Category: minerals

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A mineralogical tour of Ireland (Part 3): Connaught

Stephen Moreton (UK) In the first two articles of this series, we looked at Leinster and Munster. Continuing in a clockwise fashion brings us to Connaught. Some of Ireland’s oldest rocks are to be found here, forming the Ox Mountains. The rugged and mountainous west is dominated by metamorphic rocks and a series of granite intrusions. Inland, Carboniferous limestone prevails. Fig. 1. The four regions of the island of Ireland. Fig. 2. Connaught in more detail. Where the latter abuts Devonian sediments is found the jewel in the crown of Irish mineralogy – Tynagh Mine. This giant polymetallic deposit, near Loughrea in County Galway, was discovered in the 1960s and yielded close to a million tonnes of lead, zinc and copper. Much of this was as sulphides dispersed through black mud filling a huge depression in the limestone. This was formed by acid from rotting pyrite dissolving the country rock. Extensive oxidation and remobilisation of the primary ores produced hundreds of thousands of tonnes of smithsonite, cerussite, malachite and azurite. Scores of rarer species, such as linarite, anglesite, brochantite, native silver and numerous arsenates were also present. Fig. 3. Malachite, from Tynagh, Co. Galway. 64mm x 35mm botryoidal and stalactitic mass dug out of the tips. Sadly, collectors were slow to learn of this treasure and most was sent to the crusher. By the time they did realise something glorious was going on, most was already turned into smelted metal. A fickle attitude on the part of management did not … Read More

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A mineralogical tour of Ireland (Part 2): Munster

Stephen Moreton (UK) In the second part of our tour of Ireland, we head for Munster, which occupies the southwest corner of the island. Geologically, the rocks are mostly inland Carboniferous shales and limestones, with Devonian sandstones forming the coastal peninsulas. All host mineral localities of note. Fig. 1. The four regions of the island of Ireland. Fig. 2. Munster in more detial. Starting in County Waterford, mineral collectors will tend to head for the copper coast – a group of nineteenth century copper mines centred on the coastal village of Bunmahon. The magnificent crystallised native copper and cuprite these mines yielded in the past are elusive nowadays. On the other hand, post-mining oxidation in the dumps and sea cliff levels and outcrops has produced an array of vividly coloured and sometimes rare secondary minerals. These include connellite, langite, atacamite, botallackite, brochantite, lavendulan and erythrite. The soft, wet, blue and green substances that coat the mine walls are amorphous gels that dehydrate and crumble to powder when removed to a dry environment. They are best left where they are. Fig. 3. Tankardstown Mine, Co. Waterford. The author is examining post-mining deposits of an amorphous copper-bearing gel. Mention should be made of the Croaghaun Hill beryl occurrence inland from the copper mines. In a small outcrop of conglomerate, one of many among the scrub, patches and sprays of slender, sky blue beryl prisms occur in a quartz matrix. Unfortunately, the rock is so tough it defeats even the largest sledgehammer. The … Read More

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A mineralogical tour of Ireland (Part 1): Leinster

Stephen Moreton (UK) The island of Ireland has much to offer the mineral collector, but is relatively unknown to most. This may in part be due to a lack of published information, although, for years, the troubles in the north also served to deter visitors for many years. This series of articles briefly summarises the principal mineral locations on a region by region basis. Fig. 1. The four regions of the island of Ireland. Fig. 2. Leinster in more detial. As the island is divided into four regions, Leinster, Munster, Connaught and Ulster (Fig. 1), which are in turn subdivided into counties, it seems appropriate to cover the island in this way. As the main ferry terminals for the Irish Republic are in Leinster many a trip to the country will start here. Leinster occupies the southeast region of the island and is the driest (or rather least wet) part of Ireland. Geologically, it offers the largest granite batholith in the British Isles, complete with metamorphic aureole, Carboniferous and Ordovician sediments and a scattering of basic igneous intrusions. County Wicklow dominates the mineral scene in Leinster. Fractures along the margin of the Wicklow granite have acted as conduits for much later mineralising solutions, giving rise to lead/zinc veins. These reach their best development in Glenmalure, Glendasan and Glendalough. Fig. 3. One centimetre spinel law twinned crystals of galena, from North Hero lode, Glendasan, Co. Wicklow. Fine schieferspar calcite and dark brown sphalerite have recently been found in Glendasan, while some … Read More

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Plate tectonics (Part 5): A simple key to identifying rocks in the field

Helen Gould (UK) Following on from my articles on plate tectonics and the rock cycle, the tables below will hopefully be useful as an aide-mémoire to identify rock samples on your field trips. This table is intended as a field guide for budding geologists. Take it with you – and have fun. Fig. 1. (Right) Breccia; and (Left) Conglomerate. Fig. 2. (Right) Desert sandstone; and (Left) Micaceous sandstone. Fig. 3. (Right) Fliint; and (Left) Chalk. Fig. 4. (Right) Oolitic limestone; and (Left) Crinoidal limestone. Fig. 5. (Right) Siltstone; and (Left) Silt. Fig. 6. (Right) Quartzite; and (Left) Clay. Further reading Introducing Metamorphism, by Ian Sanders, Dunedin Academic Press Ltd, Edinburgh (2018), 148 pages (Paperback), ISBN: 9781780460642. Introducing Mineralogy, by John Mason, Dunedin Academic Press, Edinburgh (2015), 118 pages (Paperback), ISBN: 978-17-80460-28-4. Introducing Volcanology: A Guide to hot rocks, by Dougal Jerram, Dunedin Academic Press Ltd, Edinburgh and London (2011), 118 pages (Paperback), ISBN: 978-19-03544-26-6. Introducing Tectonics, Rock Structures and Mountain Belts, by Graham Park, Dunedin Academic Press, Edinburgh (2012), 132 pages (Paperback), ISBN: 978-19-06716-26-4. Planetary Geology: An Introduction (2nd edition), by Claudio Vita-Finzi and Dominic Fortes, Dunedin, Edinburgh (2015), 206 pages (Paperback), ISBN: 978-17-80460-15-4. Rocks and minerals: The definitive visual guide, by Ronald Louis Bonewitz, Dorling Kindersley (2008), 356 pages (hardback), ISBN: 978-14-05328-31-9. Other articles in this series comprise:Plate tectonics (Part 1): What are they?Plate tectonics (Part 2): A closer lookPlate tectonics (Part 3): The rock cyclePlate tectonics (Part 4): More on the rock cyclePlate tectonics (Part 5): A simple … Read More

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Plate tectonics (Part 4): More on the rock cycle

Helen Gould (UK) In Plate tectonics (Part 3): The rock cycle, I presented an overview of the relationship between the rock cycle and plate tectonics, and then went on to look more closely at igneous rocks. This time, I want to discuss sedimentary and metamorphic rocks, and review the occurrence of the rock cycle in the Solar System. Sedimentary rocks Sedimentary rocks, by contrast with igneous and metamorphic rocks, have no crystalline structure, being made up of little lumps of non-crystalline material derived from weathering other rocks. However, where they have been built up in horizontal layers, they may contain large-scale structures such as bedding. What are bedding planes? When sedimentary rocks deposited underwater (for example, limestones), they may be periodically exposed to the atmosphere due to tectonic uplift or a fall in sea level, perhaps because water is locked up on land as ice. The fact that it may take a thousand years to deposit a centimetre’s-worth of limestone places bedding planes into a context of millions of years. Fig. 1. Garnet micashist. Fig. 2. Garnet. Limestones are deposited in shallow seas, forming from the rain of billions of shells of sea animals onto the seafloor. Deposition stops if the area is exposed to the air and restarts when the sea covers it again, so a gap (bedding plane) forms. Many sedimentary rocks are laid down underwater and may contain bedding planes. In addition, the grain size, the fossils that are present and other lithological features all may vary … Read More

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Plate tectonics (Part 3): The rock cycle

Helen Gould (UK) What is the rock cycle? Usually, the first thing that budding geologists learn about rocks is that there are three kinds: igneous, sedimentary and metamorphic. These three major kinds are divided up into many different types of rock. For example, marble, slate and metaquartzite are all metamorphic rocks; basalt, granite, obsidian and andesite are all igneous rocks; and limestone, sandstone, clay and siltstone are all sedimentary rocks. What is a rock made of? Rocks are made of minerals. Therefore, particular combinations of minerals help us to identify rocks. Minerals are chemical compounds, consisting of chemical elements, which in turn are made of atomic particles. Who first thought of the rock cycle? James Hutton was the first geologist to propose a cycle of rock creation and change. In 1785, he gave a talk to the Scottish Geological Society in Edinburgh. In it, he suggested that rocks undergo processes that change them from one type of rock into another. He later developed the idea in his book, ‘Theory of the Earth with Proof and Illustrations’. He thought there was a relationship between the three basic rock types: igneous, sedimentary and metamorphic. We now know this is correct. Fig. 1. Agglomerate, Fig. 2. Andesite. However, it was not until this idea was set in a plate tectonics context that it really made sense to geologists. At about the same time, another Scotsman, James Hall, invented experimental geology. He demonstrated the crystallisation of basalt under slow-cooling conditions, and produced marble by … Read More

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Plate tectonics (Part 2): A closer look

Helen Gould (UK) As we saw last time (Plate tectonics (Part 1): What are they?), the Earth is a pretty dynamic place, with tectonic plates moving about on the surface, driven by convection cells in the upper mantle. But producing a workable theory, which combined most of the observations of geological evidence, took years. It was known that the centres of continents were extremely old, and that some areas around the continental “cratons” didn’t seem to belong because they contained completely different types of rocks. Combining continental drift with seafloor spreading and mantle convection currents produced the idea of plate tectonics, and provided an explanation for the odd rocks on areas fringing some cratons. These “microplates” had come from other areas of the Earth, where different geological processes had produced different rock types. The role of density in recycling: oceanic and continental crust The physical features of the ocean basins and continental mountain ranges are known as the “crustal dichotomy” (splitting of the crust into two equal parts), and because these types of feature are essentially dissimilar, they have their own rock types. Basalt is the commonest rock both in the Solar System and on Earth, where it forms the ocean floor, along with various sedimentary rocks deposited underwater which make up another 5% of the total oceanic crust. Continents typically consist of coarse-grained rocks related to granites, which solidify below ground. Comparing similar-sized pieces of basalt and granite in the hand will establish obvious physical differences between them. Basalt’s … Read More

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Plate tectonics (Part 1): What are they?

Helen Gould (UK) What does “plate tectonics” really mean? The Earth’s surface bears about 20 plates, which are able, over millions of years, to move about on layers beneath the crust. Some of the surfaces of these plates consist of continental crust, some of oceanic crust, some both (Fig. 1). Fig. 1. Map of the tectonic plates of the Earth. Who came up with the idea? The idea didn’t develop overnight as a result of one person’s efforts. In 1915, Alfred Wegener suggested “continental drift”, in which the continents moved around on the Earth’s surface. Arthur Holmes later suggested continents could be moved by convection currents in the mantle, fuelled by the heat of radioactive decay. Harry Hess was an American geologist who came up with the idea of seafloor spreading. In the 1960s, J Tuzo Wilson developed the convection current idea further, proposed “hot spots” and “plates” and, in 1963, Fred Vine and Drummond Matthews proved the existence of seafloor spreading using “magnetic striping”. What proof is there that plate tectonics really exists? The fit of continents against each other, particularly Africa and South America, shows that they were once joined (Fig. 2). This branch of geology – palaeogeography – has led to the detection of several ancient supercontinents and oceans. Their existence is supported by matching similar geological features, such as ancient crystalline rocks and glaciated areas, in adjacent regions of South America and Africa, and North America and Europe. Two massive continents, which existed in the past, … Read More

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Fabulous Fluorite: Derbyshire Blue John

Richard M Haw (UK) Blue John is a unique variety of blue-purple banded fluorite. Hydrocarbons or oils have been deposited on some of the crystal surfaces while the mineral was forming. These oil layers are partly responsible for giving the stone an alternate blue and white banding, best seen when the stone is cut in section. It is not known to occur anywhere else in the world and is confined to an area of about 1km³ of the Carboniferous “reef” limestones at Castleton in Derbyshire. Fig. 1. Old picture taken sometime in the 1870s, showing miners digging in the Old Dining Room, now part of the show caves. I have been involved with the public caverns here for a while and I am sure many of you have visited them. However, there are many people who have never even heard of Blue John, so the following article gives a general overview without intending to be too technical. The area Castleton is a small village located in Derbyshire’s “Peak District” between the cities of Manchester and Sheffield. The village is dominated by the ruins of Peveril Castle that was built by the Normans to oversee lead mining in the area. The scenery around Castleton forms a dramatic backdrop and the rolling limestone hills end abruptly atthe vertical face of Mam Tor. Beyond and to the north are the gritstone moors known as the “Dark Peak” that eventually lead up to the two-thousand-foot-high plateau of Kinder Scout. Castleton and the surrounding area … Read More

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Urban geology: The battery on the Sloterweg

Stephen K Donovan (The Netherlands) The city of Amsterdam in the Netherlands is surrounded by a great defensive earthwork on its landward side, the Stelling van Amsterdam (= Defence Line of Amsterdam), along which are a series of forts and batteries (Figs. 1A-E and 2). This major structure was built between 1880 and 1914. The principle feature of this defensive system is a raised earthen embankment or dyke, still imposing today although breached or flattened in many places to make way for modern developments, most commonly roads. The embankment is often flanked by two canals, one on either side. Fig. 1. (A, B) The Battery on the Sloterweg, Hoofddorp, Noord Holland, the Netherlands.(A) General view of the Battery, looking approximately northwest.(B) Nameplate.(C-E) Three views of the restored embankment between the Battery on the Sloterweg and Hoofddorp station.(C) The view southeast on the northeast side of the embankment from the R-Net bus stop (routes 300 and 310) at Hoofddorp station, looking towards the Battery. The cycle path crosses the bridge and continues away from the photographer. Note the blue tractor scraping the embankment.(D) The view southeast on the southwest side of the embankment from the R-Net bus stop at Hoofddorp station, looking towards the Battery (at the end of the path in the distance). Again, note the tractor scraping the surface.(E) The view northwest from the Battery, looking towards Hoofddorp station, showing the ‘exposure’ in the foreground, which was particularly productive of builders’ rubble, including lithic fragments.(F) Details of the granite … Read More

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Alluvial gold: A geological model (Part 2)

Philip Dunkerly (UK) In A geological model for the alluvial gold environment (Part 1), the first part of this article, I discussed how alluvial gold is found and suggested a geological model for alluvial gold deposits. (Readers are recommended to have another look at that part to remind them of the model.) In this second part, I now turn to the nature of the gold itself. Fig. 1. Gold bullion bars of 400 troy oz. Fig. 2. Sites from around the world. Gulch gold Gulch gold is the coarsest that exists in any part of a river system. If nuggets (pieces of gold weighing more than 0.1g) are present, they will mostly be found in gulches (narrow ravines), provided suitable traps are present, such as irregular bedrock. In gulch alluvium, the vast majority of the gold will be found on, or in crevices within, the bedrock. Gulch gold is often coarse and angular and may contain silicate debris, especially quartz. As examples, gold from Victoria Gulch on the Klondike was described as “sharply angular”. In the Ballarat gullies, some enormous nuggets were found and Canadian Gully yielded nuggets of 50.4, 34.7 and 31.4kg. At Bendigo, White Horse Gully, a 17.8kg nugget (including some quartz) was found. (Interestingly, of a list of 92 Victorian nuggets, 34 came from localities specifically named “gullies”.) Finally, in the Sierra Nevada of California, most of the gold is from gulches or minor streams close to croppings. Fig. 3. Old hydraulicking operation of terrace gravels, note … Read More

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Alluvial gold: A geological model (Part 1)

Philip Dunkerly (UK) Mankind almost certainly first found gold when a yellow, glint from the bottom of a stream bed attracted the attention of one of our ancestors in pre- historic Africa. Ever since, the allure of gold – its colour, improbable density, malleability and scarceness – meant it has been prized, and great efforts have been made to accumulate it. Most ancient peoples venerated and coveted gold and used it for decoration, and empires used gold as a store of value and a medium of exchange. The Egyptians are known to have used gold as early as about 5000 BC, followed by many others, including the Romans, the Incas, the Spaniards and, of course, the Anglo-Saxon invaders of North America, Africa, Australia and New Zealand. Fig. 1. Spectacular Roman paleogravel workings at Las Medulas, NW Spain, now a World Heritage site. The mouth of one of the tunnels through which water was released from a header tank is visible in the shadow. Fig. 2. Panoramic view of Las Medulas, worked by sluicing using water brought through canals up to 60km long. Though gold was won from hard-rock deposits in ancient times, most gold until perhaps 1900 was won from riverbeds, and was traditionally called alluvial or placer gold. Prospecting for alluvial gold required relatively little equipment and always attracted hardy pioneers willing to forego the comforts of society in the hope of ‘getting rich quick’. The gold they found – if they were lucky – could almost instantly be … Read More

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A field guide to Barbados (Part 6): Central Barbados

Stephen K Donovan (The Netherlands) Stop 1. Waterford District, near Codrington Agricultural Station (approx. 59º 36’ 8” W 13º 6’ 49” N; Fig. 1) The area considered in the final part of this guide is outlined in A field guide to Barbados (Part 1): Introduction (Donovan & Harper, 2010, fig. 1e) and Fig. 1 in this article. As with other articles in this series, the starting point is Bridgetown. Fig. 1. Locality map showing the positions of Stops 1 to 6 in central Barbados. Only those roads relevant to this excursion are shown (after Donovan & Harper, 2005, fig. 12). This figure should be used in conjunction with the geological map of Poole & Barker (1983) and any tourist road map. Key: abc = ABC Highway; B = Bridgetown; 1 = Waterford district (Stop 1); 2 = Dayrells (Stop 2); 3 = Harrison’s Cave (Stop 3); 4 = Welchman Hall Gully (Stop 4); 5 = Horse Hill (Stop 5); 6 = Hackleton’s Cliff (Stop 6); coastline stippled. From the ABC Highway, turn southwest towards Bridgetown on Highway 3. In the area of the turnoff towards Codrington Agricultural Station (on the right), in the parish of St Michael, examine the road cutting, starting at the southwest corner and walking northeast. This is Stop 6 of Humphrey & Matthews (1986, p. 101), in the Middle Coral Rock, just above the First High Cliff and dated at 194,000 years old. The succession shows a range of reef-related biologically-determined facies (that is, sedimentary rocks … Read More

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A field guide to Barbados (Part 5): The Scotland District

Stephen K Donovan (The Netherlands) Stop 1. Chalky Mount (approximately 59º 33’ 15” W 13º 13’ 55” N; Fig. 1) The area considered in this part of the guide is outlined in Donovan & Harper (2010, fig. 1d) and Figs. 1 and 2. As with other articles in this series, the starting point is Bridgetown. Those wishing to examine the succession and structure of the Scotland District in considerably more detail than outlined below are referred to Speed (2002). This can be complimented by Patel’s (1995) discussion of the geomorphology. Readers are referred to the glossary in A field guide to Barbados (Part 2): The coastal geology of southeast Barbados Fig. 1 Locality map showing the positions of Stops 1 to 7 in the Scotland District of Barbados (after Donovan & Harper, 2005, fig. 11). Only those roads relevant to this excursion are shown. This figure should be used in conjunction with the geological map of Poole & Barker (1983) and any tourist road map. Key: C = Conset Point; H = Horse Hill; W = Welchman Hall; Stop 1 = Chalky Mount; Stop 2 = Bissex Hill; Stop 3 = Coconut Grove; Stop 4 = exposures on East Coast Road; Stop 5 = oil seep; Stop 6 = Bathsheba; Stop 7 = Bath Cliff; coastline stippled. Fig. 2 Geological map of the Scotland District of Barbados (after Donovan & Harper, 2005, fig. 2; Donovan, 2010, fig. 3; simplified after Speed, 2002, fig. 9). Key: open stipple = basal complex; … Read More

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A field guide to Barbados (Part 4): Bridgetown and the South Coast

Stephen K Donovan (The Netherlands) The area considered in this part of the guide is outlined in Donovan & Harper (2010, fig. 1C) and Fig. 1 of this article. As in other articles in this series, the starting point is Bridgetown. Fig. 1. Locality map showing the positions of Stops 1 to 5 on or near the south coast of Barbados (after Donovan & Harper, 2005, fig. 8). Only those roads relevant to this excursion are shown. This map should be used in conjunction with the geological map of Poole & Barker (1983) and any tourist road map. Key: A = Grantley Adams International Airport; abc = ABC Highway; C = Six Cross Roads; O = Oistins; 1 = the Barbados Museum, Bridgetown (Stop 1); 2 = South Point Lighthouse (Stop 2); 3 = Foul Bay (Stop 3); 4 = Woodbourne Oilfield (Stop 4); 5 = Chapel Quarry (Stop 5); coastline stippled.  Stop 1: The Barbados Museum The Barbados Museum and Historical Society was founded in 1933. Its museum occupies St Ann’s Garrison, a nineteenth century British military prison. It is situated in the parish of St Michael, southeast of the central part of Bridgetown, behind the Garrison Savannah racetrack. The museum has displays covering many aspects of Barbadian history and life, including natural history, prehistory and maps. The library is an important research resource, containing 5,000 books, monographs and articles on the culture and natural history of the island. Articles about the island’s natural history, culture and history are … Read More

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A field guide to Barbados (Part 3): Northern Barbados

Stephen K Donovan (The Netherlands) Stop 1: Arawak Cement Quarry The area considered in this part of the guide is outlined in Donovan & Harper (2010, Fig. 1b) and Fig. 1. As with other articles in this series, the starting point is Bridgetown. Drive north from the Bridgetown area on Highway 1, the main west (or leeward) coast road, which is constructed on the Lower Coral Rock and overlies superficial deposits. Fig. 1. Locality map showing the positions of Stops 1 to 4 in northern Barbados (after Donovan & Harper, 2005, fig. 7). Only those roads relevant to this excursion are indicated (including the track to Stop 4). This map should be used in conjunction with the geological map of Poole & Barker (1983) and any tourist road map. Key: C = Content; Ch = Checker Hall; G = Greenidge; T = Trents; 1 = Arawak Cement Quarry (Stop 1); 2 = Animal Flower Cave, North Point (Stop 2); 3 = limestone cliffs west of North Point (Stop 3); 4 = Cluff’s Bay (Stop 4); coastline stippled. The First High Cliff and the Middle Coral Rock are close by in the east (Speed & Cheng, 2004). This coast has been developed for tourism and has neither the magnificent sea cliffs of the east coast, nor the impressive Atlantic breakers seen in the previous excursion. To the west, two submerged barrier reefs, at 22m and 70m water depth, are separated from the coast by a submerged wave cut terrace (MacIntyre, 1967). … Read More

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A field guide to Barbados (Part 2): The coastal geology of southeast Barbados

Stephen K Donovan (The Netherlands) and David AT Harper (Denmark) Introduction This article is the second part of a field guide to Barbados, the first part of which is A field guide to Barbados (Part 1): Introduction. The areas visited by different the excursions outlined in Parts 2 to 6 of this guide are shown in Fig. 1. All itineraries commence from the Bridgetown area and the itinerary outlined in this part is rewritten after Donovan and Harper (2002). The words in italics and bold appear in the glossary at the end of the Part 1. Fig. 1. Relative positions of field excursions described in this field guide (after Donovan & Harper, 2005, fig. 5). (a) Southeast Barbados (Part 2). (b) North Barbados (Part 3). (c) South Barbados (Part 4). (d) Scotland District (Part 5). (e) Central Barbados (Part 6). Charles Taylor Trechmann DSc, FGS (1885-1964) (Fig. 2) was an anachronism, a twentieth century gentleman geologist and archaeologist. He was an amateur with sufficient private means to dedicate his time and use his scientific abilities to make an original contribution to his chosen field of study, an original thinker with a desire to use his observations to interpret broad geological phenomena. He devoted his time to research on Malta, Gibraltar, New Zealand and, particularly, northeast England and the Caribbean. He published over 80 monographs and research papers on geology and archaeology, including at least 40 on the Caribbean (Donovan, 2003, 2008, 2010a). Fig. 2. Charles Taylor Trechmann, DSc, FGS (1884-1964) … Read More

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Agates: A brief introduction to agates in the UK

P W Forster (UK) I have many years of experience collecting and cutting agates. It was my wife who originally had an enthusiasm for these beautiful semi-precious stones and it was because of her enthusiasm that I developed an interest that has now become an obsessive hobby for the both of us. Cabinets in our home evidence the wide range of specimen stones that an amateur collector can discover. Each specimen has identification labels and is catalogued to show the date and the region where it was found. Before starting my first collecting foray, I obtained as much information on the subject as was available. To this end, I found the book ‘Agates’ by H G McPherson most useful. (This book, together with ‘Agate collecting in Britain’ by P R Rodgers, has been extensively used in the writing of this article.) From my research, it became apparent that the Midland Valley of Scotland contained many of the best deposits of agates in Great Britain. With this in mind, we paid the first of many visits to the region. We started searching along the east coast of Ayrshire. This coast abounds with small coves of pebble beaches and large stretches of andersite larvas that stretch out to sea. During the first year, we amassed a large amount of what we thought were agates, but closer examination revealed that we had collected some colourful specimens of jasper as well as some lovely quartz pebbles. This first attempt had revealed that those agates … Read More

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A field guide to Barbados (Part 1): Introduction

Stephen K Donovan (The Netherlands) This is the first of six articles that will introduce the geology of the Antillean island of Barbados. It is an expanded and more detailed guide derived from two earlier publications (Donovan & Harper, 2005, 2009). The structure of the guide will include a summary of the geology of the island (in this part) and five, one-day field excursions for the geologically-biased tourist. These excursions will introduce the stratigraphy, structure and geological history of Barbados (Figs. 1 and 2), a small Antillean island shaped like a contorted teardrop, about 34km long by 24km at its widest. Fig. 1. The principal features of the geological history of Barbados summarised in a single section at Spring Bay, parish of St. Phillip, on the southeast coast. Professor David Harper (University of Copenhagen) is looking northwest, towards Ragged Point (Fig. 2) and admiring the angular unconformity between the allochthonous Palaeogene basal complex (=Scotland Beds) and the overlying autochthonous bedded limestones of the Pleistocene Coral Rock. A visitor to the island, who wants to undertake fieldwork, should hire a car. The only other reliable forms of transport are bus and taxi. While cheap, buses tend to stick to the main routes, particularly in the countryside. However, the size of the island means that localities are rarely more than a few kilometres from a bus stop. If money is no object, a taxi driver will be happy to drop you at a site in the morning and collect you at a … Read More

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The Deccan Traps, India (Part 3): Evidence of recent tectonic activities in Deccan basalts

Mugdha Chimote (India) The term ‘Quaternary’ is derived from the Latin word “Quaternarius” (meaning “four”, such that the Quaternary is “the fourth great epoch of geological time” in the now-abandoned system of dividing geological time). It refers to the most recent period of the Earth’s history, covering a span of about 1.77 million years extending up to the present day. The Quaternary System is divided into Pleistocene and Holocene Series. The term Holocene was introduced for the part of Quaternary that contains only living species. It covers the last 10,000 period of the Quaternary. The Quaternary has witnessed some very important events of great consequence, characterised by dramatic climatic changes. It witnessed repeated glacial and interglacial periods, more so than any other period of geological history. Monsoonal wind patterns also developed during this period, and deserts were formed during the latter part of the Quaternary. Although the Quaternary recorded a few extinction events, a great biological diversity still exists on Earth. Most of the present-day species of vertebrates, invertebrates and plants are believed to have remained unchanged during this period. By now, you must have realised, the Quaternary is not a separate entity of geology. It rather refers to the time period ‘Quaternary’ and all the geological processes pertaining to that period. Quaternary rocks and sediments are the most recent geologic strata, which lie on the uppermost layers of earth and have been exposed to the least amount of erosion. As such, they are one of the most well studied … Read More

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The Deccan Traps, India (Part 2): Its geomorphology and stratigraphy

Mugdha Chimote (India) Fig. 1. The natural arch/bridge at Ahmednagar, Maharashtra (see text box below). The Deccan Traps occupy approximately 25% of the total of peninsular India, that is, the triangular shaped landscape of southern India. They traverse the states of Maharashtra, Karnataka, Madhya Pradesh, and Gujarat. The Deccan Traps are currently believed to occupy about 500,000km2 of northwest peninsular India. It is estimated that the total exposure prior to erosion (including the region beneath the Arabian Sea) is of the order of 15 million square kilometres (Krishnan, 1956) or even up to 18 million square kilometres (Todal and Eldham, 1999). The differences in estimations of the total area of the Deccan Traps resulted from the fact that, an unknown area of the Deccan Volcanic Province (DVP) was rifted away as the Cambay rift system moved south and the Seychelles-Mascarene Plateau, along with part of the DVP, migrated to the west. The earliest basaltic eruptions took place along the north-western margins of the Indian continent, that is, in the Nashik-Narmada region. Later lava successions were emplaced on the southern flank of the evolving volcanic edifice as India migrated northwards over the plume head. The last of the flows were erupted in the southern DVP near Belgaum in Karnataka. As a result, the thickness of the flows gradually reduces from north-western to southern region of Indian subcontinent. Given the massive extent and volumes of Deccan Basalts, extensive studies have been carried out over the years to better understand the petrography, geochemistry, … Read More

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The Deccan Traps, India (Part 1): The story of its genesis

Mugdha Chimote (India) Fig. 1: Deccan Traps exposed in Mahabaleshwar region of Maharashtra, India. Introduction Sandwiched between the Arabian Sea to the west and the vast Indian subcontinent to the east, the Western Ghats, a haven for trekkers and travellers, are a 1,600km long range of mountains along western edge of Deccan Plateau. Also known as “Great Escarpments of India”, the range extends from Gujarat in the north to Kanyakumari in the south. The Ghats traverse the states of Maharashtra, Karnataka, Kerala, and Tamil Nadu. They comprise of more than 39 wildlife sanctuaries, reserve forests and national parks. The Western Ghats were declared as one of the eight hottest ecological hotspots in the world in the year 1988, as the area is home to nearly 325 globally threatened floral, amphibian, fish, bird and faunal species. Famous hill stations such as Mahabaleshwar, Panchgani, Munnar, Wayanad, Coorg and Ooty are among some of the perfect weekend getaways and popular tourist attractions here. Thanks to the rich ecological reserve and tourist attractions, Western Ghats were awarded the title of UNESCO World Heritage Site in 2012. The Western Ghats however cover just a small portion of Deccan Traps (see also box: Kaas Pathar below). Deccan is an anglicised word derived from the Sanskrit word “Dakshin”, meaning south (the region is located in the southern part of Indian subcontinent). “Traps” mean step-like features. Thus, the Deccan Traps are step-like volcanic features found mainly in Southern India. The laterally extensive basaltic lava flows of the Deccan … Read More

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Baths and batholiths

Deborah Painter (USA) “Look over there!” I exclaimed as I stood on the grounds of a manufacturing plant and stared across the tracks of the Atchison, Topeka and Santa Fe Railroad to the east of the plant. I was pointing at several mountains a few kilometres in the distance. “That mountain is glowing!” Standing alongside me was James, the plant’s maintenance supervisor. “I guess because I’ve seen this for the past 14 years, I don’t even pay attention anymore” was his reply. The mountain was not glowing due to any internal source but because exceptionally light toned granites captured and reflected rays of sun streaming from behind a December cloud cover (Fig. 1). Fig. 1. The mountain glowed in the shaft of light, as the sun peeked from behind a cloud. (Credits: Deborah Painter.) The mountains looked like this for most of that chilly day and the glow shifted from mountain to mountain (Fig. 2). Fig. 2. A remarkable combination of December light and greyish-white toned granites produced this day-long glow in the Bernasconi Hills. (Credits: Deborah Painter.) The granitic mountain cluster was in Perris, a city in Riverside County, California in the USA. I have had the good fortune to visit this county twice recently on two separate and unrelated trips a few years apart. And my friend, Mike Ramsey, had been with me on both trips to this same county. He was with me and a friend late in November when we visited another friend in nearby Moreno … Read More

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geology minerals

Earth history in miniature: Zircon under the microscope

Dr Robert Sturm (Austria) The mineral zircon (more correctly, orthosilicate zircon or ZrSiO4) is an important accessory mineral in various rocks of the earth’s crust, but most of all of igneous rocks with the mineral composition of granite. An accessory mineral is a mineral comprising less than about 10% of a rock and which therefore plays little or no role in naming or classifying that rock. Fig. 1. The typical appearance and morphology of zircon crystals that have been separated from various granitic rocks. As well as its ubiquitous appearance in magmatic, metamorphic and sedimentary rocks, zircon is a remarkable mineral due to its high resistance to mechanical and chemical processes within the earth crust. Therefore, it is very useful as a protolith indicator in different types of crustal rocks (Speer, 1982). (A protolith indicator is a mineral in a metamorphic assemblage that provides information on the chemistry of the host rock, within which it had originally grown.) As a result of the repeated formation of magmatic overgrowths around older ‘inherited’ zircon cores (like the rings of an onion), evidence of several stages of earth history are preserved within a single grain and can be scientifically analysed (Sturm, 1999, 2004). Once these overgrowths have been identified, the phases of crystallisation included in accessory zircon can be attributed to geological time periods. This is achieved by radiometric dating, based on the mineral’s content of radioactive uranium and thorium, and the redistribution of these isotopes and their daughter products. Another important characteristic … Read More

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Geological field trip through Scotland: Basalts from the Isle of Skye

Robert Sturm The Isle of Skye is a part of the Inner Hebrides in the north-west of Scotland. It has a total area of 174,000 hectares and has an irregularly shaped coastline that is typical of the British Isles. Since the early nineteenth century, the island has become a centre of geological research, because rocks of different geological periods are exposed there. For instance, the gneisses of the Lewisian complex were formed in the Proterizoicum, 2,800Ma and, therefore, are some of the oldest rocks in Europe. On the other hand, intrusive and extrusive igneous rocks can be assigned to magmatic events that covered wide parts of the island during the Tertiary. This event, which took place about 60Ma, resulted in the development of the Atlantic Ocean in its present form. In more recent times, two ice ages, which affected the island 26,000 years ago (the Dimlington glacial) and 11,000 years ago (the Loch Lomond glacial), resulted in the formation of a partly spectacular glacigen landscape (a landscape formed by the ice) with sediments that are of high interest for geological research. Fig. 1. Geological map of the Isle of Skye (modified after Anderson & Dunham 1966) illustrating the high variability of rocks that can be found on the island. Impressive evidence for the Tertiary volcanism is provided by the plateau lava series (these are horizontally stacked layers of lava), mainly exposed in the north and west of the island. These extrusive rock formations probably reached a thickness of 1,200m before … Read More

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Shear zones: Natural laboratories of rock deformation and mineral alteration

Robert Sturm (Austria) During the last few decades, the interest of diverse geosciences has increasingly focussed on the examination of so-called ‘shear zones’, because the displacements between two lithological blocks represent natural ‘laboratories’, within which the phenomena of mineral alteration and deformation are clearly shown for the purposes of scientific study. Many shear zones are only a few centimetres in size, meaning that their examination is relatively easy. Others, like the San Andreas Fault, have a width of several hundreds of metres, which requires a bit more effort to investigate (Fig. 1). Fig. 1. Some selected examples of shear zones with different dimensions: (a), (b) small-scale shear zones in the Zillertal/Austria, (c), (d) medium-scale shear zone in the Bohemian Massif/Austria, and (e) the San Andreas Fault. Shear zones – definitions and main characteristics Broadly speaking, along a shear zone, two lithological units – ranging in size from several square metres to the size of continentals – are displaced against each other. The movement has to be exclusively evaluated in terms of plate tectonics and often represents the cause of earthquakes. Depending on their orientation, three main types of shear zones can be distinguished (Fig. 2): The normal fault is characterised by the lowering of a lithological block with respect to its neighbouring tectonic unit. If the face of displacement between the two blocks has only a small angle of inclination, the shearing process is accompanied by crustal extension which is most impressively seen in the Rhinegraben and the East African … Read More

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Book review: Geology of south Dorset and south-east Devon and its World Heritage Coast, The British Geological Survey

I don’t normally review BGS memoirs – they are excellent publications, but largely written for the professional or the seriously committed amateur geologist. (I have to admit to owning several, which cover my favourite fossil collecting areas of the UK.) However, this is one ‘Special Memoir’ that I am quite willing to make an exception for.

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Tauern gold: The history of gold mining in the Central Alps

Dr Robert Sturm (Austria) Exploitation of gold deposits in the Hohe Tauern, in the Central Alps of Austria, has a long history: occurrences of this noble metal were explored for the first time about 2,000 years ago. Since the fourteenth century, the search for gold has been conducted on an industrial level, resulting in the production of 130km of tunnels and shafts, with the main centres of medieval gold production being the Gastein Valley, Rauris, Heiligenblut, Fusch and, later on, Schellgaden. In the second half of the fifteenth century, all of the gold found in the Central Alps was sold to Venice, but from the year 1501, the noble metal was exclusively used for indigenous minting and, therefore, all gold mines came under the archbishop’s control. Fig. 1. Map showing the position of the Hohe Tauern National Park (green) in Austria and the main locations of historical and current gold exploration. The economic zenith of gold exploitation in the Central Alps was reached in the middle of the sixteenth century. At this time, three families – the Weitmosers, Zolts and Strassers – dominated the mining industry in the Gastein Valley and in Rauris. In 1557, 830kg of gold (corresponding to about 27,000 ounces) and 2,723kg of silver were hauled from the mines. However, 50 years later, gold mining ceased completely. The main reason for this economic collapse was the total exhaustion of all lodes of ore that had been exploited in the Hohe Tauern until that time. Furthermore, only ‘visible’ … Read More

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