and Holocene buried and fluvial deposits (Fig. 1). Cambrian, Ordovician, Silurian, Carboniferous and Permian rocks are not reported. In addition, younger granites, ultramatic and kimberlitic intrusives, mig- matites, pegmatites and dolerite dykes (Jurassic/ Cretaceous) are also found in Liberia.

The granitic gneisses and schists found in the EburneanIPan African Age Provinces are the re- worked equivalents of the formations of Liberian Age. The LiberianIEburnean transition zone has yet to be properly defined, but the Pan African Age Pro- vince boundary is well marked by the Todee Shear Zone. Major structural folding during Liberian Age tectonics is indicated in the Wologizi, Nimba and Bong Ranges, all of which are enriched in sedi- mentary itabiritic iron ores. During Cretaceous time, two onshore basins - Bassa and Roberts - were formed. Beach ridges of unconsolidated quartz sand were formed during the Holocene uplift.

Liberian geological configuration is in many ways similar to that of the neighbouring Sierra Leone, Guinea and Ivory Coast, especially in respect of the style of mineralization. Iron ore, gold and diamonds are the major economically important natural re- sources of Liberia. Iron ores of itabiritic type are being mined from the Nimba and Bong Ranges. These deposits in Western Liberia are of Precambrian sedimentary type, whereas those of the Webbo-Goe and Putu Ranges in Eastern Liberia are of later magmatic origin.

Gold is being recovered by alluvial mining along the Lofa, Gbeya, St John, Dube and other river basins set in greenstone belts. However, primary gold ex- ploration and mining have not yet been started. Promising auriferous quartz veins are present in amphibolites and quartz mica schist of the schist belts of Western Liberia. Also, in Eastern Liberia, primary lode gold mineralization is known along the Dube Shear Zone in Grand Gedeh County. Kokoya in Bong County of Western Liberia is well known for alluvial gold. Tortor Mountain and Bie Mountain Ranges of the same region have promising primary gold mineralization. Economic reasons and the urge to mine easily available gold have retarded primary gold exploration in Liberia. Exploration, drilling and sys- tematic laboratory research on rocks in the vicinity of auriferous river basins might help establish the source of primary lode gold deposits. Diamonds, too, are recovered by alluvial mining. Many kimberlite pipes and dykes are reported from Western Liberia, especially from the Kumgbor Forest region in Lofa County. Despite the presence of diamond-indicator minerals, so far no other kimberlite has been

investigated for diamond mining, except that of Mano Godua in Grand Cape Mount County.

Ultrabasic occurrences are reported in the Kumgbor Forest area - probably a continuation of the same formations from Sierra Leone. Chromite, so much mined in Sierra Leone, has not been located in the Liberian region. Systematic field and laboratory studies could reveal the location, extent and grade of chrome ore deposits in Liberia. In Eastern Liberia, younger ultramafk intrusions in Sinoe County at Tarjuh Hill have shown traces of Cr, Ni, Mo and other base-metal sulphides. Base-metal sulphide concentrates are also reported from alluvial samples of the Lofa and Sam Davis Creeks in Western Liberia. In the latter locality, tantalite-columbite, scheelite, galena and stolzite have also been found. The schist belts of Eburnean Age in Eastern Liberia are in places manganiferous and, rarely, graphitic. Manganese with ferruginous and argillaceous intercalations is reported from Eastern Liberia. Again, proper ore- grade assessment has not been made.

Pronounced shearing and migmatization in the Eburnean Age Province with resultant younger granites and pegmatites promise rewarding mineral- izations in Liberia. In addition to the minerals so far cited, Liberia has baryte, kyanite and phosphate deposits in, respectively, Margibi, Grand Bassa and Bomi Counties of Western Liberia. Uranium and rare minerals are also present in Liberia. Detailed research-orientated studies have so far been attempted on kimberlites, chrysoberyl, iron ores, phosphate rock, monazite and granitic gneiss by various workers. Similar studies have also been partially done on beach-mineral concentrates of the Liberian coast, which include ilmenite, rutile, zircon and monazite. Exploitation by beach mining techniques on the vast reserves of black mineral placers has also not been initiated. Attention should also be focused on the buried beach placers and lagoonal and lacustrine sediments.

Detailed petrogenetic and geochemical inves- tigations are needed to classify the complex gneissic units forming the basement complex of Liberia. These, as well as the migmatitic and granitic episodes of both Liberian and Eburnean tectonic events, will have to be geochronologically and structurally cor- related; and the tectonic pulses within the age pro- vinces responsible for the petrogenetic processes, including the evolution of the greenstone belts of the basement complex, will have to be established. Inte- grated studies along these lines may help to redefine Liberian geology from the metallogenetic point of view.

Opinion

The Glasgow Garden Festival* 1 promised the family that during our stay in Glasgow we should have a day free from visits to quarries, museums and outcrops. In the event, one of us spent more time looking at the rocks than he did

'Held 28 April- 26 September 1988

looking at the gardens and the helter-skelter. Rocks lay everywhere. New Red Sandstone enlivened the waterfalls in the National Trust display; some large boulders of Dalradian schists counterbalanced the

MIGEOLOGY TODAY January-February 1989

daintiness of the Japanese Osaka garden; and banded quartzite, gabbro boulders and granite/quartzite beach pebbles in the Landscape and Scenery section demonstrated the range of Scottish stone available from CED, specialists in stones for paving and land- scaping. Leeds City Council brought an authentic atmosphere of West Yorkshire by importing recycled Millstone Grit and Victorian style railings for their evocation of city parks and gardens. Elsewhere, large, rounded and faceted glacial erratics were strewn everywhere as though an ice sheet had downwasted as part of the festivities.

Whilst it is true that the event was a garden and not a rock festival, I believe that the general public would have welcomed labels showing names of the rocks together with their provenance. Visitors to Saas Fee in Switzerland may have seen the collection of labelled erradcs in the middle of the village. Similar labels would have enhanced the displays of rocks and quietly promoted the status of geology.

Britoil sponsored the ‘Time Trek’ pavilion, which attempted to tell the story of the evolution of plant life on Earth. It would be uncharitable to be over-critical of any sponsorship, as geology needs all the goodwill that it can get. However, the family felt that the selection of about a dozen display-sized specimens (not plastic reproductions), well lit and tersely labelled, would have been more effective than the over-ambitious attempt to convey so much highly structured information. Surely exhibitions are places where we catch ideas and curiosity is aroused; they should complement, and not rival, the function of textbooks. In addition, a life-sized Hugh Miller held forth high up in the pavilion, reminding the awed spectators of some of the more dramatic scenes from Gotterdhmerung! This costly exhibit, of which someone somewhere is probably very proud, was alongside a small glass case crowded with small specimens barely discernable in the poor lighting. When will exhibitors learn that the humble specimen, nicely presented and with a short text, is far more effective than expensive audio-visual displays?

Gateshead will be the venue for the Garden Festival in 1990. Let us have plenty of rocks on display but with labels this time. Perhaps British Coal could spon- sor and provide a block cut from one of their nearby

opencast pits, representing a cyclothem from the Coal Measures. Maybe there will be some natural outcrops to use as rock gardens. Certainly both children and adults would enjoy a play/picnic area that included mini-landforms - drumlins, eskers, kettles and kames, and a replica of the sub-glacial channel west of Wooler. Some Cannonball Limestone could be in the art display. At Glasgow there were replicas of German beer tents; perhaps at Gateshead there could be a replica of that well-known haunt of geologists, the Grotto at Marsden. But please, not in plastic.

HUGH PRUDDEN Yeovil College, Somerset

Geod igest Giant meteorite impact structure in South Australia

Jonathan Selby (Department of Mines and Energy, South Australia) writes: Remains of a 600-million- year-old meteorite impact structure, one of the world’s largest, have been discovered in South Australia (Fig. 1). The structure was caused by an asteroid or comet that hit the Earth during the Late Proterozoic. The resulting immense explosion threw up vast clouds of dust and debris which may have affected the global climate for several years.

The discovery was made by two geologists working independently in different fields. In 1979 George Williams, an exploration geologist for Broken Hill

Pty, Australia’s largest mining group, was studying satellite images of the remote Gawler Ranges in South Australia. The ranges consist mainly of porphyritic volcanics dated at about 1600 million years old. A circular salt lake, called Lake Acraman, caught his eye. It appeared to be surrounded by a pattern of curved fractures that could have been caused by a meteorite impact. On visiting the site he found shat- tered and deformed volcanic rock typical of other hypervelocity impact sites.

Meanwhile, Vic Gostin and colleagues from the University of Adelaide were carrying out research on

GEOLOGY TODAY JanuatyFebruary 19891 15