Geology of the Ikutha Area

Report No.1r56No."56

GOVERNNIENTGOVERNMENT OI"OF KENYA

MINISTRY OF COMMERCE AND INDUSTRY GEOLOGICAL SURVEY OF KENYA

GEOLOGY OF THE IKUTHA AREA

I DEGREE SHEET 60, N.W. QUARTER

(with coloured geological map) by J. WALSH, B.Sc.

I Geologist

Ten Shillings —- 1963 FOREWORD

The Ithuka armarea is on:one that is little t'tsitctivisited as a whole,whale. for thutighthough the railwaytummy and the Mombasa-NairobiM ' a~7\a‘:'obi rmdroad passmen- through itsit; south-westwntn—west turner.corner, mtzqhmuch ntfof it is not casliyeasily

21:56:31,accessible‘ no:exceptIt vnon thatfoot. Part of t:it Picalies, however.however, in the ntrth—um:north-west CUTTICTcorner 0fof ththe(3 TsztmTsavo Royal;\x '24] NationalNationzt‘t Park,Park and maythat} becomehcmmc better known in thathe future. The areac3 Exis nnimmt'th);noteworthy in mso far as it contains a graphite mine that has npcratcttoperated forfur a numbernt::1:hcz‘ nt‘of yum»:years, and hhasx: prnduucdproduced \L‘T‘A‘ICsome 3116103,000 tons of the:hc mineral.YIELTECI'EL Ay‘\ gsncru‘:general acu‘::tn‘;account of, ' the graphite d‘;deposits‘ of‘tt thL'this partv 01“of Kenya muwas given in a previous report (Na(No. 37.37, 1957) on thcthe SOUIZ'WSouth KituiKm ' area, which’ adjoins the Ithuka area on theit: nortn.north. Mr..‘t‘, Walsh,\\. in tht:the presentt Lt: rcpt“report, continues the account, giving details‘h‘ oft::‘ the processpz'octW: that is55' usedt , for cxtztgtti‘rt,extraction, and a proposed process induding winnowing, ‘9.whichh t c h hhasa s been 5:11»suggested= in view{cw ofcf tn:the' Ett‘tttcacute sshortage of water in the mine area. The areaand ists crossedum d by a part Mof the 'krttaYatta plateau,plateau. which i>is nncone at:of the unique features oftsa‘ KenyaK; gst‘lugeology. MathMuch :tt‘gtxhcntargument buhas centred round th’the emplagcmcntemplacement 0:7of its thinthfn cap}capping' nt'of phonolitepht’tnttt, ’t lava.lava, Mr. Walsh\\L1 21 dLihETL’iadheres ,toto the M12]valley-flow‘ fimx hypothesis and providesprt'rx'idcs a.n interestingham "t’nfl calculationlatiun to showshun that:hat xu‘qtl‘tsuch a lavalaw: mutt!could have flflowedowed the length of the plateauD3315; u tnin :1a periodpCf‘t‘ud “ofbf about seven manthxmonths.

Nairobi,Nairobi WILLIAM\VILLEAM PULFREY,PULFREY. 30th30111 33h;January, 1959.NS“) 4;}.Ag. Cummf,Commissionerfurlﬂ‘ (\H‘HL‘S(AIines (‘2‘& (JUN,Geology)

. j CONTENTS(f(')_\'l'L'.\"i'S PAGE AbstractA by: mu:

I-Introduction37~lzmﬂuiL in and General(.‘rctmrtxi'. Information.Imm'In.tLEL'\1'1 .

II-Previous Geological"L WorkWm}: ‘ . . . V. . 3

III-Physiography 4

IV-Summary of Geology 10 V-Details of Geology. . 11 1. Basement System . 11 (1) Metamorphosed psammiticpgwmmHJ sediments\J-il11illL5 13)1 (2) Metamorphosed ‘ semi-peliticscn‘ﬁrpal' atdxmcmssediments 155 (3) Metamorphosed pelitic sediments. . 18 (4) Metamorphosed calcareous sediments 19 (5) Migmatites...... 20 (6) Intrusives into the Basement System 21 2. Tertiary-Yatta Plateau Phonolite 23 3. Pleistocene and Recent 23 (1) Lavas.. 23 (2) Alluvial deposits 24 (3) Superficial deposits 24 VI-Metamorphism and Granitization 25 VII-Structure 25

VIII-Mineral Deposits 29

(1) Graphite 29{2 (2) Limestone 344- (3) Vermiculite 35.n 35U“

(4) Sillimanite .‘AA'-)‘11IJ 4 (5) Water-supplies 35U:

IX-References.IX ~t.2£‘;115€< . 36‘1)

LIST OF ILLUSTRATIONS[LLTQSTR \TIONS

Fig. I.-Physiographic-al=..Logrm1\xﬂ mapnwp , . . .‘ h .. 5W Fig. 2.-Structural map 26

Fig. 3.-Contoured stereographic diagrams ifof ”3301‘polesI» stothli;‘.L1w1:p.gn.->to foliation planes and pfof linea-Hm“- tions . . . . 27 Fig. 4.-Frequency diagram of joints 29 Fig. 5.-Graphite flow-sheet, Ganesh Claims,(Pix; KamstoiKmmun 31 Fig. 6.-Experimental graphite flow-sheet,. GaneshCmnwh ("inClaims,ﬂux Kamstoiligftaﬂ . . 323

MAPS

Geological map of the Ikutha Area (degree sheet 60, N.W. quarter). Scale 1: 125,000 at end ABSTRACT\BSTRACT The'l he reportrcport describesdcsci‘ibcs an area of approximatelyappmximaiely l.2tlll1,200 square miles in the Kituilx'iIui and MachakosMachakus DistrictsDistrims oful‘ Kenya,Kenya boundedbciundcd by parallels 2°2' and 2'2° 30'El 8.S. and meridiansrtiCi'Id'ut‘is 38‘38° and 38°38’ 30'3W E. PhysiographicallyPhi,xii‘grzlpllicall} the area[lien is divided into fivelike uriitsLiiifis (l)thell) ‘il‘ic AthiAihi river drainage,druinagc. of gentlygentlj. undulating country in the west\c‘t and south-west,R'l'illl'I-cl. (2)l1) the1hr: YattaYalta Plateau,Plateau. (3) the fairly deeplydcsply dissecteddisscclcd catiiiumcatchment Liftiiarea of thein: Tiva'liui river,rixcz‘. east618' (ifof Eh:the YattaYaiizi Plateau,Plulcziu. (4)(-H the Yamala-Kimathena-IthumbaYiimzilu-Kimaihena—llhumba range,limit and1nd (5):5) 1h:the scz'Lm—cgzvcrcdscrub-covered plainsplaim in the east,cast. grading toLo thellic crud—"lertiarjuend-Tertiary peneplain,p:ri:pla.:i. with erosion l'C‘ildllresiduals.fliS. The[he solid rocksi'uclii of the ureaarea lI-Qlfall intoima Ilircethree groups (ll(1) BasemcmBasement Qi‘ysicmSystem [Pro(Pre- cambrian),u’xmbriam. (‘k‘IlsiSllf‘igconsisting mainly of para-gneissesparrqnefisscs .withwith minor intrusions.intrusions, all 5stronglyngl J‘olfoldedLl C(l and metamorphosed,me norphoscdi (2)(21 Tcrtlnr}Tertiary \caniu.volcanics, representedI‘Epl'CSCllled by a Singlesingle extensiveex : phonoliteplmimlftu lava flflow,ow. and (3)(3'! RecentReam: volcanics,\olm' Ed, comprisingcomprisfig two flowsfilm's oiof olivine basalt.bzixui‘i. SuperficialSlipcrliciiil deposits of Pleistocene and RecentRcccn‘. age include red and black suili.soils, sewndai‘ysecondary limc~limestones and alluviuimalluvium. ThuThe petrographypctrugrapliy (ifof Ill:the various :0nrock lypcstypes isi.) Ll‘CSCl‘id‘described, amland 1h:the \Irimui‘c‘structure, mem—meta- morphismmni‘phkiii and granitizntiongranitization of thell}: BasementBasaiiiem System rocksrolls LTCare discussed.din-lived,

. Occm‘rcnccsOccurrences of esonomiueconomic iiiinsi‘alsminerals :tare described,desmibcd. with \pcia‘,special attentionzillcz‘ii‘nn to.to girthilc‘graphite, whichnigh is being worked in the area.areal 1

I-INTRODUCTIONE----§"-~"1,A£,3L3L(”HU\ AND.\\i) GENERAL INFORMATIONl\l5()R\l\'J'IU.\ Gencl'al.-The area described in this report is approximately 1,200‘ :UU \qJLm'qsquare milesIT Am in3 * "'xtent, and is bounded by parallels 20 and 20 30' S. and meridiansm 3801 (UL!and 380H‘ P30'’ i‘.E. It comprises the north-west quarter of degree area No. 60 (Kenya), and is the area covered by the Directorate of Overseas Surveys Sheet No. 175. It lies in the Southern Province, the part north and east of the Athi river being administered from Kitui and the remainder from Machakos. More than one-third of the area, in the east and south- east, lie~ in the Tsavo Royal National Park, and of the remainder, most of the land north-west of the National Park forms part of the Kamba Native Land Unit, the rest being Crown Land. There are three blocks of land in the south-west leased from the Crown, two of which are planted with sisal, by Dwa Plantations Ltd. near Kibwezi (LR 917), and by Masongaleni Sisal Estate Ltd. at Masongaleni (LR 5903). The third block, (LR 3685) at Manoni, produces sugar-cane and paw-paw under a system of irrigation which taps the Kibwezi river and a large spring near the snout of the Kibwezi lava. Approximate boundaries of these estates are marked on the map as a guide to their position and extent. The only village in the area is lkutha, which existed before European settlement in East Africa. Although small it is important as the focal point of many scattered small-holdings, and contains a primary and an intermediate school, a dispensary, a Government rest-house, several small Indian and African shops and a covered "market- building. It is the home of the Ikutha Dancers, one of the best-known troupes of Wakamba dancers. There is nowhere else a large enough collection of huts to justify even the term village, although the country east of the Yatta Plateau and north of the National Park is fairly well settled. The Athi valley, which has a perennial river and is in many places bordered by fertile alluvial flats, is devoid of settlement except at the crossing of the Kibwezi-Kitui road. Various explanations were given to the writer for this-abundance of big game, liability to flooding, and seasonal infestation by mosquitoes and tsetse flies. The latter would appear to be the true reason, since the local Wakamba have little fear of e!ephant and rhinoceros, and serious flooding appears to be a rare occurrence. The African Land Development Board has instituted the Athi-Tiva Reclamation Scheme, whereby thousands of acres of bush country between the two rivers have been cleared by tractors and bull-dozers and, by a planned and controlled system of burning. rich pastures free from tsetse fly have been developed for stock grazing. Many earth dams have been built and wells sunk for watering. It is hoped that by levying a small annual grazing charge on each animal the scheme will be made self-supporting. Grazing will be strictly controlled to prevent soil erosion, which is a serious problem in the extreme north of the area. There tracts of land many square miles in extent have been denuded of grass cover by cattle, goats and sheep, which has led to serious gullying and the loss of much of the top-soil. Serious efforts to prevent further erosion and to reclaim lost grazing land are being made by compulsory purchase of cattle to keep numbers down to a safe figure, and by "scratch-ploughing", the cutting of shallow furrows across the gullied traots at intervals of a few feet to trap rain-water run-off and to stimulate the growth of new grass. The gullies themselves are dammed at short intervals with rocks and woven sticks. Such a ploughed area is usual1y regenerated to a sufficient extent to alJow light grazing again within two or three years. Game is abundant in the less settled regions and in the Tsavo Royal National Park, particularly elephant and rhinoceros. In the National Park, particularly near the Tiva river, the number of elephant is so great that vast areas have been flattened and trampled and the ground is littered with fallen trees. Buffalo are common in the thick bush of the Yatta Plateau. In the more open grasslands around Ithumba there are large herds of zebra, gazelle and impala, with lesser numbers of giraffe, oryx, wild pig, wart-hog and ostrich.

. J -r-

Climate and Vegetation.-RainfallVegetarian. ~~Rainfall statistics.statistics, which are recorded only at lkutha,lkuthu. DwaDua and MasongaleniMasnngalcni are tabulated below,below. together Nthwith readings made at Kansu.Kanziku, tuntwo miles outside the nnrthernnorthern border 01‘of the area.area, east~nurth-easteast-north-east of lkntha,Ikutha.

------.""--.--- -T- I------i------Total Rainfall (inches) No. ul‘ i Locality I — Rainy Yearly Years I I No. of I ! No. of 1954i954 1955 i 1956 ' . 1956 Average Recorded lI>ays,1956i 1\verage Recorde Rainy Yearly I Years

lkuthalkuth‘d .. .. 17.6717‘67 * 23.5123-51 3H32 18'2718.27 I 5.5 I I .. 21.53 14.19 27.71 24'30 3T I>waDwu . 21'53 14'19* 37'71 5:52 i 24.30 i 37 MasongaleniMasongalcni . . 20.482048 * 'i‘t ‘1‘t I 239723.97 ' 4'?47 Kanziku .. .. 11.2211433 16.841684 25.7825-78 51 237123'71 I 14 I I I

*Reeorde*Records incomplete. tNot'l‘Not recorded.

The rainfall is bi-annual,biannual. with ita marked rnaxlmutnmaximum in November-December,\member—December. and ad. lesser peak in April.

The main rtgrietlttitutlagricultural cropsemps are maize,ntai/e. beanshean's and eaxxum.cassava, nithwith C(13101‘~Ollcastor-oil beansheath as 21a cash-crop. 1\A smallwall amount:znrttunt of inhaceutobacco isix grtmngrown furfor locallneal eonsuntptinnconsumption and in a few favourable locationsltseatirtnix there areWe plantationsplantu‘iivnx' of bananasbanzu 4.x and plantains.plantaini. 1WDTwo cropscraps are planted eaeheach year.year, to ettinerdecoincide with the rainfall snzn‘mu.maxima, htztbut it is not uncontmununcommon for rain;rains (and crops!crops) to failtail etsrnplctclt.completely, as the}they did in earl;early 1956,1956. when\\ hen the rainfall for March to May mt»was enntinedconfined tnto a few short showers.

MuchMuelt of the area i\is emeredcovered b].by macaciati . thnrnthorn serubscrub 'v-Cthwith .Jecusit'tnizloccasional t:‘t'[i".‘lldbbaobab trees.trees, and where game tracks:z‘aelw are fewten penetrationpenetrt inn is :1a sltmslow and dfttieultdifficult business.btﬁltlé‘si Natural water is almost :ttmpletel)completely luelxln;lacking .4 over mmtmost of the arm.area, since only the Athi1\thi and Kibwezi rivers are perennial,perennial. though water C‘dl‘tcan be found along the wholewhale length of the TimTiva river only :1a {notfoot or mi)two bclmt.below the \nrl‘dee.surface. All1\11 the spring:springs mttppedmapped ureare very small with ,thethe martianexception of that in the KibweziKihnc/i lamlava near Manoni,Metnoni. which haxhas a1a measured:nemured ﬁflowtm- nt‘of Zittttt)25,000 gullttnsgallons per dd}:day. The \\:tter'~hnleswater-holes tlthpf‘cdmapped are either

Inln the past few yearsyuan only,only. \ntgtll-l‘ioldingxsman-holdings havehum been clearedelettred on the YattaYattn Plateau titat KabeteKEiltL‘IC and Mwala,Mazda. and thethe red milsoil deriveddcrixed fromhunt thethe underlying phmmTitephonolite hmhas provedﬁrmed particularly.particularly fertile.1" 5g. However,1’1mxewr, wine:since the soil«01:7 istx' nowherenorthern ubnveabove tuntwo feet in thielx»thick- itemness and t‘t‘equerttl}frequently 16%less than tint:one tin“.foot, ~t:;hsuch \lll‘tlisman-holdingsEtttltin‘g» .ti‘:are particularlyP.11'11ctltl'i:{ tltjpcntledependent:f ,, on .ta goodgtmd and well-spacednull-spaced rainfall.ninth l..

Communications.-TheMt.ri:it‘!i.tlt‘ 71h; mainzttu't‘. Nairobi-MombasaNrtlrttrti-Xltt't‘ri‘ road and the East African railway cross the south-westM‘Lllli ..:~t corner ofat tstethe area,.mcrt. tuttiand two:, railway stations lie within it, Masonga- 3u;

leni and Kikumbuliu. TbeThe latter is a new stationstatinn built as part of a scbemescheme to increase tbethe traffic-carryingtrealhe-e'lrrying capaeitycapacity of the Single—single-tracktr gel; line by providing a passing point for trains trmellingtravelling between Masongaleni and KiKibwezi.bwezi. The main road from KibweziKibxsezi toto KittiiKitui passes throughthrough lltuthtl.Ikutba, crossingcrossing thetbe Athi river by a low concrete bridge and running across the sand};sandy bed of the TlV’d.Tiva. It followsfollows the route of an old footpath across the Yatta Plateau,Plateau. where a fault has given rise to :1.a col some 200 ft. below thetbe plateau lexel.level. There is said to hebe a Stone—ageStone-age living site near this e111col but neither the writer nor Dr. B. N. TentperleyTemperley of the Ministry of Works were able to find it.

From Ikuthalkutha a fairly:fairly good road runs east and north—eastnorth-east to Kanziku,Kanziku. with motor—motor- ableahle trackstraeks branching south to KasaalaKitsaala and Kimathena. An meellentexcellent road has been made through the 'lsavoTsavo Royal National Park northwards from Voi toto meet the latter tracktruck at Kimathena. From just south—nestsouth-west of lkuthaIkutha a nevtnew motor—traekmotor-track runsruns north:north- west to KalebuKalebti and KituiKitni as an access road for the Athi-TivaAthirlixa ReelamatinnReclamation Seheme.Scheme. TwoI'wo roads reach the Athi.»\thi1i\riverer in the south-nest.south-west, the most northerly,northerly. primarilyprimnrih a me-fire- break for ,thethe D1121Dwa sisal estaestate,t: . reaehingreaching the rixerriver 211at Mitsalani.Masalani, and the other a few miles to the south-east running to KithEnldKithiulu frontfrom I'Vldnoni.Manoni, \xhiehwhich is reaehedreached from either Kibwezi.Kibwezi, 13.1via D113.Dwa, or Masongaleni.l\1dsong;1leni.

8111161537South A.37 .‘ljtt'llh, 14.111 maps coxering the are; haw been published. sheet Maps.-Two maps covering the area have been published, sheet 0 1111on :1a se‘li‘ii‘scale 111‘of I:l :35t’l.(lt‘tl'|.250,000, dated 1912,l9l2i and Voi\'11i (EAT.(E.A.F. No. lTl-tl1714) on a scale of l:5t'H‘fl.t'tl’}tl.1: 500,000, dated 1946.11946, theThe latter map111.11) appears to be based largely on the 1912Ml: sheet “hieh.which, while good in parts,parts. eontainscontains glaring inaeetirtteiesinaccuracies which'11.hieh huehave been perpetuated on the 194619% sheet. inIn V'.C\\view of the unreunreliability'bilit:~ of these maps the whole\Vholle of the topography for the geologicalgeologieai map was taken from air photographs made:1111de by the R.A.F.RAF. in 1948.1948, controlledeontrolied by '11a plane-tableplnnevtable surveysurte}. based on surveyedstimeyed trigonometrica1trigonotnetrieal stations. The form-lines are based on eorreetedcorrected spot heights obtained by a single tineroidaneroid barometer and must there-7there- fore be considered {isas onlj.‘only apnrtwimate.approximate. The area to the north was mapppd in 195219*: by E. P. Saggerson ol'of the Mines and GeologiedlGeological Department (Saggerson.(Saggerson, 193.1"1957)* sineesince uhiehwhich time a new triangulation st1r\e\1survey has b~1enbeen made and several new stations fixedlixed near thetbe eommoncommon border.border, enabling points on the borderhorder to be fixedt1\ed withnith greater ateeui'ue}:accuracy. Itl1 will be seen ththat1t some eommoncommon pointspomts on the border differdi tlet by up to hahalflt Lta mile on thlthe two11o maps.

ACKNOWLEDGEMENTS.1\('K.\()\\‘IFD

'1Thanksh.tnlss are due toto thethe Administrative5dimini strative Otl1eersOfficers of thethe KittiiKitui a1eaarea ioi'for their assistdneeassistance dotingduring the surxej..survey, and to the Manager and stallstaff otof DwaDna Plantitt:Plantations11ns Ltd.ltd torfor helhelplp and hmpitulitehospitality.

II-PREVIOUSll—PREVIOL'S GEOLOGICAL WORK

The first :ra1eilertraveller uhowho Eeltleft a record of hishis ionrneyingjourneyingsﬁ in the.the area was [t1du'=Ludwig KraptKrapf tRzehzzrtis.(Richards, IEJ.‘1950,t‘ pp. 54-871154-83) who passed ththroughtrough thethe north-nortb-western“estern part of thethe area 111 1849. but made area in 1849, but made no mention of the geolout‘geology.D. lnlIn lRTT.1877, .l.J. .‘xl.M. HildebrantitHildebrandt MSW.(1879, pp. 7134-3-12)334-342) followed thethe Athi titerriver for several miles north—nestnorth-west from MiltomaniMikomani (possibly.(possibly Ngomano)Ngointtno) and crossed the YettaYatta Plateau to reachreueh the TimTiva somewhere rues:west of .lltntha.Ikutha. He mentions various soil tapes.types, and 112‘of the Yattr.Yatta l"111e.111Plateau he s51:says- "lt"It seems toto be 11:1an 111111;outflow111.1. t1t111l12111'1111(auslauler) of the snort.momgrt-tsnow-mountain Kenya",Kenya '

* References are quoted on page 36.

. ~~ 4

J. W. Gregory (1896, pp. 76-79) in 1893 followed the route of the present-day railway between Masongaleni and Kibwezi, where he stayed at the East African Scottish Mission whose station was situated near Dwa Rock, at the edge of the Kibwezi lava, to which he refers, commenting that its surface features indicate that it must be of recent age. He again visited East Africa in 1919, and in his subsequent book (Gregory, 1921, pp. 184-191) discussed the Yatta Plateau and the Kyulu (Chyulu) volcanoes. .

Captain L. Aylmer in 1907 travelled from Kibwezi to Ikutha, where he began a traverse down the Tiva which took him eastwards beyond the limits of the present area (Aylmer, 1908). The geology is tersely summed up in the sentence "Quartz abounded everywhere but no shale formation was seen which gave promise of coal."

Brief reports on the geology were made by E. E.Walker (1902, pp. 4-5) and H. B. Muff (Maufe) 1908, p. 21). Both mention gneisses, and the volcanic rocks of the Yatta Plateau and Kibwezi valley. E. Krenkel visited the area in 1908 and later (Krenkel, 1911, p. 257) briefly referred to crystalline schists, strongly folded on N.-S. axes, with variations to N.W. In a later book (Krenkel, 1925, p. 241) he identified the Yatta Plateau as an extension of the Kapiti phonolites, probably quoting Gregory, and his map XXI included the area in broad detail.

Fritz Behrend's map (Behrend, 1918) shows the area as crystalline schists overlain by younger volcanics of the Chyulus and the Yatta Plateau, the latter being shown as an isolated sheet.

The existence of graphite in the area was first recorded in 1940, and a good deal of prospecting has been done since then. This is discussed at greater length on p. 29.

Much of the area to the south was mapped by‘ J. Parkinson (1947), but there is a .

‘ gap of a few miles between the limits‘ of the two areas.

B. N. Temperley (1956, Vol. II, pp. 64-122), studied the lavas of the Kibwezi and .L Masongaleni valleys as part of a wider survey‘ concerned with water-supplies.

‘ E. P. Saggerson (1957) mapped the adjoining‘ area to the north, and there are

. differences between the nomenclature of rock¥ types used for his map and that of the

‘ present map. Saggerson's group Xh' (biotite-hornblendei gneisses including hornblende granulites) has been subdivided in the present area into Xh' (hornblende-biotite gneisses) and Xhh (hornblende gneisses), the latter symbol being used for rocks in which bioMe does not exceed 20 per cent of the total hornblende-biotite content. In addition Saggerson's group Xs (psammitic bands in a semi-pelitic series) is represented by Xgg (quartzo-felspathic gneisses and granulites) and in one case by Xn (granitoid gneisses).

III-PHYSIOGRAPHY

The most noticeable feature of the area is the Yatta Plateau, which is roughly paralleled by the Athi river on the west and the Tiva river on the east, each of which has its own distinctive drainage system. To the east of the Tiva system is a gently sloping bush-covered plain with poorly defined drainage, broken by a major range of hills and other isolated hills and ridges.

The Yatta Plateau is capped by lava which is considered to have flowed out on the sub-Miocene peneplain (Schoemann, 1948, p. 3), and is here taken as the datum level for the erosion surfaces in the area (Fig. 1), there being no fossil evidence of the

~ 5

'" '0,.., '00 ,'.. 2'30' 5. , Degraded remnants of Eru:l-Cretaceous surface SCALE 0 5 10 t'.ILES ~ Sub. Miocene surface (under lava on Yatta Plateau) 5 J;. J ~ (riSing from 2500 to 3100 feet a.D)

D Post-Miocene dissection ",.' r-:-:-l End-Tertiary$urface ,~ Gradientanddirectionofslope L-J (rising from 1700 to 1900 feet a.D.)

Fig. I-Physiographical sketch-mapof the lkuthaarea age of the sub-Miocene or any other of the surfaces. The spot-heights marked on Fig. 1 were read on the present-day surface of the lava, which maintains a fairly constant thickness of about 30 ft. The plateau thus falls from about 3,100 ft. in the north- west to about 2,500 ft. in the south-east, a steady fall of 12 ft. per mile, excluding the portion of the plateau in the north-west, which is tilted by post-Miocene faulting and locally slopes down to the north. Further remnants of the sub-Miocene surface are represented by well-defined shelves on the hills of the Ngunumu-Mbatu ridge in the centre or the area, and in the south-west by the isolated hills of Chae, Dwa, Kalima

If 6

Koii and Muliluni.Muliluni, which.which, while high in relation to the Yatta Plateau (perhaps.(perhaps a resultresult of post—Miocenepost-Miocene tilting).tilting), show a good accord of summit levels. Poorly marked shelves and ridges on Kimathena and neighbouring hills may also mark remnants of thisthis Rurface.surface, but.but this could not be determined with any degree of certainty. East of the Tiva drainage system the ground slopes gently toto the easteouth-east.east-south-east, and thethe extreme north—eastnorth-east is consideredconsidered to bebe part of the end—Tertiaryend-Tertiary peneplain.peneplain, which isl\ uwellell developed over much of Kenya. It lies between about 1.7001,700 and 1.9001,900 ft. here.here, and grades down to a base-level of erosion of 400 ft. in the Kilil‘i-MazerasKi1ifi-Mazeras area lCaswell.(Caswell, 1956.1956, p. (a).6). This surface is still developing by cutting back to the westward.westward, across countrycountry with a gradient of about 25 ft. per mile. In thethe south—eastsouth-east an averageaverage gradient of SS55 ft. per mile along the line of thethe Tiva river is an indication thatthat thethe TimTiva is still cutting down to this level.level. A few miles further east thethe Tiva Opensopens out intointo a flflat,at. swampy bed in the end—Tertiaryend-Tertiary peneplain proper (Sanders.(Sanders, 1963). No ermionerosion surface older than the sub—Miocenesub-Miocene was proved to be present,present. though both Kimathena and lthumba.Ithumba, and possibly other billshills on the ridge continuing north‘northwards from Kimathena,Kimathena. probably mark much-degraded remnants of the end-Cretaceous pcneplain.peneplain, which would be expected to have lain more than a thousandthousand feet higher thanthan the sub~hv1iocenesub-Miocene surface here.here, i.e. aboyeabove 4,000 ft. at Kimathena. At three points on thethe eastern slope.slope of thethe YuttaYatta Plateau between IkugiIkugi andand Wathoni shelves up to 150 ft. wide were noted.noted, at 2.2602,260 ft. at lkugi.Ikugi, 2.3452,345 ft.ft. just north of \VathoniWathoni and at 2,230 ft,ft. at \Vathoni.Wa1honi. However.However, no corresponding bevels were found on the western Slope.slope, and it seems probable that these shelves are due toto differential rates of erosion in anan areaarea of rapidly alternating rock typestypes ratherrather thanthan remnantsremnants of old erosion surfaces. Residual hills are not,not. uncommon.uncommon, particularly on the end-Tertiary surface and on the pediment between it and the sub-Miocene surface. They taketake, the form of tors and inselbergs,inselbergs. and usually mark the outcrop of steeplyadippingsteeply-dipping strata. Although the rock types in such hills vary from granitoid gneisgesgneisses to hornblende gneisses and migmatites they are always of a leucocratic.leucocratic, and therefore fairly competent.competent, facies.facies. The whale-back rock of Katolotwa.Katolotwa, seven miles north-v.north-westest of Kimathena.Kimathena, is an excellent example of a miniature insclberg.inselberg. It is oval in plan.plan, about 1,2001.300 ft. by 600 ft.ft., elongated along the strike.strike, its summit being over 200 ft,ft. above the surrounding plain. itsIts slopes are convex.convex, but vertical or near vertical at the foot.foot, where the ground is littered with exfoliation debris resulting from “onion—skin”"onion-skin" weathering.weathering, the splitting-oilsplitting-off of successive sheets of rock parallel to its outer surface.surface, with little regard to the foliation of the rock. The finalfinal stage of such weathering which still remains visible above the soil cover of the plains is a flflatat rock platform.platform, bare of vegetation e\ceptexcept where stunted bushes have gained a root-hold in joints and creyices.crevices. Such platforms are seen at Chae and Siulungu.Siulungu, parts of a discontinuous elongated outcrop scyenseven miles south—eastsouth-east of Kimathena.Kirnathena.

The Yatta Plateau has long been an object of interest to geologists. itIt is a lava-1ava- capped feature beginning at 01Oi Doinyo Sabuk.Sabuk, near Thika,Thika. as a broad plateau.plateau, soon narrowing to an average width of one to two miles and running south-south-east and finallyfinally east to a point east of 'l'savo.Tsavo, a total length of about 180 miles. Gregory (l92l.(1921, pp.pp. 184489)184-189) summed up the salient facts as follows:follows:-thethe lava cupscaps the summit of a long,long. high ridge,ridge. and nowhere shows evidence of having overflowedoverflowed the ridge or flowed oyer the plain 0 over the plain at the foot of the ridridge:a e: there isia no evidence that the lava flow ever covered an appreciably wider area thanI han that of the present-daypresent—day plateau: the lava surface falls gradually along its \vhtwhole‘tl e length.length, and never at a steeper gradient than 30 ft. in a mile. He made errors in some minor details.details, cg.e.g. "Its"lts width E. of Masonga- leniieni is between three and fourfour miles.miles . . .". Later work has disclosed that pebbles of the lava are found up to 15lfi miles from the plateau,plateau. across a waterahed.watershed, in the area west of RumKitui township (Schoemann.(Schoemann, 1948,WAS. p. 6,:6) and pebbles have also been found at

~j.::.,, . 7

Kinyiki hill,hill. south—westsouth-west of the present area,area. and J133 miles from the plateau and separated from it by the Athi river. East of Tsavo'l'savo t'Sanders.(Sanders, 1963)1963t records a gradient of 35 ft. per mile. Some variations of gradient also occur due to post-Miocene faulting,faulting. as in the north-west part of the present area. Gregory concluded that the lava flowedflowed down a valley.valley, subsequent erosion having worn down the flanking gneisscsgneisses at a much greater rate than the lava itself,itself. which protected the underlying rocks to leave the plateau as it is today,today. in this area up to 1,0001.000 ft.it. higher than the bed of the Tivaliver and up to 600 ft. higher than the Athi. Dodson (1953,H953. pp. 4-5)4-5.) has put forward several arguments against this hypothesis. He writes that it seems highly improbable that a single lava flflowow could continue to flow over such a great distance.distance, especially since the thickness of the flowflow seldom exceeds 50 ft.,t't.. and that the characteristics of the lava suggest that it must have been fairly viscous. He adds that nowhere in the south—eastsouth-east Machakos area were underlying valley deposits found. He therefore advances the alternative theory that the lava was extruded along fault—linefault-line fissures extending along its whole length and that the surface flow nowhere spread [orfor a distance greater than 500 yards from its point of extrusion. One obvious weak point in his argument is his tacit acceptance of the original thickness of the lava as 50 ft. He accepts the fact that the lava was extruded on the sub—Miocenesub-Miocene peneplain.peneplain, and his map shows dissection in the gneisses to a depth of about 600 ft. below this surface. Much of this erosion is due to pediplanation, and the main process of erosion of the phonolite must be by lateral spalling.spalling, but surface erosion of the lava is taking place.place, proved by altitudes taken across the plateau which showed the present surface of the lava to be as much as 15 ft. higher in the centre than on the flanks,flanks. It must therefore be concluded that the flow might well have been twice its present thickness. The writer made extensive traverses along and across the summit of the plateau.plateau, and looked for basal contacts at 38 dillerentdifferent localities. The base of the lava could seldom be fifixedxed with any degreedegree" of accuracy owing to the heavy bush and thick soil cover and the large amount of lava screcscree that masks the slopes. Nothing suggesting valley deposits was found.found, and often it appeared that the lava rests on the red soil of the original peneplain.peneplain, which is indistinguishable from more recent soils. (The(fhe phono'phonolite itself weathers to a bright red soil on the plateau.plateau, not to blztckblack soil such as so often occurs on phonolites in Kenya.) Fairly reliable fifixesxes of the lava base gave thicknesses varying between 10 and 55 ft.,lit... but the average is about 30 ft. The width of the plateau (excluding spurslspurs) varies from two and a half miles to three—titiartcrsthree-quarters of a mile,mile. and at two places the lava is breached along fault-lines. Both of these gaps and the area between the plateau and the large outlier of Ndoyani in the centre of the area were carefully traversedtraversed in a search for feeder dykcsdykes as envisaged by Dodson. In the two fault gaps the country rock is greatly obscured by scrcescree and huge boulders fallen from the summit.summi,t, but south of Ndoyani lava float is almost completely lacking over much of the country, and clearly no dyke exists here. Similarly no dyltesdykes of phone-litesphonolites were found flanking the plateau.plateau, and it seems certain that none exist.exist, since by the evidence of.of the rock of the plateau itself such a dyke would form a prominent feature. The S-bendIS—hend at Kabaa is difdifficultficult to explain on the dyke hypothesis.hypothesis, but is a very character—characteristic river feature.

Frequently the large phenocrysts of the phonolite show a marked preferred align- mentmerit over fairly large areas. Sixty-fiveSixty—ﬁve such alignments were recorded and almost without exception they lie parallel or sub—parallelsub-parallel to the margins of the lava at those points. 'ItIt is considered that the phenocryst alignments here indicate the direction of ﬂflowow of the lava.

The lav:-lava is visibly vesicular,vesicular. and must have had a high gas content when molten.molten, and would therefore have been very ﬂfluid.uid. Gregory considered that local vesicular structure may have been due to steam generated as the lava flowfloweded over swamps and lakes,lakes. and ifit the volume of water was notnot enough to have “frozen""frozen" the lava the steam caught up may have increased its fluidity.

- 8

Dodson'sDudum'x‘ main point,.‘ that "It seems highly‘ improbable thatHui? ita single lava-flow couldN‘L. untfmzccontinue to flew for a distance of about 170 miles" willw ”1 best"bear furtherfarmer investigation. In much of th”the present' I area, the gradient"" I of'l- the‘ surface”A ' 'C on01": which"“lfﬁh the[176 Mm.lava flowed is now as low as 12 ft. per mile, but it has been proved from bore-hole logs kept by the Ministry of Works that the Kapiti phonolite, the parent mass of the Yatta phonolite, has been tilted since its deposition, and it is clear that the whole sub-Miocene surface must have been tilted to the south-east. The overall fall from the northernmost poin, of the flow to its end is 3,000 ft. over 180 miles, a gradient of 16.6 ft. per mile, but the gradient of the mature peneplain would have been only about half that figure, or eight feet per mile. Kempe's Engineers Year Book (1954, Vol. 1, p. 146) gives a formula for the flow of water in open channels. V = C vRS, where V is velocity in feet-per second, C is a constant (usually found to be 100) R is the ratio of area of cfOss~section of the channel to the wetted perimeter of the channel 911in [hitthat g1.cross-section, and S the slope of the channel. Considering a flow of water down the"\valley in which the Yatta lava flowed, assuming a depth of 60 ft., and a V-shaped channel several miles wide (since 'the width of the channel is very large compared to the depth the wetted perimeter is virtually the same as the width), R becomes 60/2 = 30, and S is 8/5280, and V is calculated as 21.3 ft. per second. Thus a water flow comparable in shape with what the lava is thought to have had originally would take only 12.4 hours to cover the 180 miles. If, for the purpose of argument, both water and molten lava are considered as ideal Newtonian liquids, in the formula V = gdh2 sin (I.. for both 3"1) . liquids g (gravity) h (depth) and sin(l. (gradient) remain the same, so that the two .. . d density . ve1oCItresvary accordmg to - = --: ;-. The density of the congealed lava was 'f) VISCOSIty found to be 2.49, indicating a maximum figure for the molten lava of 2.4. No viscosity figures of phonolites are available, but Daly (1933, p. 72) quotes measured viscosities of flowing:17 Hawaiian3—1, : w(basaltic)x lavas\ asN lowC‘ asL 11 times‘W ‘ that of.‘ water. That felspathoidali; *Pd‘ﬂxVU v ,7 V , v ' lavas have a relatively‘ H. g lowu viscosityvu». is1x indicated41 byM figures‘1 ‘ k quotedHA: in1 the Handbookarm; 1» ofH ‘ K 1 W x < ‘ ‘ Physical Constantsx ‘\‘ w (Geo!.I.‘ Soc.L America,K H‘ Special» 11)Lt‘,'Paper No. 36,‘ 1942,J j pp. 134-135).J -1* poises Laboratory melts of Japanese lavas (at 12000 C.) AndesineL a basalt,‘ 1 . Motomoura1 ‘ 1 3 1,200T4 . , - ‘ ‘ w Olivine\ u basalt,.H Gembudox u" :1 [J 3,180~ 341 ~1w Olivine‘ “7' basalt,, x Konoura"\ u 3t 7327‘; ‘LV k . 7 Nepheline.g .l \ basalt,. Nagahama\ 1 190 (The viscosity of water at about 20° C. is .01 poises). \ ‘ Since these are‘ laboratory melts of congealed lavas it is certain that most or a11of the volatile fluxes had been lost, and the true viscosity of the original lavas must have been many times lower than those quoted. The large phenocrysts of nepheline and anorthoclase> in the YaHait phonolite clearly formed before extrusion, but both would crysta11ise at abovex ‘ 11000‘ |c., x‘so that the above figmes can be taken as some guide to the viscosity of the lava. Assuming that the original viscosity of the Yatta lava was of the order of ‘, . 1,000 times that of‘ water, jf the same viscosity were maintained throughout the flow and assuming the‘ lavaL to be an ideal Newtonian liquid of density 2.4, the flow would have, requiredx» “M_214.9 days to cover the 180 miles.

Thei» movement‘ - of lava flows at Paricutin has been described by Krauskopf (1948, pp. 1267-1283). Soon after extrusion the surface of a lava hardens to scoriaceous blocks which are carried along with the flow, building marginal moraines which dam the lava to a great extent, and also forward over the snout to be overrun by the still molten lava. Krauskopf5‘»u ‘ likens the mechanism to that of a caterpillar tractor. Tempera- w ‘ turesV in 1he lava fall only very slowly along its length, and a lava comparable with the Yatta phonolite above 100 ft. thick (before congealing the thickness in places may have

~ 99

beenhcen severalseVerztlttimesimes thisth: s figure,ltgtne asits the depth of lavalaw would fall{all as.15 the flowlion movedntmed on) would hold its heat byh3 its very\e13 bulk,bulk. aided byb3 the insulatinginsulatim,‘y effect of the cooled blocks on its surface and latent heat of crystallization givengi 3en mloff b3by individualindi3'1dur1l minerals as the lava congealedcongealed.

The actual time taken for emplacement of the flow{1033 may have been several times the estimate forfor;an ideal flow.llow. There is nothing in the microscopic d'tdetailail of the rockr11 k to suggest that a time of even se3enseven or eight hundred daysd113s would.30111ld be tooton g1great.;11. Thethe groundmass is fineltne but holo-crystalline,holo- c13stulllne. and timernever glassyglass). L15as i1it 331111ldwould be if 3er3very quickly chilled.

UndoubtedUndou ted flflowsows of lawlava of lengthslengths toroughlyughl3 comparablecomparable 33ithwith that of the YettaYatta Plateau havehzt3e been described. Barth (1952.(1952, p. 148)I48] quotes a lava flflowow along a water- gradcgrade in the western33estern part of the Grand Canyon which flflowedowed forfor 135 km. 33ithwith 21a drop 131‘of only 210 m.-a1n, :1 gradient of little more than eight feet per mile.mile It therefore seems certain that Gregoi3'sGregory's hypothesish3psothe1s is the true one,one. and that the Yatta Plateau marks the course of an old valley.3alle3. In the present area the S-bendS—hend in the plateau is very characteristic of a river feature.feature, though the reason for the double hendbend isi\ not now clear. The fault thatthzit runs north—north—eastnorth-north-east from Kuhn-tiKabaa may have ininfluencedfluenced the course of the river, though it was not traced to thethe southwestsouth-west of the bend,bend. Gregory followed his theorythe0r3 to the logical conclusion that the emplacement of the lava must have diverted the original river into two133111 rivers.rivers, one on each side of the flflow,ow. now represented by the present-dayprescnt-duy Athi and ’Ii3‘aTiva rivers. Sanders (1963](1963) has prmcdproved that the TivaTi3‘a formerly paralleled the Yatta Plateau far to the south of the sharp easterly111stcrl3’ bend near Wathcni.Wathoni, which he attributes to river capture.capture TheIhe;\th1:‘1ndAthi and Ti3uTiva ri3ersrivers showshow very dissimilar characteristics.characteristics. The Athi.Athi, which has a:1 perennial lluwflow comparedoompared to the very intermittent surlztcesurface flflowow of the Tim.Tiva, shows few of the features of ua mature river system. Long stretches are perfectly straight.straight, and shallow rapids are not uncommon. Its[ts u3‘erugcaverage gradient isis fairly low.low, h1133c3however,er. about l414 ftft. per mile in the present area.area, and the 1riverer is frequentl3'frequently horderedbordered hyby alluviuntalluvium 3'.hichwhich stretchesstretches up to it‘ll500 ff.ft. tromfrom either bank.bank, but isis generally less. ItsIts local catchcatch- mentHl'c‘l‘ll a:area::1 is gent‘13gently undulating countr3.country, and exerteven its major tributaries seldom cut down to‘.1:- sch.solidid toclrock. The MasongaleniMasonguleni river in fact meanders sharply {111‘for several miles. Inn contrastcant‘nst the Tiva'sTi3" ‘s catchment is fairlylairl3‘ deeplydeepl 31 dissecteddissected, and the river's course is frequentlyfrequent)". 1:determineder ne1l byh3 steep bounding rock faces,faces usuallyusuall3 steeplys tecp3l dipping ridges of quartzo-felspathicquart/.11 felspa1thic orcr granitoidg1ranitoid gneisses. It has ncwrthelessnevertheless cut ;a flood1l1111d plain about :1a mile 3311icwide across which it meanders. The gradient of the flfloodood plain is El21 ft. per mile.mile, but the meandering reduces the gradient of the river lJCLlbed to 12-!-133 ft. per mile. The eastward3315:3111 rd portion of the riverri3er fromfrunt the bend of capture near WuthoniWathoni is not titlii‘tltaken into accountaccoum in these calculations since,sincc. as11s mentioned earlier.earlier, the gradient shuwsshows 11a marked steepemisteepening12 herhere 111to 55 it.ft. per mile. The dissection of the TivaTim catchmentcutehme 11‘: area(11:11 is a:1 result of the riverr33cr capture. Altitudes read along the:he old course of111 the TivaT3311 south31.111111 of WathoniWitthoni show thatmat the capture redled to ita cutting-down“11111191113111 of the river-bedri3 er- l‘, ‘1l to111 200 ft.l‘t. below its original level.level, The marked meanders of the riverr13e: are111‘: an indication thatti at the river:‘i3‘er itself is1s approachingup}munching grade.grade, but the shortness of its tributaries and the low.1333 rainfall:‘rtint'all haveh513'e1.otnot yet3et allunedallowed them to cut down the cateh~catch- mentmen: area to keep paceDECO withMilt the Tiva'sTina's downcutting.dot-szicutting. The finalling] episode in the river's historyhistor3 is an accelerated dun-cutting.down-cutting, which has incised the riverri3er bed totn a depth ofc- 15 ft.,It. sweepingweeping awayand; the alluviumdllLl\:LlflT which33hih the flood-plaintlood- 11.1 11 must have contained.cuntttined. North-west' oft Ikuthalkuthrt two1330 cut-offc1.:-otf meanders can still behe recognized whose beds are now1111331 above the Tiva's1s bed,bed. showing that:h:1.t they were cut off before the:he meanders completed their incision. 'The' reason* sen for1111' this1h»: last1:11: t‘lntkdownflcuttingc‘tlliillg was not found,found. but is probablyprohahl a result of Pleistocene rejuvenation.:lLL‘3Cl1ElliclIl.

I 10

710The {1151.1third 1'i1'01‘river 51.51071".system 111'of the1110 310:1area is3: £110the [JOOI".'_‘-'poorly 11:1311011'defined 11drainage of the end- Tertiary'11 11011011111511peneplain .11111and 1111:the 1101111110111pediment 0111110above 1'1it 111in I10the 111:11‘tI‘1—01'151‘1.north-east. '1110The rivers here, the largest151111051 of1.11 which11111011 .110are the1110 31111111111Mavuko 51.11.11and Kaloboto,'1\ 111.1 1111 "401are 011711.001choked \1'3111with 5111-111sand .11‘1'1and 11.111:flow only.1111) 11.1101after 11111111354011prolonged 1".11115.rains, and 111011then 0111‘only1 11‘1'for '.-.a 1011few hours.111‘ .1115 They19101 rarely show' rock' exposures, and1111-11 the:11: nature11.111110 of:1." the1111: 011.111‘11',country 5111;011:0515suggests that111.11 down-cuttingdawn—01‘” is not keeping pace with the 11'00111:::1111;1weathering 111'of 1110the 11:1.50:1).L1‘:1;underlying 11101.5.rocks. These1'L'1051': 1'1101'5'.rivers, like the Tiva, drain towards the 001111.21central 1'reaches:11 1105 111'of the1111.‘ '1Tana,:-.:1.1. but 1'.it is1'5' not1111'. 1'11known11.1.1.1 if11' 1'.they reach that river or are lost in the1‘10 3101115plains before1101'1'11'1.‘ then.(111.111.

IV-SUMMARYIV—SLTM VARY OI"OF GEOLOGY The[1112 5.11311solid 1.11.1.5rocks underlying11‘111.‘.'1_1':'111; 111‘the "whole.11'11 " 1.10.11area :11'0are 1'11of 1.1110the 13:1Basement0:110:11 \1System,510111. 0511151013111considered .15.1\z"'h:101211as Archaean 11121g1.in age, with1'1131‘1'1111111'11‘11111115311115.very minor intrusions. For1" 1".1'11‘111015'.the most 111.1part 11101they are 0111::0'11covered 111with 511110'superficial”.11 11011115115deposits .11'of PleistoceneP05100011." {11111and RecentRc:1‘111 1‘11;age. '1Two1.1.1 5:105series .111of 1:11:15.lavas .110are present,11105-3111. .one .‘11'of 3110100110Miocene and 1110the 1111's.“?other 111'of R000111Recent I100age. Basement81.111.111.111 .‘1'11.'1..'.'1.1'.~~~T110System.- The Basement11350111011: 51.510111.System rocks11101.5 .'.'111‘515'.consist of111' 11a 5110005511111succession 01"of para-p.119.» g110f551‘5.gneisses, 111.11111,mainly 11.311111101111110hornblendic 111in :11211'110101'.character, 1111111511111:with biotite appearing :15as 1111an important1111110111." 011-35111111-1111.1..11'115'constituent towards 3110the 0.151.east. 1201.111Relatively11‘01 minor:1110111 111101.11111110115‘intercalations 111in 11115this 501205series '11.are0 1110115111diopsidic, 015111214quartzo-felspathic151131111: 5:1.1and 1;";1111111111granitoid 5411015505.gneisses, 11.1111.with 013.51.111‘1'0121105101105crystalline limestones, quart/.1105quartzites, 111.?and .1110one 5111.11.small 11.11111band .‘11'of 51111111111110sillimanite 1;11‘.1‘155.gneiss. A11All :11:are 511011;).strongly 11101211111111111115001metamorphosed {11101and 11.1-01.1.-meta-

5.111"somatized,.{011 111111and 1.1111contain 1111no 1115511.fossils 1f 11:.by 11111011which 11101they 0.111can 110be 15:30.1.dated. '1'1101'They .1150are 1255:1101}assigned to1.1 the1110 Basement'I.‘1.5'"110:1‘. 81.570111System 1"‘211from 111011'their 011151close resemblance10501111111100 11.1to rocks.‘111'1.5 111'of {11:11that 51510111system in11'. :1‘1..111j.'many 1111.10:other 11.1115parts 111'of Kenya,150:1}... 51111::some .11of 111121which have1.10 1.10011been 1.1.111dated1.1 1"by 0110111113111chemical 111011.115means 3116and 111011.11proved 1:1to 11-:be PrecambrianP1'0:;11"11.11'1;.11 31‘.in .1130.age. 11".:The granitoid51111112.? gneisses 311.1and {11.111111quartzo-felspathic1'1. 15p:11111': gneisses, 1.111111both of01' 11111.1.which :consist of little1111: more:1‘11 than111.1:1 1.11.1.11'11'quartz -1111and felspar,10151111. form1111‘1‘11 1.110the 11‘most11051 .1conspicuous" 511': :1.1:15 features1012111105 of the area, 1111.01other 111-11than 1110‘the Yatta.-'."11 1‘.1‘.0;11.1.110"Plateau. They 1110.are usually115113111. 50011‘1steeply 11dipping1.1111111; 211111and form1111112- long sharp ridges.' 5'. The1110 g1'11'1'1.1granitoid . gneisses015505 which."1.11.".'1 1111111form 111':the highest1 11.1111point 11'.in 1110the 1110.1.area, the ridge of Kimathena,'11'1‘.-:;11 are 11’1;'1mtightly folded3111011 into' the almost vertical"‘11 :111‘0core .11of :1.a syncline which311.111 has11;.” remained11'as5 :111an '01..erosion5' residual. The hornblendic gneisses'1.10155 '5 .17are0 0generally0110.; poorly exposed\11115011 um.owing‘ to11.1 1111:11‘their 0.15.."ease of erosion, and the most mafic 1110105facies 1‘13"of 111050these 1;111.gneisses,'5505. 1111.1the plagio-pingi.1~

01.15."clase .1:1‘.111hi1.1.~11‘.05.amphibolites, 30111:.“generally 170111111form negative00:111.“ 10.1.1111'0'1',features. The- Cl"crystalline111110 "limestones, 11.11110while 5011110111105sometimes 1111111forming1111: .1'\I"'1.".distinct 1‘1151111‘0positive 1'features,05. I170are 111‘10noften 111111011planed .111off to111 1110the 11:01level 111'of I111.the 51113113111111;surrounding 1.1110gneisses.5 1:5 '.1.:1.1.and, where1111010 511110111-"..',superficial 00101cover 15‘.is 111ithick, .110are 1111011often 11119015511111:impossible 1.11to 11:10.1trace for101‘ 1111:;any 11151111100distance along 11:0the 511115111101.strike. They .151111113‘usually 01111carry 5111.111small flakes11.11505 111'of g1‘11111‘1310.graphite, 3.11.11and 31.101111}locally 1110111110include 111‘or 1'overlie011.0 11011111111017.1515workable deposits 1.11of grdplmgraphite. 11111151105Intrusives E11111‘110into the 801501110111Basement 8'11:System 11101.5rocks ;110are 1'011‘few 111111and 5.11'11‘1'1.small. They include a dyke 111'of '1511150'1'10.pyroxenite, .1a 11111;:plug 111'of 111111110olivine 1101‘norite and a 51:11.few 5111.111small ;'1;j_.'111111171:5pegmatites 01111551111;consisting 111"of .111.“quartz, felsparp.11 and mica.1111011. T110Ba5011'1011tThe Basement System8151001 1110105rocks .110are 1111.11all of 50:111‘10111a1‘1‘sedimentary origin.111:1;11. '1'110The 110111113011110hornblende-biotite~311i1111':0

gneisses'1: 11.55505 1..are'0 1111,1111;htthought 10:01::‘050111to represent a 111110101100difference 111'of 00111130511composition1111 111in thcthe 0111;111:111original 50011101115sediments, perhaps.0 11.1115 .111an {11311111131100abundance 111011101110of chlorite, .15as 01111111111001compared 1113.11with 1111.150those 11111which. gawgave 1150rise 11:1to 1110the 1'1111'115horn- blende111011110"TJ 1;1101.5'505gneisses, 111111017rather 1'11than1'1 :1.a 111110101100difference inE11 11101011111131.1110metamorphic grade. 1111‘The granimidgranitoid 111111and 1.1113:quartzo-felspathic'1'171 1015411111110 gncgneisses15505. are mnsidcredconsidered 10to be of 5011:111031‘11r1'01‘1g311.sedimentary origin, and 111'1g1na113.originally 150170were 5311111sandy 51101.1strata and 1'11505111lenses in 13111.0.rrather 1111.(111_\'muddy 5011111101115sediments. 511111111111.Similarly 1110the 01"1‘5131111‘0crystalline 1§n10~lime- 5101105stones [1.101and 1131111511111diopsidic .0110gneisses550 101100.reflect 1.03local1 increases11101031505 in111 1110the 0.11calcareous111011115 01111.content01'. of 11-10the 11111;?11111original 5011111101115.1114;111:1350sediments. The plagioclase 11.111111110011105amphibolites 111.111.11.113‘probably 010'developed0111111. d f‘nmfrom 11110fine-grainedgrained 111111131111105mudstones 151111with a 101'11'C11dlow sand 0111110111..content. 1110:;They 01:0are {1111.115always 01.111101111111110conformable \13'1E1with 1‘10the 511110.strike of the ne'ghbnurfngneighbouring 111.0"1\'5'."'1rocks. TTertiary1.1.131 V1"11.".‘11111'1'.»'.~~--'1'110olcanics.- The lava1.1.13 0:111capping111111;" 1111‘the \Yatta[111.1 PlateauP310 1111 55is of 1110the 11130type 111.11that 0.11135covers 1110the KapniKapiti P1111115Plains 51111111south and south-west5.111111—11051 1‘1'of _\11'111b1_Nairobi, where0.11010 5'.it 1'5is 11.11.111.71known :15as Kapitian15.11.11: Phonolite.1’h11r1111310, It 11215has :1a 11110011111100]fine-grained ;111'111111L1111:155.groundmass, 1.11.11but 001:1carries" 1.1.1150large phenocrysts{11101111013515 offof 3.110111111-anortho- 033513clase 2.1111and nepheline.3101111011110. It11 15is 0111151110201considered :0.to be111‘ 311.11.011.11Miocene 51‘.in age.age, 511100since 11it lies105 1111on 1210the 5:111»sub- Miocene\1'1‘1."0r.: 11011013111111.peneplain. ()1'.‘:'Over 11115:most 111of 1311':the K.1_:‘.Kapiti_; 1’?Plains:115 1110the 1111.1'1ni1tephonolite \10111110:'5.weathers ‘111to .1.a 771131035black

~ 111 I

clay soil,9.11 b!but1: hereHum the1111: soil,NH. where it does11.1: \ not111,": exceedman] two‘11.. feet1'c in311 depth,1191‘. is13 f"ccfree from‘1'1‘11‘1‘. waterloggingwaist]. ggfig and weathers111.51 «1. {1'911bright red. PleistoceneP.1"v1‘111111'11't and(11111" Recent.-MuchR11 “511': Much 1311'of the111: 501511solid rock1.1" of 111:the 11".;area is1~. mantled1115111111311 with red-136: brown sandy“4.111.111 soils,<0317. mainly111111111 residual1191112111 in111 character5111.73.33? and11111.1 deriveddcr‘fVe' by113' weathering‘»\‘C‘.1t1‘1:21‘111g and.11111 mechanical111261151111- breakdown11..11....1.11. of the underlying rocks.1065;. Where drainaged1'1'111111.31 is13 poor1.1.11.1: black-cottonblack-5.111011 soils«111 s have1121‘ . developed.11:\'c:1.111cd. The' .' Athi. .E river1‘1\‘€I' has111.1% :1a narrow11.111011; band1.1111111 of01 sandy5111.1} alluvium.11111‘111111 along31.11.13 most111.11%: of its1i§1eng:h.length, sometimes5311123111165 interstratified11:11:‘1\1r:1t1ficd with“1111 thin1.11131 grey layers211.115 111‘of fine5111.: silti111 deposited151111511eu' d.during11‘: 11:1 floods.l111odfi. MostMINE of 111.the larger1.113.: rivers:‘1'1'31‘1 have111110 53.11111;sandy beds,bc-dw. but1.111: none111111: have1:1:t ailmialalluvial flats.119.165. Small1111 depositsdcpnsiw 011of surface$1114..“ limestone1111101011.: (kunkar)11.51.1111 11-; have31:11.: 133181111131]developed (11‘on 11ndand near most1.15: 111'of i111:the 11111.3:11115limestones 1111.1and .11c1‘over [bethe 111more111C 111:111.‘mafic hornblende1111111111: HERE.rocks. 1\‘.1.11.11c:c amphibolites"11.111111.'11£1cs 1111.1and 1».well-developed..\C11132d1 .1,crystalline‘ ..1111‘1c limestones.11:1‘1C511111C3. Thickness111gkncxs about1111:1111 10,000111M111 ft.11E.

ViDETAILSV-DETAILS ()FOF GEOLOCGEOLOGY\ I.-Thel.—-Tl|c BasfememBasement .jstemSystem The rocks of the 151...:1113111Basement \j,~..1c111System in1:1 1111sthis (1:011area 115'.“are .110511151classified 2:\as follows:-1111111115:7 111(1) L\1CI;U‘1‘IUFP1]USQL1Metamorphosed pumm‘psammiticﬂii.‘ smiinu‘nh:sediments- (a)1111 Quart/.1195.Quartzites. H1)(b) GranimidGranitoid gneisses. (c)(1 ‘1 Quartzo-felspathic(';>11;11‘tz..1»‘1‘c1x1111'1 gneisses=1w:\ 111111and 15141111111»:granulites. (d)(111 Biotin:Biotite gneisses.gnu-Eucs. (e)1 Bimiitc-Biotite-garnetgarnet gneix’qgneisses.ﬁ

(2)-1 'XIcIamorphoscdMetamorphosed 51111111161111::c1u1111311tvisemi-pelitic sediments- (a) Hornblende-biotitel-Ior11b1c11dc-b1111'w "11¢s.gneisses. (f1)(b) HornblendeHomblcndc uncisgneisses. 1:1(c) Hornblende-garnetHornblemie-ugrnet gneisses.nn (d)[11" Hornblende-diopsideHm‘nhlem’i: d10m1d gneisses.

(3).\11:Metamorphosed31.111 111111030? pelitic1?. 11:: sediments-\6111111cn1»A {111(a) PlagioclasePhgmdmc :1'1‘1111111111111cs,amphibolites. {1'11(b) 8511111'-L11‘111cSillimanite fJI‘LT‘1\&_‘§.gneisses.

- 12i:

(4)i—Li MetamorphosedMcimtmi'pimscd calcareouscutaneous sediments-sctiimcms (a)(ii) Crystalline('i‘tiiiiinc limestones.iimcstoncx

(5)i5i Migmatites.Migmuiiic» (a) HornblendeHombicndc migmatites.Uligii‘iﬂiiiCN. (ff)(b) BiotiteBintite ti’iigiiiatites.migmatites.

Intrusivesintrusive) intointo the Basementacrixcm SystemSixtcm comprise--L‘t‘ilipi‘iSL‘" (a)(it) Pyroxenites.Pyro‘x‘cnitcs. (b){in Olivine()ii‘v‘itie norites.iiot‘itsx. (c)(r) MinorMinm‘ acid pegmatites.g‘cgmatitcx‘.

The lowestik3\\'C§[ part of theti?L successionSUCCCRRiHI] that:imi canran be determineddetermincd with“iii! anyan); certaintyL‘Ci‘iainl} isis the Yamalu-KimathenaYumuiu—Kimuthcna syncline,iniSiii‘iC. wherewhat's it isix’ asax‘ follows:-foiimw: ApproximateA p[H-(.=,V:fm‘ 1 average[l‘v‘i‘l'ti‘i’t’ thicknessitkm'm' (feet)(jut!) BiotiteBfntitc migmatitesmigmatitm . . . V i . . . , . 500 HornblendeHui‘nbientic gneissesgnaisw» .V ,V i , . ,. ., .. 400 Hornblende-biotiteHot‘nbicm‘ivbfotitc gneisses,giicfx‘scs‘ with‘.\ith minoriniiiui' bands of tiLartzn-fcispathi;quartzo-felspathic gnciksgneiss ...... ,. . , ,. ,. 1,4001.404;! GranitoidGi‘anitniti gneissesgncissm with“iii? thintiiit: biotitebiiizitc migmatitesi‘iiigiiiatitcx , 1,200L200 PlagioclasePlagiot‘iaqe amphibolites311'1pi‘tii‘0iili‘i . , . it . U1001,000 GranitoidGmnitoid gneissesgncisscx‘ V , , i i . . , . 300

+8004,800

Thisthis group is7< $99.3?tseparated from adjoining L‘\CUSL1F:\exposures into thethc west byin} a[1 faultfiiizit which probablypi'ubtibiy downiiii‘ijxn':downthrows tnto the west,\xcst. though this wasmu: not proved.pi‘mcd. 'l'hcThe nmtnext higiissthighest rocks at:are thezinc hurni‘icmic—bintitchornblende-biotite gncissesgneisses about Ndawemwe,Ntituvcmwc. which Ciicioscenclose L1a Singiesingle band oiof urystailinecrystalline limestoneiiiNCFZUDC newnear thsthe haw.base, refoldedrct‘uidcti to outcmpoutcrop three times.times, and smotheranother neatnear the.ic (up.top, togcti‘ict‘together with a bandEmmi of hortibicndewiiopsidchornblende-diopside gnciis.gneiss, and discontinuous bandxbands ofmf granitnidgranitoid gneiss,“is. Lguat‘tzo-fcisputhfcquartzo-felspathic gsiciss.gneiss, plagioclasepingiociaic ampiii‘sa‘amphibolite:3 and biotitebintitc migmatite,i :te. thethc broadri‘uati outlinesiiiitiincx iii'of the st;succession\SfUil being as follows:-{Jim-.9177

a ' ., , 7 Approximate£1 f."[}"i“,'(i’i.73i‘ii‘t (H'é‘l‘dL’L‘average thicknessI/IEI‘AIIVH' We?!)(feet) HornbictidcinioiltcHornblende-biotite gticimcsgneisses with Qi’fmidiiilii:crystalline iiii‘icstai‘climestone .iiidand plagioclaseiiiaginuiux: mimhiboiftegamphibolites .,. . , i i . .. .. 1,5001.5m! Oiiatt/uvt'cispathigQuartzo-felspathic gnciwogneisses , , , i . , 150I50 HumbicmieHornblende-diopsideflitipside gix‘EWC»gneisses ._ ., .i .. .. . 350 Iloz‘tibiCIidchiw'itcHornblende-biotite gi‘ieiRlL‘Ngneisses ‘0»withit}: quartzo-felspathic.1it"?i'w-t'ciximtif; gi‘L‘.\\:-\gneisses 7 1,800iflUU (7Crystallineris‘itziiine limestone i. , .i . . 4m400 Hornblende-biotiteHm‘t‘,biciitir‘oiuiitc gi‘rc'xxmgneisses Igi‘Lgranitoidfiiiifliti gilCi\\c*~gneisses 101.101,000

5,2005.31“]

ThcThe remainder:‘cnmit‘it‘ici' of the BasementHawaii: :11 System§:.\'(‘:ii I'OL‘Rhrocks fallfaif‘ intoiZ'ii‘i,‘ .917:one Si‘txiip.group, characterizedQi‘LII‘ILCIEI' by Eioriibicmlchornblende gnciswsgneisses with intci‘uii:intercalations of granitoid i ggneiss,."iL‘i\\'. LiJLK.:Z\‘1'fL'i‘lipL’L‘hiL‘quartzo-fe1spathic i:gneiss216155 :1tand crystallineEine iimcqttms.limestone, antiand lesserit<>er bi.bands of hornblende-biotitedo-Lniiiiii gncis‘s.gneiss, biotitebiatiic gnmss.gneiss, horn-horn— blende-diopside"1-di0P5iL’iL‘ gITL‘i‘Kgneiss, plagioclase amphibolite, hornblendeii'i'ﬂiTiL‘I‘iLi‘C migiiiittitcmigmatite and quartzite. 13

The'1 ho prc<3ntpresent thicknessthicki11:35 of111' this1h 5 group 15is about2111121111 70,000711.1010 it.11.13111.but, axas discussed later,‘11131 7 the 1111:true thicknessthicknesg must1111151 be much less,1655. 1111:the width 115of outcropnutcrop being exaggerated by 161321311011repetition oiof beds byb). strike-faultingsirikL—i‘auiting andEnd f\)i(iifolding.ﬂg. Averaging,-\\'c:‘1gix1a out across the wholemimic area1115a lotof the out-1111‘;— L‘1‘L1p9.crops, ih-L‘the 7()_(JHU70,000 ii.ft. breaks 2.1mmdown as 150110.115follows 1.1111(not in

70,000

(1)111 METAMORPHOSEDNIEI‘AHJRPHOhLL) PSAMMITICP§ \313111‘11‘ SEDIMENTSS1ED1\11,\1S The't‘ rocks includedinLiLidL‘tl in this‘11 is section\3L‘tit3n hadtmd their originurizin in .\.,11‘1Ci\it‘1]‘13\sandstones 211111and .11";arkosesRex L‘Lm»con- tainingminim:v only0a minor111111131 amounts of ferruginousfcrwginous or calcareous minerals. TheThc dIstinL'tiundistinction betweenbLtueLm granitoidL112111ilt1id gneissesgn ifse s and111111 quartzo-felspathicquart/,1»-iei'

(a)(111 QuartzitesQ11111'13‘1‘11 True11:1: quartzitesquartzitcg are 11:11few in 1112the 11162.1.area, the1h: felspar11:19pm content of111' most11.1.2.1“ 111of 1111:the psammitic1111111?" rocks being high enough to1111111put them into the1112 quartzo-felspathicquart/m spathfc or granitnzdgranitoid gneissgneisx‘ classes.94:525. AA1111».true quartziteL11. L11 1/i11L which11.11;:1113L‘11rxoccurs 111.at UtundaniL1 1i2L.11L 111 on th-.the TivaIi\.111\L1‘.s111:L‘:111Lnriver, specimen 60/202*,00 11112* isiﬁ a11 dark grey-brown:Ircyhrmm rock: withwth numerous11121112111119 «1211‘:small 13111:“cavities coated:11..t with brown iron oxide.oxidc ItIt‘has112.15 L111.j-.':1only a vague121215111: foliation,13,»). ‘71111. which\‘ALhELh is marked1111: Lu by'j‘ the 111ig111alignment1:111 111‘of the1111: amines.cavities. In111 thin1h: 11 section.xL‘Liiun iti1 shows.hmm a.1 mosaicmm... of111' coarse.1143: anhedral1-. tii'C11 interlocking unstrained11:1311'21111L‘L‘1 quartz111111: 1/ grainsgr :n\ which enclose$11.10;: numerousT‘ill?11L‘.‘.’:‘.‘.% small9111.111 euhedral32.1'1L‘L'11‘11'. orL1" subhedral\.1E1hL‘2Lii‘z1i pale11.1‘jc pinky-brownp.11k\-L,1L1‘»\11 1.111garnets,‘nL: . and small\11‘117: grainsgzuim ofL11 augite2115" .e surrounded\UliULIHKiL‘ki by and11:11! intergrown11‘1L11‘g11111n with opaquet'aputiLiL iron111,111 L‘ai'c.ore. ItJt containsL'tintaiih 92'92 per13:1" cent quartz.quartz, with 4.4 per cent garnet,2.111191. 1 per11m cumcent augite2.11:1:itL= 21nd.and 3 per1121‘ L‘L'.cent1. iron 01‘s."ore. t Specimen' 60/187T 187 fromfront Imiwa111m- 1.11611seven 111113miles 111,1‘111-L‘1;~,tnorth-east L11of Ikutha,liai‘ is‘ light red ini colour,'1 duethis to111 staining«11.111111 byh} iron11'1111 oxide.L1‘..2LiLx ItitLt111tr1111s51‘1131‘L'2311'.contains 80 per cent quartzL;11:11‘1/ and2. 20 per cent microcline, with iron:1‘111‘. 11\2L2;3oxide and tiny2:11}. prisms:‘i‘ixP‘H 111:of :1114111‘12.“apatite .1~.as 1-...‘23sx‘111'icxaccessories.

(b)WI (,1'3111131‘1Granitoid2511" L'IIL‘KAH‘A'gneisses These are generally coarse-grained~1g-51‘21i11cti rocks,1'L1L‘m. varying1111211111; in Lcolour'1.1.11‘ 1111113..from ovumcream to1.1 pink, 11‘111‘and often. LR.carrying very small:1‘.2...i 311111111115amounts 111‘of 11‘111iL'mafic 1.11511:minerals, usually magnetite, hornblende and hfLitit-s.biotite. Foliation is 115usually11:11.11 onivonly poorly1.1011111: 111;1'Y-:Lti.marked, 2.111..and shows itself by alignment of

> * \tinihiisNumbers prefixed 60I referFLY-Lt to:11 specimens\UL'L-l'f'l.11‘5 in5:1 theiiiL‘ regional121' 1:11 .1i collection.1‘1"'_2: '.‘2:1 for11‘" Degree Shect 60 in the Mines\i‘il‘v and GeologicalCL Department,’11‘1‘011. Nairobi.\':111*oiri. 5't .-\HAll modesm quoted are.. volumetric11.11.1111. 1'i: and.11'1.i estimated.L‘ 141 J

mafic 215311115crystals 111111and .~1,1111::?111'::'sometimes by 1116the ﬂflatteningattening 41111and clonga'knnelongation L11of 111111112quartz and feIsparfelspar grains.L:' . 111In 111111thin asL'11L111section 11113this 11111121111111flattening 1*is seen 1.1to be1.1L: duodue 1.11sto re-crystallizationL‘1'L'511111121111L111 of quartz.quartz, 11ndand 111to 11a lesser CKIL‘mextent 1313115'111felspar (usually1511;111' 111iL‘1'1'1C1111L").microcline), 111L1:V:u'1.1.:11individual grains 81111111115;showing 11':as large.large, L‘1L‘11rclear 111611:blebs with1.11111 1L1rounded111L1LL'1 01111111s1.outlines, interlayered1DIL‘1’1‘d}L‘1’CL1 with111111 1111c1'—g1'.111finer-grained1s1'i anguiarangular aggregates 01of quartzLl‘. 1'17. and fL'“felspar,' 11:1.the" 13:16::latter 11x111111f.‘usually Liam-.31.cloudy and 111altered,1:11:11. 11:11.11111and all Showingshowing frasturcfracture 211111and 1::1JLz1u1a'L-undulose L‘\11nLextinction.' :1. 'l1s.

(.10. 167 (1013360/132 60.3165. (‘060/144144 (1019160/191 (11]60/139139 601143 I 60/165 I,-60/148 160/167o ' n n o 11 I % % % % % I1%' o% Quartz . .. I 55 41)40 41140 27 :525 23 12 ()1'11111L‘111SL'Orthoclase . ... [ i, 323: 1 24 (16 1011') 3528 Microcline\'11L'rL)C1111L‘ . . 3631') 7 40 4-040 5.858 60(10 55

I PlagioclasePlugiodass . .::. I 8 :525 18 (f16 10 ?7 4 Accessories.AL'LL‘x'sorim . , i 1 3 Z2 3 1 1+ 1 .. I I --.-.- (1060/167-Dwa167 DWLL Rock. 6011360/132-Ngunumu.-ﬁNnumu. 60/165-Iviani(10 1657111.":11111111611.Ithieti. (1060/144-Kimathena.144 _- K111111111'L‘1'1u. 60"191*Chamwalanyu.60/191-Chamwalanya. 60/139-Kimakimwe.(10‘1‘1397K111131L'1111we. (10.160/148-Dudini.148—D11d1111.

1 V ;L, V 1::(c) QIMLHJJJL.1’\[.‘LI1;1ILQuartzo-felspathic. A‘VL 1:111,gneisses>'MHA>' M111and‘y' granulites2’111.1.1.’!1U1\“-I I" “

ThsssThese varyvar) in11‘». 12010111colour from L'1‘L‘L1mcream to 111111.buff, L11or more 1'31‘L‘1yrarely p111kpink 1:11:or red,rcd. and {11'sare usually1511L111Y of ﬁfinernL-r gm:grain11 than the granim:granitoidL1 gUC1SSCSgneisses, and are <11111'L‘11111L‘.‘sometimes 231111113115.granulitic. They usually1511311: Showshow ghndgood foliation11111311011 marked:11L111Lcd 1‘}by 31113111119111alignment L11of 111:;‘1Lmafic assessor}accessory 111:11L.'.11sminerals. Inter—Inter- 31111111stitial reL‘rysm11izatEnr1recrystallization of quartz and fsl<11"'sfelspars as Lis§L11describedbcd 111in 111Cthe g1'ar11101L1granitoid L'nsisssa'gneisses mmstimcssometimes occur».occurs, :19as 111in SpsL'1111L‘11specimen (11115160/151 1111111from Ndnyam.Ndoyani, 11111but the newsrnewer gcgenerationazation 11fof I‘L-1sp'L11'sfelspars hsrehere 111'sare 11111111131119.orthoclase, 111.11not 1111L'rL1L'1111:.microcline, 0111:.only 511g1111j.slightly mfcmpcfthiticmicroperthitic against the1111: «5111estrongly 1111crn'1LmicroperthiticT111111; L1:'111L1L'1a'~sorthoclase 1'15of the whimolder generation. PiagiwdassPlagioclase felspars1‘c1spurx LlL‘L'are 1111all 111‘of £113the L0111p111~111o11composition of 1‘11‘JDL1LINL.{\CCC’SSUI‘Foligoclase. Accessory 11111161111~minerals 111;111dcinclude 1101'11b1sr1L1s,hornblende, biotitc.biotite, muscovite11113L‘L1111s 11'1'111iron or:ore and apatitL.apatite. $111111:Some 6:15111'L1IL‘L1estimated 111.1L1L";modes 11112117are:- .---. (.10 Z 1 1 (101 1 {1060/13013:) 60/211 60/151 I % QuartzQuark: -- %~ %-1 51.150 3727 I 15 Or1h..1s1:1.~sOrthoclase 1312 (.365 I 454< P1ag111s1z11'1L"~Accessories . . _ . . . 4.1 2!2 5 ! 5 60/2111111 Chmrtzp-fslspaQuartzo-felspathicﬂ11c g1'211111111L'.granulite, Ithangathi.1. 11211115211111 (11160/151151 Quart/w[clipathkQuartzo-felspathic gnc1ss.gneiss, Ndoyani. (11.160/ 1301.31131 (3112111711114spathicQuartzo-felspathic gnsfs's.gneiss, KK11'a111b1c11.wambleti.

~~~""" ...... 15~~

~~(d)1111') Biotite13111.11’11'1‘ gneissesf 11,1111~~

Biotite82111116 gneisses311915588 area:‘:‘ few10.1 and£11111 small31111111 in111 outcrop,1113:1011. occurring11cc111’r1ng mainly11.11111 11%as thin111111 discontinu-d1+c1‘.'1‘.11:111- ous1111s bands in111 the111C hornblende110111913111: gneisses.'. A typical11:11:11! example,L‘\'C11'11P1C.1131911111611specimen 1.11.160/1311711 from111111:

Chunguni,("hungunL isi~ a: light1114111111111.buff, medium-grained111m 1111'31-g1'1111'15'd gneissQ1c11xs containingCQ111'1‘1111111 1rI up1.1111to 1011:) per cumcent of111111111110biotite in.E11 fineﬁni- flakes,11:11:65. whose111111311 alignmenta112nn1sn; givesQ11cs a:1 marked11111111011 foliationfol1a1111111 to11 theme rock.r1111". Granitization(’12‘11111112311-1131 has caused1111113011 aC1 small-scalesnmU-walc developmentLicwl ! 11fof porphyroblastic11111'17hj1111111as11c felspars.1e1

~~(e) Biotite garnet gneisses~~

111-:The 11511);only 11cc111‘1‘cn1'e‘11occurrence of 1111this15 10:11rock type13.11: 11r~Qclarge enough to1,11 be'11-; shown1110\11‘1 1111on the map is1'1 the11“? isolatedisolatcd 1111;:eoutcrop 11fof Nd111l11.Nduni, south11.111111 11fof Ithumba,l1h11mha. which form.forms :1a 11511hill 111‘1'1‘over 10011‘11 111ft. high elongatedclan 15:11:11 along the1115 strike.51111112111In hand-specimenhand—spccinmn (60/(1301-191149) it11 is cream11121111 in 111111111"colour, speckledapccklci with111111 blackb1ack biotite13101110 11111113flakes which“111111 arearc a11g11'cdaligned in111 bands11111115 along2111111" the1111‘ foliation.111111.111In 111111‘ec11011thin section thehe biotite1111. c is15 scanseen to111 11111111111111:be dichroic from yellow3.121 1111 to111 dark b11111:brownish-green,11>11—g1‘ecn, 11ndand 1111:the gumchgarnets brightbugh: pink,pznk. much fractured,frac1111‘2d. 31111and with111111 subhcdm]subhedral 11111111155.outlines. B11111Both orthoclaseorthodasc and plagio-plagfn» clasec1396 fclspaz‘sfelspars occur.occur, the 111111“:latter of1:11 111:the 111111111193111111composition of x1111:sodic C111g11giz1xc.oligoclase. A Qccnmisecond 761111»repre- sentative,«1111111110. €11cc1111€11specimen E1060/195,{'95. is31 :1a local1111111 VELL'LEUUHvariation 111of the1hc 111111711111411x11111111:quartzo-felspathic ggneiss:18; :11at Mkongweni.Niknngwwcni. In111 hand-specimenhand-spcqinmn it11 closelyc105c1y resembles1‘sscmb1cs 60/149;('10 1411; under1.:11jcr the111s microscope:11'C1‘c C1116 the1h: fe1sparsfelspara are'1‘11'1 turbid,111.4111}. and 3111111111appear 10to be mainly111311} 111:1‘Q11‘11'111.e.plagioclase, 111311111again 111'of V1111;sodic 111oligoclase7L‘1‘11dx1 composition,c111111‘1c113111111. with1.11111 a.1 51111111small 31111311111amount of1.11 orthoclase11111111: 351* and microcline.111 1C:'11.C111‘;C.

~~Estimated1351111113311 modesr11111‘11‘x of111‘ thesethew 11111two rocks11.11115 1110:are:-~~

60/1491- 141) {11160/195195 % % (31.111111Quartz .. 3211 H35 01‘11111cl‘11scOrthoclase .1144 121 MxmdincMicrocline .. ,- + PlagioclaseP1ug111g1351‘ . . .. 17 .1144 Biotite”11.11118 .. 5< 16 Muscovite\111'x1‘1111'1c .. .. - + (1:11‘11‘1'1Garnet . . . . 21 31 Iron 111‘:ore ...... + +

~~60/14911119149 Nduni.Ndunf. 111160/195195 1x111111g111-111,Mkongweni.~~

~~1.11(2) .\11i1\\11’JR|’HIf1SFDMETAMORPHOSED S1‘\1.‘»PSEMI-PELITICﬂ.11 51:11SEDIMENTS»::‘\‘ <~~

~~The 111c1u11‘1o1‘ph115cdmetamorphosed sen11»pc1111csemi-pelitic 51111111121119sediments are divided into511.111 four 311111115.groups, viz.:-114,27”~~

('1)(a) HornbkndcHornblende-biotiteb111111e gncgneisses.;/1 {b}(b) H111‘nb1cndeHornblende gnezssugneisses. 1'1")(c) Homblendc—gametHornblende-garnet gneisseqgneisses. (111(d) Hornbicnde-diopsideHornblende-diopside gncisscs.gneisses. The (13V1S11'1118divisions tendlend to ovarianoverlap, and to10 grade towards11111111115 the 1113911111331:plagioclase :11111111111011105.amphibolites. The distinction between homb1endc-hornblende-biotiteb11111 1c gne1s.gneisses‘33 and hornblende11011111151111: gncisscsgneisses has been made where1111616 biotite13101116 211110111115amounts 1:1to -1)20 per11-31' cent0:111 of the 101total1:11 hornblende/biotitehornblende 13101118 content. All Ihcsuthese rock 13111125types are thought to haw:have dcxclopcddeveloped byt1} metamorphism11.1911111101111111:111 111of 1911111111}relatively ﬁfine-grainedns—grained sedfmemssediments 111111with 111211dmarked :a1c111'c1111scalcareous 12111116111.content.

~~. 16l6~~

11.3(a)3.2111111Hornblende-biotite.111111111'—.'1111 111’ 351653.111gneisses These occupy, 1' :1a lttxg:large region in111 the north 111"of the area and pass westward into hornblende'C‘lLDd : b.gneisses,5' 1111:the tt't'tnsitinntransition being:1being grgradualrttdtN. orcrover a width 111'of atttcropoutcrop of a quarter ul’of .1a n1ilc.ll1c3mile. They are t\;1ic:1‘1l\'typically buff .1:or grgrey:1 in colour.colour, with the mafic1‘1‘l.l.ﬁc minerals arranged in lawnlayers interspaced511: 9::::d with.11'1l1 n1t1rcmore lettcocratzcleucocratic l:t;.:"3‘.layers, giving :1a pearl).poorly dedefinedﬁned banding usually 1111on :1a 1111:fine 2:211:12.scale, though lacallylocally alternating handsbands 11pup to h.1lfhalf an inch thick were noted. Inln thin SL‘L'll'Ul'Isection tl‘lC‘the hornblende is found to hebe plcrmhrnf:pleochroic 17.1111from light to Veryvery dark r1livc—olive- green.green, and 1h:the 131.1111:biotite dichmi:dichroic from yellow to dark grccn'sh-browngreenish-brown or black.lack. The biotitebin: :1: i3is usually1131111'11} 33311::itttcdassociated with grains 111'of opaque iron or:ore and is often marginal to caror intergrowni:1..-:'5_;r't:1\1n with the ht'vrnblsnde.hornblende, to which it 311n1etin1essometimes £10113shows :1a clear replacivcreplacive relation- 3l1:}1.ship. Plagindtts:Plagioclase felsparlclspttr pre3cntpresent has the compnsitioncomposition 111'of oligoclnscoligoclase (11'or 11ligocla3'c-oligoclase- unit-Sine.andesine. lnIn 3p:::in1:nspecimen 60313-160/134 from KasasiKa3a3i much of th:the original rncl:rock material has re- CT'V'S'IHHlZCLicrystallized 1111rlcrunder prc33nrc.pressure. Most .11'of the hornblende and b31151:biotite is in wisps and grain3grains aligned:11: 1120115:along 1h:the 1'11linti11n.foliation, and much of the micrmlinemicrocline of the original rock has been rcplructireplaced by l:’1'.:-:‘later quartzartz and 11rth<1uletgczorthoclase. (351mm.Garnet, zircon.zircon, apatite and iron 111‘:ore occur :13as f;:'£:‘\\'[77,k'\.accessories.

~~Four£71111: :3tfnttttctiestimated modes,ntodcx listedH3t in o:'tl::1'order 111'of decreasing htu‘tthlcnd:hornblende canicnt.content, arc:are:-~~

I 60/175 I 60/143 60/169 I I 60/134 (1 ‘5 1 u I % I % % QuartzQuartz. ,...... 22 25 I 15 15 ()rthoclns:Orthoclase 1 . . 2213l3 I 13 22 '1919 MMicroclineicmcl i n c . , ﬁ 44 57- 5555 PlagioclasePlttgioclasc . . 1 52 S8 40 (16 Hornblendellornblcnd: . . 7 5 4 3 Biotite.Bintit: ...... 5 4 14l4 1 Acccs311ric3Accessories 1 1l 5 1l I --.-.---. I

~~(10'175~M:1511lnni.60j175-Masalani. (109'1437T11‘nn1li.60j143-Twanili. (1031697Dwn.60j169-Dwa. 60jI34--Kasasi.(5.0 1347K113g'13i.~~

(1’11(b) I.’..1rrtt',1i.“1111‘1“Hornblende 15/111311'5gneisses 'lh:The ht‘.l‘l‘|l)'.k;11{i€hornblende g:1:i3333gneisses 111':-are mposrdexposed in a l1r1121dbroad handband flankinglinking the 'l'futTiva river:'t\'::' and11111 “:1a narrowernarroncr handband :1l.along.i the Athi riverriver, thnnthoughﬂh it is fairlyfairlj. :(1‘111certain that 1hr}they underlietznclerlf: 1h:the 31:11:1'11:superficial' 3111l3soils 1:111.covering th:the whole .11of 1h:the 3south-western1111h— 11 :3161'11 1141':part ntof th'the 3:“:51'1,area. 'lh:"rTheir 1':3i3t—resist- 1111::ance 111to crnsinnerosion .73is 3.114ll.small, 11ndand :hcythey nun-M11“outcrop only in riwrriver '3"sections“1'11113 11ndand in thethe tli3scctcddissected :':1::.1.1:ntcatchment 111121area :11'of :h:the lTiva.Ra. "They 1.1111vary in :nltttircolour 110211from ht:11'buff 1.1to almmtalmost bl:1:“k.black, according 1.“.to their 1'11r11311‘.content 1E1of mafic" minerals.miners. 3, \‘x'hcr:Where :\ptt3tl.‘cs‘exposures 111':are continunns.continuous, :13as in SIECIClk‘Sstretches .11'of the1h: TivaTl‘-'[1 .1111land D'l'i“DiEu "rivers, 1'11:the :11l11111'colour 111:1:2‘:index :hangmchanges rapidly.rapidly mcrover 3ht1rtshort tli3tt5t1ccs.distances, and '11in placesI. :LL‘L‘S ['"1Cthe l‘Ouimrocks are clearly’1]. hantlulbanded 1111on “.111all 3:4scales:5 ftttn“.from :1.a fraction1‘:'.1:"f:1n 01'of 2111an inch" i1 to11) tens[611‘ Ofof 1 feet.teat. Along53.1.1112 1h:the Tiva[21:1 river"i131 11.1t'tl1-north-west11:31 .11"of Ikuthalltu llCt the 91:133.:3gneisses contain:111t1tt1i11 bandsh= .‘tlof T‘lureddishlﬁl 111111111191-quartzo-felspathic.thi: 3:13.33.gneiss, 11:11perfectly“:11“ :‘1‘111‘1'111'111al1l:conformable with the 31rf.;:strike and “hi:which, while [11iL‘5loften a3as ltttl:little :13as 1.1.1two tcctfeet tl“thick,11:A_ :ztnl“:can be traced 1111for 3:1:r21lseveral mil:miles as.3' :ltej.they :11:cut mu:successive 111:“:11111cr3meanders 01'of th:the r:\;..river. .lantctimrsSometimes 81:3these911:111ti3'bands 1'c.1:l1reach .11..1tlt'&tl3hundreds 111of feet:5: in thicknc33.thickness, 1.1whenhen they 111':are 311111111shown 1121on the 11111.1map :13as 1.111111%quartzo-felspathic20-15:13:13. or granitoid gneisses..uf33cs. S:Similarly'mila:'l1' nthcrother bands 11:-1:1and lenses .1."of 1.1;.crystalline31.... 11: limestone,1111:311111:. hornblende-diopside 571113.33.gneiss, £110.11biotite1.: ggneiss:.-.3 :13.etc. are ' 5111:119intercalated.. 11:4 133111with 1hr:the horn'nluntl:hornblende gneisses. In all the microscope:11::'n~:'1111c 315d:3slides_1‘ , wantexamined. ' 11::1, ' uthe1h;-_

~ 17 hornblende is strongly pleeehfoiepleochroic in shades of olive-green and hefrequentlyquentlx has 5111.13.small amounts of b10111:biotite 111in contaetcontact with 11.it, sometimes as :1a 1Lreplacement,placement. 111:1the latter being 1111;111‘111'edichroic from straw-yellow to dark grey-browngrey—1111111111 L1ror black. 111In specimen (103131.60/121 1l1ethe biotite131111111: occurs as shreds :11in 11L11111le11dc1111hornblende at wide angles to1L1 111:1the foliation111111111011 and neeasionall}occasionally :19as cores 1:)to hornblende masts?crystal aggregates,1221' mates. suggesting. ~'..11g here a replacement of 11101116biotite 11}by hornblende. 1h1111'1hthough in one part 1fof the slide biotin:biotite is seen 1:1to be followed 11};by hornblende 11111111111without replacement. Quartz 1‘1L1e111'3occurs in all the:11:- ilidcsslides examined.examined, and in all but one plagioclaseplagioelase lelspm‘felspar 15is an important111111.11 :11111 constituent,e1111<:111e:1 12111351111ranging in c1’1n1pnsiLiL111composition from acid aligoeloligoclaselass 111to andesine. ()rOrthoclase11111L111se felsptnfelspar oeenz‘soccurs 1nin varying 31110111113amounts in 1111.191most L11~of the seetiomsections, but 1111erc1e1111emicrocline occuroccurs5 111111,only' 1nin 111:the slide511:1e 1nin whEehwhich plagioclasepinuioelnse 1Ris lacking. 111?;This 5111113.slide, speLimenspecimen (10519—1.60/194, 110111from Kanndui.Kaandui, is 111L1e11much ﬁfinerner in grain thanthan normal and 1xis 111111151almost sehistnse.schistose. ExcepzExcept where quartz and feIx‘parfelspar have.have been re-crystallized‘stdllized 111:1the com?—consti- 111e11ttuent n11'nera1sminerals 1111all 511011show 91111::Lr1ingshattering and 51.111111strain eIfeets.effects, :15as :1a 1119111:resu},t of 311011;strong shearing.

~~MineralsMiner'- 1: present as assessuriesaccessories inE11. 111':-the 111.11‘11bl:1hornblende1:1 c gncls~~

~~SomeSnme “1111mmestimated modes are:are:-~~

~~, (10 1.0 601153 (101131 60.5310 (1060/108108‘ (101194 6030960/209 I 60/153 I I I 60/120 I 60/121 160/2101 60/194~~

[1 0/ U (1 ﬂ 0 CI‘ I % 1- % % % % . % 1 % Quartz , . 7 I 20 3J3 4(0)40 R8 40 53 (rhino-chm:Orthoclase . . 15 .7 35 S8 I :222 L) 10 Microelin:Microcline1 . . * ~~ ~7 4g35 I . I 55 i 35 I PlagioclasePlagioeli5e . . 55 62 4‘5.45 35 55 7" I 2535 Hornblendellornblen de 22 16 15 12 10 8H 8.1; IS i Biotite.8101116. . . . l1 m+ l.1 2 i T.2 2 i Z2 Other miner-L115minerals . .+. 2 1l 3 I 3 1:1"6* 32 I I I .. I I I *‘Md"'Mainly 11011iron ore. 60.1“1207K'130L1.60/120-Kisou. 6051537131111160/153-Diliu. 60.“1217Ng\\':13e.60/121-Ngwate. 60/210--Mbatu.60‘11‘2107Mbt'1tu. 601“]60/108-Muthungwoi.OSAMuthu 11gw01 . (10519460/194-Kaandui.Kmndni. (111131.1960/209-Kazi.K171.

(c) Hornblende-garnetHum11.911.11111'L'111‘1111‘ 1 Y . gneisses1:11q' 1 v The aspect 111in 111Cthe ﬁfieldeld 111‘of 111:the 110111111:11de~gar11:1hornblende-garnet gnclwcx.gneisses 1.101111%points 111to thattheir being local variations11111111511115 111‘of {hethe 1111rnblet111chornblende g11efs

x.\ 2= pink,13.2114. Y = yellow-green» :Juw-gt'ccn andmm! ZX ::= 1:11:11).blue. Diopsidetvm'tic Justin‘soccurs 0.113only in.11 (31160/188118C? \x'twherec1): it1. is1‘ L;a I'i‘uiJQfmajor CUTMUZLCTC,constituent. The n‘u‘dw‘modes ‘11.of ihcscthese I‘m‘two rocks1c» were cxiimnmt‘]estimated 1151*-as:-

60 117 I 6060/188188‘ I 60/127 1‘ u (l%, Qtttn‘t/Quartz...... v 5 PlagioclaseHugh 5130 I SO50% 41 Hornblende11011113101111: .. 15 8‘8 Biotite..Binnie . _ .. v- +‘ Diopsidc I - 18 Diopside ..: I . 18 I’UDCI‘SIhCNC .. 25 2 Hypersthene.. .. i Unmet.Gamet...... I S8 2313 Iron[ton on:ore .. .. I 2 3 I 2 .‘V --' \ 6060/127-Hornblende-hypersthene-garnet1ZTiHomMcndc—h)persthcnc—gurnct gntixgneiss,fi. Nzauni.$7;a (KO60/188-Hornblende-diopside-garnet1N8 r1lornblcntic—diopstdc-gnrnct gncf'xg.gneiss, Makaiye.\lukuiyc’. 11/3(d) [[wtnt’v'ftitu‘vt'~dit)/}\M’L’Hornblende-diopside g’igneisses‘s‘W‘n‘V The hurt]hluttisdinphornblende-diopsidefiidc gncisscs'gneisses are dark grey in cohnt‘.colour, of mcditlmmedium 0:or count-coarse gram.grain, and tun-“1;.usually fohatcdfoliated mmon a fifinens 50.110scale '0}by the concentration of the mclanocruiicmelanocratic mineralsmincmls tnin m'marked. :11 laycrxlayers. In thin section the hnmblcndchornblende is wenseen to hebe p1cnchmicpleochroic from lightIfght to dark nlixcolive green.green, and intergrownEntcrgt‘mm with\xith diupéde.diopside, faintly{dimly pleochroicplcncht'oic in light bluerblue- g‘ccns.greens. What'sWhere hypersthcnchypersthene nucttrx‘occurs it ixis strongly plcochruicpleochroic from pink to green. In \pccimcnspecimen bl.)60/157157 from NdnwniNdoyani thcz‘cthere ixis clear cxldcnceevidence of rereplacementfinement atof both diopsidcdiopside and hyperhypersthenefihcnc b}by horn‘thornb~ende,flcndc. a gnudgood daddeal 01of iron or:ore having been thrown out in thcthe process.procc s. In [11:the same spectmcnspecimen are a fenfew 11:1cflakes ofnf red-httmnred-brown biotin:biotite derived from hnrn~horn- bicndc.blende. PlagtnclnicPlagioclase OQQLN‘Soccurs in :111all thsthe sections naminedexamined and is andssincandesine execptexcept in 5pccimenspecimen 6060/118,119. from Katakolo,KLiLtknhx where it is1\ OhgoghtSC-doligoclase-andesine,fldcéfl:C. and is accmnpuniedaccompanied hf;by inthndasc.orthoclase.

~~EstimatedP‘s‘tim‘gttcd modesadCN of thcthe hornblende-diopside1mt‘nblcttdc»d[opitdc gncigneissesﬁcs arctiiare: -~~

60/16160 161 60/157(10157 60/180(\(1 180 I 60/118 I I ! i % I % % 1% QuartzQuartz.. _ . . . 7- - 2021) I 5 I - OrthochtscOrthoclase A . >77 77- -. I 15 PlagioclasePlagiocluac . . I 55 45 61 47 I Biotite..Biotite . . - . 1 7 + I HomblcndcHornblende . . 2328 23 12 12 I DiopsideDiopsidc _ , ..., 161t.» 15 4:4 I h8 I - - HypersthcncHypersthene . . I ., I 13 , , ..\8 i - , I GarnetGarnet.. ., . . 1 ~ i -.T "7 i I i AputitcApatite.. _ _ _ _,+ 1 I 1 2 I 1 2 ..L I Iron ore.. .. I 2 Iron org. . , .... I I 2 I 2 I 3 I 60/161-Koala.16 » Roam. 60/1.157!57-Ndoyani.Ndoyani. 60/180-Uthainga.601'1SO—Lithainga. 60/118-Katakolo.1 IS—Ktitukolo.

(3)(31 METAMORPHOSED.\1F‘t‘_\mJRmr:)~_¢D PELITICi’iiLlHC‘ SEDIMENTSSIiDHIEZN‘w This section wmpmcgcomprises nnlyonly tumtwo rock upcs.types, oi?of veryx'ct’y \‘ut-x'itvarying_ g aqpccts—plagioclascaspects-plagioclase Lirl‘tptOlitL‘Samphibolites andnatd <1Himanilcsillimanite gnciascs‘.gneisses.

~- 19 L . ‘ ,. (a)[1H PlagioclasePL‘LL’JL‘L'JL.LL.1L‘ LL‘t'ii'”1L‘L'.“‘LL)1L1‘.L'L,1amphibolites TheThe field relationshipsl'Ll‘lélliUUShE‘i" L52.of the plagiueinx'eplagioclase Lunphihnlitcsamphibolites L‘LitilLlcould usually hebe (1:1:determinedL'Ln'f \\with1th 50:11:some degdegreeL: 01'of eei‘tLLi:certainty, and proved that they are eL>tL11‘11‘1:'teconformable with the trend of :11L'.L1L‘1:sthe rocks in their Vie:n'vicinity,. Liror :care l‘nnnu‘found in eomm‘mahleconformable L'L1n1;LL'1.contact with other I'OL‘kS.rocks, and allall L‘LrL'are L‘L‘D‘Ll'ui‘lconsideredﬁd 1L1to bewe of Sidllltc‘l‘zl'dl'}sedimentary 01'origin.:11. As :LlL‘L'alreadyLm mentioned there is ‘LLa gLaLluatinngraduation 111Eof types between the plagioclaseplagioclnxc lphlliollIC:amphibolites hidand the hL11‘11blend-Lut‘mn‘inhornblende-bearing ﬁ gncisses.gneisses. RL‘JL'lRocks inelincluded1L1'L‘L1 in 1hi1for thLthe Ielsparsfelspars toto LJLLLLroccur 111in L11discrete1< LL'tL' layers.ill)CtS. In1n thin sLetionsecti'On the hornblhornblendeCndC is toLLlfound to l":be 13lL.L1L'nLoiLpleochroic in olive green.green, and sometimes'nmetir Shimsshows L‘lctirclear eVidenL‘eevidence of replacing Lliops‘idediopside andiin‘and/or hypersthene.hypei'stltcne. the lii'rnhlhornblendeenLL' itself sintL‘LtmLssometimes Showingshowing replacement by bionic.biotite. Nol\'.1 LgLLLLr‘Lz.quartz waxwas foundlliunLi in an}any miof 111:the \‘lELiL'Sslides and the only telspnrfelspar is plagiLaL'lase.plagioclase, varyingmi‘yi‘ig from lindfkiinx.‘andesine to lt‘LhrzaLlLJi‘itL‘.labradorite. (diet;-Calcite occurs'Occurs in specimen (l160/1591514 from MaindunduniMaindnn 11ml EHas lobatelu'tutL' patehm‘patches intergi‘mVnintergrown with hornblende. Other LuleiL‘calcic ntinei‘nisminerals are rare,rare. ben'igbeing represented"gresemcd only l3;by aphencsphene :LnLland apatite.apatite, both in Iran:trace amounts. EstimatedEatiz'itLL‘LCLi modesnudes CHI"are 3as follows:-{UHO‘ASEW

(30.3113 60/15960159 6017660/176 (111120160/201 60 145 I I 6°1 I 60/145 %”a m% “7% %L. 1'.)% I PlagioclasePlagioclnse . . L IS25 31 25 52 I 4—144 - I I Biotite..Biotite L ...... 7, I 2 ~- T 7 HornblendeHornblemlc . . . r3767 I ('1565 55 I 3x38 371“ Diopside .... . L - W- 21‘}20 x8 “- - - - Hypersthenellypcrsthene . . . '1'7 I W i I 181.8? Calcite..Calcite ,. . . . W,- 1t 77 7 . - -L ApntiteApatite...... ,L. +1 - . I . I7 SpheneSphene... L . . . . - » W- +~ 7 . 7 IronIron UTEore ...... 1l 1l -7, I 2Z l

6031B60/113-Mavia.rhltix‘iLLL 60/159-Maindunduni.6011597Mnindunduni. (.3011764Ngomnno.60/176-Ngomano. 60/201-Wathoni.01201 \Vuthoni. 60,.60/145-Kimathena.I4577K1111nthenu. (b)HI) Sillimanite51111 Edwin gneisses:21L'1'55'L’3 01113;Only on:one :‘L'nrcsentatixcrepresentative 131of this rock type was'1'.t found,{ULIUCL inin the nose L11of 1!.a plungingple LintiL‘lineanticline <1ongalong :hL'the rnilwqxrailway at Kabingo.Kabingn. It is poorly exposed in1n Lira:drainagena trtrenchesanchm and is15 not thought to exceed 31.130 ft.it in thiL'aL'mthickness, its outcrop havinghaxfng been necessarilyme: a r11). 6\exagge-:nL- rated on the map. In hand-specimenhand—specimen ((311(60/172)l,_1 it is of medium graingrain, £11111},fairly friable,fri'L'te. white .mdand purplepurpl; in Lalnur.colour, the latter Linedue to mLlL'r}powdery iron 0x1oxides.LlLs. Under the microscope1111c1‘osc0pe the:hL' sillimanitesillimnnitc is seen to occur in felted clusters ofL131 ﬁfinene fibres111nm enelnscdenclosed in a:1 quartz matrix,matrix. Associated minerals are :‘Litile.rutile, in stnh‘n).stubby prisms and anhedral graingrains,ﬂ. unhcdralanhedral 31511119grains of opaque iron ore and lesserSever amounts of translucent red iron oxideLoxide. Its[[5 mode was estiznntLdestimated 11%:(31131‘12as: - Quartz 47 per L‘L'nt.cent, 5fllinmnitesillimanite 50 per L'L'nt.cent, rutilcrutile l1 per cent.cent, ieiron minerals ..2 per cent.

~~(41(4) METAMORPHOSEDMLL \ML')RPHUSLI) CCALCAREOUS‘Ll(_’\RE;DLS SEDIMENTSSunntzzx‘rq The'1 hL L3;_\'11L'L3:only representativesC>L111E111\C5 ofat this elas~~

~~. 2t)20~~

(a) C‘t'fi‘s‘rali'ittcCrystalline iitittas'rtittt‘s'limestones These occur asis bandshands and lenses sometimes reaching 4a thousand feet in121 width of outcrop. All are veryVery coarsecourse in grain.grain, that at ()tekilOtekilawaL111 .1 h-havingting grains ctof eatcalciteeite up to 2 em.cm. in diameter. Colours range from pure white\shi te to light red and shades of grevgrey and blue-grey.blue—grey. The colour often varies:varies widely in differentditletent parts ot"of the same.same outcrop.outcrop, and could not be used as 21a criterion for identifyingidentitﬂng diset‘tindiscontinuous:n‘Ltous outctooutcropss of the same hand. Specimens fromlrom all thethellargerliirger outcrops weremm c tLtestedste Ll byh‘ '1a colorimetric method and found to contain 21a small ppercentagerccntage atof dolomite except‘RC cpt 60.1124.60/124, from Kusaala.Kasaala.

Graphite occurs in the majority of the exposures examined.examined, usually as.1s single ﬂflakesakes disseminateddissentioo ted throughout the rock,rock but occasionally in large lenses enclosed in and marginal to the limestone. At Kamstoi,Kamstoi. on the northern boundary of thethe area.area, such lenses are being worked as graphite ore. A microscope section of a hand-pickedhand—picked speci»specimen of this ore.ore, (10.1190.60/190, shows graphite in ﬂflakesakes up to 5 mm. in diameter tntergrotsnintergrown with quartz and felspar and a..associated.cinted with small amounts of apatite.apatite, pWogopitephlogopitc and a pale green mica. The graphite content of the slide was estimated as 3U30 per cent.

Other accessorytt‘ecessor;y mineralsm inertils in the marblesrntrl'iles occur bothhoth as1.s disseminated grainstins and :1sas knots and lenses which seldom reach a diameter of01 more than 10 cm. Such knots tend to stand out sharply on weathered surfaces.surlaces . One such knot,knot specimen (1060/ 117 frontfrom a.a point twottso miles south-eastsouth- east of01 the KamstoiIs: nstot graphite workings,work: ngs was\RLts sectioned and11nd found to consist mainly of cloudycloud}, fe1spars,felspars chieflyLhiet‘ly microclineitiieroeline buthut with\sith small amounts ofot orthoclaseorthoeluse andnod plagioclasepkzioclrtse (oligoclase).(oligoelnsei. Other minerals present in significant amounts are carbonate relics,relies. quartz in cle‘.tr.clear, elongated belbsbe1bs and pale brownbrmsn to colourless phlogopite.phlogopfte. Apatite,Aputite. sphenesphcne and opaque iron ore occur in trace amounts.

Quartz is ua fairlyl‘LiiFlj! eotumoncommon constituent of the l:limestones,mest mes. and chalcedony oceursoccurs in specimen 60/106no mo from l‘sluthtutyxsoiMuthungwoi. SeaSeapolpoliteite oLL'ursoccurs in large grains in specimens 60/141r10 lull frontfrom KasasiKasrtsi andnd (110460/164 tronifrom \luitttMutula,L1 and in Lla sideslide of thethe lCtUCl‘latter is associated withsaith phlogopite.phlogopitc. ForstetuteForsterite occurs “idelt‘.widely, sometimes.sometimes altering toto talc. 1nIn specimen 60/208(11] 208 from Waamata\\";t;1111ut11 on the TivaTim riverriter in the e\::‘emeextreme south-east s'cupolitescapolite is seen in large rounded grains. This last specimen is interesting itsas being particularly;particularly impure,impure. knots of greenish black mineral aggregatesaggregates. making up one third of the bulk of the rock,rock. the calcite groundmussgroundmass being pink in colour. Inin Litticladditionti111 to settpolitescapolite thethe toiiowfollow- ing minerals were identiidentifiedﬁed in this sectionzripinkishsection: -pinkish inbrownout sphene.sphene, :1p.1:i;c_apatite, (.[Ll.1ll,'.quartz. diopsidediopsidc and11ml actinolite.itctinolitc.

~~(5).1.\‘111,'v.t\;t11s:MIGMATITES~~

Inin the east of the ureaarea and locallyloci-.l't inL1 the hornblende uncimesgneisses in the centre there lif't‘are outcrops 11of rocksrock» that are c?t.‘1r:tcterisLL.characterised byin the intermixingintern isinu of itiincr.1lmineral aggregates ofi veryr} marketmarked (DIXIJSZ.contrast, inkingtaking the form of lcucocleucocratic' host-rocks1-11 \ls\ Cile‘lttsenclosing lenses and schlieren of me1anocratic-11oc1‘:itic character.churustcr‘. TheyThe”. are.1"L- characterisedL‘h.tz‘.1 . by acutelC‘ contortionsCsl'l on a small scale,SCRiC. indicativeindicrt‘..‘.e of tight foldinglohlng whilsth.l~t in a.1 semi-plasticsemi-ply. state. Where such rocks are weatheredthet‘co to s1':‘1;'u‘1:hsmooth mourn-Luvspavements ot'or lowlets rounded eros'irnerosion residuals.residuals, (isas is Oftenoften litethe 4 case, it is usuallyl ' possibletoss h. r to determinedetL r111 tn: their titerulloverall sstriker; and1.1:Li dip,dip. which than»always proveprose to be in good :1L-LL‘7“.accord withh the regiiregional1 L1 pattern.tuner'11. Th:The 11melanocraticiC‘lL't‘cl'LtllC coinpoicomponents of" these rocks"ocks are1.:‘L‘ ulu‘rtxsalways hornblendic,1‘. buthut thein; htst-host-rockrock ssometimes‘11L‘11. L's. cvntttinscontains biotiteb‘o‘ii‘L and some- ti'ti.»times 311"":Tltlct"hornblende,is. Luttland :tit iss on thisthis criterion that theyMe). 111';are divided.kli'»itic'\l.

tut(a) Hornblendet'fwt'tri‘ii't‘itnt utigtttivrmigmatitesﬁ.‘ 'lheThe lior1i‘ole1‘itiehornblende inigiitatitesmigmatites nest:occur {isas t‘tftl‘l‘t"‘wnarrow bands5:1 it's :1L‘_-.rnear the ceri:"ecentre .it'of the arc-.1area and in larger LRposan'esexposures in 11‘:the south-e1;south-east. .\lAlongoog the Tiva river‘lxcl L‘Ltsteast oiof Wathoni»\ Hum theirthere is :1a gz'udtial‘gradual transition'l h'lti‘Ofl westwards\‘\ 351‘1‘1 Ein’ls‘ il’llOinto hornblendel1 LnL «1"1L-‘s's'es.gneisses, .tndand thethe d.‘v.is..1:1division inarketimarked between the t‘mtwoo types:types 0:1on the ninjamap is on.)only upproRitnuLuapproximate. A»\ typical specimen,"me-’1. (it)60/182HI

~~I ~ !!!!!!I~~

fromI'm-1‘11 Kwambagi,KER-Lining}. is'1" coarse-grained,1 . 11111.1L‘d andL111L1‘ creamL'rcum in1r. colourL'L11L‘L1r with111111 blackbiuck inclusions1:131:51 whichWhich frequentlyL'uq11311111 take1113'; the1111: form of boudinage11L111L11n11gc31odlbodies. In111 thin111111 section the leucocraticIctlcocra’LiL‘ facies11331655 is foundoL11Lid toIo consistL'Lms:'RT. predominantlyp' 61.101111 :1L'L11. 1‘1 of felspar,1'31 including1":1L'1‘L1L'131‘g both slightly\l‘lghﬁj, microperthitic:111'L‘rL1pcrlhit1'c ortho- claseL' 11w andL111Li plagioclasephgioclas: of the compositionLL111. 11.15111 1111 of andesine.;111dcL‘L11 1L1to bebL pale11:11:11pink11:11L in1:1 colourL'L11L1L1r witb11.1111. frequent inclusions111.115.0115 of opaque iron me.ore. TheyThe): generally bavehave a suturedSuturid and embayedembnjucd 1111111113.outline, but occasionallyoccasiomdly showShow good crystalcry's‘L'Ld facesTomi in part.pm: TheTbe felsparfobpar is1i andesine.:mdcsfnc. Hornblende,Hor11b1c11dc. pleochroic111cochroic fromfrom light toto dark olive-green,1"1].\:-""cc:1. isIs thcthe predominantpredL11111111111: mafiL'mafic mineral,11111101111. and hyperbyperstbeneﬂhenc also occurs,occurs. TbeThL' estimatedL'xL1'111'LLIC'L1 modeIQJdC of111' tbe111: mafic1' '1L' parts of the:11e rock7.1.x is plagioclasep13gEL'1L'1'ass 2826‘ per cent,cent. hornblendehombkndc 47 perpar cent,Len: hyperstbene11:.pcrs‘111cr1: 12 perL1:r cum.cent, game!garnet 10 per cent.cent, iron oreare 3 per ccmcent and 'LLa tracetruce of apatite.apathy

(b){1'1} BiotitefjjﬂrrgL migmatites111.31 1‘1'(\ A. typical exampleL"L:1111L11: .11‘of tbefr." biotite1"} 111:: migmatites1111g11'111.1L'\ is1s providedproudcd by11‘ vspecimen:51 11:11 60/116111"! 111~ fromI'rorz' Ndulukye.\'=.i‘.1111LjLL'. The[71c leucocratic1cL1L'L1L1';1:.'L' componentLL1r1pL1ncn1 isix mineralogically111:1L'r11Lw1L'L111 11a fairly1'11.' . typicalL‘Lpiqal biotite11101118 gneiss,1:11:13» ,1 exceptL-'LL'L'11'. tbat11‘ the' plagioclaseL11L1g'L1L'1119: is:‘ ratherL111L'. basic,‘mm'. being113.111; of tbe:11Lco111p11111L111composition of anLiexinL'.andesine. ItI'L usuallyLL‘LEL'L1L"11‘1;. In1'1 the[11: melanocratic11:1311L1L'TL11'L' componentsL1111111L111L'11‘Lx biotite111L1111L' makes up11111131only 2I per centmm against:1 1L: 651151191'per1 cent66111 of born-horni blende. Plagioclase1’1L1L1.1\c felspar1L'51 jL'LL:' berehere isis, mostly111<11"‘untwinned,1mm; 11:1eL1 again andesine._'1Li:<'r1L SmallSma11 amountssmountx of orthoclaseL‘ClLi‘C and quartzL111.“11.2: :11

(6)1111 INTRUSIVES{\111 x ‘1» INTO;\.11 THE.11: BASEMENT1%L\1\11.\181~1:..SYSTEM Intrusives1'111'L1x .Lw 111111tbat bave1111‘“ penetrated_.1L11:::‘.1 L‘Li intomm the111: Basement11.1iL'111L‘111 nicn‘.System 1'.1:'~;~rocks 31‘;are fewfun and'L'L:1Li 1:113".small, tbose111 1.1+: recognized1';'1L'L1'L':1" ' ' bongbeing :1a 1.1;:pyroxeniteLL':111L' LL:at Makaiye,3111:1113. westL L1:of Ikutba,1k11m. L:a biotite:iiL'xtiiL' pyroxenitepg.;L1.\C111"tc :11.at Wathoni, an olivine111:'11‘. 1111'noritetL' at:11 \;1111§.1.1.:1\L1Yumba, two miles1121: s soutb-south-west\ :11111»5L1u:11-'L~.esr orof Ikutba,lkuiha. 11ndand minor acid pegmatitesL'~ at21'. various\ 111':L‘. localities,V'-;11Z1“L'<. all:11 too1.1.1. small5111111 to..1 map.may. Some83:11»: of 111.:the pegmatitespcunmtitcs 112':are not true intrusives,.1‘:\L_"\. butm: are.LrL' consideredL'.1r,\1L11:1'L'L1 berebars for convenience.L'L'11111L'111c11L'L'.

11‘(a) Pyroxenite‘." ‘ Tbe pyroxenite11.3“ :L' at Makaiye'L: ‘L‘ 11pm".(specimen.. 60/126)1211‘ takes:11.» [ENthe form1L1r111 of :1a 51"..slight ridge L"elongated along'g tbe11L strike. 1.11of 11".;the neighbouring1‘LL'1g11r1L1L.r111g hornblender1L1r11111L 11L Lg1.cgneisses,Lissa; the Sharr‘sbarp iL'LiL“junction betweenL" tbe 7.1..»two bringbeing u'Lgfossdexposed L1111jy1r1only in (1:1;-one art1z‘111small $1111.11;gully. It is Lom'dcr:consideredL1 to hebe an 111...uItra-

~~I 22~~

~~baai;basic 1111.1"dyke 111'or 8111sill 1'1".1171;11':.'1emplaced in 1111-the Basement311s1':111 l'OCl-wrocks 111‘1before1‘l'C metamorphism.. In hand-11d-~~

~~11:3?deep 1'111‘1'1111'11red-brown wantsoccurs in large 11:39:11ragged 3131155,flakes. (')lif.f1’1.'l1’:~1'Oligoclase i~~

(b) Olivine norite The outcrop1111111113" of1131' thisthfx 11.1111rock isix‘ slightly 1"1L1gg1'1'111exaggerated on 1112the map. It1'. is seen$3.11 as'.’;< large.3131: rounded1'11111'111'11 bouldershuuldch exposed1'. in red11111 soil+1111 over1 1'" a'11 roughlymug elliptical$111131 area 3111300 byh} 1511150 11..ft., the long1"; axis beinglacing align-.3aligned along11111111.: 1.11:the regional1191111111 511‘1'111'.strike. No1N0 athet‘other solid 111.11rock isls exposesexposed nearby,111.1513}: 111111and it was11.15 1’1’11'11'1111’1111concluded 211.11that thethe outcrop1111113" represents1‘1'111‘1'3511115 ‘1‘111'an eroded V‘Q'Cavolcanicﬂlc plug. 11‘;In hand-specimenh' n11—spccir11cn 1191.1(60/154)1.; the rock1‘01'11' is13 \1'11'very coarse,1'11111‘x1‘. 1111111dark greygm; and111111 black,1‘13: 111311.with no [mugtrace of 111511111211foliation The mostmm: mm;conspicuous11111115 11111131211mineral is '1’:a black migamica in plainsplates 11pup 11"to 2 cm. in diameter.Ll:' 1119111. It1t is not11111 \‘et':'111.'11lit11‘.vermiculitic. In.111 thin1111. section\.'.'1i.1:1 1111‘the mica111119. is plcochrnicpleochroic from vcz‘j.’very palep.111" $11.19.straw yellow3. 1"11011' 11.1to light brown1111111. 11 or111' green,21211111111and is1'\ 1'1mx1'1'11'1‘1'11considered to be phlogopitephllagap‘tc 11111111rather that.than 11131113111biotite. OlixincOlivine occurs1'1'1‘111‘x: 11‘in 11:11"large anhedral113111113511 crystals,1 talx 11111111much fractured1':".1.".;11‘1"d 211111and {111111111111frequently rimmed by1:1: \1’1very pale green 111'11111'111'.hypersthene. This1h 5 11'.hyperstheneCINEhL’EIC 111\:<.does not11111'1'111'37appear I.)to hebe :11}an alteration' product;‘1‘i.1'1i.11."_ after[11'11'1‘ olivine0111\1111: 111.11but rather11121111 a.1 reaction1111-1511'n product bc111’61.between th:the early-crystallizedcarlvcrjuxllizcd 1’111‘1‘1111'olivine and 2111'the residual tiagma.magma. AlmoStAlmost colourless1'1111’1-1rl.'s< h}p1'1'.'cross- hatching 1C1‘111Ilo1'i‘nireminiscent .11of microdine.11311111115111), R1111:Refractive1't1'1'c 1111’cindex 111'1'1'1‘1123naii1111xdeterminations show51'111' it 11’1to be ::a sodic<111;i1‘ oligoclase.11lig1.1.111<1'. 11It 1110111112;includes mammany p:prismsl.\111\ of111 apatite1.11.1111: and blebs1311111,; and «gymsstreaks of calcitecalcite. Thatl'hut <111'hsuch a 511(111‘sodic 181511.11felspar 5111111111should occur01'1111' in:n :1a rock:‘(11'11’ buxi.basic 11101111".enough to containamt: 1111a largeurge propor-propatv tion111m of Mixingolivine can1".111 131'be explained by the $111.11separatingat mg out1111. 111of calcium11111111111 fmmfrom 1"'igin;1ll"originally basicbask felspars1615;121:5‘ in1n the mursccourse of111' metamorphismntctu1111.1rphi.\111 thus raising them11211.1 to111 :1a more sodic«’1111. $1.15status 1111::(the prawn:present Oligociasclioligodase), the 11111111111calcium being 1:11'55111111'21211recrystallized axas caldumcalcium cat‘bot‘mie.carbonate. B}By this 1'ria1'ri1.>11criterion alone it is 1'0considered:1xi1’11'11'1’l that11131 :11:the 111111rock was 1'111:1;11'C1’1emplaced before the :anclmhmconclusion 111'of the regional 1111;11111.1r'7h:5mmetamorphism and gt‘ahitizuti1’111granitization 1’111111'of the BaxsiBasement1151111 System 51:11:.rocks. The mad-3mode of111' the rock,1'01"}; estimated from 1.1121111111411111:examination of Notwo thin septum.sections, is plagioclasepl'1’1gi1’1cl.1<.1- 1815 per cunt.cent, pWogophitephlogophite 22 per cent.cent, hypersthenchypersthene :1120 pctper cent.cent, 111111.11"olivine 3\38 per cent.cent, 2121511111:apatite 1 per cent,1111!. calcite 1 per cent,ccnt. iron11'1’111 oreme a:1 trace.t1..1'1'

(c) 11]."111’11'Minor 111'1'11'acid p1'g1121111f11pegmatites 'l'woTwo occurrcn.occurrencesec of masscoarse pCLpegmatitesqmutiics were“1'1: noted,noted but in neither case 1011-111could 211011their relationship 11)to the surrounding511': wounding nSusgneisses 1'1'be stab}established.1311111 :pcciSpecimenﬂnn 61.1160/178 was11212 mkcntaken from large float11.11 blocks on th1the 51111195111slopes of 1.11.:the Yatta\311 .1 PlateauPl;;t:a near Masalani.312151111": 11It unmiszsconsists 11fof wh'tcwhite quartz.quartz, p.111"pale cream i'clx'parfelspar 1.119331111111211(determined 1111;711:111'1111'21111‘microscopically as 2111111121albite) and lesserlesxcr 11111112tamounts 111"of 13311111:biotite 111111and muscovite.1111511111112 111."The 11101119biotite occurs in fractured...131 .1red platesplat 1'~' up Into 5 cm. in:11 dia..:.diameter,'1't . and 15is slightly vermictiliifc.vermiculitic. The muscovite11111:."m 1131115flakes never curedexceed :1a few millimetres'1 1:111" < NoN11 other0th: minerals:111'111'1‘1113 \11'1'1'were wen,seen.

The1’- Rewndsecond '.'\'.’11:1pl1"example isi< amused.exposed in :11:an 11111old prospecting[3113\‘p1‘c111‘13' 1351pit at h'ianm‘ii.Manoni. The 1111]):only minerals' 2115' 51:11seen \"c:1'were 1113mm.quartz, 2'1'1191'111'felspar and 1.131111511111111vermiculitic h'cititcbiotite in 11111109flakes up to 15 cm. in diameter. Thelhc pegmatite'1'1'131111: lies111'» under11111111 :1a Carercover of red m1“.soil up:1: to1111 three feet in thickness,thiqkncsx and ‘hits relationship1‘1..1ti 11111 to1111 1.111"the 1'-,111nt:‘1country 711‘.»rock could..11:11 not111111 be1'1 determined.

~~~ 23~~

Specimen 60/123 from Mongua was taken from a pegmatite seen cross-cutting quartzo-felspathic gneisses. It is cream and pink in colour, and consists only of quartz, felspar and biotite, flakes of the latter averaging 2 em. in diameter. It'1'. isE5. one1.1113 of111' many1 stringers511i21g31'5 that111.1.‘1 reach1351311 a'.1. width1.11.131 of.11 up=.:'.. to 30 em. growing outwards from knots1.:11115 up to11.1 1 m. in diameter11.111. 311:: the‘1113 stringers~11:11:31'~. narrowing1111773111111; with\1 increasing distance from the11713 centres.33112235. They'1" , J. show51131-11: no1111 displacement11'2511111331113111 of1‘11 the‘.113 31.111111r'1-r'country rock nor any chilled3 margins,'.115. and111111 are1113 replace-131111133— ment111CF'1 pegmatites111% formed1111" by re-crystallization in situ1. 1 of111 the minerals.1 of1 '1' the1113 country31111111"; rock1113-. under:1211'i'..:' the influence131131133 of metasomatizing agencies.

In1.11 the1.3.3 bed3311 of111' the111-3 Tiva111-11 river1'i\c:' at:11 Munyuni\11111'111'11i where1131311: the1113 hornblende1111'11‘11'1311113 migmatite1:1ig111.111'-.C is15 cut311.1 by113. minor111111111: shears,1113113. replacement111.113'1'311131‘11 veins131115. in11 the111 11 form1-11' 1'1 of1.11 stringers5.. 1111315 follow111111111 the1113 shears.51133.73. They1113}: seldom5311111111 exceedcxctcd 20 3111.cm. in111 thickness,111131. 11355. but11-31 are1173 unusual1111119113- ini11 111.11that they1111'"; consist"'11-.'153

2.-Tertiary-Yattal.~'l'ersi;erv—‘1 :11111 PlateauP13131111 PhonolitePhonulite Y In hand-specimen the phonolite of the Yatta3. Plateau,P1113511]. which corresponds to that of the Kapiti Plains south of Nairobi, is dark grey.\ in"1 colour1.11.0111 with‘ a grain so fine as to be barely distinguishable to the naked eye, supportingx '1 ' 171711713. numerous1|-.|1:131'1’11_\ phenocrysts;.11'1311.13 .51 of111 white'.1/nit: anorth0clase in elongated'd plates[35.11133 and111111 a:1 '.13531'1111111'11111lesser amount of1'11 nepheline.1'3p11315113 in111 short5111111 stubby5:111:11); prisms, almost black in colour3111.111.“ and1.1.1111 with16111 a7175.3resinous111.1115 lustre.1115173. Anorthoclase.3111111'11111-.‘1.15.3 phenocrysts91131111313515 were‘1‘. CFC" seen\L‘L’ that measured as11~ much111:3:1 as:15. 8R em.3:11, in:17 length,1:111111" but the average"37:113 is1; about3.111111 2.5‘.7' cm.3-1.". TheT113 nepheline11311311 ..13 phenocrysts1111311113. A." seldom1311111'11 exceed31133311 1 cm. No difference in texture or phenocryst content311'1i31'11 was11.15 noted71.11311 along{111mg the' whole' length' of the flow in this area. Hollow vesicles are rare but are sometimes seen with a thin coating of.11 zeolites,3-" tentatively. identified as natrolite. Smaller1:11.131" vesicles\'3\:."'3<. up1.111 to:11 3:- em.31'1'.. in11 diameter111.111 . completely3011111131- filled121-311 with1:11:11 analcime,..111'.11311113. are.113 fairly common. A typical'1 specimen,51113111131111:-60/129131‘ from1111111 Nzauni,\za' 111i was111m sectioned5331:1121'311 :11111.and, besides1331135 anorthoclase1p 11." and:11711 nepheline,1131.1‘73'" -3. anhedral111.1111 and.1111] euhedral31.113.“ microphenocrysts[11.1111131321137} 515 of111‘ olivine,.115 2'113. usually111.1111 rimmed:111'. by cossyrite,1215532111.. . were.. identified..1....:.:-....!1113!.1.1 7The11131.7, groundmassT consists3.1.5.111. 1 of.11f a.; felted1:1 11.11,. ....mass.13151 of1-..1 laths1.111% of1" anorthoclase, small crystals of nepheline, cossyrite pleochroic13 from5:11:11 brown1717 to black,2111135. kataphorite1311131117111": 3 pleochroic11331111111313 from10:11. very1.3:"; pale1141: green37331" to purple,11111113. and.1i"1'1 laths1.11315 and'1113' 3113.112:angular grains11711135 of.11 deep1.33p- green1’31'3311 aegirine..'1 1111113. AugiteAugiw also.11-111 occurs,113312175. pale11:113 green11.'33.'1 or1:1' brownish-green1.1:'.11111'.511—grc311 in111 colour,311111.111". often011311 simply531111111; thinnedtwinned on1111 100.11111. Other(11.1137 mineralsr1"-ir.13:'.115 identified31-31-11i1131! were1.1.373 a'.1 few1311' small3:13.11 anhedral311111211141 grains$731115 of11.1 apatite,311111. 3. small

~~Err-Pleistocene3.-Pleistocene and Recent~~

~~1 (1)1'1! LAVAS1.11. 1.5:~~

."_3311113Specimens15 1:11of the1113 ('17.:Chyulu111.1 3.1\'.1\lavas 117.11:that E11'113'jflowed 11.11.1111down 3111:the Kibwezi13-11311 and 5113;11:114313111Masongaleni 1.117.- vallevs11.1I1'.._1\ were'3 “1111111311sectioned (60/1681115' :11111and 131160/170)1.71.11 ;-.:1.?and "33111111111identified 71;as 1111\51'13olivine 1311571315.basalts. 111In 1113the 113111field' 1111;the lav-.15lavas {1173are 11:11;dark blue-grey111131;"3} in111 30111111".colour, very11' . 5.3:.1ri'113311us.scoriaceous, "111111with :1a 11113fine g:'a.1111|.11'granular 131111113texture ;and- 1111lxisib13no visible phenocrysts.[111311-113 5:5. Zcoli:35Zeolites 2113are rare.rare, and 1111373where 53311seen lining 13351-3135vesicles were 'identified as natrolite.- In 1‘1.thin'1 533113111section the 7113115rocks 311113151consist 1.11of :1a 1111—2731133!fine-grained 11111.1»holo- crystalline groundmass. supporting5:11: microphenocrysts711'3171111113-z1'11'11» 1.1.1of 331236311euhedral 11111.".olivine3 and 53711353135sometimes prisms91351115 of.11 plagioclase.11112111312113. MincralsMinerals 1113;21:11311identified in the 15111122111111.1751groundmass 111373were y‘agioclaseplagioclase 111'of 31111111151101“.-composition An".-‘\.'1. ,, 1_2111L1C5'i13—1:111(andesine-labradorite),11 (1111131. olivine,' . very 11313pale 131173311.green :11augite3 111111and :13octahedra_ 111of magnetite.11133123112. In1n H1331i1111section 60/1701111 1..) 1111;the olivine1111.. phenocrysts”131111311515 211-:are 111411marginally11:111} 211131331altered 1.1to iddingsite.31111111511813.

~~L "~~

~~Ll-24~~

The'lhe chemical composition of the KibweziKihnczi lavaJava has been determined as follows:--ioilons: Norm Per centrem Per cent Si02.oSiO; t. . V 43,7343-73 or . . . 10-57to'57 Ah03A130; . . 13,40l3‘40 ab . . V 4-734'73 *Fe203*l—"e305 V V . 15.0715-07 an . . , 14'7514-75 MgO.MgO.. . . to.21lO-Zl ne t . 14.20l4-30 CaO .O' V . 9-589'58 di . . 26'5916-59 Na20NagO . . 3'703-70 01ol . V . 18.5218-52 K20K20.. V . . 1'801-80 mt .. 5-335,33 Ti02..TiOw. . V 3-0l3.01 il 5-H5'78 MnO . V '20.20

~~100.70l 00-70 100.47100 47~~

~~Anal.Anal East African_-\t‘rican Industrialindustrial ResearchResearcn Board,Board. Feb.,Feb. 1951.[95L~~

* Total iron as Fe203'r270, From more recent,recent. as yet unpublished analyses of Chyulu lavas the ratio FeO:FeO:ITe7VOVFe203 is assumed to be 7117: 1 No felspathoids were recognized inin the thin section and the presence of soso much nephelinenephcline in the norm is remarkable. Presumably there is (Allan alkaline mesostasis thatthat is not easily detectable.

(2)(ll ALLUVIALALLL’VIAL DEPOSITSDeposits Along the course of the Athi river are alluvial deposits consisting mainly:nainlt of fineline sand with occasional thin layers of grey silt marking seasonal floods.lloodsV 'l'hicknessesThicknesses of alluviumLtlluvium up to 12 ft. were measured,measured. but the base was seldom.seldom seen and it may be much thicker in places. The alluviumalluviutn stretches for up to 500SUD ft. on either side of the river in the north-west,northwest, narrowing southwards to less than 100lot! ft.It. Yer)Very thin unconsolidated gravel sheets sometimes occur,occur. consisting of poorly-roundedpoorly—rounded pebbles of Basement rocks and phonolite. Many of the other titers.rivers, notably the Tim.Tiva, Mavuko and Raloboto.Kaloboto, have sandy bedsheds buthut no flankingﬂanking alluviunt.alluvium. Panned conceit-cconcentrateses from the Tiva rivetriver showed (after removalrctnoval of quart/.quartz, felsparleispar and biotftetbiotite) magnetite, ilmenite,ilmenftc. garnet.gamet, horn-horn— Hende,hlezide. diopsidc.diopside, :‘utiie.rutile, zircon, apatite and sphene. InIn addition to all these minerals a concentrate from the AthiA:hi river lspecitnen(specimen ott'l60/ 165a)ﬁifal showedshorted ita high proportion of epidote,epidoicr which is veryVery rarerare in the rocks of the presentpresen: area.

t(3)3 l SUPERFICIALSt: war in \L DEPOSITSl) EPOSI Is Over much of the area a red or red-hrownred-brown soil,soil. usually sandy.sandy, has developed by weathering-z-thering ofot the underlyingunderlting rocks.rocks. \treWhere the vegetation cover has been removed by overgrazingmererazing or poor husbandry gullying quickly develops and leads to barrenharren sandy areas. The phonolite of the Yatta Plateau weathers toto a non-sandy red soil very similar in appearance to that derivedderiwd from Basement System rocks.rocks Crops planted in the:he few places where“here the bush has beenheen cleared on the Plateau showshtm that this:his soil is more fertile than those derived from Basement SysiemSystem rocks. In areas of poor dru‘.drainage typical "black-cotton"black—cotton“ soils are dexeiopcd.developed, hard and fissuredtissut‘cd when dry but tenacioustenacrous clays when wet. In most of the black soil areas examined a discentindiscontinuousous layer oiof nodules ot‘of secondary limestone occurs at or a fenfew inchesinches- below the surtaec.surface. UnlikeL’nlikc the red soils.soils, which support a thorn-scrub vegetation,x'cgett‘ttiozt oftenottcn dense.dense, the black soil areas are always open grasslandgrassland. Over and near tt'tllc‘l‘ttpsjoutcrops ofot‘ the more calcareous rocks secondary limestones are.are widely developed. They'l'he}. are usually bidbuff or pink in colour due to iron-staining,iron-staining. and often include angular fragments of ct-artxquartz and lick;felspar.- "theyThey seldom exceed a tenfew inchesinches in thickness.

~~.. -~~

VI-METAMORPHISMHIM}: AND.'\.\i) GRANITIZATION(;I{.t.‘\i'l If. 'x'l'!{)\ Like many other areas of Basement System rocks in Kenya the sequence of events appears to have been firstly deposition of sediments, secondly widespread folding with accompanying regional metamorphism, and lastly granitization. That the granitization was later than the regional metamorphism is proved by the formation of fine-scale layering of re-crystallized quartz and felspar, seldom showing strain shadow, between layers of cloudy, fractured and strained minerals. The abundance of orthoclase or microcline felspar in these later layers, compared with the felspars in the older portions of the rock, show that the metasomatizing agents were potassic. Where granitization has been locally more intense felspar porphyroblasts developed and grow laterally into the minerals of the surrounding rock without noticeably displacing them, showing that the porphyroblasts grew by assimilation of the existing minerals, without significant addition to the rock mass. At the highest degree of granitization the rocks invaded attained a degree of plasticity that led to the contortions and swirlings seen in the migmatites and finally to the obliteration of most of the original sedimentary characters of the rocks, and giving granitoid gneisses. The metamorphic grade of the area is high. and is characteristic of the almandine- diopside-hornblende sub-facies of the amphibolite facies (Turner, 1948, pp. 87-88), a typical mineral assemblage being hornblende-plagioclase-hypersthene-diopside with or without quartz and orthoclase. PlagiocJase is commonly of the composition of oligoclase or oligoclase-andesine. Hornblende is sometimes clearly secondary to the pyroxenes but sometimes apparently in equilibrium with them. Where biotite occurs it is usually secondary after hornblende. Further evidence as to the high grade of metamorphism is given by the occurrence of sillimanite in one exposure, the alumina content of the original sediments being too small to allow this mineral to develop elsewhere. In the crystalline limestones actinclite, forsterite and diopside are commonly developed. The appearance of these minerals is in good accord with metamorphism to the stage of the amphibolite facies. VII-STRUCTURE The main structural features of the area are shown in Fig. 2. The general strike of the rocks is west-north-west to east-south-east, and is in good accord with many of the other Basement System areas of Kenya so far mapped. In the extreme south-west the strikes trend east of north. The lack of exposures between this corner and the remainder of the area makes correlation of the two parts difficult, but for reasons stated later the writer considers that there is a gradual transition between the two strike directions and not an abrupt change such as might be caused by a major fault. Foliations are readily determined in most of the exposures, and are marked mainly by the preferred orientation and alignment of mineral grains and the tendency for mafic minerals to occur in well-marked bands. The foliation invariably parallels the junctions of the different rock types, and marks the bedding planes of the original sediments. Lineations are marked by fine-scale puckering on foliation planes and the alignment of elongated minerals and mineral grains. Six hundred poles to observed foliation planes were recorded in the area (excluding the south-west corner, which is considered in detail later) and were plotted on a Schmidt equal-area stereographic net and contoured (Fig. 3a). The concentration of poles along the great circle;; indicates a single system of folding with (3 as fold axis. The mean strike, shown by the maximum point of concentration, is 3400. with a westerly dip of 550. It will be noted that the mean strike differs slightly from the direction of the fold axis, which is 3340, owing to the plunging of the fold axis to the north at 10° from the horizontal.

~~.. 26~~

In the same part of the area 173 lineations were recorded, and the contoured pintplot isi~ shown in Fig. 3b. The maximum of thisthis diagram correSpondscorresponds to a mean lineation direction of 330:3300 with aa northerly plunge of 12"1ZO, The regularregular extensionextension of contours into thethe Southernsouthern part of the plot ﬂshowsows thatthat thethe southerly—plungingsoutherly-plunging lineationslineations belong tnto thethe same system.system, and indicates:indicates thatthat thethe variatiomvariations inin plunge result fromfrom minor ﬁflexures.exures. 2'00'52.00'S. I" " 8J " -u'oo'E t', \ v f5~ "..- .., -,v \ \~" 1$ ~r'\ ,;111 ans- "r;1'1 ' IKUTH‘.IKUTHA\ -- us: ,4 ~ '\:t¢’ ... 11 \'

~~,.~ ~~~~

~~, /' \ ~V .r I 10 I '~i \~~

~~59"", I~f U\ .11'\1' $4M, V \ " /' / ‘ \\/'/' Ndovoini yo u! / / < " ,,1' 'i / /.. 51;?\ IS-b‘?~ ,y)(r/y" Mb.Iu ~ / / 11~.. ~~~~

~~1O~ \/“ti. .\:".5 u“ “7“;~~

~~'fr-\1 1 \“ \,, 11"'~ '\ '1. ‘0'" MulilunlMM hm '0 v: ”t :5 \ 'm.'"' I"~ ‘ 2'30'2‘30'5S SCALESCHE 5 0 5 5010MILESMilES L ‘ '-' L Il~~

v I, v v VolcanicVolcan-c rock:rocks ,4"..'’ Fault:F.ults wtthwith d‘FBC.‘0direcllon. ﬁ 0ofg _..pa4mdownthrown4..a,ow Dv v /" '3“» I Surf.ceSurface tracetrace of ..iilAmi pl.neslunex of ~ synclme;synclines P -;;::-{/5 RelatweRelative movement Itong.Iong fauhsfaulls

~~"\\J ,4 SurfaceSurﬁcz tracetrke atof axtalIx'al plane:pl.nes 05of ‘ p‘~ \ anzctmesAnllclines -::?',7'7” Fol,alionFclatton dipdtp~~

~~x'" DirecliOnDwechon . .ndInd plunge 01of \ """-, \ heathen:line.tiOns \f Vertic.1Vernal foli.tion(shaman Fig. Z—Geueraliyed2-Generalized~~

~~.;;.... 27'3':~~

~~N N ""~~

~~(a) (b)~~

~~N N~~

~~°)1'~~

~~(c) (cI)~~

~~Fig. 3—(‘0ntoured3--Contoured Ktcrugmphicsterographic diagrnnhdiagrams of poles to infoliationﬂation planes and of lineatiomlineations in the lkuthaIkutha arm:area:- A~~

(a) Poles tolo foliationfoliutiun planes,p1une~. Ikuthalkutha area ncludingexcluding the south-westmum-“eat corner. Six hundred readings, contours at 1.1, 5.5, 10 and 15 per cent per one per cent area.

(b) LinwtiouS.Lineations, IkuthuIkutha area excluding methe south-nestsouth-west corner.comer. One hundred and sew-My-seventy- three readings,readings. contours at 1.1, 10.10, 20 and 30 per cent per one per cent area.

{C}(c) P0195Poles 10to fOIiHIiOIIfoliation planes.planes, south-“estsouth-west cornercomer of the Ikutha area. Smcnty-oneSeventy-one readings.readings, contours at 1.1, 5.5, 10 and 15 per cent per one per cent area.

IdJIJncntiomV(d) Lineations, south-we»!south-west cornercomer 01‘of the Ikuthalkutha arc-.1.area. 'I‘nenty-threeTwenty-three readings.readings, contours at 5.5, 10,Ii). 15 and 20 per cent per one per cent area. 28

The good accord of the axis of folding of Fig. 3a with the lineation maximum of Fig. 3b is a clear indication of monoclinic symmetry in the rock structure, and proves that the lineation is a true b-lineation marking the axis of folding. Thus the structure -. of the area is the result of a single major tectonic phase in which pressurel‘ ‘ was applied along west-south-west to east-north-east directions, the [ItoverturningL' ‘lL'IHiH; of folds to the east-north-east indicating that the pressure was applied from the west-south-west.L'x1—;.,

Figs. 3c and 3d are contoured stereographic plots of poles to foliation planes and lineations respectively observed in the south-west corner of the area, which is delimited roughly by a line from Ndovoini south-east to Muliluni. Fig. 3c, while showing a wider spread of points than Fig. 3a, shows a marked concentration of poles along the great circle 11.",and again shows a single system of folding with [3 as fold axis. The axis of folding here is 012°, with a plunge of 12° to the north. Although only 23 readings were made of lineations, the plot nevertheless shows a good maximum, again coinciding with the fold axis and proving homogeneity in the rock structure. The contours of lineations are elongated towards the west of the diagram, suggesting that the lineations swing over gradually between the south-west corner and the remainder of the area, without a sharp break.

The major fold in the area is the Kimathena syncline. with a near-vertical core of granitoid gneiss and biotite migmatite which has, by its compacted nature, resisted erosion to a much greater extent than the flanking gneisses and now forms a steep- sided ridge rising to 2,000 ft. above the plain. In direct contrast the Kitui anticline to the west has been eroded at the same rate as the flanking rocks and makes an ill- defined feature. The Kitui anticline was so named by Saggerson (1957, Fig. 2.), who traced it northwards through the South Kitui area and correlated it with an anticline terminating at Kitui township (Sanders, 1954, Fig. 3). Other lesser synclines and anti- clines occur between these .two major elements and in poor exposures east of Kima- tbena ridge. Several minor folds were mapped in the extreme south-west. Between these two areas of folding is a broad belt of predominantly hornblende gneisses in which onl} one fold was recognized, the Yumbuni syncline. The regular westerly dip qf this band of hornblende gneisses would, if undisturbed, represent an original thickness of deposition of upwards of 70,000 ft. The writer considers that the width of this outcrop is rather due to repetition of the beds by strike-faulting such as the fault followed by the Ngunumu river in the centre of the area. Faults seen cutting the Yatta phonolite on either side of the Ngunumu fault may indicate late movement along Basement System strike-faults. Further evidence of such faulting is shown in the rocks of the line of ridges from Ngunumu to Mbatu. All the thin sections of rocks from these ridges showed characteristics of shearing in the plane of the foliation, though only cross-faults were identified in the field.

E. P. Saggerson, who at the time of writing this report was mapping the Simba- Kibwezi area to the west of the present area, informed the writer that plunging folds parallel to those in the present area reach to within a few miles of the common boundary, the meridian 38° E. .

The evidence therefore suggests that some of the crustal shortening in the hornblende gneisses was accomplished by reversed strike-faulting. However the existence of one big fold (the Yumbuni syncline) and the folding on either flank of these gneisses suggests that other folds do occur, but have not been detected. In the folded areas minor folding is common, with amplitudes of 20 ft. or more down toa few inches, always parallel or subparallel to the major folds, and marking puckers imposed on the major folds. Prominent joints were recorded at 68 localities, all vertical or near vertical, and were plotted as a frequency diagram, Fig. 4. This shows a marked maximum at about

~~~ .n"" ~n.-... 29 ~ ~ ~/'l~~

Fig. 4-Frequency diagram of joints in the Ikutha area. Sixty-eight readings GO°, at right-angles to the trend of fold axes, representing ac or cross-joints, produced by relief of stresses set up during deformation. Such joints would only occur upon relief of confining pressures such as would be produced by unloading, when the rocks would have lost any tendency to plasticity and would be able to adjust themselves only by fracture. Where such joints are large and relative movement has taken place along them, whether at the time of their formation or subsequently, true faults are produced. Most of the lesser faults of Fig. 2 are para11el to the ac joint system and are genetically related to the ac joints. VIII-MINERAL DEPOSITS The only minerals of economic importance which were found in the area are graphite. limestone, vermiculite and si11imanite, though only graphite is known to occur in sufficient quantities or of a suitable grade to warrant exploitation. I.-Graphite Graphitic gneisses occur as lenses and impersistent bands associated with the crystalline limestones crossing the Ikutha-Kanziku road and extending into the area to the north. In his report on that area Saggerson (1957, pp. 35-43) has given an exhaustive review of the prospecting and mining done up to 1953. Briefly, the first record of the existence of graphite was by E. V. Kinloch, then of Masongaleni, who does not appear to have pegged any claims. In 1942 and 1943 E. R. Wright pegged claims over one of the marble bands, these claims being abandoned in 1944, after their transfer to Raw Materials Development Ltd. C. R. Stokes-Fair held a single claim between 1945 and 1948. No commercial production was made from any of these claims. In 1950 R".O. Johnston erected a protection notice which was a110wed to lapse, and in

~~I .. 30~~

1951[951 D.l'). K Hamilton i‘lr.C..-\.(E.C.A. Geologist) and D. Hobden tProapeetori(Prospector) prospected thethe area for the Mines and Geological Department and proved the existence of economic deposits.deposits, Inin the same year claims were pegged by BewiekBewick Moreing 8;& Co. and by Shah Vershi DevshiDeVshi & Co.,C0,, both of whom carried out extensive prospecting. Bewick Moreing & Co'sCo.'s claims finally lapsed in 1955, and at the time of writing (January.(January, 1957)195’?) only Shah Vershi Devshi & Co,Co. harehave workings in the area.area, on claims at Kamstoi. In Win1956 this company pegged most of the kntmnknown graphite area under an Exclusive Prospecting Licence which is bounded by the triangle of roads joining Ikutha,lkutha. Mutomo {to(to the north of Ikutha)lkutha) and Kanziku. Shah Vershi DevshiDershi &é: Co.Co., unlike other Kenya producers.producers, harehave not concentrated on the production of graphite for crucibles.crucibles, for which the rather exacting specispecificationsfications are based on flakeflake size,size. but on the production of graphite for surfacing moulds and for lubricants,lubricants. where the main qualiqualificationfication is high carbon content,content. Owing to the acute shortage of water near the workings for most of the year [bore—(boreholes have been sunk but their yield is small) the flotation plant has been re—sitedre-sited at the Tiva IiVCI’river near the road-crossingroad—crossing south-“estsouth-west of Ikutha,lkutha. and the milled ore is carried by lorries the ten miles from the mine.mine, the dried flotationflotation product being returned to the mill for fifinalnal processing.processing, details of which are as fretyet confidential. Inin August.August, 1956,1956. the fiflow-sheetow~sheet for the mill was as given on Fig. 5. A new milling and separation technique, still in the experimental stage.stage, the tints-sheetflow-sheet oiof nhichwhich is shown in Fig. 6.6, has been devised by the corttpanjs.company. 'lheThe important feature of this nextnew system is the winnow.winnow, developed in its present form in the laboratories of the Mines and Geological Depart-Depart ment,ment. Nairobi. After preliminary milling the graphite is blown from the gangue,gangnc. giving a product which approaches 50 per cent graphite. With further screening and winnowingtainno‘oing the graphite content can be raised to 80 per cent itif necessary. The flflotationotation unit is;is to remain at Ikutha,lkmha. but much less transport will be needed to carry the enriched ore from the mine. At a later date it is intended to move all the equipment for final processing to the flotationflotation plant.plant, so that the finishedtinfshed graphite can be carried straight on to Kib‘o'ez:Kibwezi for railage.rdiiagef"* Graphite from RamstoiKamstoi commands a ready.ready sale to lubricant nmnut‘ac‘.titers.manufacturers. The run-of-millrun-ot‘vmill graphite reaches 80 to 82 per cent carbon content.content, but after iinu‘.final processing and screening three diticrentdifferent products are obtained.obtained, 93 to 95 per cent carbon.carbon, 80 to 85 per cent carbon and 65 to Til70 per cent carbon. The 93 to ‘4595 per cent fraction has been further treated chemically.chemically, on a laboratory.laboratory scale.scale, to obtain a carbon percentage of 99.5.99.5 per cent.cent, but a good deal of work requires to be done before such treatment can be regarded as an economic proposition. .»Att the moment onl)only 21a limited market exists for muchsuch :3.a high grade product. Production began in 1952.1952, and records in the Mines.Mines and (icologscalGeological Department show that thethe output s?ncesince then has been:-been1~

~~YearYour ' Ore Production ' Percentage ' Va1ue*Valuett i‘VIilledMilled RecoveryRecover)~~

I I I I ' TonsTURN TonsTrim I £5 l9521952 .. . .. I 400 3!31 I 7-757'75 1,406.406 l9531953 .. .. , 1,870LHTO I 127 i 6.2727 I 5.7655,765 .. I 1954 . , . , . . No\o production due to reorganization of mill

1955l955 :: I 1,671 168l68 ! 10'0510-05 6.636,653 I 195.61956 001 6,551, 492 '7'51 5 19,680 1957 I 1957 I 10,791 I 942 I 8,73" 39,847 .. I * EstimatedLiwli ticlcd ‘-.ll‘.ic‘value basedhosed l‘I'lon averagei g pricep i.;' ofi‘ii‘ gmp'i‘iugraphite ~‘sold in that year.

* 1Finalt processing,‘rttcssftf‘c hasl l\ been ccarriedt'tti .ioutt at thei flotation plantt out sisince.i.c late.ttc 1957.it.“ #

~~f'o, 31~~

QUARRYQUARRV J W JAW CRUSHER \\ / t 7 JAWset toC:U?,HER3/ ’ "'" + / L "'— ”*—F “ ’7 J 4 , CC...“ J J set to X l——> 5otJ fOtJ /2 J~- J ~___.___/ Y . 7, _ ‘ J ‘ v Jr— —<— ‘ k C , 1 V f 00 ‘y 0L_,~ 50; 10* 4t» ‘ , vﬁ w4.317“: ” V f “ \ .. if m, 7-3;-) '/ O:QoLii Jj‘fs'oj} o#1 4 «9:; “J, 7 4, A C77 5T I Y V V l ‘

K A a 7;: 747? "CC/JV 4 dQ01L, , J 50: 1019—» 'CP‘JSH'NGCRUSHING TROMMELTROMMEl vt v ROLLS set SCREENS -10+5°*40-53; -5041:-50* to 15”16-iF UteThe 5L50*7 3C'een3screens are FJoatedFloated . concentn’cconcentric around Wethe *earately/ EC:10#0 screersscreens ~ra7\\

ROUROUGHERH: F? FLOTATJCNFLOTATION CONDITIONERVCR?0 CELLSCEllS 77777 ! Vrfiifii if ’ x / ? Tailingsr‘ A1»;7 uJ nJ / Xi W {3'4to dumpu3:9 \KR fiCCNDfTONERCONDITIONER *VV/ A aJC/H0 7 . // 4 A/lx/

~~6SCROSSFLOWCROSSFlOW " FLOTATIONFLOTATEON CELLSCEllS~~

~~80-827._-'/Cc. DRIER «#M FiLTER «———' ‘ i-[~~

~~193.951093-957. C. F'akeFlake '"‘ a PM“FINAL 3‘ROCESSJNGPROCESSING ; 80-80785;;85 ZCC. Fine flake+ ANDno SCREEMNQSCREENING F“ J‘a‘e‘ 65-70%(35707.; C.C FmesFinestJ.~~

q, ~Can‘Car be webroughtug‘m _Cup 5:to 911-93596-977. CAc. / t‘CanCan beBe broughtbroug‘nt up :0to 90%90? C.C Fig.1' £14. 5~-CM[JJ!HL'5-Grapbite ﬂflow-sbeet-Ganeshinvxilwl Gumml (JLUIIN.Claims, K‘JmktniKamstoi

~~. 32 QUARRY / ~ ~ // ~ /JAW CRUSHERS set to Y2 "~~

~"'~l"' < I~ SCREENS I +10~ I +10*

~~14 J & HAMMER MILLS~~

~~t ~ ->-+ ~ ~ TROMMEl I ~J 1 =SJSCREENS~~

~~ROUGHER FLOTATION CONDITIONER@ CELLS i -?f&><~r-;;' Tailings // ~_--'L /' '> to dump~~

~~1 i l~ ROD MILL I~~

~~I i i r ~CONDiTIONER i ~ I ~/V1'Vj~7T)7i;7i'j I I /~)t:~k'~ -, L-/J" >/L-/~~L/ ~~ i 2 CROSSFLOW FLOTATION CELLS~~

~~FJLTER185-90ZC~~

~~96-9710 c. FiNAL PROCESSING AND SCREENiNG 80-85 % c. Fig. 6-Experimental graphite flow-sheet-Ganesh Claims, Kamstoi~~

~~.. -33‘u 'u~~

In11'. 19561950111the1‘ graphiteC71"1:1111111‘ was shipped<1'111‘L11‘1i in:11 roughly111.11.11.11; 112111111equal 1111311111115quantities 111‘of three gradm.grades, 9‘393 1.11to 95U." perp131 centccn' 11111.11carbon,‘ 11 901111 to111 92 per;.‘1‘1‘ 11'111cent carbon11.3'1.‘ 11. and'11:11111111111under H11180 per111:1 cent1‘1‘111 carbon.1".11'b1'11.

Messrs.3168515 133111111Bewick MoreingMar-sing &1Q Co.,.. in the‘11"1111course.' of their1111‘i1' prospecting,.11<111‘21111g. 1‘111cut 11'1‘111‘1'165trenches across1.111135 the111:1'11‘11‘marble ‘11"1‘1'1‘1‘19outcrops 511117.11south of'1 Kasaala,1x.1:'.'1:~.1'.; . 111111and uncovered11111'm'cr'311 bands. of granhi‘cgraphite gnuisx:gneiss up to111 four11111:' .feet11‘1‘111'1in thickness111111-313.“ on.11 the1111: western‘-‘-‘1.‘\1C1'1‘. flank1111111 of111‘ 1111‘the more:11111": CLi~11ET1'ylfI11Ceasterly of the two 111111113.bands. These11165: g111‘1101seen at:11. surface>LL.1;..'~1‘ 111.1:due to1.1 5.1111soil 211111and scree 1111‘1111gcovering from 1111‘the limestone111165111116 ridge,17111111 111113111but it is' probable11.1111: 1111.11that 1151'1‘111'11111‘115discontinuous bandsbaI1d< 1'1'or 11‘1151‘9lenses 1.11of 111411111graphite:1: gnugneiss\ occur 51mg,along 1111‘the length.611; 1":'of :51:the limestone1'111'1‘.\:1'1‘1'." 011113111.outcrop, and 111:1].may 1'1‘1‘111repay further1'11r1hcr praspecprospecting.ﬁng

Appendix1.1111141111111' on11.11 0112111111.Graphite ReprintedRep: :c11 1'1'1'111'1from "K;"Kenya Trade1:".1116 [11‘11and \11pp1i1‘1'.Supplies", 0.1111127.October, 1958,111155". 1‘11.pp. 2713233-234.11—1.

~~KENYAKE.\\ A GRAPHITE~~

UntilL': '1 fairly5:11:11 recently,[11:11:15. the1111" world's'11111'111K graphiteg1'g1p11i11‘ requirements1'1‘11111:'c1111"11:~ were'.\c:"1~ met1111‘: chiefly1'11'1‘1‘1} 111by Madagascar."‘1'.1d.1"11s1'.11 and11111 Ceylon,(c. but1‘11 it1'. is1': the111: hope1‘.1.'1r‘1‘ of1‘: Messrs..\1.':,~.\1\. Shah$1.111 \‘crshiVershi DevshiDewhz and (711111—Com- pany Limited,.1111 of1'11 Thika, the111: company1"1'1:11p.11‘.1; which12111111 hashr» developed1131111111911 111.‘the production11:11d111'1111:1 of11f local11'1."=.'11 graphite, that some. of these.1:'~'.‘ requirements1131:1111'1‘1111‘311». may1“;i". in1:1 future11111111: be11.: met11:1‘1 byhf. Kenya.11011119..

The KenyaKC:1\;1 graphite1‘:‘:1pi111: mine11111: is15 situated#1111111 in:1' the- Southern811111111111 KituiK1111 District,[1511111. 17517.‘ 2111';miles5' 1111111from Nairobi,\".11'rv11‘: on51:1 the1.111" road1'1 to:11 Mombasa.\11‘111‘111151 Many$13.11;; difficulties111 11.111111?) were111.1": experienced1"\1.11‘1"1':111‘1‘11 bybf; the111‘ company111111131111. whenwhc‘n. the1116 mine111.11: was\1. .1; started,511111.11. chief.11c among1:1111J11g these1110‘." being1.133113; lack1.1111 of112‘ water.1.121111. 11111::Three 111boreholes1132111111 were11'1‘1'.‘ sunk$1111}. at the1111" site'11}: of1‘1 the1311" mine,111111;. without1.1111111: success,1111.68 and water11.511111 had11111 to111 be brought in' drums1171.111; from1'1'1'11'1'. the‘11.: Athi311111 river,11'11‘1'. 252" miles111I11‘x away.11'. Communications1111111111111. were also.11») a'11. major problem 1. and9.1111 when“1:113:11 breakdowns11r'1'.-..L.11111\1"< in1:1 machinery111:1.'11f11C1'1,r occurred1‘1"; it was necessary for messagesmovaggx to111 be1:23 sent31:11: to111 T!11...'1Thika, 1651115 miles 3.11.1}.away, by1.13 'car1 over1111:.“ very11'1" 1".111bad roads.1‘11'111is Two11.111 or111‘ three111rcc 11.115days usually1111111111 elapsedC111}. 11‘11 before1111111. information11111113111111 11 regarding1131:1211 the breakdown1:41.11'111'11‘1 reached113111113131 Thika111.1111 and211111 it11 was“Hi 1111311then necessary111:1.s for spare51.11111- parts3111 1» to11'1 be171‘ procured1‘1‘111‘1 from Nairobi.1\‘.11mr.11 before'm‘f: 1‘ repairsram could11.111111 be‘1‘: 1"carried1611 out.1111?. Conditions improved11111‘r11'.1'11 when.11 1'11 the1111" company found101::11'1 water11.:111‘1' \‘~'.1.~was avajlablefrom1111111111110 from 1111‘the Tiva'1 ha river, about 12 miles south of the mine, and moved their. refineryreﬁner} to11.1 the1.111: (1‘161‘riverside.1C. A road12.11.11 was constructedC01151111CIC11 by1" the1111‘ company.‘111‘11111111. which1.2113411 reduced:1‘111111‘11 1the1: distance11:'\.'.11‘.1': henwwbetween 111:the mine1113111: and11:11.1 the11’11‘.1‘1111';‘r:.refinery from1mm 1212111to eight.‘g11i1'11i1c'x'.miles. Each111.11 year,3.11:; nowadays,111.111; :1p1tz'11\1111:11:13.approximately 15,0005.111.111 1111.15tons of111‘ ore111‘: are transported11 111131111" 1‘1 over5" this1111; road10311 to11‘ the111.2 refinery.:1'11":'1.. The1111‘ company.'1"111.1p'. has also(11911 installed11:31:; 11011 radio-telephone1':111111—1:;ep1111111‘ communication1‘1‘111.1111::1.. 11:1 between1‘C1'11'1‘1'T1 the1111" mine"1' and1.1111 ThikaT113113 and111111 breakdowns1:111 111.1111 11~ :11:are 111‘“now usually115211111111 dealt(113E111 with1121111 in1:1 (1a day11111 111‘or two.111,11.

A.—\ further111111131 problem11:'111.11'::11 arose117050 in111 111111that 111:1“1;during the:111‘ rainy11111111 seasons56115011< the1111" levels1611315 of111 .'\‘.11EAthi :‘1'1'1‘1'river £11111and 111::Tiva river1‘f\".‘1‘ rm:rose to11'1 \11‘1‘1such 111‘.an 1‘xi1‘111extent 11.1that the1111‘ 511111road from11111111 111:the 1111:1311refinery to 111C111”)the railway 51211111111station :11at Kibwezi1x11311171 133111111:became 'flooded1111011611 and impassable.1:1;‘11Ssi11‘11z‘. ThisI11f< 1111111111}difficulty “11‘was :171‘1met 1‘}by 1211‘the building111111.111‘4':r 1“.by Kenya11111111 (11119111111111Government Ofof a{1 bridge111111;: (111:1over 1111'the Tival'1\.'1 :‘111‘1‘river 21111.1and 111by 1111‘the 51‘11‘111x'.‘~improve- ment1116111 of11111:the roads.0365 in£11 the111$ district.'11:: TheseThis: difficulties1'1: ’ 1 71111311;having been111. overcome,111.9:1 '11111' together1.1131“. her with111111 1111151‘those 1‘11:of 1111:1115obtaining.111.1 labour1 31.1." 11111and :11111'11111xiladministrative. staff.1111 for1111' 111111such £111an 1\1}1C11C11isolated ddistrict, the 1‘1'1111111‘111}company .11111111111‘11continued ‘1'11to 1161:1111.develop 1111‘the mine111111: with.11“: 1.111an 11111143121131:unshakeable faith111151 11‘.in 1111:the 1111.1future11: 111of KenyaKarma graphite,graphﬁc. which11111111 can'1’ :1 bear11::11' comparison311111111511. with1.11111 gri1113111egraphite produced1111111111'1‘1'1 in.11 most717081 1111121other parts1.11111; 111of the111C world.111111111. .ElectricE151ri1'11: plant111 has11m been113:1: f.‘installed" in111 replacementrcpl'. .'1."1.11‘:11: of.11 1111‘the diesel 2111:1124engines prcx;pre'jiously1111s '1 1113.11used :11111and testing:1. .11‘;‘;17.111:\‘apparatus 11.1.1has recently 11cmbeen 11111‘1.1.'11..'1‘11..introduced. ProductionP:'11111:.'11'1.111 beganb19111 on1‘11 a:1 small5:11:111 scale«11110 in111 1953,'15.. but the necessity'. 11"1‘for training:t'ﬁinir'" 1111‘the labour1.11.111111‘ and the111: "trial“1r.-. and error" 1 methods11 1111111\ which1111311: had‘1‘ ..‘ to be adopted,11.11‘1'1'1‘11. rendered unprofitable11111.. J. 3:11.11: the1111‘ first11131 years\63: S of111 operation.11pcra11. ByB} dogged1111gga11 perseverance,psrspmr production11111 .111on '.1a 10111111011131commercial sea}:scale began11:14am in 19561"- .' 1'1 and1'1. 1‘ 1'1 smaH' . ‘ exports:\;;11'11'1‘.~ of1.11 graphite1111111111 1‘ were achieved‘1111‘1315-1'1in that1‘1.11 year.1:111 AnJ\11 overseas11131313115 tourmur b3.by the PrincipalP1‘111c1p'1‘13 1.of the1‘ company:13111p11113: 1:51.resulted in good business contacts and production was Steppedstepped up 111to 10011111 11.11%tons 11a month.1111111111. Exports rose in ]957, the111: principal1111111111111 p11r1‘1...1sc1spurchasers being as111111111115:—follows:-

III 34 Short tons United Kingdom. . 44.8 Australia 11.2 \Vestern Germany 196.0 United{INS States of America 168.0 Japan 290.0 Indiamm . ‘ , . .7 730.2 L'mzmaCanada gmiand Newfoundland\‘mmunumnd __ H h .V .. 1.31.1 The future looked bright when, at the cmiend (Wof 1957,N. 'Lm‘;there wasH LLa suits?sudden <1slump in world demand for graphite. This state of affairs umcontinued into 1958 and Mr. C. D. Malde, the youngest Director of the 5011‘.dcompany, decidcddecided I»to adset nutout omon L1a worldv tour to study the trend of world3"]d t1:[u‘k6t>.markets, into m;cmake H"new \business«mm gutmm‘m‘contacts .XI'WJ.and, thereby, to estimate the future of' the graphicgraphite 311mb"industry in Korma.Kenya. HisHj‘x {nur—four-month tour,H' took111ml: him to most European L‘xcountries,INNER.“' (:LU‘IJCLHCanada, the[1’18 Unitedlini“ : States~< of America,\11161; . .IJapan1 iU‘dand India and he reported on his return that he had received['3‘ the LJ‘JHL'E:utmost k'\r"\‘}“‘:jco-operationan and assistance from the Governments of all the:11: gmmm'wcountries he visited. 'l'h:The dug,demand for good quality graphite in the United States of ,\AmericaM. ,Nis ”‘13great‘ and there2‘; isR cxci‘levery hope that a , ‘ ~, « ‘ part of this market, at least, will be available to Kenya. Germany‘ .lull“ isI“ LLJW‘also ‘1‘.an important market for graphite and it is hoped that Kenya's sales "‘-to 7‘11}that L‘country, also, will increase. The company employs 250 Africans Midand czghieight Asians*‘x>7;1:1\ 4;;and mmnow jnrguproduces approximately 1,500 tons a year of a wide variety of gr'uphm'graphite u:of {xxfromm 50 per cent to 98 per cent carbon and ranging from fines through \miLHsmall flakes1‘1: \L‘\ Wto 1:113:large flakes.flil‘hC‘x The graphite produced is used for lubricants and paints Aidand in fwmunijjm.foundries, 5._1\\~,\u;‘}-c«.glass-works, r:etc.; orders to specification are also met. Kenya'sRaw 4} own urnuannualfl requirements are lowV at around 100 tons. If the demand from:1 moverseasLJ‘ «.1» 3m;increased,xw. production could be stepped up to 2,000 tons a year. Every effort is being made to reduce the cost of production whilst maintaining the quality of the product and :hcmthere is'5 ‘everycm chance that the future of Kenya graphite will justify the faith and determination\muflm‘. ~h‘wn‘.shown by the promoters of the Kenya mine in the early stages of its development.flpmcm.

~~2.-Limestonel—Limestcm‘~~

All the larger outcrops of limestone were1‘ [wk-Qtested :uto g;-determine their calcite: dolomite ratios,{ \ the‘L’ method. used beingH colorimetric, the[hc ratiosm‘iu obtained being to the nearest fiveI\_ per‘3 cent.»_ ‘x ResultsE‘)\\ ‘ \ LUare as,_\ follows:-{1" '»\ ~ RatioR” Specimen“n, «'1 No.‘u Location Calcite,1,,»1]‘;"TC. CCaC03Edit") : Dolomite,iLtiuI'niIC. (NAM;CaMg ECO(CO.),,‘I u60/124a u Kasaala 100: u0 7 60/1387w Kimakimwe 95: 5 w60/142 Otekilawa 95: 5< “H60/164 Mutula 95: 5 Mil60/184 Kasaala 95: 5§ 60/201 Utundani 80\(1 : 20“1:5 Analyst-Mrs. R.R, Inamdar.It‘mrniar'. A ratio of 95: 5 is equivalent to a magnesia (MgO) content of about 1 per1721' ccn!cent, which is within the maximum allowable content in raw limestone' 3"1L‘11L’ usedMM. for'T'I‘ Portland.l’nrtlnm‘. Cement. Only random samples, however, were taken for 13155:these drag-zdeterminations, and before the limestones as a whole could be proved suitable for cement-making:1m1t~11m.}.. Hg w.systematic sampling along and across the strike would be necessary to prove that the low magnesia content persists throughout the outcrop. All the occurrences are near to existing motor tracks or can be ai.easily l'czighcdreached by new tracks. For small scale cement-making or for burning for quid‘mwequicklime brushwood.15}! w and fuel is readily available at all the outcrops.

~~...... U)~~

~~35I»~~

., _.3.-VermicuIiteV Vermicuiitc Vermiculitic biotite occurs in a massive pegmatitem‘ilc exposedupuwui in an old prospecting pit about 15 ft. square and 8 ft. deep at Manoni. The[ ‘1: pegmatite lies under about three feet of red soil. and no indication was found as to its extent. It was discovered in 1944, and reported on by the Mines and Geological Department. The bulk density of the raw Lgmuvunprocessed; material was found to be 56 lb. per cubic foot, and that of the best quality expanded material between seven and nine pounds per cubic foot. While this indicates a fairly good quality the quantity available seems to be far too small to warrant exploitation. Flakes of vermiculite up to six inches in diameter were seen, but always in single crystals, and the percentage of vermiculite in the pegmatite where exposed does not exceed 5 per cent in volume. Thus even if the pegmatite proved to be of large extent the cost of extracting the vermiculite would far exceed its market value.

4.-8illimanite The only occurrence noted in the area was in a quartz-sillimanite gneiss exposed in drainage trenches along the railway about a mile south-east of Kikumbuliu Station. The thickness of the outcrop, which is exposed around the nose of a plunging anticline, is apparently no more than 30 ft. From a single microscope section the percentage of sillimanite was estimated as 60 per cent, but the small extent of the ore reserves so far indicated makes the occurrence quite uneconomic despite its ideal situation on the railway.

5.-Water-supplies The Athi and Kibwezi rivers are perennial, the former being fed from the Nairobi area and the latter from springs in the Kibwezi lava, which carries a considerable volume of underground water from the Chyulu hills (Temperley, 1956, Vol. II, pp. 64- Ill). There are two major springs, one at Chae, a mile west of Dwa rock, and the other at Manoni near the snout of the lava, both of which are extensively utilized for irrigation and stock-watering, and the former for the sisal factory at Dwa. The smaller lava tongue at Masongaleni also formerly gave a perennial flow to the Masongaleni river (Temperley, op. cit., pp. 112-122), but this flow ceased almost overnight in 1918 for reasom which have not been determined, but which Temperley considers most probably due to diversion of the ground-water flow by faulting. (An earth tremor was recorded at Masongaleni on 17th April, 1918.) Water can be obtained by digging a foot or two into the sandy bed of the Tiva river. A few minor springs and water-holes occur in the area, but all have only a small supply and are liable to dry up within a few weeks after the rains have ceased. Bore-holes have been sunk on Dwa and Masongaleni sisal estates, and at Ndovoini, four miles north of Dwa Rock. Records of these bore-holes supplied by the Ministry of Works, Nairobi, are as follows:- ---"------_.._--"---_. "------I No. Locality Depth Water Water Yield per Struck Rest-level 24 hours I 'I I Feet Feet Feet Gallons C.33 .. 400 ? 42 8,640

~~C.34 I Masongaleni .. .. 254 ? 4 11,520 C.73 I Masongaleni .. .. 313 40,200-230, 37 24,000 , 300-313 ("myC.640 : Ndovoini\Idcxx‘m] .. .. .‘.. 335 315m.“ 45 I 64,800~~

(“.HC.l004ﬂLa—i ' I? milcs N. 567 280I‘M I 250 12,000 I Dwa Plantation (2 milesN. of Masongaleni). , C.IOO5 Dwa Plantation (2 miles N. i 190 I 150 150 7,200 of Masongaleni). I (bitter) I ------. --" ---- 36

In1:1 Teccnlrecent years :many"111;. dams111m 111111:have 11:61]been 1111111built 11.1to 1"1'113121..:11'_=impound waterM1161 11‘;in <:.1.s;1:1.11seasonal :'i'u.1'xrivers £11211and siz'cams.streams. 'l‘hcyThey :111are ,mostly«11;» 111‘of 112-11911.earth, 11.:built by.by 1111111hand ‘11311123‘labour 11or “1111with L110the :1111‘aid of '51a single tracim'.tractor; and the111C 11111111311majority 11fof 111cmthem 111'111hold 11:11:11water farfor 1a g1'1w11'1c1'ablcconsiderable panpart of [hethe :1year.' ' ."x".At Ngaii.Ngali, mum—eastsouth-east of Ihcthe peak[31:11; of Kimathena,kimm'F 1 11:11and 1"1‘.on the KatakoloK111113111 rwerriver ﬁfiveve miles sm11'11-suum—easzsouth-south-east 131“of Ikutha,lkmha. corwrwcconcrete dams11.11; 3111:have been built111'}; 51.11%across csteep.sided:ep sm‘ed channels. Five and a half 1111125miles 2111111415!south-west 111‘of lkutha$111111 .1.a 1'31“,very large1111'": stone-faced1.1.141 c1111:earth dam-' 1'1 has'11; been built.built LACI'D‘S'Sacross 111$the DiliuD1311 1212::river. A1At 111Cthe 1111ctime 111‘of Wethe <.11'\.:;.survey this 111.111dam was' 1» not ~.-completed,amp 611311 11111but when filled'5111125. it is cxpemcxiexpected to10 31111111111111impound 21.111}.many millions1111111111Q 111of gallons113 111of 11311101.water. The buildingbuilding. 11fof 11111111391:numbers 01'of 5111131small dams is .‘Ic:11.‘1'_~.'clearly the right1"h1 11111111011solution {1'2to the water problem in the area.areaJ "13.1111both from the {1513caspect of low[1m cm1cost ;1111‘1and highugh 13mpn1'lianproportion 13fof SJCL‘ESHsuccess so5'0 far attained. It11 has often been proposed13135c 1.1111that 1311115dams \h11'11dshould be13:11:11l1built across Thethe Tiva'li\1 river,river. but1.11:1 at{11 pmwntpresent this111‘: wouldv. 1.311111 111pc1t‘appear 10to be rather".‘1111c1.‘11unnecessary,11; .1: «7111'ssince ample111‘11311‘ water for1111' domestic111‘11'1‘511: use1152 and stock«1.1”. watering\\'L‘11C1‘1:1g is' available' in the sandy bed withmth :1a minimum111112113 01'of 11'digging.ggmg. x\.A 111111dam 11011111would 11').very 5131111soon silt up,. and. digging would againin 13;:be necessary toin reachrc1.‘h the water111111 held 1:1in 111;the {11111sand behind the. dam, unless the retaining wall was Ofof '5:1sufficientﬁ:.-1'crtt hash:height 1nto 1111Mallow sub—surfgc’csub~surface water:.' " to be piped to troughs or tanks.

~~IX-REFERtENCESl ‘g—IQLF I RLNCL‘~~

Aylmer,.-'\y]:11c1‘. L.,L. 1908.-"Captain1908. ”(111111111 Aylmer's\umwﬂ Journeyl '11' ,.\' in111.119the 1Country13.111111. \.'~1.".E1South 01111:of the T1111Tana River,R1131: E1xtEast Africa."M11111." Gm],Geog. 11211111,.Journ., Vol.\1i11\\\11.XXXII, pp.131. 557W,55-59. Barth,Barth. T. F.F, W.,W” 1952.-'Theoretical11:15:. "T}‘..u.11'€:31111 Petrology."Pct:

Behrend,ﬁch'V‘I‘d. F.,If. 1918.-"BeitrageWIRE. I'Lgc «1.11zur geologischen ErforschungF't)v'~;'f11::1g 11:1:der DeutschenDultsn Schutzgebeite." Caswell, P. V., 1956.-"Geolo.gy of the'.‘11;*\11.‘1114Kilifi-Mazeras112111;» Area."$11.1." ReportRCDUTI No.N11. 54.34, Um}.Geo!. Surv., Kenya. Daly, R. A, ]933.-"Igneous Rocks and the Depths of the Earth."

Dndwfli. f (1.. 1.1 Dodson, KG;, ]953.-"Geology. of the South-east Machakos Area."311:1.“ Report19.111131 No._\3, 25,‘11 113.1Geol.. $11132.Surv., Kenya. Gregory,Urcgu'}. J..1, W.,‘-\ .. 1896.-"The"Hz-c (311-11Great RiftR11: Valley."\L.I.'1c;..” 7 . ,. “1."?1921.--"The”1111‘ RiftR11”. Valleys\Iflh'fim 1.11.1and Geology(11.111111; of EastF.1d Africa."Afmca.”

~~Hildebrandt, J., \1.M., ]W71,879.-"Von”‘1 11:1 M111111x1mg111:11:11Mombassa nach Kitui."KEE1:1.”[.—1.Zeitschr. d. (icxcilsgh.Gesellsch. f. 11:11}...Brdk., Vol. XIV, pp.pp 32]-350.13611~~

Krauskopf. K. 3.,B., I1948.-"Lava1 1 191.1 MMovement1». -1 1 [11at I.Pari‘1;;1.1.31“cutin \1f'11;;1.111‘;.Volcano, Mexico."T\L"\1\"1.u Bull..‘a‘ f1.52111".Ceol. ,‘mSoc. America,1 11.1 \111Vol. LIX,I 1\ pp.1313 121171267-1283.‘21"

Krenkel,K'cnkci. E.E., 119117773135111';;.I<<111'!911.--"Geologische BeobachtungenH..'1'1f1;f11111'1g:11 in11"1 BJ‘1’..~‘.'1'1*J~1L1‘1‘§'11‘LL:Britisch-Ostafrika." ” _\'.N. 111.1113.Jahrb.. Min.‘21 1 (1Geol.1‘1. P.1'Pal., 31.3], 1'13pp. 147-31”243-267. --, 1925.-"Geologie Afrikas." Vol. 1. Muff (Maufe), H. B., 1908.-"Report relating to the Geology of the East African Protectorate." B.A Protectorate No. 45, Cd. 3828, H.M.S.O. Parkinson, J., ]947.-"OutIines1’ ;1i.11‘.:‘s of ‘the Geology of the Mtito Andei-Tsavo2;\u Area.".‘;\1:1. Report No. 13, Geol. Surv., Kenya.Kcnya,

~~Richards, C. G., 1950.-"Krapf,"K1'1‘111‘. Missionary\11»1'~"1;_11r;. 111:1and Explorer."[3'17v EastE1151 Africanski-1111121 Litera,ture2.1331111111': Bureau.~~

~~Saggerson,‘ . E.[1. P.,P11111957.-"The"'11:: (imiwg;Geology of thcthe SouthKnuth KituiK1111 Area."Area.” ReportRepm". No.N3. 9737, (1'30“.Geol. 81111.Surv., Kenya.km 11.1~~

~~---- 37~~

~~Sanders, L. D., J954.-'The Geology of the1h; KituiRug Area."Area." ReportR39“; No.INK), 3m.30, (jg-[‘1‘Geo!. Surv.,\ Kenya.~~

" '., 1963.-"The:LI‘I'13,'”¥'N (Jk‘gjj‘Geology L‘li‘of theL91; \IUf-SVoi-Southﬂ‘llh YattaYMZ‘J Area."\121,H Report{{6}}i No.\‘. 7—"54, (”anyGeo!. '\U,11‘\,.Surv., Kenya.Kcnw. Schoeman, J. J., 1948.-"A"A (:xcmugw'ca}Geological ReconnaissanceRecogmaiswmc nfof 1h:the AreaArc». West\\ cu afof KituiKm: Town- ship." Report No. I4.14, Gm},Geo!. Surv.,Sung Kenya. TemperIey, B. N.,\,. ‘34t1956.-"Report7"Rupwr‘t anon Ihcthe Klbucii-(f'uuwKibwezi-Chyulu 1954-19551‘95 Groundwater Investigation."~41," Minim‘;Ministry nL'of Works,\\ 01'5“. Nairobi.Nairobi, deparizlxu;departmental report. insultTurner, F.II LJ., 1948.-"MineralogicalFMS. "N1a'mmgmzti am?and SH'LQZL‘I'LNStructural EvolutionEwlmkm of the Metamorphic Rocks." Memoir 30,11‘. Kiwi.Geo!. Kat.Soc., America.\JUCTiCIi. Walker, E. E., J902.-"Report”R; ' :" mmon Incthe (unwingj.Geology NJof the1}“; EastEShI Africa\"t‘kd Protectorate." Africa No. 11, Cd. 1769, H.M.S.O.n.

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~~. ....~~

~~Sanders, L.E‘ D.,1).. 1954.-'TheEli—C "H (.xx;Geology ofnl' Hi;the KituiMED Yum."Area." Reportcwr: No..'\n. 30,5U. Lind.Geol. Surv.,‘~ Kenya.~~

., M33,1963.-"The"Exp; Lun‘ug;Geology wof the1'32 Voi-South\‘ui-Sninh YattaE Area."\xm." Report 1\No. 54,7—K mgui.Geol. \H‘t,_Surv., Kenya.Rana. Schoeman, J. ..J., 1948.-"AW—Wr "A bcmuugiuaiGeological ReconnaissanceRcuunnai»armc n1of 1h:the AreaA'mt West\\ CST. of KituiKim: Town- ship." ReportRzpnx“. No.\‘m, 14,i4. gm,GeoI. Surv.,Sump. Kenya.Kama. Temperley, B. ,\'..N., 1956.-"ReportI“~‘5h. "cflt': \mon 1hrthe Kibwezi-ChyuluKlbxxc/d-(hjwm 1954-19551:51.71‘263 “mmGroundwater1"“ Investigation."‘ Guilty“ Ministry31.113111. mof Works,\\ Grin. \fNairobi,flz'mbi. dCPILI'UI'JmZL-Lldepartmental :‘cpmt.report. "Iw‘iwi‘.Turner, 17F. J.,I. TWA.1948.-"Mineralogical"\Iu‘wu‘ahgfiui uzwand SU‘LgLLAI'LUStructural EvolutionHmhlkm a}:of 1h“the Metamorphic\1 Rocks."R wink." Memoir\ICI‘nnir 51:30, (mm,Geol. Max.Soc., America.Mncrica Walker, E. E., 1902.--"Report"Kg 9:: m,‘on the151v Cs;Geology103'. Hfof if);the hmEast Africa\i‘vmx Protectorate."Pmlcgiur‘gih" Africa No. 11, Cd. 1769,'~ J7 H.MS.O.H}; 31";

~~G.P.K. 1278(60)-lm-3f63~~