The ries kessel, Germany: An example of meteorite impact as a terrestrial geological process

The ries kessel, Germany: An example of meteorite impact as a terrestrial geological process

Geoforum 12/72 91 time the value of northern resources would have risen oWi% to the rundown of alternatlve sources of materials within the continen...

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time the value of northern resources would have risen oWi% to the rundown of alternatlve sources of materials within the continent; crudely, that the Americans would be In more urgent need of wood pulp or petroleum - or for that mater, water - than they are now, and these thirps would acquire 1 ~city vdue. Between these conflicting claims upon their rrsources, the provtncial governments bear a heavy responslbiltty for decisbn.

homestead of today (250-300 hectares, in place of the historic 65 of the quarter-section) is uncertain, is there any justification for the outlay, in public land and taxpayers’ money, involved In continuing the homestead policy? What future has agriculture In the farther north? At the agricultural research statlon at Beaverlodge, estimates of potential agricultural lands in the north and north-west run as high as 7.5 million hectares, compared with a present area of crop and fallow (mainly in the Peace River country) of 1.8 mHlbn hectares. The northern research stations have a long and honourable record of breeding plants to match the environment, but the estimate of potential arable lands is based on sketchy soil survey and very scanty, perhaps non-representative, climatic data, while there Is a [email protected] absence of coordination between the agencies which offer the farmer tech&al dvke on one hand and economic advke on the other: between the agency which tells him what con be grown and that which suggestswhat will pay. Even if we accept that there are physkal limitations on prlculture, it is still necessary to add that experience in other areas where remoteness is a factor suggeststhat, for agriculture to be economically viable, it should be developed where, and only where, there is clear assunnce of market opportunity. In practice, there are two ways of securing this. One Vito develop in relation to loco/ markets. On the whole, this has not happened In the area visited by the symposium. The other - perhaps the only other -way is to develop specialisatkn within the range of crops permitted by the environment: to follow the example of, say, Prince Edward lslrnd or Northern Ireland and produce not srmply grass but grass red; not simply potatoes, but seed potatoes. Of this trend there Is some evfdence in the Peave River region: them is room for more. Under what institutional arrangements should northern resource devebpment take place? The question is not a new one; in Canada, after all, it goes back to the first French fur company charters and nowhere has it been more hotly debated than in the Prairie Provinces, where the Social Credit parties have added new dimensrons to it. Two basic issuesare involved: (1) the question of alienatbn of resources versus leassingand (2) the question of rate of development. Undoubtedly, the quickest way to open up the north is to permit llknatbn of its rey)urctl to all comers, and 10 to attract the Iwest possible flow of capital and labour to the frontier. In the Canadian context, thh means makhtg ovar the natbn’r resources to what Canadians am ddkataly twferrlw to these days as ‘*bC International corporatbns” but whkh, in plain Iaaupe, are American firms huqry for new supplks of raw material. It is they which hava the means forgeneratl~ rapid development in the north, altwh Canadians react abruptly to this reallsatbn and probably subrtitute for debdopmrnt *Mle word with more ColOUrfUl ow#tona& This mactbn seems to be automatic; it is justifiable in thr lmt of past orpcrience but prob&ly does much kss than justice to the corporation of today, which is bound by provincial [email protected] I# aml commltted to rbntlflc tree farmh& Nmrthebss, It forms one aspect of the tyled relatbnshlps between Canada and the USA. There are those who argue that Canada rhould opt for the lower rate of development whkh Is knplklt in a policy of keeping the Americans out. They would accept the resultant lack of immediate employment opportunities (the present lack of such opportunities is surely an anomaly in a frontier situation? ) on the grounds (a) that development, when it did occur, would be firmly in Canadian hands, and (b) that in the mean-

4. What is to be done for, or about, the Indian and Metis populatbn? The comfortable statistic that Indians make up only 1.1 % of the populatbn of Canada is brought into focus with a jolt in the northern towns: Wabasca, at the end of 110 km of dirt road with no other desthutbn, has 2,400 Indians and Metis in and around lt; Fort Chlpewyan on Lake Athlbasca has 1,700 and Fort Vermillbn about the same number. Such concentratbns of people In tha northern forests must necesnrily assume some type of MwtriaJ or service+ector employment in the area; tf It Is rbsm (and in the places mentbned It largely is), there is no alternathmto the welfare line, a line that leads in many cases stwht to the nearest bar. A few groups are suuassful ~ulturalists. However, the problem here Is that the Indians were permitted, by tmaty, a choice of reservation lands and they gemrally chose lands for the qualitkr they then prized, such as #ame capacity, and not for their gkultural potantlal. Thrn again, there are rqrncks at work, such as the Human Resourcsr Development Authorlty, operatiN from a base at Slam Lake, Am, where a pibt development project covers a populatbn of 22,000,45 % of it Indian. But even the yency reoogntses that, In areas which luck short-term development potentlrl, the only sensible course is to duate the lndtans up to a pow where they can move to somewhere etse with at least a fair chana of hokliq their own. And perhaps the most depressiq aspect of the problem is the total lack of any solution in the minds of the white populatbn whkh mans the frontier, and which will talk loudly and long about a great future for the Northlands but admits that, for the indtgenous population, it can see in that future no prospect and no share.

Peter HORN,

Heidelberg+

Introduction Fatls of stony or iron pkcas, preeumaly meteorltes, whkh have been observed as coming down from tlm sky, are recorded&en in ancient literature. A well mnticatd faJl of a metealte is that of Ensishelm tn Alrru (France), where a stone of 127 kg fell on November 16,1492. But until 1803, when the L’Abk meteo&e was obsewed fallla and subsequentJy manred, there was a lot of skeptkhm about the true nature of such “rocks out of the sky”. Today there b a separate disclpllnc among science whkh Is calkd meteorltks. In historical times only relatively small objects have been colliding

+

Dr. Peter HORN, MaxPlanck-lnstltut ftir Kernphysik, P. 0. Box 1248, D69OO Heidelberg, Germany (W)

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with the earth. The lugest single meteorite found so far is that of Hoba West in Southwest Africa, which when arriving on the earth’s surface (the time is unknown) weighed ,approximateiy 100 tons (GORDON 1931). Today them ii increasing evidence tha& also very large extraterrsstriai object have been arriving on urth with cosmic velocities, causing the formation of explosion craters due to impact up to tens of kilometers in diameter. Their time of fait dates back to prehistorical or geological time. in the beginning the recognition of such structures as the msuit of hypervebcity impact of met-rites was based on the discovery of meteoritic fragments around them and on the obrrvatlon of unusual petmgraphic features and meiting products in the surmundings and underlying rocks now bsiieved to be the msutt of shock metamorphism. This shock metamorphism is distinct from normal geoioglcat metamorphism in re$pedt’of pressurn, temperature and time - ths former two might be M) order of manitude higher during shock metamorphism, whereas the timespan might be as short as microseconds instead of millions of years as in geoiogicai metamorphism. A major discovery concerning the criteria for impact craters was the isoiatbn of Coesite (CHAO et II., 1961) and Stihovite (CHAO et al., 1962) from the Coconi~~o undsuwte et the site of Meteor Cmter, Arizona. These two hiih-pressure polymorphs of Sio, were prevbusiy not known from natural occurences. Subsequently more and more-independent petrographic indkators of shock matamorphini have been found - diowing the recognitbn of an increasi~ numbrr: of unusuai structures as impact craters. These mostly empiricai investipntbm in the fields of geoiogy, mineralogy, geechemis&ry and [email protected],wera supported by laboratory and f&Id experiments which aiiowd some insbhts into the mecbanicai pmcess of crater formation and into the temperatures and pressures which wem reached.

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After a compilation given by SHORT and BUNCH in 1968,52 structures were known on carih to be of most probable impact origin - the largest of them hating a diameter of 61 km (Manicouagan, Canada). An additional num6er of craters has been identified since then. One of the largest welt. established meteorite craters is the Ries - Kessei in Germany with diameters of 22 and 24 km. This structure has been a suspected meteorite crater since 1904 (WERNER). Alternative explanations warn discussed in detail until SHOEMAKER and CHAO (1961) reported the occurrence of Coesite in breccias from the Rier The Ries - Keseei (or Basin) is famous in that it isgeotogicdly mapped in detail, welt preserved and because the metoorite impacted into a welt defined sequence of mainly triasic and jurassk sed&mentsoverlying a differentiated metamorphic.crystaiiine basement - aibwing the reconstruction of the mechanism of crater excavation. This article summarizes the most important results of Ries investigations, recently documented in a symposium (PREUSS and SCHMIDT-KALER 1%9) together with some mom recent msuits. The Ries crater in Germany The RieeKe& (Fig. 1) is a fiat nearly circular topographic depression at the northern scarp of the Suebian Aib. The outer rim of the basin bar: a height of about 180 m in, the south and 80 m in the north mlative to tha basin plane. An inner mound with a diameter of 1O-l 2 km has the shape of a horseshoe open to the north-northeast and is built up by 11iochtonousorystaiiine bmccias which occasionally risa above the take sediments. The basin is fitted in its inner part with tats M&cene.iake sediments to a depth of about 350 m; these are covered by Pleistocene perigkcial and aiiuviai deposits. Towards the morphobgiul rim of the crater the fillings consist of clods of brecciated sediments.

Fig. 1 Ries Crater in the Swabian Aib, Germany (view NE). Rim of crater in foreground is covered by forest. Cumulus casteiatus clouds stand above fields outside crater rim (Luftbiid A. BRUGGER, Stuttgart; courtesy of innenministerium Baden-Wiirttemberg, Nr. 2/l 4605 and 14607).

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As indicated by selsrnic measurements there is underneath the lake sediments a Suevite layer of at least 400 m thickness which grades into shattered crystalline rocks of the basement which unnot be distinguished from undisturbed basement outside the crater from a depth of about 2 km on (ANGENHEISTER and PDHL, 1969). Measurements of the gravity field in the Ries-crater indicate its filling with shattered rocks and fall-back breccias (JUNG et al. 1969). Outside the Ries-Kessel itself the ejecta consist of sedimentary breccias (“Bunte Brekzie” = colored breccia) and of giant boulders of Upper Jurassic limestones which evidently have been radially transported carving grooves on the underlying bedrock surfaces not unlike that of a glacially abraded surface. Ejecta material of this type can be found as far as 15 km from the crater rim. The distribution is assymetric due to post-Ries event erosion which was more extensive in the north. From the rim outwards the fragments within the “Bunte Brekzie” become gradually smalkr while there is a tendency for younger constituents of the sedimentary sequence to be farther away from the Ries. On top of the sedimentary breccias lies a very unusual breccia, the so-called Swvite which was deposlted in one sfngle event as indicated by chilled margins only on bottom and top. It occurs in patches, but very likely covered the area as a continuous layer just after the Ria event. It consists of a finegrained matrix of glass and a clay mineral of the montmorlllonite group (v. ENGELHARDT et al. 1969) which may be of secondary origin. Within this matrix crystalline rock fragments with varying degrees of alteration are embedded. Different stages of shock mrtamorphism can be recognized; von ENGELHARDT and STGFFLER (1968) distinguished four stages (Table 1). Sedimentary rocks occur very rarely in the Suevite and account for about 1 36 as a maximum. Very charactcristk for the Suevfte is the occurrence of glass bombs (“FlEdidle”), which have a somewhat aerodynamic shape with sizes from 2 - 30 cm. These glasses are evidently shock melted. Mineralogical thermometers indicate temperatures of more than 2000 “C during the formatbn of these glasses (EL GORESY, 1964,1965; v. ENGELHARDT and ST&!FFLER, 1968). Within shock-fused gneisses El GORESY and

DONNAY (1968) found a new allotropic form of carbon Chaoite - indicating temperatures higher than 2500 “C (WHITTAKER and KISTNER 1969). After the deposition of the Suevite the slowly cooling glass from within the layer crystellized in part to form minute magnetite crystals. Upon cooli~ past the Curie point (580 “C) it preserved a remanent magnetisatbn which was in that time inverse to the present earth’s magnetk fkld (POHL and ANGENHEISTER 1969). Material that has been ejected on balltstk trajectories might be represented by erratic upper Jurassic bbckr ( (“Reutersche Bliicke”) which are found occasbnally up to 50 km south of the Ries rim - one blocky mass &out 1000 kg in weight was found even 150 km east of it (HEROLD 1969).

The age of the Ries crater Stratigraphical consideratbns (DEHM 1961) date the Riesforming event as Upper Tortonian - an us which was confirmed by K-Ar and fission - track ages which yielded 14.8 f 0.6 m.y. and 14.0 f0.6 m.y. respectively (GENTNER et al. 1963, 1969). Structures genetically related to

the Rfes-Keseel

The same stratigraphkal age was found for a nearly circular basin about 40 km southwest of the Rks crater, the Stainhelm basin. This basin is distinct from the Ries in that It is comparatively small (35 km in diameter) and shows a central uplift. Planar elements in quartz grains from within the sedimentary breccias have been found (GROSCHOPF u. REIFF 1969). But still the beet indicator of shock In this area are shatter-cones of limestone whkh are accepted to be indicators for astrobhms (metoorke impact scars, DIETZ 1968). Shatter cones develop most likely in shocked limestone, but are known from other rock types, too - they are “striated cup-and-cone structures” (DIETZ). The we coincidence of Stelnheim and Ria basin with the evidence of shock metamorphism in both structures supports the idea of a doubiett cratering event - either by two distinct meteorites or by fragmentatbn of an approaching meteoroid under gravitatbnal forces (STORZER et al. 1971).

Table 1

!

Characteristk Deformatbns and Phase Transitbns

Stages of shockmetamorphism in rocks from the Ries crater.

R&dual Temperature OC

Fracturiql Plastic deformation (diaplectic quartz) and feldspar)

250-300

Stage 2

Phase transltbns (Diaplectic glasses of quartz and feidspar, [email protected] p-s of Sioa)

2BO-3BO

500-550

Stage 3

Sekctlve melting (Normal glassesof quartz and feldspar, highpressure phasesof SlDs)

1200-1500

6OO- 650

Stage 4

Meltim of all main rock-formiq minerats (Unhomogeneour rock melts, glass bombs)

2000-3000

+

1000

After von ENGELHARDT

Volatllizatbn and STGFFLER

(1968)

-

5000

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@!

Tektite area (high frequency)

q

Libyan Desert (Iass

+

Mikrotektites (deapsea drills)

e

Bontonite glasses

q .

Tekttte area (low frequency) Single tektites

0

Crater glass

0 impact craters, generally xcepted as brirp true meteorite impact craters; if doubtful craters are added the mai densities of enters would rise by a factor of about three. The uneven distributbn of craters is mainly due to tho geoiogtii ago and history of the underlying rocks and to the fact that vast land areas have not yet been carefully surveyed for the presence of impact craters (for references see teat). Fig. 2. Geographkai distribution of impact craters, toktites and related glasses (ages in brackets: my = miiiion years) 1 Australian and Asian tektites with mkrotektites from deep-sea drills (0.7 my). 2 Ivory Coast tektites, mkrotektite from deep-sea drill and Ashanti crater or lake Bosumtwi (1.1 my). 3 Moldavites, Ries crater and glassesfrom the bentonites in the Molasse-trough (145 my). 4 North American tektites (34 my). 5 Libyan Desert glass (27 my). After Gentner et al. 1969.

Other impact craton gonetkaiiy related to tho Ries event have been mported very recently by ILLIES (1%9), RUTTE (1971) andSTORZERetai.(1971)fromareasnorUmastoreastofthe Rios crater. Judging from the criteria these authors use, the Stopfonhoim-Kuppei (STORZER ot ai.) is very probably an astrobiem.

Ries, Moldavltes and Bentonites Now we have to discuss still anothor phenomonon whkh has been correlated with the Ries event..Thir is the appearance of glassy dropiots found in Upper Tertiary sediments in Bohemia and Moravia 250 km and 400 km east of the Ries crater. Potassiumargon age determinations by GENTNER et ai. (1963) showed that the ages of the impact glassesfrom within Suevites and of the Moidavites agree with an uncertainty of about a miiibn years at 14.5 miilbn years. This age coincidence supported the idea of COHEN (1961) that both glasseswere formed during a single event and wore doposited at groat dbtancesone from another. Theoretical evaluatbns by DAVID (1969) whkh based on the physical, mineralogical and geological observations made until then in the Ries area showed that upon impact of an extraterrestrial body (in the case of the Ries event a meteorite or comet

with a diamotor of about one kilometer) large amounts of silicate vapor must hwo been gonerated which upon cooli~ condensed to form glassy objects or tektites like tho moktavites. Com(wabie condensates (mkrotoktite) might be represented by theglasses found within the bentonltes in the large molasse trough south of the Aib - the ages of which again agree within the limits of error with those of the Ria event and the Moldavites (STORZER and GENTNER 1970). Impact cratering as a world-wide phenomenon The coincidence of these ages is not lccklentai, which is supported by the fact that similar processes must have taken place elsewhere on earth at different times (GENTNER et al. 1970). Figure 2 shows the location of another crater which is associated with crater glass, tektites and microtektites (No. 2 in Fig. 2; see caption). The intention is also to show that terrestrial impact melt and impact vapor - condensates are distributed over large areas of the earth, for two of which the associated crater is known. The geological age and the time span covered by these impact events is only 34 million years which is very short compared to the age of the formation of the earth’s crust (-3.7 billion years). Since one knows from impact crater statistics on the lunar surface that moteorite collisions with our planet were much more frequent in

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the past and the bodies were much Jarger, one can imagine that impact cratering and associated effects must be considered as important geoJogical processes on earth (see also Fig. 2 and caption to Fig. 2). The meteorological and hence the ecological effects of such sometimes truly catastrophic events have not been the subject of studies yet. In the case of the Ria event they have been reported only by way of intimation (WAGNER 1965; ZEBERA 1969; DAVID 1969; RUTTE 1971).

POHL J. and G. ANGENHEISTER (1969): Anomalien des Erdmagnetfeldes und Magnetiskrung der Gesteine im Niirdlinger Ries; Geologica bav., 61,327-336.

Acknowledgement

SHOEMAKER E. M., and E. C. T. CHAO (1961): New evidence for the impact orbin of the Ries basin, Bavaria, Germany. /. geophys. Res.: 66,3371-3378.

I thank Mr. Bob Cliff and Ms. Urmitzer for help in preparing the English manuscript.

References ANGENHEISTER G., and j. POHL (1969): Die seismischen Mcssungen im Ries von 1948-1969; Geologiu bov., 61,304-326. CHAO E. C. T., E. M. SHOEMAKER and 8. M. MADSEN (1961): First natural occurrence of coesite, Science; 132,220-222. CHAO E. C.T., J. J. FAHEY, J. L. LITTLER, and D. j. MILTON (1962): Stishovite, SJOt, a very high pressure new mineral from Meteor Crater, Arizona; /. geoP/rys. Res., 67,419-421. COHEN, A. J. (1961): A semiquantltative asteroid impact hyp thesis of tektite origin; /. geophys. Res., 66,252l. DAVID E. (1969): Das Rks-Ereignis als physikalischer Vorgang; Geologica bav., 61,350-378. DEHM R. (1962): Das Nordlinger Ries und die Meteoritentheorie; Mitt. Boyer. Staatssamml. Paltint. hist. Geol., 2,69-87. DIET2 R. S. (1968): Shatter Cones in Cryptoexpiosion Structures. In: Shock Metamorphism of Natural Materials. Edited by B. M. FRENCH and N. M. SHORT, pp. 267-285, Mono, Baltimore, Md. EL GORESY, A. (1964): Die Erzmineralien in den Rier und Bosumtw i-KraterGlgsern und ihre genetische Deutung; Geochim. cosmochim. Acta, 28,1881-1891. EL GORESY, A. (1965): Baddeleyite and Its Significance in Impact Glasses;J. geophys. Res., 70,3453-3456. EL GORESY, A., and G. DONNAY (1968): A new allotropic form of carbon from the Ries crater; Science, 161, 363-364. GENTNER W., H. j. LIPPOLT and O.A. SCHAEFFER (1963): Argonbestimmungen an Kaliummineralien. XI Dk KaliumArgon-Alter der GJ8serdes Nerdlinger Rkses und der bohmischm;ihrischen Tektite; Geochim. cosmochim. Actu, 27, 19 l-200. GENTNER W., D. STORZER and G. WAGNER (1969): DOSAlter von Tsktlten und venvandten Gliisern; Noturwissenschoften, 56,255-260. GENTNER W., 8. P. GLASS, D. STORZER and G. WAGNER (1970): Fission track ages and ages of deposition of deep-sea microtektites; Science, 168, 359-361. GORDON 5. G. (1931): The Grootfontein, Southwest Africa meteoric iron; Proc. Acad. nut. Sci. Phi&d., 83,2Sl-255. GROSCHOPF P. and W. REIFF Geologica bov., 61,400-412.

(1969):

Das Steinheimer Becken;

HEROLD R. (1969): Eine Malmkalk-Triimmermasse in der Oberen SiiBwassermolasse Niederbayerns; Geologica bov., 61,413-427. ILLIES H. (1969): Nordlinger Ries, Steinheimer Becken, Pfahldorfer Becken und die Moldavite: strukturelle und dynamische Zusammenhgnge einer Impact-Gruppe; Oberrhein. geol. Abh., 18, l-31. JUNG, K., H. SCHAAF and H. KAHLE (1969): Ergebnisse gravimetrischer Messungen im Ries; Geologico bov., 61, 337-342.

PREUSS E., and H. SCHMIDT-KALER, eds. (1969): Das Ries. Geologic, Geophysik und Genese eines Kraters; Geoologiw buv., 61,478 pp., Bayer. Geol. Landesamt, Miinchen. RUTTE, E. (1971): Nwe Ries-8qulvaknte Krater mit BrektknEjekta in da Skdlkhen Frankenrlb, Siiddeutschland; Geoforum, 7,84-92.

SHORT N. M., and T. E. BUNCH (1968): A world-wide Inventory of features characteristic of rocks associated with presumed meteorite impact craters. In: shock metamorphism of natural mote&/s, edited by 8. M. French and N. M. Short, pp. 25% 266, Mono, Baltimore, Md. STORZER D., and W. Gmtner (1970): Spaltspuren-Alter von Riesgl&ern, MoldavRen und Bentonlten; /her. Mkt. oberrhein geol. Ver., NFSZ, 97-l 11. STORZER D., W. GENTNER and F. STEINBRUNN (1971): Stopfenheim Kuppel, Ries - Kessel and Steinheim Basin: A Triplet Cratering [email protected] Earth Planet; Sci. Lett., 13.76-78. ENGELHARDT W. von, and D. ST(SFFLER (1968): Stages of Shock Metamorphbm in the Crystalline Rocks of the Ries Basin, Germany. In: Shock Metamorphism of Natural Materials, edited by B. M. French and N. M. Short, pp. 159-168, Mono, Baltimore, Md. ENGELHARDT W. von, D. STOFFLER, and W. SCHNEIDER (1969): Petrologische Untersuchungen im Rks; Geologica bov., 61,229-295. WAGNER G. (1965): Ober Bestand und Entstehung typischer RiesGssteine; jber. geol. Londesamt Boden-WUrttemberg, 7, 199-222. WERNER E. (1904): Das Rks in der schwiibisch-frgnkischen Alb; BI. schw&.‘Aibverein 16,153-167. WHITTAKER A. and P. KISTNER (1969): Carbon: Observations on the new allotropic form;Science, 165,589~592. ZEBERA, K. (1969): Geological effects of comet and large meteorite impacts on terrestrial and lunar surfaces; Vgstnik &edmho bstavu geologicklho, 46,57-64.

Farming in Society Towards the Year 2000 Anton J. JANSEN, Amsterdam* In launching Plan Europe 2080, the aim of the Europan Cultural Foundation has been to stimulate the study of four of the most crucial questions facing mankind today. One of these questlons was originally formulated in the statement: “Everywhere the technicll revolution is transforming rural life and causing an exodus to the towns. What help can be given to rural youth and an agrarian society in the throes of change? ” Since the first round table meeting on this project (Amsterdam, 1966), the scope of this study has been widened from rural youth to rural society. Up to September 1970, when Project 4 became operational, the name most frequently given to the project was “Rural Society in the Year 2000.” *

Anton J. JANSEN, Fondation EuropCenne de la Culture, Jan van Goyenkde 5, Amsterdam - 1007, Netherlands. Mr. Janrn is director of this fourth project of Plan Europe 2000.