Geology


“The Colours of the marbles of Ireland are almost as numerous as those obtained from Italy” (Wilkinson, On the Marbles of Ireland, 1845)

There are several coloured native stones displayed in the Museum Building, however, interestingly, there are no true marbles present.

What is a marble?  In terms of commercial stone, a marble is any stone that is attractive and susceptible to a polish, which may include limestone, true marble, serpentine, even granite.  However, in geological terms, a marble is a metamorphic rock whose parent rock was a limestone.

Carboniferous Limestones, Kanturk, Co. Cork. Water colour by G.V. Du Noyer (1866) GSI

Carboniferous Limestones, Kanturk, Co. Cork. Watercolour by G.V. Du Noyer (1866) GSI

Geological Timescale

Limestone is a sedimentary rock, consisting mostly of carbonate (calcite CaCO3, which is calcium carbonate in crystal form, or dolomite CaMg(CO3)2, which is calcium magnesium carbonate in crystal form).  During metamorphism limestone is altered by heat and/or pressure and a resultant marble is formed.  During this process the original limestone loses many, if not all, of its original sedimentary features and fossils.

Tip:  No true marble will contain fossils

The carbonate is recrystallised to produce an interlocking granular mosaic of roughly equal-sized crystals.  Non-calcareous minerals are also metamorphosed and new mineral assemblages created.

The majority of the coloured stone types in the Museum Building are polished limestones or pseudo-marbles.  Irish coloured limestones formed during the Carboniferous when Ireland lay in tropical latitudes and was submerged by a shallow sea, creating a carbonate shelf with enclosed deep water basins.

Limestone occupies one-half of the entire country of Ireland, approx. 15000 square miles, and covers the large central plain of the Island.  Irish Carboniferous Limestone has an outcrop area three times that of the United Kingdom.  The light blue areas in the below maps represent Carboniferous limestone.  The abundance and variability of Irish limestone explains its extensive exploitation for building and decorative purposes throughout the Victorian period.


Descriptions of stone in the Museum Building


Dimension stone, stone floor tiles, polished decorative stone and carved stone are exhibited in the Museum Building.

Each of the stone types in the Museum Building are described from both surface features and where possible from thin-sections.  Based on the lithological characterisation inferences on their original environment of formation is provided where possible and quarry locations are given where known.  The age of the stone types are expressed either in absolute time in millions of years (Ma) before present, and/or according to the geological period when they were formed.

Decorative polished stone used for interior columns, handrails, string courses, decoration above central arches and exterior roundels

Armagh Limestone

A pale reddish-brown, mottled, compact limestone containing thin red clay seams.  Fine-grained iron oxides, probably a mixture of haematite and goethite, tint the matrix.  This relatively unfossiliferous limestone contains dark deep red to brown patches, which are sections through fossil shark teeth which were generally dome-shaped rather than being serrated like in many modern sharks. In thin section this limestone is seen to contain bryozoans, foraminifera, crinoid fragments and algae.
Utilisation: One full column and three half columns on ground floor.
Locality: Armagh, Co. Armagh.
Age: Mississippian (Lower Carboniferous c. 330 Ma).

Clonony Limestone

A pale brown, sienna-coloured limestone composed of micrite and a low percentage of fossil material (wackestone).  It contains many fossils including visible crinoids and some cephalopod molluscs.  The latter, related to the modern-day Nautilus swam in surface waters and controlled buoyancy through gas-filled chambers in its shell which can be seen in some examples in this stone. In thin section various other small fossil organisms can be identified: bryozoans, crinoid ossicles, ostracods, small brachiopods, and molluscs. Original cavities in the lime muds are infilled with white sparry calcite and thin clay seams as well as veins infilled with grey calcite and also a series of wavy lines called stylolites.  These formed as the rock became subjected to pressure such that some of the lime began to dissolve along these horizons.  Stylolites are not strictly linear but have a three-dimensional shape which is highly convoluted and as such they do not reduce the cohesive strength of the limestone
Utilisation: One column on ground floor.
Locality:  Clonony, Co. Offaly.
Age: Mississippian (Lower Carboniferous c. 340 Ma).

Galway Black Limestone

A black, bituminous, fine-grained limestone that is relatively unfossiliferous, and which takes a good matt polish.  On the surface colonial corals, solitary corals and brachiopods are visible.  In thin section the stone from the classic quarry at Anglingham on the shores of Lough Corrib contains numerous unicellular foraminifera but little shelly fragments whereas the Merlin Park stone quarried east of Galway city contains algae corals, bryozoans, a variety of foraminiferan species, and amorphous opaques. The presence of algae indicates that the lime sediment was deposited in very shallow water. This stone was particularly popular from the early 1800s, and utilised for chimney pieces, columns, staircases, and even furniture; one bed at Anglingham was termed the ‘London Bed’ on account of its popularity there.
Locality:  Galway city (either Anglingham, Menlough or Merlin Park Quarry), Co. Galway.
Utilisation: One half column on first floor.
Age: Mississippian (Lower Carboniferous c. 340 Ma).

Kilkenny Black Limestone

A black, bituminous, fine-grained (micritic) limestone.  It is highly fossiliferous, containing fragmentary solitary and colonial corals, crinoids and large productid and other smaller brachiopods (seashells), as well as bryozoans, foraminifera, calcispheres and opaques seen in thin section.  The Kilkenny stone was well-known in the 1850s, and had been quarried at a number of localities on the River Nore just south of the city by then for over a century. It was exploited and utilised commercially in a large number of Irish and foreign buildings.
Locality:  South of Kilkenny, Co. Kilkenny.
Utilisation: One half column on first floor, and black to grey fossiliferous floor tiles (small in inner hallway and large in outer hallway).
Age: Mississippian (Lower Carboniferous c. 3450 Ma).

Cork Red Limestone

A lime conglomerate that contains rounded white blotches representing the original calcite pebbles, surrounded and supported in a red clay-rich matrix.  This rock type was deposited during the earliest part of the Carboniferous 350 Ma ago in a shallow water marine environment quite close to the shoreline. The dominant red colouration results from haematite, an iron-oxide which was derived from the erosion of the red sandstones that underlie the Carboniferous succession.  Following lithification, the rock  underwent some slight deformation due to compression caused during the Hercynian Orogeny towards the end of the Carboniferous.  This produced stylolites due to dissolution with concentration of red clay in wavy bands that cross-cut some of the pebbles. Calcite veins also cut the stone fabric and the stylolites. This limestone is sparsely fossiliferous with crinoids visible as circular traverse sections as well as longitudinal sections of crinoids stems in both the matrix and the calcite pebbles in hand sample, and fragmentary brachiopods, bryozoans and foraminiferans in thin section.  Cork Red Limestone became particularly popular from the 1850s onwards and was extracted from a number of quarries in Co. Cork, the largest being at Midleton and at Little Island from where stone of slightly different appearances was raised.
Locality:  Co. Cork.
Utilisation: One full column and three half columns on ground floor, two full columns on first floor, seats either side of main staircase, internal stringcourses, balcony rails, triangular insets to panels above central arcade, and insets in external roundels.
Age: Mississippian (Lower Carboniferous c. 344 Ma).

Connemara Marble

This marble (or strictly an ophicarbonate) occurs as green, white and sepia varieties to pure green varieties with occasional bands of black or grey.  The colour is determined by the extent and diversity of the coloured minerals present, varying from white to green with increased serpentine content.  The groundmass is typically calcite, which is a white to grey colour (sometimes with traces of pink), and dolomite, which is a creamy white, and in which minerals produced during metamorphism 463-475 Ma ago create decorative bands and blebs of colour.  These minerals include a diverse complex suite of minerals including amphiboles, serpentinised olivine, tremolite, chlorite, talc, mica, diopside, phlogopite and chalcedony.  The foliated and serpent-like texture is imparted by folding of original sedimentary banding, and secondary foliation.  The pale green variety shows folded layers of sepia and green minerals.
Utilisation: Five full columns and four half columns on ground floor, two full columns and two colonettes on first floor, two piers supporting central dome span on upper portion of inner hallway walls, handrails, triangular insets to panels above central arcade, and insets in external roundels.
Locality:  Light variety – Ballinahinch, Co. Galway; dark variety – Streamstown, near Clifden, Co. Galway.
Age: Precambrian (c. 600 Ma).

Castle Caldwell Limestone

A pale grey fossiliferous limestone, rich in disarticulated crinoidal debris.  Crinoids are stalked members of the Phylum Echinodermata that also contains sea urchins and starfish, and although they are animals they are often called Sea-Lilies.  Crinoid stems comprise stacks of circular or oval-shaped ossicles, and are mainly represented in this stone isolated, as white circular transverse sections, but some longitudinal sections of stems up to 3 cm in length are also visible. This is a grain-supported packstone with a micritic matrix.  Similar crinoidal-rich decorative stone is also known from Clerhane, Co. Offaly and from North Wales and Derbyshire. The latter were known in the 18th century locally as ‘screwstones’ on account of the crinoid stems. Not seen in thin-section.
Utilisation: One half column on first floor.
Locality:  Castle Caldwell, near Belleek, Co. Fermanagh.
Age: Mississippian (Lower Carboniferous c. 330 Ma).

Mitchelstown Limestone

A ‘reef’ limestone containing many cavity infills, termed stromatactis, up to 10 cm in length. These are filled white large crystals of sparry calcite that contrast with the black colour of the bulk of the rock, which is composed of fine-grained lime mud called micrite.  Stromatactis generally have flat bases and irregular flame-shaped upper portions.  Some thin pale grey veins crosscut the limestone. Not seen in thin-section.
Utilisation: One half column on first floor.
Locality:  Mitchelstown, Co. Cork, possibly from a quarry between the old workhouse and the town.
Age: Mississippian (Lower Carboniferous, c. 345 Ma).

Donegal White Marble:  A white, very pure calcitic and crystalline, fine-grained saccaroid marble that first came to prominence when displayed at the Great Exhibition of 1851, at which time it was compared to similar stone from the Greek island of Paros.  The extent of the marble outcrop in small and is surrounded by the Donegal Granite – this body was responsible for metamorphosing pure limestone into this marble.
Locality: Dunlewey, near Dunfanaghy, Co. Donegal.
Utilisation: Triangular insets to panels above central arcade, and insets in external roundels.
Age: Metamorphosed during Silurian (c. 400 Ma).

Scawt Hill Marble:  Stylolitic pale yellow to white marble.  Originally a pure Chalk limestone deposited during the latter part of the Cretaceous geological period 66-77 Ma, this became metamorphosed approximately 50 million years ago when the chalk was altered through contact with a vertical intrusion, or dike, of the igneous rock Dolerite.  This also led to the development of a suite of hydrated calcium-silicate metamorphic minerals including Larnite, Portlandite and Scawtite which were described from this locality for the first time.
Utilisation: Insets to exterior roundels.
Locality:  Scawt Hill (or Scaughthill), north of Larne, Co. Antrim.
Age: Cretaceous (c. 85 Ma).

Lizard Serpentinite

 A distinctive dark metamorphic rock with a blotchy red and black colouration.  It was formed by serpentinisation when water was incorporated into the metamorphic causing event on a dense suite of igneous rocks gabbros and peridotites that made up parts of an oceanic crust that was later uplifted to the surface.  The rock is composed largely of the dark minerals olivine and pyroxene, with veins of red and green oxidised serpentine minerals.  The stone became highly popular in the 1800s as a decorative stone and promoted by several London-based companies.  It continued to be worked until the mid-1900s and fashioned into small objects for the tourist trade.
Utilisation: Two full columns and two half columns on ground floor.
Locality:  Lizard Peninsula, Cornwall, UK.
Age: Devonian (c. 400 Ma).

Stone used for flooring

Carlow Slate  

A fine-grained black slate.  This was originally a mudstone deposited in deepish water, that was subsequently metamorphosed during the Caledonian Orogeny (mountain-building event) that resulted from the closure of the Iapetus Ocean when two continental masses collided approximately 400 Ma ago.
Utilisation: Triangular insets to panels above central arcade, and black floor tiles (small in inner hallway and large in outer hallway).
Locality: west Co. Carlow or eastern Co. Kilkenny.
Age: probably Ordovician (c. 450 Ma).

Pen-yr-Orsedd Slate

A fine-grained smooth slate, pale purple in colour with occasional oval-shaped olive green reduction spots.  Originally a deep-water, fine-grained sediment deposited c. 500 Ma during the Cambrian, these rocks were metamorphosed during the upheaval of the Caledonain Orogeny in Silurian times c. 400 Ma ago. Quarried from a major slate district near Snowdonia, Pen-yr-Orsedd was the last quarry to close in the 1970s.
Utilisation: Framing tessellated floor of inner hall.
Locality: Pen-yr-Orsedd quarry, Nantlle, North Wales.
Age: Cambrian (c. 500 Ma).

Yorkshire Flags

A well-bedded, well-sorted fine-grained sandstone composed of angular sand grains with a small amount of mica crystals cemented with iron oxide.
Utilisation: Large squares in tessellated floor of inner hall.
Locality: Yorkshire, England.
Age: Carboniferous (c. 325 Ma).

Creme Sintra Unata Limestone 

A pale cream-coloured fine-grained oolitic grainstone containing some fossil material including algae and mollusc fragments.  It exhibits a moderate porosity.
Utilisation: floor tiles in hallways and main staircase.
Locality: Pé da Pedreira, Santarém, Portugal.
Age: Middle Jurassic (c. 170 Ma).

Scotch Stone

A mudstone in which grains became lightly annealed and hardened through slight heating by an adjacent igneous intrusion.  Often used for hearthstones in fireplaces, they were also utilised as whetstones for sharpening tools and razors.  Also known as Water of Ayr Stone.
Utilisation: hearths for fireplaces and beneath stoves.
Locality: Ayr, Scotland.
Age: possibly Carboniferous.

Dimension and freestone used for exterior and interior walls, internal carvings and arches

Caen stone ashlar blocks and carvings above the arches.  Alternating pale Portland Stone and red Mansfield Sandstone arches.  Portland Stone carved capitals.

Caen Stone:  A fine-grained, cream to pale yellow coloured limestone, which is generally poorly fossiliferous. In thin section small shelly fragments together with spherical ooids are seen to form this grainstone.  Porosity in some Caen Stone horizons is quite high while in others is low, and this variation resulted in issues with its long-term durability when used for exterior work.
Utilisation: Interior ashlar walls of hallways, carved tympanum above doorway, some internal carvings above archways and on the stairs, fireplaces.
Locality: Normandy, France
Age: Middle Jurassic (c. 170 Ma).

Portland Limestone:  An extremely pure limestone, pale greyish-white to cream in colour.  It is fine-grained, oolitic (being largely composed of tiny calcite spheres called oolids), and fossil rich in certain beds with oysters, bivalves and gastropods dominating and some algae which indicates deposition in shallow water.  Portland Stone which is a grainstone, formed in marine, sub-tropical environment where it formed a number of distinctive horizons or beds. The stone used in the Museum Building is probably from the fossil-poor Whit Bed which provided the premier freestone from Portland.  It is capable of being worked and carved in any direction.  Highly regarded as a building stone and worked for at least a millennium, it was widely used from the early 1600s in England and was favoured for many 19th century buildings in Dublin, and is still quarried today.
Utilisation: Exterior window frames, capitals, string courses and carved elements; interior capitals and polychromatic arches.
Locality: Isle of Portland, Dorset, England, Co. Armagh.
Age: Upper Jurassic (c. 145 Ma).

Mansfield Sandstone

 A fine to medium-grained dolomitic sandstone composed of small angular sand grains which are cemented together with iron oxide.  These sandstones were deposited in terrestrial, arid environments probably as dune sands, and beds which were horizontal may have internal laminations called cross-stratification arranged at low angles to this primary bedding.  This stone became popular with Victorian architects striving for a polychromatic effect, and major resources were located in Nottinghamshire and Cheshire, England which yielded pale red stone, and Dumfries, Scotland from where a deeper purple coloured stone was quarried.
Utilisation: Coloured stone in internal arches.
Locality: Probably Mansfield, Nottinghamshire, England.
Age: Permian (c. 255 Ma).

Calp Limestone

A dark mud-bearing limestone with thick beds containing internal thin flat or convoluted laminations.  It was probably deposited in deep water marine settings from sediment that was carried from shallow water down-slope by turbidity currents.  Calp contains a small volume of the iron sulphide mineral Pyrite or ‘Fool’s Gold’ which may form small cubic crystals.  Generally poorly fossiliferous it contains some brachiopods, bryozoans and foraminifera.  Often small thin white veins of quartz or calcite cross-cut the foliated fabric of the stone. In the Museum Building large blocks were tooled producing a rough conchoidal surface.  In Dublin it was first used in the construction of Christ Church Cathedral in 1190 and later was much employed in Georgian Dublin and in Trinity for the ground floor story of the Old Library.  Roads were paved with the stone but it proved unsatisfactory as it decayed quickly and yielded mud, earning the epithet ‘Dear Dirty Dublin’ for the city.  The term ‘Calp’ was coined by the celebrated Galway chemist and mineralogist Richard Kirwan.
Utilisation: Basement walls and concealed internal layer in ground and first floor walls.
Locality:  Possibly Rathgar or Lucan, Co. Dublin.
Age: Mississippian (Lower Carboniferous, c. 350 Ma).

Leinster Granite

A fine-grained intrusive igneous rock composed of crystals of clear, glassy quartz, white feldspar, and two varieties of mica: black biotite and silver muscovite. Crystals are approximately 3 mm in size, closely interlocking resulting in a porosity of low to none, which is confirmed in thin section.  The stone blocks on the exterior average 20 cm in thickness, and were given a smooth chiselled exterior finish but were not polished.
Utilisation: Exterior ashlar, internal secondary staircases.
Locality: Ballyknockan, Co. Wicklow.
Age: Silurian geological period (404 Ma).

Roof slate

Super Bangor Queen Slate

A metamorphic rock that was popular and widely used as a roofing slate. It came in a variety of colours, purple being the most common and green less so. Vast quarries in North Wales were worked during Victorian times to supply the rapidly expanding metropolitan areas in Britain, Ireland and further afield. 
Utilisation: Roofing slates.
Locality: Wales.
Age: Cambrian (c. 500 Ma).

 

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