Building-stone resources in Eritrea: results from introductory work in the NGU-EGS co-operation programme

NGU-BU LL436,2000 -PAGE 183

Building-stone resources in Eritrea: results from introductory work in the NGU-EGS co-operation programme

TOM HELDAL&ERMIASYOHANNES

Heldal,T.&Yohannes, E. 2000: Building -stoneresour cesin Eritre a:result sfromintroducto ry work inthe NGU-EG Sco-

operationprogramme.Norges geologiske und erse kelse Bulle tin436,183-188.

Several buil d ing-sto ne quarries and prospect s in Eritr ea werevisite d during Septem ber 1998, as apart of the Norwegian-EritreanERINOR co-operationproject.Thebedrockgeologyof Eritreacomprises a varietyof rocks,of which Proterozoicgranitoidsand marbles are,at present ,considered tobe mostinterest ing for building-stone produ ct ion.Neith erthemarbl edepo sitsnorthe granito idsvisitedareof suchexclusivequalitythat theywould obtain highprices on the internat ionaldimension-sto ne market.How ever,several deposits seemto be of excellent quality forlow-cost produc t ionof slabs,tiles andother produ cts,and thus representanint eresting resourcefor domesticprocessing .

TomHeld al.Nor gesgeol ogiskeundersekelse,N-7497Trondheim, Norw ay.

Erm ias Yohannes, Eritrean Geolog ica lSurvey,Asmara,Eritrea.

LEGEi\J)

QUATERI\ARY

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marinetransgression

• Assa b PROTE ROZOI C Post te ctonicgranite Metagabbro

Syntectonic gra nite and granodi orite Marble andothe r meta scdiment ary rock s Verylow gradevolcanic andsed imentary rocks Low grade metamorphicrocks(basic metavolcanics. chloriteschists,etc.) Medi um grademetamorphicrocks (amphib oliteschist)

o

m

olistostrorneassemblages

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2 - Ker en (po rphyri ticgranite) 3. Westof Keren(gra nite)

4· Dckemhar e(g raniteandgranodiorite) 5-Gogne(marble)

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?NEO GEN E.PLEISTOCEN E AND 1l0LO C ENE

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~Sedime nts -

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200 Kilomcters

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Fig.1Simplifie dgeologi calmap of Erit rea andlocalisation of building-stonedepositsdescribedin thetext.

NGU-BULL436,2000 -PAGE184

Introduction

Inspiteof its relatively small size,Erit rea exhibitsa diversified geology which can generally be grouped into three main Iithologicalassemblages. These arePrecam brian metamor- phic and igneo us rocks, Mesozoic sedimen tary rocks and Cenozoic volcanitesandsedimen ts(Fig.1; Kazmin 1972).

The metamorph ic basement,belongi ngto the Arabian- Nubianshie ld,coversa major part of the country. The Pre- cambrian of Erit rea includes a variety of lithological units, ranging from high-grade (Archaean?) gneisses, low - to medium-grade,volcano-sedimentarysuccessionsof Neopro- terozoic age to pre-, syn- and post-tectonicgranitoid int ru- sions of Neoproterozoic age.

Although the Palaeozoic era was characterised by peneplanation and virtually no sedimentation,transgression oftheInd ian Ocean led to depositionof sandsto neandlime- sto nesuccessions inMesozoic times.This wasfoll owed by uplift and subseq uent developm ent of a thick, laterit ic weathering profi le.During EarlyTert iary time,an extensive period of volcanism wasinitiated, starting with the plateau basalts of theTrap Seriesand follow edby several events of rift related volcanism andsedi mentat ion.

Regard ingbuilding-stone ,both marbleand granitehave been exploited indust rially sincethe Italian colonial period. In parts of the country,there are strong traditionsof usingstone for local housing ,especially granite ,slate,limestone and vol- canicrocks' .After Eritrean independence in 1993,a national stone industrywas estab lished,anda sophisticated process- ing plantfor dimensio n-stone,locate d inthe town ofGindh a, isnow produci ng tiles and slabsforthe domestic market . Severalmarbleand granit edepositsare now being exploited and prospectedthroughout the country.There are,howeve r, fewgeological report s covering building-stone resourcesin Eritrea.A brief int roduct ion to the subject howe ver, was given byBradley (1995).

As a part of the Norwegian-Eritrean ERINORprojecr',sev- eral depositswere visitedduring September 1998 (Fig.1).A nationalinventory of such resourcesisexpectedto becomea partoftheErit rean Geological Survey'sact ivit ies in the years to come.In this art icle,adescription andpreliminaryevalua- tionof some of themost im portan t build in g-ston e deposits is given,includ ingthemarble deposits near Gogne in SE Erit- rea and granitoid deposits south and north of the capital, Asmara.

Industrial and market considerations

Dimension-stone differs from other types of mineral resources by the variety of its uses and markets.Stoneprod - uctsof excellentquality may readily beproduced usingsirn-

1. Suchtradit ions arealsoknow nfrom theEthiopi anhighlands, see Walleetal. (2000).

2. The ERINORproject is aimed atincreasing theErit rean Geological Survey's(EGS)capability of investigating the country'smineral resources,and involves institutionalco-operationbetweenEGSand NGU in different fields related to geological mapping and evalua- tion of mineral deposits.

TOM HELDAL&ERM IAS YOHANNES

pietools and manpower,although the mining and process- ing may equally involve the use of highly sophisticated machin es.App licatio ns vary from crudely shaped blocks for local housing to polished slabs claddingskyscrapers in the citiesaroundthe worl d(Shadmon 1998).

For small-scale,local housing product ion ofstone, it is important that therocks can beeasily workedwith a mini- mum of labour, and that the dep osit s are fou nd close to where thesto ne isused. In Erit rea, we find examples of such product ion ofgran it e(Sout h-Cent ral highlands),basalt(Cen- tral Highlands)and coral reef limestone (East Coast).These deposit s shareacom mon feature, in that the rocks can be split orcut easily into cubic or roughly shaped blocks.

Raw block export isthe most common way of getting accessto the interna tionalmarketfor dimension-stone.The blocks are mainlysol dto countrieswhich have a largestone- processing industry, like Italy, Spain, Portugal and some countriesin theFarEast(Contietal.1990).A sta ndardsize for trade blocksis2.4 x 1.2 x1.0m(ca.8tonnes).Most supp liers will beaskedfor even big ger sizes,up to 30 tonnes,to feed the bigframe saws inItalyor elsewhere.However,the need tomove suchheavy pieces of rockmakes demands for sizea- ble investmen tsinquarry machinery.

Theblocks shouldbe well-shaped with no veins,cracks or inclusions. Forsmall blocks and blocks with such imperfec- tion s,the price maybe reducedby 50%or more.Pricesof raw blocks are,furt hermo re,highly dependent on the colour and st ruct ureofthe sto ne;rarecolours suchas blue,pure white, pureblack, yellowandemeral dgreencandemand consider- ablyhigher pricesthan moreordinary colo urs(see alsoWalle et al.,thisvolum e).

The interna tio nalmarketfor finished products works dif- ferently. Here,the customers are either the end-users of stone product s or stone traders. The largest marketsfor fin- ished st one in the world are USA,Japan,the Middle East, North -Central Europe and someFar-East countries.Although the pricesfor rough blocks vary a lot according to stone type, size,shape andquality,finishedprod uct s show fewer varia- tions, duetothe'smoot hing' effectof the quite significant costs ofstone processing.There is,however,an increasing tend ency forconsume rstodemand quality contr olsystems from the supplier, and thelatt er sho uldatleastbe capableof meeting the int ernati onal stan dards on size and flatness deviati ons, uniformi ty, polish, etc. As a result, advanced processin g machinery becomes a necessary investment for thosewho wouldliketo begin exporting fin ishedstone.

Inmostcountries,including Erit rea,there is a market for tilesand slabsfor localconstruction.In general,tolerances on unifo rmi tyand other technical specifications are somewhat wid er than forexport product s, due to the customer's prefer- ences forlocal material.

The logist ics conn ected with exploi tation of building- sto ne(quarry technology, inf rast ruct ure)thus depend on the int ended market. For local housing and constr uction, it is important thattheext raction costsare low,necessitatingthe needforrocktyp es withexcellent workabilityproperties. For export markets,the colour,pattern and block size are much more importan t. During regional invest igati ons of the

TOM HELDAL&ERMIASYOHANNES

dimension-ston epotenti al,focus should therefor ebeplaced, at an early stage, on themarket pot enti alfor thedifferent rock types.In thisarti cle,wepresent aprelim in ary evaluat io n of market aspects,as wellas of thetechni calqualit y of rock typ es and dep osits,based onfieldrelation s and bycompar- ing thedeposits wit hot her rocktyp eson theinternati on al market.Testing of physicalprop erti eshasnot been carried out, butwill be a natural part of moredetail ed,follo w-up st ud iesof depo sits.

Eritreahas, incont rast tomany ot her Africancountries, a relat ivelygood infrastructure wit h roadsprov idin g access to large port ion s of the country . Portfacilit ies on the Red Sea coast pot enti ally offer rap id accessto inte rnat io nal market s.

Marble deposits

Marbl e occurswit hin theProt erozoic metasedim ent ary sue- cession s,and themarbl e-bearing unit s are presenttwo dis- tinct zones.The west ern zone defin esa NNE-SSW st riking belt from the sout hweste rn corn er of the count ry tow ard s theSudanese borderin thefarnorth (Fig. 1).Marbl edeposits areknownfrom the sout hweste rn(t he Gogn e area) and cen- tralpart s (t he Akord at area;Bradl ey1995).ln1998,theform er areawasincluded in thepreliminaryinvestig ati on s.Anoth er zoneoccurs between Dekemh are and theEthiopi anborder (Fig. 1), and deposit s are known in theDebriand Kerzekert e sub-areas. The Eritrean marbl es are essenti ally fine- to medium-gr ained,calcitic and of low region almet amorphic grade.In theweste rn zonethemarble scomprise several col- ourvariet ies, from pink andwhit e to grey and black, while predominantl y greyand blacktypes are describ edfrom the easternzone (Brad ley 1995).

TheGognemarble depo sitsoccur wit hin a mica schist unit closetothesmall townof Gogne insout hweste rn Eritrea.

The met asedimentsare locallyintruded byapre-orsynte c- ton icgabbro.Onecom pany(MarGran) has several quarriesin the marbl edeposit,most of them atatrial stage. Theblocks are transport edby trucktothefactoryin Gindha.A new road isunder const ructi on,and in thenearfuturemostof thedis- tanceto Gindha will bepaved surface.

Themarbl edeposit consistsof intercalating , ste eply dip- ping layers ofseveral commercialtyp es,ofwhich themost

1

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.

->:..

Fig.2.Prospectofwhite calcitemarb leintheGognearea,SWEritrea.Dia- mond wirehas been usedforvert icai cutting.

NGU-BULL 436,2000-PAGE 18 5

com mo n is greywit hwhiteveins. Othertyp es includewhite marbl e wit h grey and black veins(Fig.2), blackwit h white veinsand impure, silicate- bearing pin k marb le. These sub- typ es generally occur in thin layers, not exceeding 10-1 5 metr es,but thelayersmaylocallybe thickenedin fold-hinge zones.The marbles areessent ially fin e-tomediu m-g rain ed, but are locally coarse-grained, especially ap proachi ng the gabbro.Thus,the increasedgrain sizemaybe alocal effect caused bythermalmet amo rph ismrelated tothe int rusio ns.

Joint s are generally closely spaced in the area, especiallyin the depositsclosetothe gabbro,where problemshavebeen experienced in obtaining comme rcially sized blocks.Here, filled jointsarealsovery com mo n,givinga networkof black andwhiteveinsonpolishedslabs.

Conside ring theGogn e areaas a who le, the possibilityof explo it ing severalcom me rcial typ es of marbl e wit hinalim- it ed area makes itinte rest ingas adim ension- ston eprospect.

Som e of the variet ies,especially pure whiteand blackmar- bles,maybe of interestfor expo rt market s,whilst the grey and pink varietiesareexpecte d tofetch lowprices and meet st rong competition from a number of marble -p rodu cing countries.The maingeological challengewould betolocal- isepart s where theincid ence of jointingisless, and to obtai n moredetailed information about the sizeand geomet ryof the different commercial layers.This will invo lve det ailed fieldmapp ing com binedwit hst udies ofaerial phot ogr aphs.

There isa parti cul arneedfor aninte rpretat io nof the st ruc- turalpatt ern,localisingfold -hing e areas where theprobabil- ityof finding thickand massivedepositsis conside red tobe greater.

Igneous rocks

The bedrock geologyof Eritrea comprisesa variety of plu- tonic rocks of diff erent ages,ranging from Proterozoic, pre- andsyntect onicgranito idsand gabb rostoLateProt erozoic/

EarlyPalaeozoicgranite,granodiorite and peralkalin egranit e post-d ating the major Proterozoic deform ation and meta- morphic events.

Thepre- and syntecto nicint rusio nsoccur essent ially in the cent ral part of the count ry,wit hinatriangl edefin ed by thesouthw esterncorner of Eritrea,theDekemh are areaand the areanorthof Nakfa (Fig.1).Rocktyp es are predom in antl y gneissose to slight ly foliated granite s and met agabb ros.

Sincethebuilding-ston epotenti alof met agabbr osisrarely of particularinterest¹,thefocushasbeen on thegranit oid s.One such depo sit wasvisitedin 1998.This occu rsslig ht lywestof thetownofKeren,and istargetedbytheMarGran com pany as apossibleobjectfor future exploita t io n.The area is easily accessibleon paved roads.

Thegranite isfoliat ed, 'gneissose',and formslargepeaks and hillsin thearea (de posit no. 3in Fig.1 ).Boulder weat her- ing isnot common at thislocalit y; rat her, the hillshave amas-

1. Metamorp hicgabb rosare,in most cases,highl y fracturedandcon- tain retr ograde mine ralassemb lages,causingtherocksto beof considerably low er valueon the dimensio n-stonemarket than their non-me ta mor phicequivalents.

NGU-BULL436,20 00-PAGE186

Fig. 3. Exposur esofhill-forming ,pre-tect onic granite tothe westofKer- en.

Fig.4.Outcropof pre-tectonicgranite westof Keren.Not eaplit ic veins and nearlyhoriz ont alexfo liationalongfoliationplanes.

Fig.5.Splitsurface of boulder offine- to medium- grained graniteclose to Dekemhare.Notewedgi ng ho lesmadebyhammer andchisel and stainedweat hering profile(approximately 7-8cm in thickness).

sive and rounded appearance(Figs. 3 and4). The granite,at the st udied locality,is medium-to coarse-grained,grey to pink,and the nearly horizon tal foliat ion is defi ned by ori- entedmicas and aggregates of quartz and feldspar.The con- tent ofmicrofract ures seemstobe high, and becauseof this, thegranit ehas adeep,stained weat hering profile.Thus,one would expect that both water absorption andpermeabil it y would behigh erthan theaverage for granites.Suchmicrof-

TOM HELDAL

s

Fig.6.Granite columns used inlocalhouses nearDekemhare.

Fig.7.Vertical spli tsurfacein giant bou lder of granite close toDehem- hare.Note too lmarks ontheto p of the bou lder.

racturepatt ern s arequite common in foliated pre-tect onic granit es, and probably represent a 'proto-cataclast ic' response to medium/Iow -temperatu re strain.This texture could also be the reason forwhyweat hering attacks microf- racturesand grain boundariesrather thanjoi nt s, so that the typ icalboulder weatheringthatiscom monin more'sound' granit es does notform.The granitecontains abundant aplitic veins and biotite-rich inclusions. Both these feat uresand the

TOM HELDAL&ERM/A S YOHA N N ES

foliation developm ent might possiblydecreasetowardsthe cent ral partof the granite(we wereonly lookingat themar- ginalpart ). and forfurt her investig ati onone sho uld pay spe- cial atten t ion to thesecentr alpart s. Such granit es arecheap materials on the internation al market, and, furtherm or e, granites containi ng abundant micro fract ures could some- timesbetoo'soft'and perm eab leforthe product io nof thin slabsforuse astiles.On the ot her hand,theygenerallyhave excellent sp litt ing prop ert ies and may be very usable for spl it-sto ne for local housing and pavin g sto ne.Sporad icpro- duction from sim ilargranites is known from northern Nor- way; ot hercom parab le typeson the market include 'Rosa Delicato'and'Ara Cinza' (Brasil).

Post-t ecton ic granit oid s occurin several placesaround the count ry, varyin gfromfine-tomedium-grained granites and granodio rites, through porph yritic granite to the younge r, peralkalin egranit es in the sout hweste rnpartof Eri- trea(Fig. 1).Thelatt ertyp ehasnot yet been invest igate das a source forbuildin g-ston e.

Occurrenc es of fin e-to medi um- grain ed granito ids are known from anumber oflocalit ies.App roxi mately mid way alo ng the road betw eenAsmara and Keren (de posit no.1 in Fig.1),thereis acircular shaped graniti cpluton of predomi - nantl y grey,medium -grained granit e.Along the roadside, the granit e ispartly alte red to green saussurit eg ranit e. This green colouris conside red to be inte rest ing in thedimen- sio n-sto ne market¹, and the granite has been samp led by MarGran.How ever,the spacingof joint sistight,makingthe ext ract io n ofcomme rcial-sized blocksdifficult.The reasonfor thisisthatthe alte rat ionof thegraniteisprobably associated wit heit her fracture zones or the pluton 'smarginalzone,both areaswhere highly fractur edgranit e should be expecte d to occur. Furthermore,fine-grained pyrite is ab undant in the granit e,causingrusty stainingevenon quitefresh surfaces.

Fine-to medium-grained pink granite (Fig.5) and grey granodi oritecover alarge area closetothe town of Dekem- hare (de posit no.4 in Fig.1), easilyavailable by pavedroad.

Boulder weat hering is common.Thesegranitoid shavefor a long timebeen subje ct to tradition alextr action by sim ple tool sfor localhou sing (Fig.6).Both theongoing quarrying act ivityand housesin the area bearwit ness to the excellent splitting properties of theserocks.Even wit h simple tools like ahamm er,achisel andsome plugs,one cansp lit 4 metr e high boulders (Fig.7).With the introductionofslight ly more advanced industrialmethods, andgoodknow ledgeof gran- ite'rift'and'grain',itshould bepossibletoproducelargevol- umesof blocks andot her granit eproduct s (kerbs, paving - stones,etc.)at lowunit cost.Jointspacing varies in the area, and there are scatte red,small hills wit ha wider jointspacing thanthesurroundings.Further investig ati on s should focus on theseprospects.

The Dekemh aregranit es are quite exception alin the way that a wide rang e of granit eproducts,from poli shed slabs to kerbs and bricks,canbeproduced atalow costandwit h low investm ent in quarry techn ol og y. However, such granit es

1. According to MarGran repr esentativesandNGU's experiences with similar rocktyp esinNorway.

NGU-BULL 436, 2000-PAGE 18 7

Fig.8.Porphyriti c grani tecloseto Keren.Thelargestphenocr ysts seen on thephot omeasureapproximately 5cm.

and granodiorites are generally low-pri ceprodu ct s on the rawblockmarket. Ot her,similar typ es on the inte rnat io nal marketinclude severalvariet iesofScandi navian,Spanishand Portuguesegranit es.

Fine- to medium-grained, pink granite sim ilar to the Dekemh are type also occurs in the Nefasit area (Bradley 1995),betw eenAsmara and Gindha,and pilotquarryin ghas been carriedout by MarGran.

Severaldeposits of porph yrit ic graniteareknown,includ- ing thosein the KerenandAdiQualaareasclosetotheEthio- pianbord er sout hof Dekemh are (Bradley 1995).The former deposit s werevisite d durin gthe1998fieldwork,andapaved road passesthrough thiscircularshaped pluton (deposit no.

2in Fig.1).Thegranite is slight ly foli ated wit h large(upto 7cm)pink togreyfeld spar phenocrysts(Fig.8). and isremi- niscent of cert ain other porphyriti cgranit es on themarket (MUlier 1990). suchas'RojoVillar' (Spai n).'Anger Gut en'(Et hi- opia). TexasPink' (USA). The abunda nceof phenocrysts and their colo urvary, parti cul arlyperpend iculartothe foliatio n plane.Assimil ated xenolit hsand biotite-richlenses and pock- etsare quitecomm on; apliteand pegm atite veinsoccurspo- radically.In the area,there seem to be several inte rest ing prospect s, especiallywit hi n less jointe d'pockets' ofgranite form ing small hills of boul d erdeposits.It is,how ever,impor- tanttopay specialattent ion tothefeaturesinfluencing the hom og eneity of thegranit e, suchasthedistr ibution of phe- nocrysts, xenolit hs, veins and pegm atites, in any furth er investiga ti on,since thesemay varysignifi cant ly oversho rt distances.

Other rock types

There areseveral other rock types in Eritr eathat could be of potenti alinterestasbuilding-stone.Thelarge occurrencesof low-grade pelitic rocks wit hin theProt erozoic successio ns includedepositsof black slat e, but thequalit y ofsuch depos- itsfor industrial exp loitat ion isnot yet know n. With in the ultram afic com plexes in the northern part of the count ry,an obvioustargetforfutureinvestigation s would be greenser- pentin it e andsoapsto ne.

NGU-BULL436,2000 -PAGE 188

For local construction,bothlimestone andvolcanic rocks are exten sively used.The town ofMassawa,for exam ple,is almostcompletelybuilt of coralreeflimestonefromnearby deposits.Basaltand diabasedykes are quarried at a numbe r of places in theCentral Highl ands,and thereshouldbe possi- bilit iesof findingtuffand ignimbrit edeposit s.Such resources rep resent a considerabl e potentia lforlow- costsup plyofan excellentconstruct ion mat erial,and couldbefurt herdevel- oped.

Conclus ions

On thebackground of thesmall num ber of depositsvisited duringthe1998 fieldwor k and basic know ledge of others,it maybeconcluded that the bedrockgeologyof Eritrea can offer avariety of rocktypes of interestas sourcesofdimen- sion-stone.However, few of these would clearlybe consid- ered as exclusiveandtherefore willnotdemand high prices ontheinternatio nal market. Onthe ot herhand,severalofthe depositshave excellent properti esthat make themsuitab le fortheprod uction ofa widerange of stone productsatlow cost,bot hon an industrialscale and forlocal housing.

In this context,the nationalstoneindustry'sstrategy of processing a variety of Erit rean rock types for domestic and, ona longerterm , international marketsraises int erest ing per- spectiv es forthe exploitation ofanumb er of depositson a small-scale basis.Especially the marb ledeposits in the south - easte rnandsouthw estern partofthe country,as wellas sev- eral of thepost-tecton icgranit oids, would,in the short term, make the most obvious resource potentia l. In the longer term, development potent ial existsfor several ot her rock types.Fin ally,the extensivedistribut ion ofbot h'soft' and

TOMHELDAL&ERM IASYOHA NN ES

'hard'rocksthatcanbe extractedandprocessedwit hsim ple techn ology could contribute sign if icant ly to the country's fut urerequirement s forcheap constructionmaterials.

In the future,EGScanhaveanimport antroleinthe eval- uatio n and characteri sation of dimension-stone deposits.

This includes the charact erisation of rock units during regional mapping (colour,text ure/structu re, joint patterns, generalappearance),sampling anddetailedinvesti gati onof depositsin co-operationwit h the stoneindustry.Inadditio n, the EGSlaboratori es can be helpfu l wit hpetrog rap hicexam- inations and testing ofphysicalprop ert ies,especially if the laboratories are equipped wit h facilit ies for the testing of bendingand crushingstrength.

Ackno wledgements

Theaut horswould liketothankthe MarGranCompany for goodco-o p- eration during fieldw ork,and EinarTvet en (NGU) and ChrisSpencer (BRGM,Orleans)for their construct ive reviews ofthemanuscript.

References

Bradley,F.1995:Erit rea:developing the sto nesectoris aprio rity.Marmo Macchine Internatio nal B, 24-28.

Conti,G.,Mannon i,T.,Montani,C,Pinzari, M.,Pucci,R.&Ricci,A.1990:11 marmo rnelmondo .Industri ae commercia dei materiali lapidei.Soci- etaEdit riceApuana.253 pp.

Kazmin,V.1972:Geolog y of Et hiopia,Explanatory notesto Geological Map of Ethiop ia 1:2,000,000, Geological SurveyofEthiop ia,Addis Ababa.

Muller,F.1990: tntem atio noleNaturstein Kartei.Ebner VerlagUlm.

Shadmo n,A.1997:Ston e:an introduction.IntermediateTechnologyPub- lications,172 pp.

Walle,H.,Zewd e,S.&Heldal,T. (2000):Building -stone ofwest-central, central,east-centralandsouthern Et hiopia:deposits and resource pot ential. Norgesgeologiske undersokelseBullet in,this volume .