest evidence indicar_s that some, and, perhaps roost, (,f the andeskes were erupted during the late 1\9ioc enc and early Plioccnc (Chaney, Condit and Axelrod, .1944). However, it (cast some of the Sierran ar.dcsitcs were cmplac'ed no later than the ca.ly Oljgnccnc (Clark and Anderson, 1939). Thus if the andcsitic rocks at Orovdle, yucs(ionabiy referred to the N -1011 - ten Formation, actually bclun�, to the later andcsitcs, the "older basalt" may be upper Miocene to lower Pliocene. But if the andesitcs at Orovillc arc of the earlier period of eruption (cf. *P.ceds Creek andcsitic rocks" of Clark and Anderson), the lower agc limit of the basalt must he extended at 'least as lo\\t as the early Oligocrnc.' The writer \vas enable to locate a.,y vent within the ar•,t front. which the 'older basalt' mighr have 'bccn extruded. The prescnt regional distribution of the lava and the variation in thickness from one place to another Suggest that the source Wray 1\a\ -c bccn to the cast. Turner (I896, pp. 614-615) noted that "it is nr6hable that the older hasalt Issued at a nutiibcr of years, bt : the masses about Onion Vallcy Creck lin Plunias CtiUnty, 35 miles cast and north from Orovillc Table Mottntainj and the cxtenSt\'e sheets in the Bidwell. arca .about Black Rock Creek, on the ,\Ioorevillc. ridge, etc., may perhaps have come from a single orifice near Onion Valley" ForroticNewl The Tuscan Formation usually rests on a sitrfacc of low relief which has bccn dcrclopcd on Eocene or older rocks. In sc\•cral places, however, the Tuscan is underlain by coarse fluviatilc scdimcnts of post- l:occoc agc. These sediments contain debris derived largely from typical bedrock formations, and thus forth a mappable unit, readily distinguished frelm the overlying Tusc2b volcanic rocks and intercalated volranic sediments. ' Subsequent to the completion of this manuscript, several papers pertinent to 2 consideration of the agc and corrcla- tiun of the "older basalt" have bccn published. to 19`9 D-irrell (1959, 19592) published the results of an extensive study of certain of the basaltic lava ilou.•s (pre- ciously tnapped as "older basalt" by Turnc.) in the north- ern Sicrra Nevada. Durrell 2ssigned these flows to a nc\t•ly n2nud stratigraphic unit .211cd the. "I,ovcjoy Fonn.nion", which he believed to be upper F ocene or lover Oligocene (p. 214-216). Dalrymple (1904, p. 14), on the other hand, o mined an absolure (potassium -argon) agc of 23.8 million years (lower t\liocenc) on rocks hclow the "oldc. basalt" (Love- joy Formation) 2r Snuth T;1blc \lourtain. Using this de- termination and 2 date (22.2 m.)••) obtained fr"m strata �buvc the I.nvcioy Fonnarion near 11112irsdco, California, W'jr)-millc (p. 15) conclu"Ic" that the "older hasalt" o: Lovejoy Fomt2tion is lo•.t•cr \liuccnc (Arikarccan). Mi -' CALIFORNIA DIVIsurx of MINKS neo Geot.rx:v Bull. 1,84 The cerin N.e-t: Era Formation is proposal for these nuviatilc deposits. The Ncw F.ra mine (recently re- named "Jack and Jill, mine'•), which lies near -the head of Dry Creek (s cc. 1, T. 21 N., It. 3 F..), is Considered to be the type localiry.. There the con- glontcratc and intcnccddcd finer-graincd sediments cover a comparatively large arra and arc better ex- posed than in ::nt• other part of the quadrangle. The central part of the section is wcll-exposed in an old hydr.iulic-face on the New Eta property, and the re- maining portion abovc and below may be observed at the portals of adjacent mine openings. In areal distribution, the New Era Formation .is comparatively unimportant and crops.out only on the wail:. and floors of steep canyons, where the base of the overlying Tuscan Formation has been exposed by erosion. Undouhtcdly, however, much of the "Tuscan inthea northwest quarter of the arca is underlain by strata.equis;alent Cu the New Era. The most important outcrops of . the .formation arc in the upper part of Dry Crcck. A somcct•hat larger arca is probably under- lain by the . Ncss• Era along Butte and Little Buttc Creeks, but outcrops there .arc quite scarce, nn,l the contacts with. the underlying and overlying forn►a- tions may be mapped only %6thin rather broad limits of error. Thc.forination is also exposed in srt:^il • areas on the cast edge of Jordan Hill, on the west side of Gold Flat, -,nd at several points just west of the West Branch front -Cape Horn to the mirth edge of the arca. North of the Orovillc quadrangle several stream channels 'and the sediments contained therein Jinvc been preserved beneath ihc-Tuscan beds. These have become known-npinly through the many gold -placer mines which have been opened into them. One of the hest known examples is the Magalia Channel and its tribucarles which Lindgren (1911, pp. 90-93; pl. 14, p. 84) described. Lindgren traced this channel for about IK miles front Centerville, on Big Butte Creek, .north-northeast to a point cast of Barnum Hill. Che .sediments the:iisel%-cs were briefly described by L.ind- gren,,'but their relationships to the adjacent rocks strongly suggest that they arc correlative with the New Era Formation. Coarse pebble- and cobble -conglomerate is the most Characteristic rock of the Nesr Era. Its dominant color is light reddish -brown, . the color, of the matrix, set against which are dark blues and greens of the con- tained clasts. Bedding is rather ill-defined, and is usu- ally obvious only where finer-graincd sediments arc interbedded with the conglomerate. The conglomerate is generally, rather ill-sorted in respect to both size and composition of clasts present. The pebbles and cobbles are dominandy subangular, a ;csscr amount are sub - rounded, and a few arc angular or rounded. Over a ,•quarter of the pebbles have a flattened, discoidal shape. The cobbles, on the average, arc less than 3 inches across, but cobbles of S or 6 inches arc. very common. Almost all of the pebbles and cobbics are composed of ntctanu�rphic rocks derived fr(nn the ••licdrock series"_ The nowt characteristic type is a bluish -gray, fine- graincd ampltihohtc, similar to rocks of the Scott type which occur in place east and north of Jordan. Hill, in ahc northeastern part of the arca. The amphibolite alone account,. for about a quarter of the clasts at the type locality. and is vcry abundant at all other locali- ties where the formation is recognized. Slate and slaty chert, typical of the Cal:wcras Formation, arc:nearly as common, cchilc massive chert of s•arving color and strongly weathered serpentine occur in somcv.-hat lesser amounts. Other rocks typically represented in minor quantities arc white quartz, quartzite, weathered chlorite schist, altered dense basic volcanic rocks, and meta -gabbro. A.fc\v pebbles of diorite and quartz dio- rite arc present in const outcrops. Subangular col.)bles derived from the "older basalt' are rare ac most locali- tics, big their presence is significant in the dating of the New Ezra sediments. However, at the old Welch hN•driulic mine, about a mile north-northeast of Pentz, 5 feet of. coarse pebble- ro cobble-conglome. atc di- rectly underlying the basal Tuscan beds and probably belonging to.the.Ne\s' Ezra Formation contain ibundant fragittents of the "older basalt'. No other recognizable -Tertiary volcanic rocks were found as clasts in the con.gloiitcratc. The niatrix of the conglomerate con- sists of reddish -buff, uncemented, friable, medium- to coarse-grained sand. Where the conglomerate rests c bedrock and the contact may be observed directly', the Intrcr fest. feet arc seen to consist dominantly of coarse, angular rubble clerked from the immediately undcrlj'ing formation. The sandstone and siltstonc interbedded .with the conglomerate constitute but a minor part of. the for- mation. They occur in beds rarely over a fcsr feet in thickness. The sandstene is the same as that which serves as a matrix for. the conglomerate. Ic is reddish - tan, massive, uncemented and friable. It is dominantly medium- to coarsc-graincd, but grades locally into very coarse-grained types. Sorting• with respect to size 'is moderate, and only a minor percentage of admixed silt is present. The. grains are sub -angular to angular and consist chiefly 'of quartz; feldspar, biotite, and dark dense lithic fragments are present in lesser amounts. The siltstonc is red, massive, and firm, and contains -a notable percentage of sand. Flakes of wcath- ered biotite and small chips of ycllo\vish-,,vhite plastic claystone arc locally abundant. The New Era is everywhere overlain by the Tuscan Formation. If parts of the New Era ever existed which were not protected by Tuscan strata, they havf, since been eroded away. At the type locality of the New Era, the contact is conformable, and the topmost scdi- ments of the New Era grade upward into the basal volcanic sandstone of the Tuscan. Apparently the ear- liest Tuscan sediments were added to the already over- loaded New Era streams and the sediments of the two were deposited together. The New Era �dr,2mige was �)IttlwI1.LF. QUADRANCI.F.—CRFTAN disrupted and soon volcanic (Tuscan) (M)ris was dc - posited in ovcrnwhelLning quantities. The gradational relationship bct%irccn the Ncxv Fara and Tuscan title. not exist over the entire arca %there the town arc in contact, howc,cr. At the old Welch hydraulic mine, for example, the contact bct%wccn the two is conform- able though quite sharp. Near Parish Gimp, about one- half mile northeast of the Welch mine, several thin conglomerate beds containing abundant fragments of the "older basalt'.' arc interbedded with volcanic sedi- ments in the lower part of ncc- Tuscan. The Tuscan there clearly abuts against a gently southwest -sloping bedrock sutiace (Fig. 37). These ctlnglonlcrates may represent a sporadic rent=:al of sedimentation of the New Era iype at a time when the -hulk of the dctrirus entering the area (either as f tiviatilc sediments or volcanic mudll6%-s) had come, frnm typical •i -ascan sources. SW NE Figvre 37. Diegrommalie rection showing relolionship of Tuscon stroto to tonglomerote 6edi contoining frogments of ''older basalt" near Welsh hydroulk mine and Parish Comp. (Ti=lone• TI=Tuscan; cg=conglomerole with (rogmenb of "older basalt".) in the eastern part of the arca, the New Fra Forma- . cion is underlain with profound unconforntity by the "Bedrock series". Along Dry Creek it is underlain in part by bedrock, but at the south end of the outcrop arca the Ncw Era abuts, with disconformity, against the lone strata. On Butte and Little Butte Creeks, exposures of both the New Era and the underlying Chico sediments arc so poor that the stratigraphic rela- tionship between the two cannot be 'discerned. it is probable, however, that the two are separated by a slight unconformity. North and Nvest rtf the mapped area, the Chico. strata are known to be overlain nncon- formably by the Tuscan, and since the Ncw Era is in comformable and, in part, gradational relationship with the Tuscan, it follows that the New Era as well might be expected to overlie the Chico unconfnrm- ably. ,ecausc most of the New Era Formation occupies WWam channels, its thickness usually varies rapidly ong strike. The thickest section was observed along the Qwest side of, and below, the thin cap of Tuscan strata which occupies the western part of Gold 63 Flat. Here, thc.New Fara Formation is about i00 feet thick, and extends for approximately 3000 fcct along strike. At the type locali(y, the Nc%y Era mine, the formation reacl;cs a maximum thickncss of t)0 feet, gradually thinning otlt hath to ilio north and south. The strata there appear to he in a broad, shallow, southwest -trending channel, whose width is approxi- matcly 6000 or 7000 feet. Tile thickness of the New Era on Butte and 1-ittle Butte Crccks is estimated to be around 00 feet, and docs not appear to vary markedly within the mapped arca. The Ne -,'Fra Formation is believed to represent fluviatilc deposition upon a floodplain and in upland stream channels, In the eastern part of the prca our - crops of the New Era continue for comparatively fC shodis[aticeti nIong strike, and appear to Itave filled shorr segments of channels which were cut in the un- derlying fledrock and acre subsequently blanketed by the-•fuscan volcanic. deposirs. The overall. coarse na- ture of the New Era sediments and the fact ihai they appear to have filled channel -like depressions suggest that they. represent fluviatile deposits. To the \x-csr, along Dry Creek, the sediments occupy a channel Iniich. broader and relatlycly more shilllow than an\ fotm.l in the arca to nce northeast. Farther west,. on Bultc, and Little Butte Creeks the formation docs not appear to have filled any definite channel and shows no marked %arlitjon in thickness throughout that part of the arca. There, the formation was apparcntly laid down on.1 broad -floodplain to which sediments were furnished.:by streams fltlu•ing in the channels ro the mirth and east. These deposits are probablycrintinu- nus with diosedescribed by Lindgren (1911, p. 91) .1T Centerville, on Big Butte Creek, about 3 miles north (.f the north edge of the arca. Lindgren bc!icrcd that these accumulated at the lower cnd of the Magalia channel, apparently in a broadened channel or upon a fltlodplain. The composition of the pebbles and cobbles present. in the conglomerate indicates a provenance largely of bedrock metasedimentary and nlctaigncous rocks not unlike the rocks that arc at present exposed in the higher parts of the area. Flows of the "older basa:t", probably continuous with thitt .which caps Oroyi)Ic Tillie \•fountain, \were present in the source arca, and these wFre at least in part eroded and contributed a minor amount of sediment to the New Era channels. Many of the quartz pebbles found in the conglomerate. were probably derived largely from Eocene quartzose sediments which presumably cropped out in part of the source area. Mechanical, rather than the chemical, weathering appears to have dominated in the upland region which furnished dctrirus to the New Era streams. The strongly weathered serpentine pebbles may have been derived from an area where nce pro- ducts of deep chemical decay of the pre -lone period had been, until that time, preserved bcncath a cm•cr of Eocene sediments and 'older basalt". 64 Tt sae a o.. ' t BEDROCK Figvre 37. Diegrommalie rection showing relolionship of Tuscon stroto to tonglomerote 6edi contoining frogments of ''older basalt" near Welsh hydroulk mine and Parish Comp. (Ti=lone• TI=Tuscan; cg=conglomerole with (rogmenb of "older basalt".) in the eastern part of the arca, the New Fra Forma- . cion is underlain with profound unconforntity by the "Bedrock series". Along Dry Creek it is underlain in part by bedrock, but at the south end of the outcrop arca the Ncw Era abuts, with disconformity, against the lone strata. On Butte and Little Butte Creeks, exposures of both the New Era and the underlying Chico sediments arc so poor that the stratigraphic rela- tionship between the two cannot be 'discerned. it is probable, however, that the two are separated by a slight unconformity. North and Nvest rtf the mapped area, the Chico. strata are known to be overlain nncon- formably by the Tuscan, and since the Ncw Era is in comformable and, in part, gradational relationship with the Tuscan, it follows that the New Era as well might be expected to overlie the Chico unconfnrm- ably. ,ecausc most of the New Era Formation occupies WWam channels, its thickness usually varies rapidly ong strike. The thickest section was observed along the Qwest side of, and below, the thin cap of Tuscan strata which occupies the western part of Gold 63 Flat. Here, thc.New Fara Formation is about i00 feet thick, and extends for approximately 3000 fcct along strike. At the type locali(y, the Nc%y Era mine, the formation reacl;cs a maximum thickncss of t)0 feet, gradually thinning otlt hath to ilio north and south. The strata there appear to he in a broad, shallow, southwest -trending channel, whose width is approxi- matcly 6000 or 7000 feet. Tile thickness of the New Era on Butte and 1-ittle Butte Crccks is estimated to be around 00 feet, and docs not appear to vary markedly within the mapped arca. The Ne -,'Fra Formation is believed to represent fluviatilc deposition upon a floodplain and in upland stream channels, In the eastern part of the prca our - crops of the New Era continue for comparatively fC shodis[aticeti nIong strike, and appear to Itave filled shorr segments of channels which were cut in the un- derlying fledrock and acre subsequently blanketed by the-•fuscan volcanic. deposirs. The overall. coarse na- ture of the New Era sediments and the fact ihai they appear to have filled channel -like depressions suggest that they. represent fluviatile deposits. To the \x-csr, along Dry Creek, the sediments occupy a channel Iniich. broader and relatlycly more shilllow than an\ fotm.l in the arca to nce northeast. Farther west,. on Bultc, and Little Butte Creeks the formation docs not appear to have filled any definite channel and shows no marked %arlitjon in thickness throughout that part of the arca. There, the formation was apparcntly laid down on.1 broad -floodplain to which sediments were furnished.:by streams fltlu•ing in the channels ro the mirth and east. These deposits are probablycrintinu- nus with diosedescribed by Lindgren (1911, p. 91) .1T Centerville, on Big Butte Creek, about 3 miles north (.f the north edge of the arca. Lindgren bc!icrcd that these accumulated at the lower cnd of the Magalia channel, apparently in a broadened channel or upon a fltlodplain. The composition of the pebbles and cobbles present. in the conglomerate indicates a provenance largely of bedrock metasedimentary and nlctaigncous rocks not unlike the rocks that arc at present exposed in the higher parts of the area. Flows of the "older basa:t", probably continuous with thitt .which caps Oroyi)Ic Tillie \•fountain, \were present in the source arca, and these wFre at least in part eroded and contributed a minor amount of sediment to the New Era channels. Many of the quartz pebbles found in the conglomerate. were probably derived largely from Eocene quartzose sediments which presumably cropped out in part of the source area. Mechanical, rather than the chemical, weathering appears to have dominated in the upland region which furnished dctrirus to the New Era streams. The strongly weathered serpentine pebbles may have been derived from an area where nce pro- ducts of deep chemical decay of the pre -lone period had been, until that time, preserved bcncath a cm•cr of Eocene sediments and 'older basalt". 64 • (:.a1.11-11utNIA Un•tsut', ot•• hl,.Nfs AND Gror.rx:v kitivo tcnt :end deposition of coarsc. debris in the stream channels and uprnt Ilio flondptairt were brought to a halt by the invasion in late I'lincene (Tuscan) time of oycrw•Itclrttittg quantitic% of volcanic mudflow material and clmcly related volcanic sand and gravel. The extent to which the disrupted streams furnished detritus to be mixed with the Tucan sctlintcnts is not known, but apparently the quantity was negligible. since non- volcanic pebbles and cobbles arc rclatiycly scarce in the Tuscan. No fossils were fount) in the Ncw• Fara Formation by the \•'ricer, so that the determination of its age is based upon stratigraphic relationships and upon the lithnlogy of the Ncw Era scditiicnts. The youngest formation ovcrlain by the Nc\• Fra is the lone (mid- dle Eocene). It is evident, however, that the Ncw• Era occupied a broad erosional channel cur in part into J lic . lone," suggesting that a considerable time. gap existed between. ilio periods of deposition of the two forivations. This inference is strengthened by the fact that the. t%t•o units; respcctiycly, represent deposition in markedly contrasted environments. However, the most importmit line of evidence in establishing the age of the Ncw Era is the presence in. -that formation of fragments of cite. "older basalt", showing that thc• Nev., Fra definitely post-dates the."older basalt". The "older ebasalt" is thought* to be at least as young as early Plioccrie, so that the New E.ra is probably- no older than early Pliocene, and may well he middle or late Miocene 1 -lie Ncw- Ira is overlain by the Tuscan Formation (tipper Pliocene) and is, therefore, clearly older than clic Tuscan. noted above, the contact between the two is gradational in at least one locality, and at others sediments, identical with those of the Ncw Ezra, arc intcrbcdde.l with normal Tuscan strata in trio lower part of the Tuscan section. These .relationships strongly suggest that the age of the New Era docs not differ markedly from that of the 'Tuscan and thus ata►• be entirely or in part late Pliocene. Unnamed Rhyolitic Pumice Tuff Several miles cast of Orovilic, light-colored rht ' tic tuff and pumice -lapilli tuff are exposed in roadcuts along the Oroville-Forbestow•n highway. These rocks a,rt.nearly flat -lying and clearly belong to the "Super - .r+; cni series', but thele age is knmcn only`'wiihin tauter broad limits. Thcy ;nta% be in part eguiva- Icnt to the "tuffs of Oroville" described by Lind- grcn ( 191 1, pp. 36-27,: 90). However, this term seems inadequate for several reasons. First, Lindgren's definition of the "ruffs of Otovillc" was rather vague, and it appears that Lindgren may have also included �certain parts of the andesitic roc.*s questionably re- . Since the preparation of this manuscript, the "Older ha,alt.. has bccn shown to be probably lu\wcr Nlioecnest, that the lower agc li:mir of ill- Ncw• Fra must be cxtcOd,•u to i\lioccnc. )lull. 194 fcrrctl to the Alchricn Formation by the present writer. Sccond, the tomo has little regional strati - graphic significance and docs not appcar ro have bccn adopted to any apprcciahlc extent by other writers on Sicrran geologi . Finally, the name "Oroville" \vas pre- occupied as n stratigraphic term priur to its use by Lindgren; the name "Orovillc boils" was used for the Monte do Orn Formation b\.• Fontaine (1900, p. 342). No formational name is proposed hr the presxnt writer for these rocks, principally because of clic un- certainty regarding their exact stratigraphic position and bccat.sc they arc of such.limitcd areal distribution within the quadrangle. It is possible that the exposures in the Oroville region do not rcpresent .the maximum development of this unit in the Sierra Nevada, and that there exist localities better suited to the study and naming of these rocks. Regarding the "tuffs of Oroville", Lindgren (1911, pp. 26-27) stated: Alring the foothills [of the Sicrral frust Rear River to Feather River a series of light-colored tuffaceous rocks are expnscd in plac•cs, although the formation is generally cov- ered by larer Quaternary gravels or by the red. snil.of the valley . near Orovillc rile)- arc extensively developed and farm the low flar-topped hills which flank the river on the south side for a distance of 9 miles below Oroville. ... The tuff extends under the present alluvium of the Feather River and forms ti,e bedrock of the area now worked so extensively by dredging. It is a compact light brown material, containing in places pebbles of metantor- phic rucks and also small w liitc f ragments of -pumice which are hound to consist if volcanic glass; locally. these fraF- picnts arc werry small and the tuff looks more like a com- pact clary. Rorc holes tin fret deep have been sunk in it in the flood plain below On wills without fitidin;.• 'different material. On the road m bVyandottc from Orovillc similar mtatcrial outcrops in the Inv foothills undernca'th the Quat- crnary gravel up to elcwatiuns of about 400 feet. .. . The bedrock relations at Orovillc indicate that this series was deposited on the even slope of the older (Neocene) forma- tions, bcforc the modern canyon of the Fcather River had been excavated but after the earlier lone formation had been greatly eroded. The only tuff known by the writer to be exposed in.. the hanks of the Feather River below Oroville is that just cast of Thcrrnalito. This is clearly andesitic in composition and was mapped as ,Mehrtcn (? ). It is, however, lighter in color and finer -grained than .most of the other hlehrtcn (?) andesitic rocks .ex- posed in the arca. Thus, from Lindgren's statement regarding the "tuffs of Orovillc" at the Oroville dredging ground, it would appear that he included some of the hichrteti (?) andesitic material, as well as the flivolitic pumice tuff described here, in his dcfini- tion of the "tuffs of Oroyilic". The principal rock type composing this unit is=itric tuff, often intezrstratificd with which are thin layers containing abundant lapilli of \•bite putnicc. •Che ruff is characteristically light-colored. ilosi of it is white, light gray or pale buff, though certain beds are locally discolored by an orange -brown ferruginous stain. A1uch of the formation is thin -bedded a65tile indi- 0 { V �• r V �• r