._~..." 1.,'- ~ . .'"
II
I
wREPORT No. 2S
;.__..,.__-_’:
COLONY AND PROTECfORATEPROTECI‘ORATE OF KENYA
GEOLOGICAL SURVEY OF KENYA .
GEOLOGY OF THE SOUTH-EAST MACHAKOS AREA
DEGREE SHEETSHEET 52, S.E.SE. QUADRANT (with coloured map)
I f ' by
I R. G. DODSON, M.Sc. , Geologist
I I
III II
) 19531953 II mPRINTED BY THE coyEkmmqrm,GOVERNMENTPR.INTER,NAIROBI Price: Sh.Sh.6/506f50 ,t III II !~, --.- CONTENTS PAGEI’\GE I.I. Introductionllzll'uduclion . . 1
II.I]. PreviousPrenioux‘ GeologicalGavfugical Work..Wurk 1 J
III.III. Physical Features.features . . . 2t
IV.IV. Summary DI"of Geology660.10g} and Gotf-lne'cnlGeological HistoryHistm'} ofu ' theIhc Area.'\I'c:': 51)I V.\'. DetailsDCIEIIIS ofUI' GeologyGeolog} 7 1. BasementBewanrn SystemScm 7 (1):1 : Meta-sediments\Ii‘iél-\C\III‘I':L‘I‘:E\ 8 (2):1 J Migmatites.\Iiglnuulcs . 9 I31(3) IntrusivesIrrrusjws .. 13 (4)I43 Mineral\‘Iincral beltshclls 14 (5)I15: Metamorphism\Ietamnrphim: 15 (6)[m SnaicturaStructures. . 16 2. Tertiary-Turtmrg.i\Yatta:m:=. PlateauPI.1‘.J:.E.: phonolite.I‘II‘HI‘UIIYC... 17 n.3. PleistocenePICNHL‘L‘HC LICITUKILEHdeposits. . . 17 (1)IIJSECIRIC‘[11\.Sediments 17 (2)9.2) LavasLu 4% 18 4.-‘r. SupctficirslSuperficial deposits.dC‘flUMh . 18 VI.\ I. Economic|3cunomic clugj.Geology 19 1.I. Minerals\1jner;ll< 19 2. Walt“Water.. . 19 VII.VI 1, ReferencesRCIL‘TC‘ICCQ 20
LIST OF ILLUSTRATIONS“LIB | R \ I IONS Fig.Hg. 1.I Erosionbrown“. surfaces.we Illgu . 3 Fig.Flg 2.3 StructuresSIE‘LICIUZ‘C“ and mineralI‘HI‘L‘I :l beltsDC :\ 15
MAP'\I ‘-.!’ Geological(jeuioglcal map ofml thetil: south-east\L‘LélII‘Eihl Machakos\Iuclmkc-a area::ucur scale\cruic II::125,000" ‘: [NJ-U . . at end Foreword
Thel‘he report on the south-east paztpart orof the Machakos.\'liiCl1iil\US area is the ti.thfifth report published since the Geological Survey of Kenya was expanded with the assistance ot'of the Colonial Development and Welfare\Vclfare Vote. The southern part of MachakosMuchukos arealiI'CLt proper.proper, to the west, has also been mapped and :1a report will be published in the near future,future. while a report on the mapping of the KanzikoKan/iko urea.area, on the east.east, is in preps-tition.preparation.
Unfortunately,Unfortunately. the reconnaissance search of the southeasternsouth-eastern Machakos\‘lachakos area has not revealed the presence oiof numerous small mineral deposits such as ureare knownknmyn in the country south of MachakosMach-altos town. Itit appears unlikely that the area will prove to have any economic importance in the future.future, so forfar as mineral deposits are concerned. It is.is, however, the site of the MaltueniMakueni land settlement scheme (ind.and, as such,such. is Fman area of the greatest importance in the Colony.
Thethe area is traversed by the well-knownwelllnown phonolite-coveredphonoiitc-coyereo Yatta‘r'utta plateau. The lavalaw hashas always been regardedregarded as the remains:‘ezrmins of an1111 extensive flowfirm- that found:‘nund its way down a valley in middle Tertiary times.times, but Mr. Dodson has discovered evidence that suggests a diiTerentdifferent origin. He believes thutthat the lavalax-"i. L“-C:'l1L“.~overlies at: zone(one Oiof faultingiitlting of Tertiary age, and that it is unlikelynnliLcl} that at any partpert of the plateau it flowedflowed for more than a short distance. He also considers thatlhzit flowsflours moulded by whenvalleys are rare.rue.
Nairobi,Nairobi. WILLIAM\N'llllA \‘J PULFREY,[’ULIJREY. 2nd September,September. 1952.I952. Chief Geologist.
ABSTRACT
The area described is bounded by latitudes 101' Eu30' S..5. and 220 Hit00' S.,5.: and by longrlongi- tudes 37‘370 30"30' E. and 38380 00’00' E. Apart fromfront the ruggedrugged ground tilongalong the western boundary,boundary. the areaurea consists of flflat,at. low-lyinglow—lying country. Remnants of two former plains represented by summit levelsiE\'Cls of hillshlils are.v’e recognizablerecognizttbie and aL‘. Pleistocene to Recent plain appears to be still in ait stage of development.oexelopuiern, A short accounttic-count isis givenpitch ofol' the physio-physio- graphy ofor thethe YattaYetta plateau and :ta tentative hypothesishypothesis of itsits origin,origin. diil‘erentdifferent from the classical theory.theory, is tiller-ed.offered.
The rocks described comprise: [it(1) (meissesGneisses and scnistsschists ofot' the Basement System.System, with abundant pegmatitepeginatitu and quartz veins,veins. (2)(2: Ancient basic intrusivesintrusiyes including amphibolites and a;t pyroxenite,pyroxenite. (3)i3] Tertiary phonolite of the YattaYutta plateau,plateau. (4)t4) River conglomerates, soils, gi'zirels.gravels, etc..etc., (5)r53 Pleistocene olhine-basalt.olivine-basalt.
"""""'"'... GEOLOGY OF THE SOUTH-EASTSOUTH—EAST MACHAKOS AREA I-INTRODUCTIONI—INTRUDL'CTION The area described in this report is the south-eastsouth—east quadrant of.1: degree sheet 525.; (Kenya),(Kenyat. bounded by meridiansmortdians 37°3"" 30'311' E.1-_'. and 38°38 00'[i'tl’ E. and'.I.11.L1 byb1. parallels11.11.111.313 11"° 30'311-" S.'1'. and 2°3 00'110' S.,8.. of01' which the1113 extent is approximately1;;1L1:L1xin1'..tei_\' 1,2501.251.“ square miles. '11::The whole1111013..of".tltc.the area is nativenative reserve,reserve. the portion eastezzst of the AthiA1112. RiverR5137 being' part of thethe.K'KituitL'.i district,L11 st 'iL‘t. and thatthat westnest of the river part of the Machakos.\luc'.1-.t.-..1s district.Li'.striL' Where inhabited thethe;areaL'_ isl"; occupied by Africans ofol' the UkambaL'lx'anim tribe. Cu:tivationC.'L=.'.?.1‘.:111o11 ofL1: crops and.11'.L1 stock-keepings'LoL'k—keepi 111: areL111? practised in the northern,northern. western and,21nd. to a'.1 lesserless-3: extent,extent. in the easterni.’.t-Jl'l sections.sections DueD' 1L: to the erratic rainfall,1'Ltinr'all.gei1eralgeneral povertypovert} of the soil,soi|.:11dand theth-3 primitive111' 1111:11'1- agriculturalL 1'.|'. 11.".12 methods111311.oLi.' used,ttscd. crop yields)fcids are low.10“. Recently attention has.'L'1s been'ITCCE': focused:L1L'.1scL'. on.. ".the south-eastern" portion where, under the control of.11' the African.-'\l'1"1'C.&L1’1 LandLune SettlementSedicnmnt Board,8-.1.-.'... the Makueni settlementsettfesnent scheme is being developed. The object of01' the scheme is to10.3resettle'3!t" L. members"l"; CTT-“CI: of0. the UkambaCkamba tribe,ts'ibc. hitherto livingliving inin the congested localities':oc:1‘1i'.ies inin the western'LL3s‘I'11 partpas”. of(11' the1:1.- area.'.1.t Bush-clearingBllsh-Clearing and cultivation schemes have been put into practicepl'L-‘LL‘. cc underL111'L1e1' the supervisionopen-131011 of01 EuropeanFur ope-.Ln ofofficersficers to prevent11.-event the do:deteriorationerio: 11111 of0." $1.11:soil .1sas had.:tLI. her-hencehappened in‘11". neighbouringIlClLIn'l'K'Jlll'lllLf cultivatedi.' l'\' {[L‘Ll areas.‘.'..t3L15. It["1 appearsL.p'L"Ll1'>1 that131.1: the.3 L’ energeticCllt‘lgt. Il.C efforts:s ofo‘ those1.3 responsible will make1111156 possiblepossible an increasing amount ofot' settlementsettle-rcntiin1 .."3:-.sareas uninhabited.211111'11 Sibltft'. tilltill now. The area was geologically surveyedSt1r\'c}etl betweenbetween. March.\‘11L1'L‘h. 11ndand September,S-cgétcrtt'ncr. 1950,L1'5t.‘1.\1.-';thwith the object ofL1I' preparingpreparing. a reconnaissance map and in :111an attempt to discoverdisL‘Ln-L‘r mineral111't-L‘I'al deposits,deposits. or geologicalgeoiogical conditions favourable:‘ox-onrehic to1L1 mineral occurrences.QCCLI‘JI'CIlCLb. Unfortunately,t..".1;'.11't.11‘.;-tL"1}.thethe workworl' that it was. 5 possible to carry out in the time availableanoilable did not revealtor-3:1: any..:1§.' deposits of111' economic value.\Ltltie. Mapping was carried out on11:1 tracingsti'LLL‘inL'rs ofot' aerial'.'1L‘:'i'.1l photographsnt‘.‘tL'1jI"'-'.‘ and'-..'1.;" on0:1 fieldfield sheetss (scale[scale approximatelyLLDDI'OXll'ITctTtil} 1/64,000)1 (14.0001 based onL111. the 1/250,0001. 25LJ.LI'['Jt.‘I military map. _. Machakos\lzchakos E.A.F.E No. 16371113? (1945).{1945). Later the informationintorination obtained was replotted1e111otILLL'1 on the four-1ti: 1/50,0001'5. preliminaryprcl1'111ir1:L1'_\-' plotsplots ofL11" sheet SA37/H-IV.SAB? l—l—l\". With‘.\"it11 a:1 singleszngc exception,.3\L'L[1‘- .L111. namely11:'11..3‘._. in1:1 the1113 caseuse of{11' the YattaYetta phonolite,phono'ite thethe accurateacccratc demarcation of junctionsj'.1nL':io11s and' contacts..mtacts provedprotec- impossible. The reasonreason forfor thisthis is duect1L firstlyfirsti'y to an.".n insufficientirisut‘rici I:1t number1'... " ofL'.I'. exposures,L-.\p-.'.IsI.1rL'Is. and:111L5 secondlysecondh. because of the gradation of.1: the various rockroe}. typesttgrt's of the Basement System53'ste'1' o."of which the area is largely[E1 rg'ely composed. Acknowledgments:.«t('1'.1.1.11x'1‘1311gr111r31zrs: The'Ihe writer wishes'.1-1511:3111 to express his thanks tor-.1 the:‘131L1ltiLc1'sofficers ofoi thet".1L MakueniMel-{tieni settlement scheme for whose kindness.ndncss 21ndand asassistance..;1'.1L'c hehe is greatly:13' indebted,indetd. and to thethe District CommissionerC'onnnissioner ofoz' KituiKEtL1i forto: helphelp and'.=. :111 hospitality.'r'os} i. '.-.211'-..
II---=.pREVIOUSll—PRI \ IOL 5 GEOLOGICALGE'.0]. {'.GI( IC. \l. WORK Little geologicalgeologicn‘a work has been carried out in the streetarea previous|1:'I:\'io.1s to the presentpresen'. survey. J..1. W. GregoryGregor} (1921,1.192], p. 186)*1861"" gave anEll] account ofot' the:he Yatta1 ' Plateau ascribing' ' its origin to ata volcanic flowflow which followed the course of0" a:2 valley,‘ concluding, "It is therefore clear that either the existing bandhand of(11' lava231:1. isi'.' the1313 remnantr31: of a wider sheet oro." it must havehave been confinedconfined like a'.1 stream..m betweenhet'fi'een the'.lic bankshair-R's of.'1!' a valley". An officialofi'icial reconnaissancereconnaissance of the Makueni.\l.11tt!eni districtdistriCI. wasmas carriedL' out in 1938 by C.C.$.Hitchcn,p1'in1ttril}S. Hitchen, primarily to investigateinvestigate the possibilitypossi': ll1t\' of undergroundnode. ‘-L!El(l waterMiter supplies.sLipplLe~ A tentative,ICE‘ItflléVfi. idealidealizedized section was included in11'; hihiss 11111.1tunpublished:ishIzcl:report,'1-‘zt. representing.'e_.1.'..senting th'the geologygeoéog} of the area betweenhetwoen the 37°3.7" meridiann‘erioian and11nd thet.1e Yatta3 -=tt== Plateau.P..“-.ttc He considered the crystalline schists and gneisses of what11113.! he:1L calledCELllC‘Cl thet'.13 "Machakos"‘\1 mos Metamorphic\1et211'..1:' :11: Complex"(.'11'1‘.}‘l3‘.' (formingt'fotrning the hills on the west of the area) to be of'.'1: sedimentaryL oI. .-ent'.'11"_\' origin. In11'. supportstir-port of'.1' this assumption he described the local existence ofor crystallinecrj.'s:.'.. .line limestones“stones and graphiticg..-.ph:'tf.. schists, sugg~tingsuggesting the latter are metamorphosed carbonaceous or oil01' shales. The abrup-abrupt changechenge of topography east of the Nzaui range he ascribed to a correspondingcorresno'1c'i11g change ini ‘- '~~--, '. .-, * References are quoted on p. 20. o. ~
2
the geology.geoiogy. He concluded that the MakueniMakiieni area is occupied by3'} an ancient granite mass which contains occasional "rafts"“rafts" or "roof“root pendants" ofo‘.‘ highlyhighi metamorphosed and recrystallized sediments.sediments, generallygenerally- similar to these of the MachakosMachakos hills. The granite was described as a biotite-microlinebiotitc-rnicroline type,type. displayingdisp'ay ing a11 faintlyfaint; gneissose structure. Inin 1945 W. H. Reeve,Reeve. while engineer—geologistengineer-geologist in the Public Works Department,Department. carried otitout a hydro-geologicalhyt'iro—g-e1lonic11l survey of the proposed MakueniMukueni settlement area. He remarked on the abruptbrupt change of topography in:11 the vicinityyieinitx ofoi‘ the:hc westernWestern boundary: "The“The most noticeable feature ofoi the topography of this region is the abrupt change which occurs east of a11 line connecting roughly,roughly. Nzueni,N/ueni. Unoa,L 1111.1. Nthangu and21nd Ikua".lkua" Concerning the local geological structure Reeve agreed u'Ewithth the outlines previously suggested by Hitchen. The only published work on the geologygeolottgy- of'1t‘ neighbouring areas111'e is by J. J.J_ Schoeman (1948),1219513). who described the quarter-degree areaurea immediatelyit'nniediatel} to the north of the present one. ComparisonCon1n1triso1 of the present maprm }.1 with Schoeman'sSchoen111ns reveals certain differences in the identificationidentification and classificationclassification of some :1.typespcs ofot rock. Thethe chief difference is one of opinion on the origin of some of the more leucoereticleucocratic bands of gneiss.gneiss, Schoeman regarding them as of magmatic intrusive origin while the writer is of opinion that they representrep. esent an advancedadyanced stage in the metasomaticmetasoniatie alteration of sedimentarysedfrnentary rocks in situ. Other differences between the two maps are in terminology only.only, The differences in the symbols usedused is due to recent standardization of symbols used in East MricanAfrican oofficialfiiciai geological publications. The following correlation table may be found useful:-useful .' -7
Area westnest of Kitui T1111Townshipnship AreaAre-1'1 south-eastsoutheast 111'of Machakos\latch'akos .l.J. J. SchoemanSchoeinan R.R, (j,G. Dodson
SgSJ Peliticl’elitic and semi-peliticsemi-pelitie schistsschi‘ts and11nd "I gneisses with\‘Hlil granite.':r'1rni1e injections.injections, : Biotite gneisses”r. eisses liable to local varia-variet— f>\_'s'XSI tiontinsi in:31 mineral composition and SpHp Pelitic and11nd semi-pelitic schists and21nd bandedhundcd gneisses.gncis~es, gneis'SCsgneisses withoutuithout granitegiztntte injections.iifiicctions. .jJ I j’Xs'XSI/ (Compact,ouipact. resistant bands of biotite gOg“ Granite ortho-gneissortho—gnciSs as11s closely-'~sp11cedclosely-spaced l( grieissgneiss.. sheets. - ‘ I.lXnaX1111 Granitoid(,iranitoid augen gneiss. Hs Hornblende-schist.Hornblende—schist, 1 7\h Amphibolite.Amphi: "IlJLE‘. gCg Semi-calcareousSemi—calcareous gneiss.gnciss. I: Xsdd Hornblende-Hornblcntio anduno diopside-richLi'it'ips1de—rtcli ' gneigneiss.1. Phkl‘lik Phonolite. TvpTap Phonolite.ii'honoiite.
III-PHYSICALII i—PE I YSICAL FEATURESFEATL' R ES
Three erosion surfaces can be distinguished in the area,area. two[1111 being represented by remnants only (Fig.I'Fig. 1).ll. There'l’here is no fossil evidence of the ages ofoi the sur.‘surfaces,ees. but because of their elevations and conditions they are considered to represent the end-end» Tertiary, sub-Miocenesub—\‘Jiocene and sub-Cretaceoussub—Cretaceous peneplains recognized:eengrized byl1}. Shackleton (1946,11946, p. 313) and DixeyDiwy (1948,(1948.1'lgs.figs. 2 and 3)3] in Northern Kenya,Kenya. ..ndand later by SchoemanSchoernen (1948,{1948. p. 3) immediately to the north of the present area. The most youthful surface.surface, a broad,broad. relativelyrelatixely flat plainpiaizn lying south-eastsouth—east ofo: the Machakos hills and extending to the Athi River, occurs at an altitudeal12111de' of between 3,000"1000 and 3,3PO3.300 feet and appears to be still developing. It is apparently extending by -aa process of lateral development,development. away from the principal drainllgedrainage channels,channels. suggesting'suggesting that it is (ormingforming by pediplanation (King,(King. 1951,1951. p. 47).£7]. Along thethe. courSecourse of thehe KeiteandKeite zinc.- Athi rivers still younger plains have developed, though in all instances they are confinedconfined to small stretches of the valleys. 3
1.~O 5,
w [. ~="="""'" ~~" 0 " " ,<.':',': :-) r<') .<-\. ""=,,,~,,== "',',' ',' '/ 0 ”ji] , .. ' .., r- i"x V.f'e\. ""="""") "" "", K 'BAONE‘E , ", , ' "'\ r<').5? K IBAONEA 5335 , ',' ,: ,', :- ',: :'\~"", , , ' , , ' .
w
, ' , , , ' , , . , ,. ,,' , ,
. 6000' Surface CretaceOU$Cret aceous I'? Sub-Miocene? ? ~ 3800' 5urface - Subreocene
CJ 3.300'1.3; SurflJceoerflfice ~ PleistoceneFla-5:056:78 to Recent. c:.J (Kapitlan.- '3:- Phonolite)Pnorc -' ‘ -' — (Miocene) Scale aL-. :-5 lomite:>Io 711.195 FIG. t.-Erosion1,—«1-Irosinn surfaces in thethe south-east Machakos area.
The monotony of the "3,OOO-foot""3.0004001” bush—coveredbush-covered plain is broken only by thethe. inselberg remnants of an older plain.plain, now represented by the tops of hills such as Makueni.Makueni, Katambua.Katambua, Kianga, Thabu, Poi.Poi, Mandoi and MMubau.uhau. The form orof these hills varies,varies. though generally they are narrow rounded stumps and occasionally fiflat-at— topped.toppcd. The uniformity of their heights indicate that they are relics of an older plain surface, now almost completely destroyed by erosion. This is further substantiatedsub$t3ntiated by correspondingcorrespoqding platforms on most of the larger hills inin the western part of the area. A fifinene example of such a remnantrel:nn~t occurs on Nzaui where.where, approximatelya.pproximately half-way up the eastern flflank,ank. a well—dewell-definedfined shelf occurs. The surface is also preserved beneath the phonolite lava-lava-flowflow that forms thethe. ‘r’attaYatta plateau. '
.. ~ ~..., - -
4
. Finally,finally, higher flat-toppedflat—topped hills near the western boundaryboundari of the area correSpondcorrespond in height,height. and are the remnants of the oldest recognizable peneplainneneplain in the area. There are only four of themthen: within the limitsiiiiiits of01' the1h;- area:-.re'- (Ikua,tlkt;.-.. Ikangabi,Ikang‘bi Kitondo and Nzaui),\‘zaui‘i. though.though a large number of hills further:‘uriher west possess correspondingcorrespo= ing summit levels,lciels. and are the remains of the same erosion surface. As would be expected,expected. rock'ock typesopes havehax to a great”tea: extent influencedinfluenced the topography.topograph}. North of the KeiteR :2c RiverRiV'ct fortot example,cmmple. where therehorc are more resistant gneisses,gneisses. the surface is far more irregular than usual. The abrupt change in topographyaopouraplt along a north-south line parallel to the western boundary may be attributed firstlylirstl_- to the abundance of resistant'esistant granitoid gneissgueiss massesmass-es which form the hill ranges westWest of the topographical break,break. and secondly,secontii‘x. to the situationsiti nlO‘fl of the main channel of thethe drainage system in the south-eastern corner of the area. BecauseBecame of the distance from the principal river,river. the hill rangesranges- in the westWest are as yet unaffected byin. the formation of the most youthful erosion surface. The topograph}topography is well 'Llli-sSL'CL’L'.dissected byin; .ia maturenuitzir: draui'igedrainage sisten;system of which the Athi'\[l"i is the principal riverrixer and alone has a perennial flow.now. Thelhe Tiva,Tim. Keite-ThwakiKeite—Thwaki and Kikiune rivers seldom flowflow for more than a fewten months in the year. Indications are,are however,however. that in the geological past conditions were"were far wetterWottct' and the riversrixers biggerhigg r and more persistent. The drainage pattern appearsappc .-‘s to have undergone striking changes.changes, GregoryGregor} (1921,{1921. 13.p. 188)185i tintednoted the possibilitj.possibility of the existence ofot west to east flowflow of rivers before the emplacement of the YattaYrttta Plateau,Plateau. considering that "some“some of the west-eastwest—east tributaries ofot the Athiisthi maymag. follow the courses ofor riversrners that flowedfloat-ed eastward acrossacroSs the site of the YaUaYatta into the deserts north of the Sabaki. Thus"has the Keite-TwakiKeiic—Tis'aki mayrnai behe the original heaLLstt'ean‘shead-streams ol'of the Thowa".Thowa‘. The outline of the YattaYtttta Plateau lends strength to this suggestion for,for. on the easteist bank of the Athi opposite of the Thwaki-AthiThwaki-Atlit confluence,confluence. ait spur of phonolite projects,projects westwards at Kiliza. UnfortunatelyUnfortunatel} the basal contact is obscured by dense vegetation and transported material soSQ that the search for llutiatilcfluviatile deposits under the phonotilcphonolite is difdifficult.ficult. HowereHowever, it is possible to esiablishestablish that the phonolite in this localityiocalit) is .1l‘|.1ttlabout twice as thick as itat is immediatelyiinmediatel} to the north and south. FurthermoreFi'.rtl‘.erniore on the eastern s:deside of the plateau a corresponding spur occurs,occurs. cuiungcurving round from an easterly direction to an approximately northern direction.direction, ie.i.e. towards the:lie direction of the TivaTira River.Riier. It seems probable,Probable. as Gregory(ii-egor} suggested.suggested, that the Keite-Twakilx'cite—T'aaki originally continued on a west-east‘.\est~eas‘. course~course. -perhaps to join the Tana River;Rl\-L‘r andand - it is likely that it was at that time the principal rite}river in the area. inIn support orof [histhis possib;possibilityit}. is the fact:act that while the present- dayda) flowflow of the Keite-Thwaki is far less than that of the Athi,AWL its wide meanders and large alluvial deposits indicate a more mature riverrixer course. Following ruethe1 emplacement of the Yatta Plateau phonolite,phonolite. the drainage systems}\'.lCT‘:‘l underwent profound modimodification.fication. Itit is suggested that the Keite~ThwakLKeite-Thwaki, obstructed h}by the phonolite.phonolite, was diverted southwards,southwards. followingfoliowing what mustnitlst origoriginallyinaill}; havehuh: been a narrow \alle)valley running in the direction of the Kiboko.Kihoko. Extending from the northern boundarylTOImsr} to the soutli»e;-.ster:isouth-eastern corner orof the area.area, the Yatta PlatcauPlateau contrasts sharpixsharply with the surrounding plains. ItIt extends forto: mansmany miles both to the north and sot.thsouth tilof the :.tieaarea, forming,forming a prominentIprominent landmarkland-mark for .ia distance of :‘spprtnimatehapproximately 170l‘t.‘ inilcsmiles, ttomfrom 01Ol DoinyoDoinxo S::ibSabukii\' in the Thika' hisa areatreat to the GalanaGuiana valleyvalle} east of Tsato.Tsavo. The plateau has .1a s..iLtta—eastcrl}ssouth-easterly slopeo ‘L otof twentwenty-sixti-stx feet to the mile between the northern boundar}boundary orof the ur-‘:1area and \zekakia.Nzekakia, alterafter which it falls at tippi'o\irratelyapproximately twelve feet to the mile I'Cir(Gregory,rego 1921,292]. p.{3. 187).l.‘ Into contrast with its cxeneven upper surface.surface, the itmarginssgins of the plateau are“re highhighlyl\ irregular and furrowed» trrowed by deeply incised valleys. Itslt~ width ranges from app:osim:-.teapproximately1' 5 miles to about 400.400 yards. Ignoring local exceptions the estimated thickness of the lawlava: capping is about 35 feet. ' Owing to its eelelongated,ligated. gently sloping torrn.form, GregoryGregori 21921.3.(1921, p. 186)186‘. ascribed the formationtormation ofot the plateau to a lava-flowlav..gnu» which had fol1ow~tollow-‘ed the course of a2 \alle\valley. He suggestedsugge.sted. that the lax-alava had protectednroteL ed the underlying gneiss fromtrom the erosion that had '\
5
lowered the contiguouscontéguoes rocks to the presentpresent surface.5111'1'11ce. In11'1 summingshunning up111'.- the111L: casetse for101' a11 valley1.:Lile3' lava-flow1:1‘s-1111w he stated,51:11eLl. "The"the locallocal irregularities11‘1'eg111a1'it1'e5 in gradient are,are. however,however. insignifi-insig1 cantL‘ . comparedc.1.1111'.‘-.1'ed to10 the persistence1ct'515tL-1‘1ce of01' itst5 downwarddon-1115:1111 slope530p: for1'o:' itsits lengthlength of 1801811 miles. This1"1115 fact[act combinedco111hinecl with its1L5 narrowness'.1' 1' ' indicates" mates that111111 the lava11'. '1 ofol' thethe YattaYLLttLL PlateauPlat'eutit‘lo'11-edflowed down a;1 river valley".\-';1‘1-:}"'. This'1 5 hypothesis11111111110115 was1.5.1.5 later.1316.“ supported511111.1L1rtcL1'L1}.1.lby J. J. Schoeman (1946,{1946111.p. 6). It11 fails,la 11:1 however, to satisfy51111511 severalSL‘l'al requirements.1eqr.iiien1c11t5. It1'1 seems.5 highly improbable111111o'tobehle that a'11 single:' gle lava-flowl'..'-.LL-1'io\-. couldeot.iLi continuecL111'11'11.L: to flow- L---.. for_'o.:' a;' distanceL5151211'L. ofL1. aboutout 170 miles.n-E‘1e.'. Moreover,Moi'eoV'ei. the thicknessthickne55 of thethe flowflow seldomseldLit1aexce-L'Li5exceeds 5051'] feettest andLind the'1'.'1L' possibility. 311111 of aLi lava-flow1. . flow of01' that dimensiondim-311511111 continuingL:oI]ti11L'.-i=.‘.'-1 for111." such5iic1'1 a distance111111;: seems5ee1115 yet1c: more111L11'e unlikely. In addition the internal propertiespropeitie5 ofL1. the phonolite. onul: must:11L251 be considered.CL11151L|e1'.3L'.. To enable the growth of0.: phenocrysts,phenocrysts. frequently1'1'eL111entlf; up1111 to‘111 a'11 size512': of111' two inchesine'nL'5 such511cl'. as115 occur in the YattaYetta phonolite,phonolite. thethe parentpéil'Bl’it magma111111511111 must11.11151 havehate undergoneundergone slow cooling ini1: a:1 reservoir beforebefore emplacement. The partly cooled lava1.11:1. supporting5.111111311'11111:1 numerous111;:11e1ou5' phenocrysts,pheno' 5'15. may,1113.31: in theLite writer's opinion,opinion. havehave beenbeen fairly13111}- viscous.\'iscoi1'.5. Examination12".111111111‘111111 ofat the phonolite in thinthin section revealsreveals a'11 fine-fine- grainedgiaihed groundmass,groundrntiss. indicating111d. ' 1 rapid cooling ofoi the lava3.1.1 subsequent51111151113111 to extrusion,againc.11‘t'115ion 11gL11in suggesting'i:1" that111111 it couldcool-1". notno: haveiiLixe flowedflowed far.5111.111In the eventL1-.e;1t of a'11 lava-flowlava—11111:. followingIoélnwiitg the course. ofa" 21 valley1111112111.:the lava1:11.: would invade1:1\.1Lic12e1the tributary1.: iT'U \_' valleys‘L '51e§.5 for a1'1 short511ml distance. Afit spur511Lir of phonolitephonoii 'Le doesL1oe5 correspondcL1:1c5pL111Li approximately11111. :11.i111a1e11 with" theL'1e Keite-ThwakiReitcil'lmaki valley,valley. but at:11 no other lateral5111131111 valley\11. ‘L‘ is15 there:11c1‘ce1;dcnceevidence of a.1 branch111111 ' of theEh: flow.Ilo'11-.1-'\lil1ot'Althoughe basal111151110111111L'tr-contacts of111 the lava13111 werewete examined at:11 twenty-nine1111-:e1tL»nine localitiesioeal‘fi. no3111. underlyingtinderiying valley\'.1l.e}' depositsdcpo5it5 were11'c1L' found,found. the sub-phonolitesnb—plwnolite floori‘luo: being1161212 invariablyi111 111131115 biotiteljuiist1te gneiss.1:1 e555. Since it appears improbable£111;':1..1h'.'1'11le that the Yatta1.1115 lava1:111: flowed down a valley an alternative explanation13\1':l'.1n:1tiii111 of the111:: mode ofL11 emplacementci11:l.1.L'L'-1:1ent is15 required. At several localities adjacent to the outcrop ofol‘ the phonolitephL1= the underlying' gneisses were found to be brecciated along".|L111.-_1 lines1:11:15 parallel'Ltllel with" their" strike, which is also parallel to the elongation of111 the phonolite.JJl1L111L1litL..-'\tAt some localities1c.1'.i11L'.5 asL:\ many111.1111. as five lines of brecciationbreeeiution wereHerc foundI'oiinai within half" a mile111i:e of the plateaup':3L":LLl edge.L'o' 3.. None\oi‘c of-... these planes of dislocationdislocetiolo containsL1 1111:1115 phonolitepltt1i1-1l11L 1y gt,c5tcL; [1:11; "‘-5 oi .1 51.1111c1-l1eit along the but it is suggested1': that they represent the marginal parts of a shatter-belt along the1 axis ofL11 which'1'.- F 11 phonolite1111011111311 was extruded . through a series ofL1: feedinglee-ding fissures£15511:'e5 concealedL011LL...LL1 beneath laterallELlCl'Ell flows.11111.15. It11 isi5 consideredco1t5iLie1'cLl that111.11 surface5L11'1'LLL'L' flow11:11.1 did not extendexend forlot a11 distancedistence greater than 5005111] yards1111115 in the area'111‘c'11 concerned,concerned. the1.19 flow11.111. being principally1.1.1'ii1cip.1‘.l}.-' in 11a south-51.11111 south-easterly5L1'Lttl‘1-e'LLstc:'i_\-' direction. Factors1511611115 difficultLtttliLLi: to1.1- accountLLL'L'L1I..1'1 for.11.:' in1:1 thetite hypothesis|1_\p-.1‘:|:e.515 of01 aL1 valley5 '!i . lava-flow,lata—flow. :15as detailed above.above" are adequately'11deL1'1111‘ely accounted for1'01 by113. the hypothesishf;pothe5i5 of111. an11.1 1 extrusionc.‘-.‘trt;5i11.:1 fromlJUEli a'11 fault-line111'..Il:—i11 :" fissure.1:55111e. If-: the suggested5'.. ' 1 original‘ .Latl drainagedt‘Etinjtg-e pattern (p.111.. 4)4.! is15' accepted,accepted. the presence111e5enee of the long,long. narrow,narrow phonoliteplioiiolite outcropotit'op can onlyeniy be explained€111.11: 11131: by't‘.-}‘ aL1 fault-line1'.ii1lt-line fissure11.55111'e erruption.critigtion. The'1 '.|L‘ lineline of111 emplacemente111plLLce11teiit of the phonolite would then correspondco1re5pond with the strikeStine of111' thethe country-rock,cc:1111:1:1-'—1'L1L‘1L. but liehe contraryL'Lint'i'nr'} to11.1. the trend:‘end of1.11 the drainage. Perhapsl’erhttps thetie onlyon!) exampleemmple ofL11 a.i valleywhen flowlion isi5 at:11 theshe spurs5111115 of01' phonolitephonolitc which'-.1'|:iL'i1 correspondL-nr;'::.1~1~11.ti with the suggestedLiege ed original(11113111111 coursec11111'5'e ofml the111-: Keite-ThwakiKcitI'imtiki river.I'IVL‘l'.
IV-SUMMARYi\'- "S! 1'1‘1‘1‘1191’1 OF{15" GEOLOGYGEULHG‘. AND.‘5.\t) GEOLOGICAL(.11): ()(lICAI. HISTORY OF THE'IHE AREAARE-t The area:11=.'.'1 is15 mainly1111111113; occupied by11} rocks:'LiL"r\5 of111 the[.16 Basement811.51.1116111 System,5151-3111 . thoughthough lavas1.11115 and:1;1.L' sediments5eLii111c11ts ofat much youngerLounger age are also111511 present.11"e5c11t. The approximate.ippi 051111.11L agesLtge5.L11"111eof the various1111111115. groups are as:15 follows:-[oliLm'5:—~ 4.4.. Recent-Soils,Recent—Soils. gravelsgiiivels alluvialallmial depositsde11115it5 etc.L-t'L'.
3.be Pleistocene-Basalt.P *‘ Basalt. 2. Miocene (?)-Kapitian('31—Kttpitian phonolite.phonolite 1. Archaean-BasementAt‘chaean—Basement System.
Basement81151611101111 System.-S'y.s£e'111,—'iThehe predominant rock1'11c'1x of115' the BasementBe5e1nent SystemS\'5[en1 is aii microcline-111ie1'oclitie— rich biotite gneiss of01' which the mineralminer i compositioncompositim. remainsrem-11115 constantL'r.111.=1t'.-111'1 over1111:: a.1 wide areaarea. TexturalTextiirttl differencesLiifl'erenL attributed1111;11:11‘1511 to111. aL1 greater|' or1‘11' lesserlL:15".;. degree(.L' " ofL11'111-1h1.1'z.1:1'onmobilization account.1L'eot1111 for1111' a series of rock types of similar mineralin . e1". composition ranging"1. Q1131 from110111 banded'1"- .Lled gneissesgneisse5 to homogeneous granite. East14.2151 of01' the YattaY'111:1 Plateau a'11 broadbran-Ll band of111' gneiss”11111551111111with a:1 notable content of lime-bearinglime-bearir minerals1:11'11e1‘r1'15 ctossestheei'L. the 11113.area. Muscovite-bearing.‘t'iLA'L‘tn'iii': wing rocks are fairly1:11-11. abundant,:1I111nLl-CLI‘1t. particularly in the western portion of111' the area. Associated.-'-\5.5L1L‘iated with the litt'ge5tlargest
., 6 body of rr1115-.‘L1\'Et':muscovite schist is a band13411111 of11!' pyrite-bearing[1:1'1'I‘LI—h-LL-11'i11I; muscovite1111151211113 quartzite.1.31- 111111111- Amphibolite occurs mainly1:1i11l\' as115'. aL1 large bodyl. _. with15".1i31 aI3. very\e1.' 613131111313elongated o111c1'outcrop in1I.1'1ethe proximity112-3311111111 131'of 1!1-L;the Athi River. Kapitian Phonolite.-'.—-T"‘.L‘The lava.11‘11-1 isii a' coarsely porphyritic type readily identifiedident' “:13: in13.1 the{1.11: field. Its position as'.1:5 a'.1 cap1:11;) on11:1 the Yatta Plateau makes it a conspicuous feature in the eastern part of. the area.'.1I'e'L1. No75.11211;underlying‘1. fossiliferousi'oL-L sediments were found that would have assisted. in.511 an.'111 exactmac: determinationLlct'LI1'11‘ 13111111111 of its age, but on analogy with similar lavas in.I‘. other0111.31 parts11. of'KLIr:Kenya it1'1 isi5. consideredL-L3. ' 1.'I to".13 be'13-- probably Miocene. Pleistocene.-AHemw I .-.-A basalt[3.1152111 lava flflow,ow. which originatedL1:' from a minor111i1111i' volcano\L1I1;"1'1L1 in13.1 the:11: adiaL‘entadjacent quarter-degreequdrter—{legr' 21: 5111.131sheet .11-.'.;area .1:1on 1.311:the west1113.51 (52 S.W.), enters theI" ’ pi'Cf-{CD'Lpresent 'L1‘L'L‘E'.area forEU." L31a shortshon distance. Recent.-ARL'L'L-1'1.I‘.-—A large111131: proportion131'1'311L1Ytioi3; of(”ii the area is blanketed‘ by a'.1 thickth'- L'lL' mantle11111111.16 of1)] residual11'. EIdLI'Ji or,11.". in1:1 some 1:351:51.cases, transported1:1:15p.31't-.IL‘I. soils..5131! 'll1'eThe typicalt soils are sandy and contain‘ little121111: organic matter.ma ttcr. At ti.a few localitiesloc11'1i1ie5' restricted developmentLie"310131116111 of black cotton soil has111-13 taken place. The sequence of131' eventseven-.5 ini:-1 the1111‘ geologicalgcn'iwgicm history ofI.I" the111:: area1."-:1 isi.5' summarized.51'.:11:.11'L1r1'zed in the1h: following table:- 1. Deposition of sediments of the Basement System"I.-.‘1':-1 in :1a 5:135'15.subsidingI guosynclno.geosyncline. 2. Basic intrusive phase. 3. Orogenesis, of which four phases can be recognized:izLILi : (a) Metamorphism. (b) Alkali metasomatism.31141115 "1. (c) Partial mobilization'.11'11:1L1'1.."'3..5'-.'-131'11'111gei1ti11lco111pi'e55ior1.during the main phase of tangential compression. (d) Injection of quartz and pegmatite:LlLliL‘. veins.\L’il'lri. 4. Vast period unrecorded in the rocks."ii5 of the[Ell-L” IEiI'CELarea. 5. Erosion culminating in the development of the end-Cretaceous ('1) peneplain. 6. Rejuvenation of drainage leading to the formation of the 5.1b-sub-Miocene.\li- 1' .L‘ peneplain.j.3.-c11LI: 7. Faulting with. the extrusion of the Kapitian phonolite, lL3.IiL;I\I1cdfollowed E'-_'1by i'eL-ii'i.rearrange- r1-1LI11:ment oIoffi Eli-L3the principal11111331.: drainage system. 8.33%. DevelopmentDe11:l1‘.1‘i-'11e'.1t of tilt:the er1ti—Ec:ii..i'_\'end-Tertiary peneplain. 3).9. Minor volcanic10‘1-.‘-.111:.L3 11311111}:activity. Basement81111811115111 .S'j.'.i'.FLr_’.-'i:System sedimentation.111.I"1.IL'1I:.'1LI.'11.111‘ Judging from'1'1'13113. 111ci1'their present[31135313111 13-.1i131L3r11mineral 01.1115'1i21111'1311constitution it is co:concluded1011111dcti that the original 5‘.LILj121‘.-L‘1115sediments of the BasementBus-3131mm"; S'x5'1cz11System i:1in 1i1i5this arc-.1area Lonsi5tedconsisted mainly of 51315111111111:felspathic 511111;!sand- stoncsstones 111111and 3111-1125.shales. The 5c111111c:11-.11"_1sedimentary nature'.11'L3 L>:'of 1'r1LIthe rocks 15'is cor11'1r‘11zeu'confirmed bvby the rounding L‘I"of 1111.151most 111'of 111::the z.zircon-1111 or5'5111i5crystals i11in t'.'1them.11. ZircomZircons 5:111:11separated 33.11in 31131111heavy 111:mineral'111:r1ILl L1311L‘e.11‘1-—concentra- tionsii-L'.I115 fromE':'L'.Ii1".I randomfund-3:111 samples511111p|$5 ofL‘-!' Basement13311531116111 rocksFUSE-{:'1 5'110showed:-.1L‘L'i:-— Per cent:e-‘L‘s Well-rounded'1-1'01111cied zircons2111:0115 . . . . 78TS Slightly.113} rounded"1I'Lii'11.l1:LI§ zircons/_i." .'L.I1. 5 9L,- UnroundedL ::1 11111111111 zircons/_i1'1311:15' ...... 13J} No‘5'13 definite1'11:1’i1:itLIn conclusioncor1L'Liz5iLm was"I5. reached:LI:;L'f'1I:L.' concerning31111.1.- the relationship of 1.'1LIthe 10112111133foliation 11‘to thetne originalL‘Ii'igi11alp.csplanes ofoi stratification.5.1111111111111011.
Basic30.11"; IntrusiveI’IL’J’H 5 '.1’ PhasePL E1\i' The occurrence of1.I.' metamorphosed{11:13: 91:39:11 basichasao intrusionsi:11:'1.=5'1:.1115' among:I:113.-L3.11g the1311: meta-sedimentary1111:111— sedimentar‘. rocks is apparently ' an.111 example2x21111131: of131' the131::often11:11 recordedrec-1.0.511:basic intrusive1111:115116 phase131311.533 whichwhiLh follow1'01'low deep burial and fracturing' ' of111" the lowermostlLt311L-1'111L1s'. strata in- a geosynclinal‘ trough.gymiwh Orogenesis1’).I'11I-_ c.3411 The'l"1c presence|1:'LI.I.LI11L'LI of111' kyanite1v I'L-111L1'(and of sillimanite".'"i‘ ' in the" adjoining"".I"1i11g 1113.151areas) and H11:the deidomi-fi- nancei11:i11.'c of rocks:'L'Jclx'5 that11115.1 have1111111 suffered5111'. 131.5 strong alkali' ' metasomatism over0'1'61" those1110.511: thatIhaL havel3-r. merely undergone isochemicalisocl1o111ical change151111193: indicates111111511115 that. the sediments. 1311111I1115 L111ciand basicb11513 intrusions11121115113115 were involvedii1\-L1l\LId inE11 a11 orogenicL11'1'1gc11iL‘ cycle. 'l'hcThe progressiveprogressive lossE053. 131'of [EIL‘the banded.5.-11111131." structure5111113511 e in the[It-u me1115;111111iiz1:11metasomatized rocks is believedbeiicxcd 111to beL31: due 111to partial1111111111 J11L'i'riil1211tionmobilization under11111131 dimmeddirected "
7
pressure_'_‘.lt3>SUt‘i: andell-Ed tott: indicate:tttttc.:te thatIll-J. the pressurePtESSL."-' reached its peak at a late stage in the process ofat metasomatism.:7'.:1a..~'on':ti5tt:. DuringDt.zt*t_; the finaltint-J. phasep't. of compression quartz and pegmatite veins were injected into the rocks.rocks. Post-ArchaeanPEN—Arr flat (”m HistoryHfst‘urjs There is no evidenceexidenee ofol' the geologicale ':e;l history of the area"SJ. for102' "..".c.the periodpelted betweenin". I. .',‘, __,:r',\.‘_.,.. thethe ArchaeanArehaean orogenesis030216116515 andend thethe upliftup 1. that ledled tom thetlic formationto: . “ ‘UIT of31 thethe end-Cretaceouse1_e.-Lt:.l_i»'..w_.\ peneplainpcneplaln (Shackelton,IShzxckelton. 1946,l9—‘etfi. p. 2).21. The period_ .loel ofGl tectonic quiescence inin which the:rte end-Cretaceousend—Cretaceous peneplainpcneplLLln was formed waswqts followediollnwecl byh): regional uplift,uplift. when1.x 1'.C"l rejuvenationt'c“n\ert;tti;t;'t ofot' thethe drainage systemHystcm caused rapid:‘zthid erosionewsion with the:he consequent destructionde (2)'_VJ Migmatites-\llgmatltex— tut(a) Banded gneisses; [M(b) BiotiteBlotite gl'lCE-WESIgneisses; (c) Granitoid gneisses; (d) Granitoid augen gneisses; '5'1Cil'anltt35.(e) Granites. (3t(3) Intrusives-lntt'LlstveR— (a) PlagioclaseE’legloeleue .4.:nnl'tih:tlitea:amphibolites; (b)th‘ Amphibolites;.‘tmphlholizes: (c)1: t Pyroxenites;Pyroxenites: in"?(d) Pegmatites.Pegmatltee. AmongAriana; the meta-sedimentsmeta—sediments the crystallineer“ limestones, 1:10the etu‘izizeequartzites and the muscovitentttscmite schistsschls‘ts Sl‘lOVkshow no signSign 01'of havingliL‘M't had their original bulkA compositionemt'msitiun changedcm: \. layby alkali 'netasnmuttsm.metasomatism, though the diet:diopside' and hornblende-bearing" —lm:_tt'lttg gneisses,gnelesea. which\xhteh contain 20211 toIn 51150 perpet cent of01' .tll;t~.'.lalkali felspar,leisj‘u“. may owe their present composition partly to‘. 1 the addition.:(1d':[5.021 of material.materiel. UnderL.’ the head migmatites are classed the remaining rocks atof ectll't'entery'sedimentary orgoriginn the bulkE“ composition_ . . of which appears to have been changed...‘.1'l_'._‘.' t by Ltlktlllalkali metammatismmetasomatism mtland tt't'espeemeirrespective of whether_ the 'host and the introducedmtmetteed com- ponent are megttscopieall}megascopically distinguishable or not.not, .. - l 8.91 .11(I) KimMETA-SEDIMENTS1—5L111x1nw1x '..1'I1’.1"11:-(a) Limestones. -A.51 I1:1.‘1‘0‘-.\‘narrow lens1131‘s of11 crystalline limestone extends from":11111 the['11: South$101.11.? Machakos‘LE‘..-L‘?1:1kos 1.1115:districtL‘: into.1110 the north-western11:1 .. . corner1-‘nL‘: -ofL". the11‘:..‘.“’c‘-area close to the Tawa-Meu :‘01111.road. Other 1“.associateddated bands apparently11111111611113. terminate1L‘1'1111nL1'Ic hCE‘LHCbefore reachingI‘L1‘..C1‘.'. the boundary. Micro- scope913L111: examination2x11111111. 11171 of01" thin sections of01' a:1 specimen (52/404)452 411-11 revealed:cvc.:. a compact, medium- grainedgrained. marble211111111: with a1.. small5111.111 percentagepet‘s-3 111: 30 ofot' iron1. ore111: and.‘1111‘ muscovite as accessory L‘nns‘tiumnts.constituents. This limestone,11:11c5‘1011c. 11101113.along 11111with othersolhcx in1:1 the13-16 adjoining area, was sampled and analysed by 1h:the 11291553111111.1111:representative 0‘1of Lia (701:1:Company‘11. 1T1ICIC\[:;_iinterested in:1 the.1 possibility11115.51. of cement manufacture. ThoughJihOnh the results L11of [hethe 111.11}.analysis 11‘;haveE.’ 1111‘not been5-3::1 :1111L‘L‘made 111.11available it is understood that 1111‘:the limestone 2011111111;contains 100too great .1a percentagepercentaLrL 01of 11133111511magnesia 1'111for 1.1::use 111in the 11111111;..1c1111‘-cmanufacture 111'of PortlandP.11‘El1111-d L‘cmcnt.cement. An-‘\11 E11111;intensive >SLI'Cl‘11JNL’Lsearch failed to. reveal any LL111'151continuation1'111-321 in 111.:the area‘- of the 1.1-3well-defined'. 11:11belt 111'of :11c11111111rph1Lmetamorphic rocks.rocks, 111L‘l1.1‘including crystalline1;1lli11c 11111951311112:limestones :111L‘:and kyanite..1111: schists‘L‘l11:1< which L.pp]‘03€h6§approaches the norm—111191.111north-western L‘L.corner from 111-:the west. (b) Quartzites.-Thin1.7111111 bands51171.5 111of L1121:1211L1quartzite occur at Kilala and Kitonda‘rx;101‘.L|.1 L111.on the west-3111western $11.11:side L15of the [111311.area. More reastuntresistant {.1to weathering than the surrounding biotite gnusa.gneiss, 111Cthe L111111:.".1cquartzite 11:1.»has protected11-1: the' underlying rocks forming Kilala hill. The extensionL1\1211‘.51'1111 of1.15 thems band5.1.111 south-west<.11:.r.-\1=:LI of Kilala is obscured by the alluvials of the Keite River.Rher. - Microscope.‘~11L1‘1‘19LL1 examination1111111111011. of a slide (52/374) revealed that the rock has a compact, equigranular4:111:1111111111111 texture,L. and3:111 cnconsists mainly of quartz but with appreciable amounts of pyrite,[131113. 1.311111:kyanite and.11 DTLJS'L‘U‘LHE‘.muscovite. 'lThe pyrite occurs as small to medium-sized pyritohedra. 11 “1111:While i‘.1in 111Cthe hhl}Ll~H"L’CllTlCllhand-specimen 111.:the rock| 0.1. h. 1111pLappears:11‘\ 11017511non-directional, under 1111‘-the microscopemicrnac. it is seen that the muscoviteI‘ . .L‘L1131c andL: kyanite crystals are orientated in parallel planes. The approximate..1;11‘.11e mineral11111191111 0.111191%.composition* is as follows:- Per cent Quartz.Quartz ...... 76 MuscoancMuscovite 1111:."-and kyanite«5.111110 ...... 17 PyriteP9113 ...... 7 West\\'L1.~.1 01'of the area,{111311. elevenEleven REITiplL’xsamples from.‘ 11111 the1h: continuationL‘1‘1111in11:.1iu11 111'of the:E1c 1.31111‘12116quartzite band 11cmwere taken‘11:).by B.11H.B:kc1H. Baker for:'-.1r gold1:11.11 assay. The' highest yield obtained was‘x 1.06111.11 111.1%.dwts. 3131per 1.1.1ton, while110 four Ofof the1:1c 1L151 "'-~-~ ~ 9 €11(i) Diopside-bearingDir');i.\s'de— EL-{H'F' gneiss.-A dark blue“he rock,rock. cumcontaininglining \is.b1cvisible garnets, constitutes the:hc bulkb ofUr the“SIC band in its southern"n portion,poriim. specimen 52/427:2I 5‘ beinglficing representative.1- Thin sections5-3;. 11$ reveal“STEM a'. compact, equigranulari]Ll"'." texture with a certain .":'I!‘)11.:‘.1amount of fine interstitial material. TheI'hc estimatedC‘LhflJTSLi compositionC-‘IH' -‘ is as follows:- Per cent Quartz~~ ...... , . . . . 25-~ Ol‘thociaseOrthoclase ...... 21 PlagioclasePki'iliOCIil‘ . 1318 Diopsidenil-(111815: , . 17! GamerGarnet 7. 7. .. . 15ifi Magnetite,\Ll ’ is. :!mcni1cilmenite and :151._1‘:i['capatite . 4:4 ExtremeEurcmc strainStmin effects,ufieax'. obviousnbvu' ' in most""SI of01‘ theific constituent minerals are attributed to [hethe factJ‘au that [Eu-ethe \pcspecimen was takenu :1 from 1a band of rock. occurringdo in closerims; proximity 10to a:a muzone of brecciation. The quartzu. Hurt: grains are rounded,kl. due(1L: to[0 the5h: rollingmliin; thatIlla! accompanied <11shearing,, which also resulted in the production'i'_‘[10i‘. ni'of interstitial materialmEICE'iJI composedcompose-6 of qllLli'TZquartz :r.and felspar surrounding, . ., the larger grains."22:2. Thel'hc orthoclase.Ji'i'm-:l.:se isiS‘largely' alteredriltcred [-0.-to scricilcsericite, the replacementrcpmccnlcrl' havinghair; developed along7 Cl'CLHL:cleavage planes,planca and albite twinning:winning in thcthe plagioclase111.1;1'Lm'nic hasin beenI‘: .|,I|:'almost completely(UP-m}: 3| 1-. obliterated.n-I-lii-cndcd, Th:The camlpmjt'compositionfl,‘ HFof the pagioclase'n -i’)c]:13u was £‘SJIYK1‘1?L]Iestimated to1n lie‘Ic betweenher-ween An20 and An30 I'(oligoclase).oligochaw .1 DiopsideDiopai-Jc occursuugm : as irregular clustersChlalfi‘lS u;of palemi; gi'ccngreen grainxgrains. The game:garnet occuzwoccurs ini": trapezohedra1i ::pCZL)E".c';1 and is larger than in";any other:-l}‘;:-,I ofA.“ theMic constituents,:- varying betweenbaiwecu 2 mm. and 4 mm. Inin L1.L.[‘.!-.‘!L’i,diameter. UnwralOptical .Ju‘iand c.chemical tests indicate.-__1':1h::1that it is1.x grossulariticWms‘uhu'i‘. I" though'houh ”'1'the presence_.II'3\'-:i_IEr: .3‘.of 1.071iron and»suggests2w that it contains a proportion.0“ {21'of {hrthe 1.1-1]andradite.IL} {3 m‘or niialmandinenut-din; molecule.'::::.:C.I|\.. i'EEI(ii) Hornblende-bearingHr-rh'r'lhlfu'g-J 1:43“;- gneiss.-In11. theIhL' \Icmii;vicinity of Umuk [11.2 Hill,Hn'i, :lacthe gnu»gneiss isix ch;charac- terizedlcliZi‘LJ byh--. an:-,n abundanceum. inc: of hornblende,§]-.,'-l'i]:5‘-]C]1L:I<. g-andu" a.a totalE.‘-[L'.! .1550cabsence of diopside.(Lap-{dc A.-\ punicu.particularly, resistant ridge .:iof thislhix rock"Ld ;"~.r::,:,| (2):2: MIGMATITES_\iiL3\I.-'\!'!'IL'K. Various writers, for example Holmes (1919) in Mozambique, and Shackleton'ECEU'W (1946)[I'M-i1:n1 and Schoeman (1948) in Kenya, have described gradational series representing increasing degrees of granitization among the migmatites. In the present area such a series:5 hash.. again been recognized, and for descriptive purposes, is divided into the following. JG stages-S'K'IL“ (a) banded gneisses; (b) biotite gneisses; (c) granitoid gneisses; (d) granites. All the granites in the area are included with the migmatites as the writer considers that they_"r}1"'3are all‘2'. of metasomatic_ origin' 1 andam. developeducxchpcxl in situ. Transitions from one rock type Into maths-ranother in the fiseriescrics can be most-'T.0‘~Ii clearly observed in the south-western corner ofaftthe3 area. 1010 (a) Banded Gneisses.-Banded schists and gneisses, with alternating11.111111: bands[111111.15- rich:'1L‘J1 in11‘. felsiciec and mafic minerals, constitute a fairly-1'l;' small proportion._ - -.of the=‘11111L'rocks of the 111211.area. They are 11:55less resistant125151 :1111 111to 31115.erosion011 1112111than ‘1hcthe <11he1other gneisses211315865 .1111;-and 111311their 11111131211115:outcrops are frequently followed by valleys.valleya. O'-'1»\-11gOwing to the heterogeneous1.211115 nature1111111”: 111'of therhe banded- gne155c5,gneisses, weathering is seie-selective. The mafic bands are “cathcmdweathered 1'11at .111an earl}early 5121ge.stage, 13.313211while the1116 felsic'cisic bands"'11115' remain relatively unaltered,l1,111111e1'ed. producingpruduc' 11a ribbed 11:11-11.effect. The individual1I]{ll\-'lLlll bands" vary from less than an inch to several feet in [hi-ehthickness.:55. 3111111."Minor foldingI'1'1l1iii1g 1:5:is 11|"L‘11often See-:1seen and frequently felspathic augen have developed 111111111within 1111:the 111<10"‘1e11distorted 111:1i'1emafic 'bands. The most abundant. ferromagnesian constituent is always11311-1135 111111116.biotite, though1110111411 where concentrationsconcentratinm 111'of e11.Lcalcium1:11;- have been sufficient, hornblende 11'111and 111L11151dediopside h;1‘»ehave also den.developed.Ioti. SpechnenSpecimen 5252/331,331. 111ke11taken 1111111from L1a 11111|15mafic band11.11111 31151east 111of ‘5111111.Nzaui, "1115has [hethe following10110115 1ag1p1'L1x1111111eapproximate c1111511t1111011:vconstitution:- PerPm- rcent6.111 QuaQuartzr1! . . . 7 3f,26 FelsparFelipar 11111111115(mainly 11"1orthoclase)|1nel;1\.L- :- . 37A? BiotiteBiohte . . . . . 28 Diopside{Jiopside 5.5 Sphene . . 2tIf. Magnetite,.\l11g11e11§e. 11131111113.apatite, (111L111zircon Itll SperimenaSpecimens taken from theihL Matiliko\11: 1. 1-311 .11'L‘1;area cL111‘111-1:contain .111an 11pp1'cL'iL1biL’appreciable proportion11111110111011 111of 111.1111.—horn- blendeblemle and111111 11.11:have 1111c1'111elinemicro cline .15as [hethe L'lnmimani'dominant felspar.1.11. No5511 {hopsidediopside 111'or 51111L‘11esphene was,11.15. however,however. 10111111found in them. In111 central.contrast, the I'elsn:felsic bands113111.15 seldom 01111111111contain more than 10I11 perper cent 01'of 1111111112,biotite. Speci111e11Specimen 52-33152/323, from110111 the 301111south-westernwestern 00:11::corner 11l'1‘11eof the' area isi5 .1a 1111;1‘13L'l1ne71‘1chmicrocline-rich type with the following mode:-modui Per cent(an! Quartz 21 Orthoclase 2.727 MicroclineN'Iiumcllne 38 Plagioclase.Plagiaclase . 5 Biotite3101.110 . . . 3 Chlorite,Chiarhe. spheneSphene and11111.1 muscovite111 115cm. ite 32 CalciteCalciie 4 Analyses.~\11111:.5'e5 11?of 511111111.“similar rock‘srocks 311wgave much11:. the same results5 except for thethe presence 01'of calcite, Whichwhich 11:15was 11111not recorde?recorded 111in :1=1j1any other ipecjmenspecimen 1:1kc11taken from' that locality.lUCZililk. BothHmh calcite'1:.‘3.1: 111111and el1lo1'11echlorite appear :11to be alteration:112‘ 11i1111 pproducts. In slide 52! 323. 1.111111both prochloritepmehlorilL 11:11.1and 1w111111111epenninite were identifiedidentified and in several thin sections examined, flakes111-135 111'of biotitehniilu 1111:-werere found|1 1111111to be partly"1‘13 :1llL‘1'CLaltered to chlorite. In111 the1111: vicinity\='L"1*11';. 01"of Meu,Men. the composition of the banded gneisses-»L‘5 tends.111'.115 to111 be”e morerim-re basic.“1153:. The felsic‘i'efkiie bandsI11 15 :11‘care narrow and, in comparison with the11: banded11:111L1:1| gneissesgneiswcs elseelse- where,where. com-1111':contain '1a greaterenter proportion of plagioclase felspar. In the (11111dark bandshands horn-11.1111— blendeblend: representsrepresezns .111an :1pprcL-1'1-hlzappreciable proportion of the mafic minerals,5. while qucirtzquartz seldom5e111o11‘ accountsaccoums forl'-:11' meremore than."1.111 10111 perre: centL‘e- of the total composition. The writer considers that61:11 the banding of these rocks represents-L1L:115' L1:original113111111 1111:1'51111111alteration 111'of arenaeeLr'arenaceous and argillaceous layers which have been emphasized by ll1e:--1?1'11'Ll.1c111111the introduction 11!of :1IL'.alkali'1 felsic13:1 1c material1‘1111t-L‘ along the psammitic bands. As no chilled zones in the felsic materialmaterLd 01'or “baked""baked" contacts 1'11in thethL mafic111.11; material have been observed, it is15 £18.11."clear that(11:. 1“, the' introduced' ."-dL.'L'-.. felsic constituentsL .1..l had1.111 .1a 1temperature similar to that of 1131111112.]invaded E'Lrock.t:-'._ A mixing of the two contrasting layers took place in varied degrees, the more1111110 advanced:11 11101.3 stages of mixing leading to members higher in the granitization sequence. ExaminationE31L'1'111111-11111 of slides prepared from specimens taken within mafic bands at increasing 1'11}:distances.cli. \JL‘h away from contacts with felsic bands show that "mixing"mixing" decreases towards‘ds their[huh centres. During the early stages of metasomatism,‘C'liibor'l‘nKTLHJ‘J. migration1111;41'1111'111‘. wasas conconfinedfined to111 thealte felsic1}?" coustiluehts.constituents. ~-- ~= 11 1'1'1181'111‘1‘11:(b) Biotite (1'.-'11)"Grwisses.-By. ..——B}- far151' the"he: 1.1101112:greater portion_1L of the '.1.1'L".earea 1xis <1L"011111:1.1occupied byL‘I‘ 1111:1111—homo- geneous,1361120115, leucocratic,11111000111110. biotite11101111: guesses.gneisses. '1'113'11'Their 111121.0"11mineral (2011.110911111'05composition varies to'10 (-1a certain06:1:1111 0.10111extent :30“according11111111 1010:0111};to locality, 1110though a uniform11111101111 Ldistribution1011 01'of biotite' is '.1a 0035101111.constant feature10111070 1111111111011.throughout. The biotite gneisses""5 are believedhe.11:1511 1-1to have11::\-c been" - derived1":L1 from110:1: 1‘19.the banded. L "1101gneisses and- schists which, undergoing more advanced mixing of the ingredients developed0011::'0pcd :1a 511111101simpler structure with even distribution of the dark minerals. Evidence of L1ca'1\'.'11'10..derivation from5'1'1 banded13111-110'11' gnugneisses. can be seen in the. "141103."ghost-structures". of banding-_ within masses 01'of biotite111011‘1L-J gneiss.011619). 1Furthermore,'.1111101'1'1101'6. L-contacts 1.10111'013'11between 111athe =:'..1'1two rock-types10011—131103 11::are invariably'111':17'1ab1\_' gradational,1.11.11.1.:1110".1.1 though'-, _Ic_‘: the111: 1:01contacts‘ are \311'11111seldom L'.‘confined“1.311 to121'! particular ' horizons;1101' rather, the occurrences '01"of 150-1banded' 3- 1.1.101“gneiss 011110;“appear ‘10to "110"float" within the masses of biotite 3116155.gneiss. BiotiteR 1'11: gneisses13111318385 of1111' three11111.10 types1111:: occur in the area, each being characterized.'11:.‘-'.1L :i'iZCL'I' by'11:. the1110 predominance111'0L11'1111111'1'111'L'0 of01" a.'1 different11111610111 felspar.._ 51.. Some are rich in1:1 microcline,11110100110. indeed,1:111:011. the' larger' proportion11111130111011 of()1 1110the biotite1.1101110 1311018qgneisses in the area are composed'1 I i _i‘1 l."‘.L L1 essentially68-1. '.'.111'1'1 1);"of microcline,1'1.'-1L‘I'L‘.3"11:. quartz1111:1111 and biotite.11101116. In111 the['11: more111010 granitized_ 11"1' . gneisses the amount=..111.1=.:11[ 111'of microcline"111.1. 1: 1110 15'is :11111'apparently'1'K. 1111‘ proportional111'01101'11011211 10to 1311:the loss1-: 111'of foliation.1111 Being.3 fairlyI':1:I'- resistant- :1111 to11.1 weathering,'11:.111-9 :11111. microcline11110101.;1111‘ constitutes1.011511111103 an2111 appreciable11111..-9160111111 0 proportion{1111110111011 of01' the1111: lightlight detrital(161-1111 minerals1111110131 of1100:11' local river1'11'L'1' sands.$111115 In11 specimen 5"52/326,3211 from"111.111 Dinaa1‘" - hill,' " the microcline:1 occurs as'15 largeI grains, partly11.1111}. or111' wholly11511.11" _ replacing orthoclase1:111:21: and plagioclase.\s, By_ contrast,L'-.11111= specimen1‘3L'i1:1L-11 52/324 1110:1'(from 501111141101south-west of0f1\'zz11'-.11.Nzaui), in111 which perthitic intergrowth:11lc1'51'111'111 is13 frequent,521-111-110 . shows.1-.\\ replacement1':}?1111312‘1161’11 U1of microcline111101001111: 11::by plagioclase.}11:g10L‘1:1~3L'. In1'11 addition'..-:11: to 1110the L'011x'11111ez1lsconstituents 0110:1111.:1‘H1111=111L11already mentioned the11131003grocks 0011111111contain 011:1:‘1/quartz .11111and 51111511113115?subsidiary :111‘11'1-1amounts of01' 311111111.garnet, 51111-3110sphene, Z11Ci"1zircon and.11I1'1'::apatite.. WhileW111];- not1101 as:15 abundant0111111111111 :1\as 1110the rnicrocline-rich11101001111041 types, orthoclase-richL'.'1\L'—"1L'1'1 biotite111111116 gneissesg11C1SSC> account1100011111 for1111' an2111 appreciable{1110100111110110111-13portion of111' the111-: total101111 biotite1130 gneisses of01' the1310 :'=."c:=..area. In111. the vicinity151111113. of KibaoneK111110111: the1110 felsparfelxpu in111 them11111111 is25 almost.111111'w1 entirely12111110} orthoclase,-' - >6. with1&- 111 only11111} occasional1.1L'L‘1151011111 grains5.1111115 of01' plagioclase11131111101050 and 111130011110.microcline. Biotite8101110 gneisses011015505 111-1111with 111:1plagioclase1:100 asx 1110the 0111:1'chief felspar.'cixp'n' 1111'are 311111.11110111}sporadically developed.developed; for.111 01121110]:example, 1'11in 51100131111specimen 52/367,312536.117. from11:1 the11:0 south-eastern~1.11‘1"1v-"=Rfc:11 01111111corner 111'of 13'":the :1"L".1_area, plagioclase11111511001350 1.5111(An,,) 1 comprises1.1111111 1:10.. theIhL bulk 111of 111L"the felspar.'1'ee1 1' J- F while occasionally apatite accounts for up to 5 per. ‘" centL'C'T of(11' the:1‘.L‘ subsidiary‘.. minerals.111111_'1':1\ (c) Granitoid Gneisses.-AnA11. advance1011012 in:11 the degree of granitization":1ti/..-5111111 already:J.‘ ".‘L111'. described leads to the development1':-.11 of111" granitoid1.111111? gneisses from:1'0111 the11101111111:more primitive131111111110 types.11113.. The essential structural differences between biotite 51110155gneiss and111111 granitoid1.1' 111111111 gneiss11110155 is1. Tab-.-the disappearance of foliation in the latter. A general orientation0211:1101 of the micas persists.perish. however, yielding a slight gneissic effect. Due to the more intense' introduction of' felsic1915'. material, the percentage of ferromagnesian constituents in1'1. granitoid0|. gneissSS. is15 often(11.011 less'.-. than that in a normal biotite111111110 gneiss.411L133 Having11.11.19 ..c.developed from11111 biotite11101110 g1:gneisses565 the1111' g1'.111'.'10"11granitoid gneisses contain, however,€\"L"I'. the1111: same characteristic01111121010151..- minerals.11111L-.:11.~3 in111 appru-ianutclyapproximately the same5121111: 12 proportions. In 52/391,52 UL au. granitoidfluid gneiss‘ncmx from{:CH" T31;the north-western"wivrlb-axis'mc'vn L“:?1\_‘1'corner ofL" 15.:the Arm.area, the proportionspmpm'iions of diopside,diopside‘ epidote,cpldotia andLind sphene(phi’ni‘ coincide.\ with thosec of adjoining‘ rung biotitei‘u‘im gneiss:‘g'm (52/590).f": 5: ‘1. . L1,.Of the.itc felspars, microcline is generally, but not always,~ more‘ ‘ ‘ abundant‘ than ortho- clase,iw. while“‘t plagioclase is frequently absent. In only onenu instanceAmt .V was~ plagioclase found to\ 31‘.-be theih: predominant fespar. In specimen 52/384, from\ the Ikua-Kitonda. . hills, no microcline‘ (Cline was recognized, plagioclase and orthoclase~ beingL‘CZ‘ g present\x, ‘ in the approximate ratiomin }3: 1—2. The content of quartz is uniform, the average_V beingx ‘g about‘ 13 per cent. (d) Granitoid Augen-gneisses.-East‘x. l of.y the‘7 Yatta‘ Plateau‘ a7 broadfix ‘ band of augen- gneiss is traceable from the northern boundary in a south-south-easterly direction for a distance of about twenty-five miles‘ ‘ to a‘pn‘utpoint where,n V » owingk H13 to deep soil overburden, surface geology- V is completely obscured.V _ No\ ‘ kcontact'11 ;¢ betweenW mcl this\ rock type and the adjoining.;~.1jm:m.1:g granitoidgmnimid gneissgnaw wasH observed,M. mmand itsm positiony‘mum inm the111: granitization9:5 sequence isI~I“‘1;l,‘m'not clear. Megascopically', , .‘ the gneiss has a coarse augen:\ structure,~ ~ consisting., of eyes of felspar lying5.: ‘_.; ini‘ a(fifine-grainedt): mottled groundmass.a“, Frequently‘ - 7 a rim‘ of minute flakes of biotite surrounds\L :m'mndu theQM: large: felspar eyes. Microscope“1w g m k examination~. 1 V of thinx" section reveals a predominancepredommamcc ofot‘ perthite, microcline-perthites—pc‘fl‘m beingMy]; more abundantLb than‘ 1. orthoclase-“L perthite..x: 5 There111:: q A.is, however, apparently1:. no‘70 segregationxx: x: of the‘ different‘ typeswg~ of‘ felspar. Replacement-:\l;;c::ncnt ofA the p~rthiteH has been effected by‘ worthoclase‘ and, microcline,12g «11):. both of whichwinch havehznc beenmgr: themselveskm 1 who slightly replaced by nmyrmekite.5 ‘ u 2- WhereK‘. ’m' perthiteF‘kHL‘M‘JJ doesLiam not constitute. ‘l 11L: the2.1;: bulk“: ‘71,:of the felspar microcline accountst‘ for‘ up1:;‘1to 440 per,‘ “'1cent of thej: total‘1 mineralmum .xi composition,C'JE‘LEEN‘uiJ‘i'. and plagioclase is frequently".11: absent,ubiéfm or present"#53:; mayonly in2". minute'1‘ ": quantities.rams. Replacementcflgcr» by quartz“11 hasiv» led12:: toI») a‘. patchymu", appearanceappcmmcc in nus:most cgaagdxcrystals ofn1 felsparn-Mm in spemncnspecimen 52/411. Dark mineralsmil:;.1:i« in the rock are. k biotiteP a w ‘ andM hornblende1 W‘M “‘Iu and,M1. lessL,“ abundantly,A? spheneinh. w andav ironr w, ore.V‘s, Apatite\‘M k is~ also« accessory.”Mvm (e) Granites.- The ultimate‘H{J““3\stage of granitizationg“? ‘A‘ is the transformation': ~"\L;I w ofm hetero-“Lg! geneous psammiticmmm and‘ heliticHCMTM rocks, into homogeneousw'y'p , ,~ non-intrusivewtmzm 1‘ 'u; granites.:i‘AHHLN TheHu- chiefc'm-ci structuralI.[" :i differencemirr'e‘“ between such granitesy 1; and\ the relatedf granitoid-gneissesL: 1‘ _~ ,‘D isp the' lossIm- of orientationm'icngit. _ of the., g micas.» » Contacts‘ ,1» between" ,- igw the two types1,»- are1g invariably. gradational.5:»Ld.\1:mm1, The{'h: possibilityfinsszbilg of' the" existence ofn: intrusivemtmmw granite5111mm in1H them: area must, however, be considered.ww’x'dcrcd"The most likely example is'9 undoubtedlyL,‘ 1b: theTi- granite3 forming the tor at Campi- ya-Mawe,,-\I,. justR1 off the Makueni-Settlement MakinduK road. Due_. to the surrounding residual mantle:| (1‘: v: ofvi soil, which'1 extendsCucmh to' aL. considerable,nlgumhg depth,gum u the[A structuralx: :LZJ‘Z relationship(4.. 0.1%“) a:of this granite:Mume and:md the surroundingsur'ommimg granitoidgnumoid. gneissgflclsi isi5 totallyInLM} obscured.wnmrcd‘ ItI‘ is‘5 considered,mnsiduc.,.. how-Inna, , , .7 ever,up that{H11 Campi-ya-Mawe1 .m1} .Lr‘ULHKL‘ isi~ a‘ continuationgurmm ‘ {on of‘ a ridge of non-intrusive7 *. granite further south-west.n \ITh-W’cfl, The rockH . (52/350)“W has a medium*'\7:mw,1111 :nto finciwtmie‘fine-grained tcxtuz'ctexture anduni nis gcomposed:mrosm m‘of my]:equi- dimensionalwmeusionué grains,gush ,. giving131115;: 1h:the impression.mr‘r’csunn HM!that itN has11.15 been50311 deriveddam “J fromMu ag; psammite.mmrw‘dic. Asx» iti: contains3““1111: an overwhelming preponderance of felsic3:51; mineralsHM‘TK‘Mm the’H‘ rock"»\ may'7 be“C "31'?"termed an1‘ aplo-granite.,‘i‘1‘.1»_:[.1 In thin section it is estimated that the mineral constitution is:- Per cent Quartz.Quartz ...... 21 Orthoclase.Orthodase ...... 28 Microcline.‘Jicrocline ...... 44 Myrmekite\151’mckitc ., . . . . . 4 Biotite,33mm. muscovitemuscmxtc and.mu magnetitewmgnelitu 3>1 SimilarSitiihu rockrod-i occursnqcun due east,Lust. at.xt Mathemba,KILLt‘r‘m‘q though410:9.1 it:. containscinn‘l‘h a greater percentagepctulifi‘jgc of mafic minerals.mincu'y. North-westNot-wcrl ofm Kibaoneixfh‘.::‘nq aL‘ body of granite extends2mm“: to,m. and includes,,nc.u;.:>. the1E1: hillHIE ofL11 Ndutu.\dutu. It{1 isi\ more basicb.1515 thanIhm theI'm Campi-ya-Mawek. type.we. Microcline\Em‘ .‘ r anduni othoclaseathoclnsc are:c subordinate\L:bx’\:‘di11.1ie to.0 plagioclasep1; (An2o)' and hornblende, diopside and sphene account for approximately..;)p;x,\\1m¢:c1; 28 perp»; cent of the total mineral composition. The contact between this granite and the[he surroundingaurmmndmu gneissesgncissex is1~ gradational.yumuticngl. i" .... 13 13.1(3) INTRUSIVEShstaustvea (a)11121Arrrplribolirvr.Amphibolites.-Corresponding—Cor'1'e:~.pondln~__1 approximatelyapproxirnatelv with the course of the Athi River a largelarge body of amphibolite,asnphibolile. with a:1 maximum width of approximately half a mile,mile. extends across the area for nearlynearh 40411 miles,miles. lying parallel with the strike of the gneissesgnelsses on either side. SeveralSexeral smaller bodies with similar alignment occur nearby in the northern part of the area,area. The discovery 01'of relic doleritic textures in the largest body of amphiboliteamphiholite at the crossing of the Machakos—KituiMachakos-Kitui road over the Athi.-'\thi River provesproxes the magmatic originor in ofor the largelarge body.bod}. Itsits continuation further north was described by SchoemanSchoentan [lLJ—iv-S.(1948, p. 31)."1l'I daas a11 meta-dolerite111e11a—Lloler'1te sill because of the lucklack of bandingbandingI and owingOWing to the sharp contactscontaos that it displaysdisplats with the flanking gneisses. J OwingOuing to their more:1'ro:e felsictelsic composition the smallers 1111ller bodiesbodies are described as plagio-plagio» clase amphibolites.arnphlbolites, The differencedilTerence betweenbetw en the large"ge and the smallersinalerl bodies may be attributed either to differentiationdill'erentiation within a11 common magmatic source or to more extensivee\te.'1si\'e metasomatismmetasonratisn‘. of the smaller bodies due to their proportionatelypropor'tionatelv greater marginaln‘ ginal area. JudgingJudeing fromtrout the thin sections preparedprepared from specimens taken at various localitieslocalities along the largehuge occurrenceo'LL r'.r e'nee ofol amphibolite11111pl1iholite its mineral composition is,is. as a whole,~xhole. fairlylairlt uniform.111niform At contactscont:1Lts betweenbetween the gneissicenei ie country-rockcounti't rock and the intrusiveint 11si\e amphibolite partialpartral or completecompl-Lte digestiondz: 'stion ofol acid rock has,has. however,hon-etch caused a departure fromlrom thethe normal composition.compositro-i Tongues ofor" the intrusiveintrusive rock frequentlyirequentlt contain an abnormallyabnorinalh high percentagepe reentaee of(11 felsictel sic minerals.111incrals. , InIn the northernmost exposure ofol‘ amphibolite,amphibolite. due to incomplete alteration,alteration. originaloriginal. doleriteLlolerite texturete\ture isis partlypar'tlt retained. Elsewhere the rocks are schistose and medium-n1edlttn1~ to coarse-grainedcoarse—grained with occasional segregations of coarse-grainedcoarsevgrained hornblendehornblende developing withinnithin fine-grainedfine—grained portions.portions, A bandedbane effecte‘lect is occasionallyrice-.zsiL1nallv caused byl‘l- the presence of hydrothermallyhydrothermal} deposited quartz veins.verns. The amphibolite differsLlitl‘ers from the plagioclase-amphibolitesplaL'ioelase»an‘rphibolites mainly with regard to the proportion ofol‘ plagioclase (An3o)Min _1 present,present~ which seldomseldom. accounts forfor more than 15l5 perper cent of the total composition. While normally predominant,predominant. hornblende in specimen 52/3715221?] isis slightlys‘igbtlt subsidiarysubsidiarj~ toto. diopside. Epidote and clinozoisiteclinozo site are constant accessories,:-.cee ssor'ies. occurringoec111'rit._.1 inin the approximate11ppr'1.1.\ini.1te proportion ofoi’ 4 to 1.l, A.-'\ limitedlimited amount of quartzL1111ut/ is present and is of relativelyrelatieli evenexen distribution.distribution, The plagioclaseplagfoclase amphibolites:unphibolttes are representedreoresented by'9} two specimens. NumbeJ:52/401,Number 525401 taken from a thin lens on the western bankbanlx ofol the Athi-"\thi RiverRoer in the northern section of the'the area,area. isis, notably flaggyHuge} inin structure.structure, TheThe plagioclase,plmros. which has31:13 an approximate composition An:,.,A11 . makes[111s up about111111115 404-H per cent ofof. the rock,l‘OL‘h. the remainingremaining1 minerals being blue hornblende,hornblende. epidote and sphene. From the Thwaki-AthiThw;1ki—.-’\thi intersectionintersection a-1 narrownarron outcrop ofoi fine-grainedtine—grained plagioclasepiagioclase amphibolite extends forto." a:1 short distance along the AthiAthi inin a northerlynorth-cl) direction. Dark green hornblendehornblende constitutes about half of the totalto:al mineral composition,composition. the remainingremaining minerals in order of abundance being:beine: diopside,diopsfde. epidote,eoidote. oligoclase,oligoclase. calcite,calcite. sphene and zircon. The plagioclaseplacioc e is partlypartlv replacedreplaced by scapoliteseapolite which in specimen 52/3855.25385 is a1'. strongly birefringent (N1N =-- .035).035) type,type. and isis probably thethe calcium seas.apolite.polite, meionite.metomte. GametGarnet occurs in111 the plagioclaseplagioclrase amphibolites11111phlholites only locally buth'.1t occasionally consti-consti— tutes as much asas. 15 per cent of the rock. Quartz similarlysinfilar'it' is of uneven distribution. (c)11] Pyroxenites.-AtErr-(1.11311true—At the road crossing between OkiaOkla and Kilala an outcrop ofol pyroxenite in the Kyanzibi river-bedriver-bed extends down-streamdown—stream for a short distance. Owing to the alluvium which\v‘nieh flanks the stream,stream. the relationship ofor the pyroxenite to the surrounding rocks cannot be seen. ItIt is considered,considered. however,however. that it extends to Kilala hill and from therethere across to the western slopes ofor" Kitonda,Kitonda. where a pyroxenite hashas invadedinvaded a pyriticpvritic muscovite-bearinginitseovitc-bearing quartzite.quartzite, 141wr< Specimen 52/376,53 375. from1'1m Kilala,1512111115is composed c1.1echieflye11} 111'of hornblende,1:111 'nhlende. u:'uralitized111'! augite, garnet and an sm,all5111117 quantityquantit} of plagioclase013' c1115e felspar1051111. largely1a.:gely altereda11ered toL'1 sensite.sericite. In contrast with the outcrop in11'.- the river bed,bed. the pyroxenite in thet e vicinity\icinin 01'of KilalaK1131: is'15 considerablyc altered.a'.tered. Specimen§pecimen 52/379,5.". 3""; taken from the outcrop in the KyanzibiRyanzihi river-bed,"oer—bed. is15 dark green andend has a medium-grainedn1edi11n'.——2.'ainL-1.1 granularL1r=1r11=-".111 texture. Microscope examinationexert-1n 111111.:e1-eatedrevealed thatth:1 1 it consists predominantlypredominant.) ofoi partlypart1} alteredaitered augitenugite and11nd hypersthenehypersthcne crystals.L115 .5. The hypersthenehf' pet'stitene is=5 pale green to neutral coloured,coloured. with faint1'.-.1111 pleocroism,11Le<1crt11srn and containsL01111n5 lamallae111111'1' of exsolved clinopyroxene.clinom'roxene. Both augiteatigtte and hypersthene113persthene have been replaced to a:1 variable1:1."i'111le degreeLegfee by a11 palepole green hornblende. StronglyStrongl} pleochroic brown11101571 biotite is15 the chiefL"=11L". accessory mineral,mineral. Lindand pyrite occursL1L"-:'L11.5 in small anhedral:11111ed:1-1 grains disseminateddisscn'-' .1ted throughoutth L1111"ot'.t the rock.roclx. ThereThL’TL‘ isi5 a:1 small51111111 51111011111amount of01' clear.clear, interstitalinte:'sti quartz. Hypersthene111.11e1'5thene isi5 the[1'.-e most:1'105: abundant mineral in a.1 thin section of01' specimen'5pecimcn 52/390.52 390. Owing to its11$ greatgre.'1t variation14.;1'11111011 in111 metamorphism thist1115 pyroxenite is believedhelie1ed to10 behe pre-orogenesispre—orogenesis in111'age.Age (d)111:1PEgNFdft'Ft‘S.iPEL-l[111111113Pegmatites.-Pegmatite 11ndand apliteEtplltc veins,veins. while fairly abundant.abundant, are largely1:11 "‘11 150:1con- centrutedcentrated in particular loc:1lities.Tfielocalities. The heaviest concentration 01".‘Lg1'1111tL5of pegmatites L5is in the south-western quadrant.quadrant, the hills1111150of Unoa,L n01: Nzueni.\1.uLhi and DinaaDinar: being11e1:=12 we1l-dissected'-.1L'.1-Lii551:1;ted by11) pegmatite 1e1n5.veins. The veins'1 L-inS are 5e1d11z'nseldom concordant with foliation111111.11011 plan-L'splanes in the gneis5e5.gneisses. TexturallyTexturall} the pegmatites varyat y from tine-fine- to relatively coarse—grainedcoarse-grained .1111?-and occasionally large phenocrystsphenocrr351" of0-." felspar'el5p '1r occur in1:1 the finer-grainedtiner» '1 3d types.:1'11e5. TheyThe); are.1:L' essentially quartzo-felspathicqttiu't/L1~t'e15 pathic in1n composition,compo H1L3[1.SOET.6some containingco :;11 illllllg additional minerals though others are composedcotng‘osed entirelyLntu e1} of01' felspart'elst'ur and11nd quartz.-'L1u;1r. [.1118The most common typet1'11e is15' micro-1111c.o~ cline-rich,Cline—rich. containing up to 90911 per cent microcline.microeline. QuartzQuortz veinsLeins are more abundant1111111111511: than quartz-rich pegmatites,pegniatites. which rarelytardy carrycan} accessoryaeee550ry minerals.minerals, Of the additionaladdinoml minerals,1111ner=.'115. mica is15 the most111051 widely di5tr1huted.distributed, and is15 usua1ly115111111} musco-:1'1115co- vite,'-.itc_ occurring either :15as 51113.1.small di5seniinnteddisseminated flakesflakes or as:15 “hooks""books" up to:0 1110two inches'.:1Ll'e5 in diameter. BiotiteBiotitc is15 less abundantabundunt and has11:15 developeddB\E lo'1ed onl}only as:15 isolated books.books. Specimenbpecmien 52/348,52 345'. from theroe Keiteheitc RiverRivet north1111115 of11f UnoaL'nou hill,hill. contains an1111 appreciablenor-reciuhle proportionpros-0111011 (22123 perper cent) of1'1: magnetite.EildgnClLll'L‘. The'1'h: pegmatitepegtnatite from which it was15.15 taken consists chieflyc‘ ofL] I microcline,microclinc, with“1th a.1 small amount of quartz and subsidiarysuhsédtat'y ilmenite,11:11enite. zirconzu'con andLind apatite. (4) MINERALBaTsBum Subdivision of the migmatites,mign'mt'tes which constituteLon5titute the major11151111 part0.11 11:of the Basement11::5en"cr-t System3'stem rocks isin this nreu.area, has11:15 been larg@lylargely based1111.5ed on structural Lindand 11:\1L1]’31textural features. AnyAn} attempt to dividedit'de the rocks into31110 types[ft 012.5 solelysolelj. on011. the assemblagesn5seml'11L1ge5 of minerals:1'1iner5115 init: them111-1:1 would have led to little distinction ofo:- 1111‘ietiesvarieties th=‘o'.1g';10utthroughout the men.area. Cert'LunCertain minerals,minerL1l5_ however,howmer. ;1r'eare occasiomillyoccasonnli} moremote abundant.111:111111111 than usual in particularp.1:‘tieu'_'.1: belts'oelt5 IFig.(Fig. 2),2|. 01:11:12owing often no doubtdot.'ot to compositionsL'011p051tion5 inherited1. _ from:10111 the 1111original1'11 sediments.«dimer-.15. The following111110511112 areLire the characteristicCharactertstéc mineralsminerals: that havehate been distinguished:1115t:n-::11:5hed: Microperthite.MELT: ' AsA5 describedde5L:'51): above,ohm e. aLi broad bandhand of01' nugen—gneissaugen-gneiss extendse\tend.5 in111 a:1 north-north-west1'.orth»north-‘-.se5t to 51111t11—5o:.1th-e:~.5.south-south-east direction east of11:' ‘.hcthe YattaYuttu Plateau.Plntcz‘zu. ThisT1115 nugen-g.augen-gneiss'1. ' is15' clinructerizedcharacterized by111 the presencepre5ence 111'of microperthite.ttticropertitiz-L: Diopside.D111pL51'LIe. ParallelPL 112' “111-with the .11.augen-gneiss,311e155'. a.1 band111:1'L'.1L"1‘.of gneiss,' rich inIt: calc-silicategale-silicate minerals,minerals. is1'5 typifiedtypified by'0} the uppeLu'LmL‘eappearance 01'of 111111151110.diopside. At311 some localities 0thc'other dark minerals are predominant,predominant. for1111' examplee " hole garnet in the111-; southern5111111‘1crn part11.11": and. hornblende'1.”11-3-1111:- in111 [arethe northern portion Whitewhile 5:11.11}small .11amounts11.115 111'of epidote,31311012. Ant/511:zoizite and1nd clino/oz'uieclinozoisite are erratically developeddewloped throughout the1'.-e band.bttitd. DiopsideUnix-Lie is, however,':r. consistently01'115151ent1} present.'e5ent in111 much of01' the band.1.13.1111. Garnet.Gamer. ThreeTht'ce unrelatedL1111'C"'.CL1 garnet-rich belts can be'11; tracedJuiced in' the area. The most11110.5! southerly portion of01' the calc-silicate-rich0:11;» gneiss frequentlyL-L'.:L':ttlj. contains'L'o.. .' up.111 to 20211 per11c." cent of01' garnet.starnet In111 theThe Nzaui.\'7:.1115syncline1:11:11nc a:1 1'21"mafic band1110. composed:1‘11‘111'51‘11 chieflyL‘hicil} of garnet,gornet. hornblende and;1.-mi biotitehtotit; is1'5 traceabletrace;ble on both sidessides: of111' the mountain.mount ' .14112'111:Finally Nzueni\zuenf hill11111 isis. capped by1713' a:. melanocratic.'nelanocratie garnet-bearing.'net hearing biotite'“intitt' gneiss.21113155111211This garnet belt extendsextend-.5 forto: sOmesome miles southwards but the abundance'. H1:11.:lance ofot' garnetLamLt is subjectsuhjeet to1.1 extreme variation. Sphene.\'_r3."1€.'r!é’. A broad beltHeh of gneisses211' in which. "1:11 sphene.5; 115.1 1e is1» consistentlyL't-n525.'-e1-._. present-L nt forms'10::115 the KibaoncrMalueniKihaone—V‘lalueni range.1.111521'. Southwards,Southuu'ds. a'.1 further11.111101 spinene—richsphene-rich belt,helt. parallel1‘31'1'1'11". with that of:11 ~..-'iI "1 15 Kibaone,Kibaune. embracesBEllbl‘dUL‘S the[he hills oful' Makueni,Masticiii. KiangaKidllfi'd and Katambua.Kutambtga. The concentration at:of sphenesphere in this belt‘nclt is,:.~. however,hmxcxcz somewhat\021‘.:""~‘v'h£tl patchy,patch}. the:11: heaviesthcmicst concentration-;n:1ce-'it:'ation being in'.‘:‘i the1119 vicinity\icinit} of01' Kianga. " '" [V7] Yalta phonolite '\ Foliation dip 1---1 Micro -perthlte- i:-:-:-:IDlopslde. hornblende Minerai \ Lmeatlon direction P.fT1:;~lbo"le BellS ~ I. . ., Garnet W,, Sphene ,_|: '\ Synchnal axis. Scale C‘ r'i r, 0 5.{Pg} 1.0O:.'miles FIG.Ht. 2.-Structures2 i‘elrlli'illrtfi‘s and mineralniin;r;t! beltslit-Its inin the areaUrK'fl south-eastmuIIi-caxt of:uf' Machakos.\1;!L‘1]:lhn~. (5)€51 METAMORPHISMMLIAMURPHIKM TheT‘tc gnci.gneisses and schists of the area1 havelint: undergonezintici'gonc utiitsitiLconsiderableuhlc metamorphism.titatamnz'phimi. The widespread and consistent occurrenceiCECC of01 biotitebLtfltutnis notno: diagnosticdr men; ofat the low-gradeiow—gmdc metamorphismmetamorphim'i of thelit.- ciasntcclassic biotat;biotite 193‘.zone, the hiotitcbiotite beingi'rcmg a gee-ngreen variety\cmctj. stablestable. undue:under higher grades of metamorphism.:ttef=tiio:'i‘.‘t”t:t. The index mineral almaudinealmandine is dew‘topeddeveloped ale-mgalong p92.par- ticular EIQI'iZOHH‘.horizons, in the assemblagedfii‘ni‘; " hornblende-oligoclase-almandine.:ib'icndc—oligocimc ‘sximamti The absence of0: kyanitekyanfls from“Font mostHTJS of the rocksruck: is possibly to be attributed to a relatively' low aluminaalunitn: content in the origtoriginal:1 I sediments.sedtsitcmx. .Staurolite was not found and, as the rocks aree gencruiligenerally poor in iron,iron. is not to behe expectedexpected. ~ 16 In111 consideringconsidenng the[11: degree ofL11 metamorphism,mc1.1'1‘.1,1:‘pm.9:11. however,'11L‘1'11e1-L‘1‘. the1:1: useL196 01of the111:. facies1.10.135 6111.95111-classifi- cationL‘ation is perhaps of111" greater 12111112.value. In111 contradistinctionc1.1111r11d'19:111:tio:1 to the:11: typical garnet amphibolites.11‘13‘111171‘111L‘9 of01 the amphibolite.11111‘111111011te facies,13113139. Korjinsky1901111519) 1T11"1‘.e.".(Turner, 1948,19415.11.p. 87)11—1 has described. a1. 11;.r1.‘\cnr-'.‘e.pyroxene-bearing11-; garnet 1111711111131111:amphibolite sub-facies.xLIb—Iguics The majority1111101511 of01' amphibolites1119111 1‘.:1.11L:9 L11in the111: present3‘11‘::91.“.11 area11:51 belong11911111; :1to that sub-facies9113‘ 144169 and111111111111:have the{11:1111following.owing assemblage:119961111113 .1111‘111‘11'111L‘almandine-diopside-hornblende.7.211.111.9111:- 11.1‘1111‘1‘1L‘1‘Lic_ In111‘smaller111.1 11L‘1 bodies the[11: more typicalLypi 111.19:1111‘1.111 (6)‘: ‘1 STRUCTURES513116111219 The length 1.11of Nzaui\111111 hill11111 coincidesc0111.:L1c9 with1.11111. a;1 north-north-west-south-south-east111.1:[1-111i:1 11 91:1.111‘2791‘11111196191 91.117syn- clinalL‘1111C11 axis5.1.19 which“111:1: is1.9 defined110111311 partly11:11‘11‘1. by1.1;. the111.: occmrcnc:occurrence of:11 similar bands11.111119 of.11 mafic,1.9.115. garnet-5:11:10? and‘..mi sphene-bearing9p1‘1a-1‘Lg1 1:1 gneiss1.11:2» on1:111 opposite171111091113 flanks11.1mm: and11:11 partly;‘..: by the orientation11"131‘11..‘1L111 of.11 the1h: foliation.1n. Easthas: of111' the111 8 syncline9341:1111: the111,1: gneissesgt113199L‘9 have11.11.2111:the same91:19 strike9131 as the‘11“: axis,11.19. a.1 direction(iit'cqtinn that111111 persists111:1‘91919 with11.1111 little111 [ 1c variation\111‘11111111“ to1.1 the1.11: eastern:19191‘11 limits1111:1119 of11.! the1111: area3.1.1 and111.1 the1.1: north-eastern11111‘1hv.:.191c‘1‘1‘. section.motion. In111 the111C northern1101111 part of' the western‘.1‘e9‘1c1:111:1:.:.':.‘margin, however,\L‘:‘. the:11: strike91711.: de'scribes11c9L‘1i:1:9 an(:11 arc.1:: concave:11CL.-.‘.L‘ to:1.» thethc west swinging from':L1:1‘. north-east--south-west:1:“111—c .591 at.11‘. Kilala19.1.1111. I:to north-west-110:121-113917 south-east911113—3191 in£11 the1:11: north-western111'1 1‘1-‘1‘.1.‘\[L 17 corner.CO"? 8.". Except in1111116the south-western9111thwc912111 .‘111113‘corner dips.i1;‘1‘9 are.11: in1‘. general to the west,1.1.1391. at:11 angles111g1L‘9 varying1.113.11‘ from"13:11 2021.1 to1.11 6011.11 degrees.11:15:: In the neighbourhood‘1111L11i of‘1 lkua on.111 the‘11.: western11:91:71 margin1'“1.1!‘gi11 the111: (111‘dip steepens91ccpcns from1:11:11 about‘1111 40411 to11:1 about 65 degrees. In the south-eastern corner of the area dips.11139 are.11": eastward111.11g at angles varying from 30 to 60 degrees. AtA: the1119 few localities10.1.1.1 1:1L‘s where“111318 it11 was measured the1h: direction.i1r‘L‘tr111‘ of1.1" lineation11:1L‘.11111“1 is19 closec1119: to10 340°,3—1‘1 . theI11: angle.111‘_.‘1‘e of131' pitch3‘11L‘11 being113111; generallygsnsf about 20°_ to11‘ north-north-west.7111111.“.1"'.}1»'1.91:9f An.\:1 important1111110113. .‘1 structural feature of the area is a brecciated zone in3:1 the[31: gneissesgnassn running close to and11.11L1 parallel11.11“: to the Yatta phonolite.1110:1111; For the purposes of111' the present11169311: report11:31:11 it11 has1..s been11:.‘11 interpreted3111:"1‘ as a fault system:1 51921. and it is believed that the11:81‘111‘181'1“’31‘.movement was“.15 horizontal1701120111411.1.111!along vertical"' ' planes. The'1 most[1.051 intense..I brecciation has“.19 produced7‘1.‘L11.1L'-‘:I phyl-p111}- lonites.onims with111111 a slate-like structure and a fine-grained mylonitic texture..::'c. The11:: brecciationbreL‘C‘LL‘tj-fi: in11 less13.9.9 mylonitized:11\‘11‘.1111mi-;L1 types11.135 is15 often011L1'L‘L...;_1111.:‘_1‘1Ierecognizable onlyU11 by microscope examination. During ,the1116 initialmm :11 stages of‘11:: crushing9111111; less1:99 resistant1:919913111 minerals11.11 were fractured,7-5.; particularly1911111111 .1\ around their1.3}: edges.udgc: QuartzQuunz grains remainedDc relatively:e1:.."1‘.‘e1_\‘ unaffected, though{1:115:11 strain91.1111 effectscfiect9 in1:1 them(1‘1611‘ arem. notable11.1;11‘1 111in (“1ethe rocks1:1c that111., occur near‘1e.‘-.- the11.: limits111113 of the zone1.5111: of11. brecciation.:11‘LL‘Cif1tiot1. 17 2. TERTIARY-YATTATLR‘It.\Rt'—Y.-\TTA PLATEAUPL.“ FAU PHONOLITEPHDVULITF MegascopicallyMegaseopicall} the phonolite of the YattaYatttt Plateau,Plateau. which corresponds with the KapitiKupiti phonolite of Gregory (1921,[1921. p. 99) is a fine-grainediincgrained rock with medium-sizedmedium—sized to large phenocrysts. The texture of the groundmassgroundtnass variesyaries slightly according to the posi-posi— tion in the flow both horizontally across and vertically,x-ct'LLcitll}. the phonolite from the centre being coarser in texture than that at the margins.margins, AmygdalesAnn-guides and phenocrysts account for up to 20 per cent ofot‘ the total volume ot'of the rock. The phenocrysts vary in size from 10ltl min.mm. to 50 mm.HUT. the axe-rageaverage being about Fll30 l“mm. in length. AmygdalesAmygtlalcs are corttpttt'ttrcompara- tirelytively rare. Microscope examination of numerous thin-sections ofol‘ specimens taken at randomrandom between the northern limitlimit ofol' the area and the Kitui-KibweziKittii»hihn=ezi road south-eastsouth—east of the area reveals that there isis little variationyariation in the phonolite.pltonolite. The phenocrystspheuocrysts consist almost entirely of anorthoclase and nepheline,ncphcline, which occur in the approximate ratio ofoi 7:7 : 2. Those ofot‘ anorthoclase are invariably1fl\'li]'l&ibl}-' larger than the nephclinenepheline crystals,crystals. and in hand-specimenhandspecimen can be easily distinguished by their whitish porcellanous appearance which contrasts with the waxy,was}. dull-green appearan<;eappearance ofor the nepheline.nephelinc. Some ofor themthem are slightly replaced by zeolite,zeolite. and occasionally.occasionall} they are almost cnttrelyentirely altered. OlivineUliyine phenocrystsphenoCTysts areare rarerare and small.small, and partly replaced bybf» carbonates. The amygdalesarttygda'ies are composed predominantlypredominantly, of calcite though zeolite and kaolinite are fairly common in them. The relstcfelsic minerals of the groundntassgroundmass are anortitoclascanorthoclase and nephcltne.nepheline, and the maficmalic minerals are aegerine,acgerinc. aegerine-augite,acgerinesaugite. augite.augite, kataphorite and eossyrite.cossyrite. Numerous small flakesflakes of kataphorite occur in all thin sections examined. (ossyriteCossyrite isis equally abundant and occurs as minute crystals disseminated through the groundmass. Patches ofol‘ analcinteanalcime are occasionally seen. 3. PLEISTOCENEPLEtsmt‘FVr (1)l l} SSedimentsedinreittx On the broad flatsflats along the course ol‘of the KeiteKcite .tntland Athi Rivers allu\ialalluvial deposits consisting of variousvarious grades ol‘of sand and clay withWith occasional pebble-bedspebble—beds frequentlytrequently reach a depth of thirty feet. In the vicinity ol‘of KilalaKilttla scxcra‘.several pebble-bandspebble—hands occur.occur, each showing remarkable rounding ofot’ thetzie individualindividual pebbles.per-Hes. The sands and pebble beds are usually well consolidated. No fossil:ossil remains were foundtourtd in these sedimentssediments but,but. on comparison with other areas,areas. theythe}. maymay. behe consideredconsidered as of upperupper Pleistocene age. Approximately half-wayhalt‘ax-uy upup KilalaKilai: hilIlt;ll a somewhat discontinuous horizon of pebblespebbles representsrepresents sediments deposited during earlier,earlier. probablyprobably. lowerlower Pleistocene times. Along the banks ofot‘ the Athi RiverRite: in the southern section of the area the gneisses are intermittently covered by cemented gravelgrioel containing usually well-rounded,well-rounded. ill-sortedillvsorted pebbles in a dirty-whitedirty—while matritt.matrix. The gravel is tentatively considered as of upper Pleistocene age. The matrix,matrix. which is compact and fifinene grained.grained, is predominantly cal-cal— careous,careous. with lesser amounts of kaolinite and quartz.quartz distributed locally. A root-likeroot-like fossil was foundfound in it but identificationidentification proved impossible. The pebbles vary in shape and rockrock. type, the most common being of sub—angularsub-angular to rounded quartz. Rounded pebblespebbles of Basement System gneiss are fairly abundant but none deriyedderived fromI'rorn the Yatta phonolite were recognized.recognized. . -- r. 18 Heavy minerals,minerals. separated from ten specimens ofoi' the giii‘vdlgravel yielded3-.iciiled the followingfolio-Ame assemblage::issembiiage : -— PerPr: cent("E’JH HornblendeHornbiende ...... 19l9 Diopside ._ ...... _ . 15 Epidote ...... 13 Magnetite ...... 10lt‘i Haematite ...... 8S Clinozoisite 7‘ GametGarnet 6n Sphene 5 ilmenitellmenite 5 Biotite 4 Zircon 4 ApatiteApatitc 2I StauraliteSttturalite . . . . 1' TourmalineTottt‘mtil'lte + Sillimanite\Ii|::ti.1ntlc + RutileRi.ti|; . 1i (2)I 2 i LavasLat ta' i” SeveralSeierai volcanicvolcnmc cones liehe a littleliitle outside the south-westernsouth—'«t'estertt corner ofoi the area,dice. .indand numerous transportedtranspotLed boulders of law.lava deriveddct'tt ed t'iomfrom them are found into the cist-east- ward-flowingward—fiowing streams.streams A.v\ smallSllldil portionnotion of ;:a single flowtin-w extends acrossLacr'oss the westernxvi-stein boundary into the area. A spcczmcnspecimen was obtained from thetl': iteignhutti'm;neighbouring LilL‘.i.L1!l{larea, and foundlountl to behe :.a porphyritic olivineolixine basalt. It contains inclusions of Basement SystemRtstem rock up to threethiec inches in:11 diameter. Microscope e\iiexamination.-in'.;tion o:of a thin SCL‘llUHsection ofol specimen 5252/42942)? revealed medium-:iicdimi sized euhedral augiteaugitc phenocrysts,Phenocrysts. olivine phenocrystsphenoery’sts and scattered felspari‘elspar phenocrysts.phenocr} sts. The groundmnssgroundmass is tinegi‘ainedfine-grained and composed ofot plagioclasepltigioelase felspar,felspnr. magnetiteintignetite andnot? pyroxenes. The leisparfelspar is lzibtedoritc.labradorite, apprtuiniiatelj.approximately An...3m 4. SUPERFICIALStpt‘to lt l.-\L DEPOSITSDiiu'isiis Inlo the south-easternsouth—eastern portion oiof the areaurea cxtcttsixeextensive weathering«withering has produced residual:estdual and;'-i -.‘:c:i\ionnll}'occasionally transported soils olof considerable thickness. Elsewhere dexelopmentdevelopment o1of soils has proceeded to a lesser extent. Generally the soil produced is aLi reddish to yellow gravelly type,type. the redrec. coloration being due to an overalloterell stainingstaLiiEng byh} iron-oxide.iron»o.\idc. This type of soil is essentiallyessentidllj.’ acid,acid. supporting a sparse vegetation ofo1" scrubsC’uh thornthornhush.bush, and:ant: attempts at cultivation seldom meet with success.SllCCCSb. More advanced weatheringWeathering producesprotii.ccs grey to chocolate-colouredchocohtccoloured soils,soils. devoiddew-lo ofot gravelgmtel and invariably more fertile.fertile. A fair:air measure oiof success hashzts beent‘c-co obtainedobiutned in the cultivation of such soils. Deterioration ol‘of these soils is rapid.rapid, for underundei .ntpropceimproper husbandryhusbandr} the Ulgtilllc‘organic content of the soil is soon lost.IOst. FurtherFtitthei lencningleaching producesprod-Lees .ta \klZlLshwhitish mil.soil, the litfinal product being a.a barren sand almost detoitldevoid ol'of vegetation.\egetaziozt. Inlo the \iciriitivicinity ol‘of the casiceasternn boundary soil deterioration has :tdxancedadvanced to the extent itwhenhen little can hebe L'lllil\alt2(lcultivated in the sandy expanses. Along the summit plainplatn ofoi the Yatta\‘jtte PlateauPldlC.-..l occasional«custom-.3 patches ofot soil havenine developed. They are dark red in colour.colour, and are probably more fertile than those deriVedderived fromfrom the surrounding basement rocks.rocks. Finally.Finally, accumulations of so~cailiec1so-called "black cotton" milsoil occur in areas ot’of itiaerneiinternal or ill-deill-definedfined drainage. The soil is a fine-grainedtine—grained black,black. clay-like soii.soil, which when dr}.dry is hard and fissured by contraction cracks though when wet it is stick}sticky and extremeig.extremely boggy.boggy, The black colour is due to .111an unusually high org.utieorganic content. Numerous rounded, calcareous eoncretionsconcretions are characteristic of the black—cottonblack-cotton soil accumulations. The concretions are derived from lime-chargedlime—charged Waterswaters which.which, under suitable conditions.conditions, precipitate calcium carbonate.carbonate, the nodules probably representing accumulations mount?around 19t9 aLt nucleus. The nucleinuclei. usuallyll‘iliall} consist ofol‘ grains of sand,sand. oro." occasionallyoccasionuil} ofot' particles of01' organic matter.:natter, No concentric layers are to be seen in the concretionsconeretions and only seldom are the nuclei apparent. In numerous specimens of the concretions unidentifiedunidentified fossil remains were seen. They consist ofot‘ small,small. hollow, nodednodeti rods that may possibly be either fossil rootlets or worm burrows. VI-ECONOMIC\-"I rECONOMIC' GEOLOGYGE()L(}G\ 1.]. MINERALSMmetuLs During the investigation of thethe, area described no mineral depositsdepostts ofot' importancetl‘ltDOTiiittc-c werewere. discovered. Twenty-sevenTWenty-seven heavy mineral concentrates were collected fromt‘rom river sands,sands. alluvial and soils,soils. but on examination they failedfailed to reveal the presence of anyen} mineral ofoi economic value.\.:-'.|'.te. SomeSon‘s yearstears ago fineline crystalcr}st:it aggregates of magnetiteIttdgttctitu u"were recoveredrecovered nearnear Nzaui and a certain amount of prospecting was carried out. ItIt isis probable thtttthat the c:}stalscrystals were obtained Fromfrom :1a pegmatitepegmntite and workable supp'iessupplies :lr'eare unlikely.tllll-{l~. The small body ofot‘ limestonefitnestone in the north-westernnorth—western cornercomer otof the area,.it'L’d. whileWhile fairlyfulfil}. extensive furtlrerFurther west,west. hashats too highhigh ait magnesia content to bebe suitable for"or the manufact-‘tttitlllitacti turetom of cement. Occasional deposits 01'of low-grade knolinkaolin occur in the westicent“;tlwest-central partpttrt ot'of the .ircn.area, situated between the hills oiof Ikua[knit and Kitonda. The presence of high percentages ol‘of iron-oxide and silica make the dcposftsdeposits Ynluelcss.valueless. The only known occurrence ot‘of kyanite in the area'5.c consists of at; kyanite-bearingMamie-bearing quartzite,quartzrte, which enters the areuarea from the west for an distance ofor" about.tbout a mile and a:1 halfhot? near Kilnla.Kilala. The thickness of the bandbuild is approximately 505H feet but it contains onhonly a:t few per cent of kyanite. Nol\o sign of any kyanite concentrations were found in it. 2.1 WATERW'ATER A hydro-geologicalhydro—geological investigation of the Makueni settlement area was carried out by the Public Works Department in 1945.1945. Following the investigation a report was prepared recommending several borehole sites. Mention was made of other potential sources of water supply including the following:-i‘ollowingzi (a)tttt sub-surfacestih—stirt‘ncc dams;dams: (b)th] surface dams;dams: (c){Ct wells;wells: (d)Id) infiltrationinfiltration trenches.trenches Inin due course a series ofot‘ boreholes were sunk and the records“ccortis of the Public Works Department show the following results:-results: « BoreholeBorehole I Known as I Depth Rest Testedl'estet‘. Quality of Number.\'L:. DC? Level1 et el Yield Water ] 1- FeetFee! G.P.H.(LRH. C.397 No.1..\lti. l . . . , 508 7 Dry C.398 No.2..:\lo_ I . . .._ . ._ 47747"? Dry C.414 it -.. -ff 442 155ISE- 1,060 _ Mubau .. 1,060 .1. . C.482 Jr \"lUDul. . . _ . 443 155[55 1,060LOGO }Vi-‘hngntlySlightly saline. C.437 442405'05 5.050 1,580.53 ' . l, UnoaLi'tlUit .. "...... }1k :05 (JD 11-522 Lif (it‘tttl, C.446 "- 405 60 1,525 jGood. C.454 Hunter's Camp ... . 2742T4 53 1,000 Good. C.461 Woti ...... _ ..., 1781738 2525 3,6003.600 Slightl,Slightly brackish. C.474 Makueni Hill ._.. ..._ 134134 34 860 Good. C.488 Malibani .. 40240?. 44'4 860 Good. C.500 ThabuThabtt ...... , 220230 30 1,0801.080 Brackish.Brackish C.518 Poi _ . ... _ ... . 432433 54 790T90 Brackish.Brackish. I Poi . 20 In the northern and eastern portions ofoi' the area eonditionsconditions appear fari'ar less favour-tavotir— able for the sinking of boreholes. With the exception of the Athi.Athi, all Stt'L‘tttt‘tSstreams flowhow for butout a shortRhOI't period each year,)car. following the seasonablesettsoriable rains. Water is,is. however,howewr. easily obtainable throughouttl'iroughout the year by digging in the sandysand). river-beds.l'l\"e"l-bf2(l5. WaterWitter is obtained inli'l this wayhe.) by the localtout. African population,population. who seldomSeldom needheed to dig deepernecper than three feetfee-Z ini't the sand. To increase the water supplies it ists suggested thattho. the most important stepxtep is the construction of dams,dams. especiallyespeciall} small,small. high-walledhigh-w;dled types.t_\ pas. The desirability ofot' building I numerous small dams in preference to a few large ones isl’a obvious.obxious. Perhaps,Perhaps. most important is the facttact that small damstlnms can be built relativelyrelativel} cheaply and without mechanical aid. The advantage of depth in the dam is that the smaller the ratio of01' surface to volume,volume. the less water will\xi'l behe lost by evaporation.emporation. VII-REFERENCESVIIVAREFERENCES Baker,Baker. B. H.-"GeologyH.7“Geology of the Southern MachakosMachtntm District"Di 7., - -.. 1951.-"Southl95l.7”3outh AfricanAl‘ricztn Scenery."Seenetj." Edinburgh. Reeve,Keene. W.W'. H.,H.. 1945.-"Report1945.7"cort on the Hydro-GeologicalHydro-Cicologiené survey\lJHC} ofat a portion of0: the proposed Makueni settlement area,area. MachakosMuchztttm District." Unpublishedl'npubiislieo‘ report,report. Public Works Department.Department, Nairobi. Shackleton,Shackleton. R. N.,N" 1946.-"The1946. ~“The Geology(Jeolog') ol'of the Country{. aunt?) between Nanyuki\(infiuki and Maralal.".\l.tr:tl:il," Report No. 11,ll. Geol. Surv.Sun. Kenya. Shand,Shand. S. J..J., 1943.-"Eruptive1943.~“Eruptire Rocks." London. Schoemau.Schoeman, J. J..J., 1948.7":31948.-"A geological reconnainxttncereconnaissance ol‘of the area Wentwest olof Kitui Town- ship". Report No.l\'o, 11,ll. Geol. Surv.Surx. Kenya. . . Temperley,Temperley. B. N.,\i 1938.-"Thei935.e"The geologygeolog} olof the countrycountr} around Mpwapwa."Nlp‘.\.ip\t-:z." ShortShofl Paperl’ape" No,No. 14.H. Department[Department ot‘of Lands and Mines,Minex; Geological(Jeologienl Division,Division. Tanganyika. Turner.Turner, F.r. J.,.l._ 1948.-"MineralogicalW45. A“.\'lineralogie.tl Ltlliland Hll't.L‘ltll'Lllstructural evolution cifof the metamorphic rocks." Mem.N'Iem. 3t}.30, Geol. Soc. America. (j.t’.K_G.P.K. 2th}:-2002--{;OQ-llj52-t’~t'|t]—-ll 5: