Previous SiteNext Site

Summary Full report
Scawt Hill
Rec. Number:1130File Number:  
Locality Type:Crag, Crags Status: ASSI
Grid Reference: D338090 Centroid
County: Antrim District:Larne Borough Council
Period:Tertiary, Cretaceous
Stages:Eocene, Palaeocene, Santonian
Lithostrat:Lower Basalt Formation, Ulster White Limestone Formation
Site Description

Dolerite crags of Scawt Hill, Co. Antrim; the curving vertical joints parallel the plug margin; Cretaceous Ulster White Limestone adheres to the plug contact in the NE gully; foreground shows part of main scree.
Occurrence of new and rare calc-silicate and related minerals. Genesis of alkali basic igneous rocks by limestone assimilation.
The summit of Scawt Hill (Grd Ref. D337090) rises some 30 m above the general level of the Antrim Plateau to a height of 378 m O.D. It may be seen along the East Antrim escarpment some 5 km WNW of Ballygalley or 19 km NW of Larne. The hill marks the outcrop of an olivine dolerite plug which intrudes the Cretaceous White Limestone and overlying Tertiary lavas of the Lower Basalt Formation. A high temperature and low pressure thermal metamorphism at the plug contacts has produced unusual calc- silicate mineral assemblages in the limestone, and assimilation of the carbonate rock has produced a sequence of alkali basic igneous rocks just inside the old volcanic conduit.
C.E. Tilley, in a series of publications, notably those in the Mineralogical Magazine for 1929, 1931 and 1933, was the first to appreciate the nature and potential of the dolerite-limestone contacts. His writings inspired many other workers to find similar rock relationships worldwide; his interpretation of the associated alkali basic rocks also prompted a revision of the petrogenesis of similar rocks in many other countries.
The dolerite plug has a roughly circular outcrop some 270 m x 180 m. Its steep cylindrical contact is defined by the east-facing precipitous cliff (30-60 m high) where curving joints parallel the outer contact, and is well exposed in two steep gullies at the SE and NE corner of the hill.
Dolerite plug in contact with Cretaceous Ulster White Limestone in SE gully; a thin tongue of limestone extends along the base of the dolerite crag above the grass-covered scree and is cut by pyroxenite rock.
The exogenous zone: The Cretaceous White Limestone forms conspicuous crags south of the plug and extends as a thin veneer on the steep slope of the hill. The pure carbonate rock, free from detrital materials and containing only flint nodules, passes into a coarse marble towards the dolerite contact and within ~60-120 cm from this contact changes to a tough silicate assemblage. This exogenous zone of thermal metamorphism is best developed at the foot of the SE gully. The silicate zone may reach a thickness of 60 cm and is variable in its mineral assemblage; the principal rock types found are:
Spurrite-rock (with or without calcite)
Larnite-rock (with or without spurrite)
Spurrite-gehlenite-merwinite-spinel-rock (with or without larnite).
Larnite, named after the township of Larne, was the first recorded occurrence of calcium orthosilicate. It is never found in direct contact with calcite, spurrite always intervenes. Magnetite, perovskite, wollastonite, xonotolite, gyrolite, afwillite, okenite, pectolite, cebollite, bredigite, plombierite, rankinite, hydrocalumite, portlandite, and ettringite, some of them new and many of them comparatively rare minerals, also occur in these silicate assemblages. These formed when solutions from the dolerite magma enriched the contact zone in silica, magnesia, iron oxides, and alumina. This metasomatic process varies locally as solutions of different composition soaked into the carbonate rock.
The flint nodules also show progressive metamorphic and metasomatic changes towards the dolerite contact. At first the flint (chalcedony) recrystallises to granular quartz and a reaction rim of wollastonite develops between flint nodule and the enclosing limestone. Some lime migrates and forms stringers of wollastonite within the nodule. Nearer the contact a more fundamental change occurs and sheaves of greenish xonotolite crystals with associated wollastonite prisms rim the nodule and it may completely replace the chalcedony. Green pyroxene, hedenbergitic diopside and aegerine may occur and the mineral okenite has been identified among the recrystallised quartz crystals. Close to the igneous contact the nodules are replaced by pyroxene assemblages, dark green augite-aegerine and wollastonite, the latter sheathed with pectolite; melilite altered to magnetite and cebollite also occurs. The larnite- spurrite rock surrounding the altered flint nodules may show a Liesegang effect with a series of concentric rings enriched in spinel minerals.
At the NE gully
NE contact, showing limited outcrop of limestone/marble; calc-silicate rocks occur near top of gulley.
the metamorphic and metasomatic changes are more restricted, but several centimetres of larnite rock develop. The flint nodules encased and veined by this calc-silicate material show extensive hydration to mineral gels such as plombierite.
The north contact of the plug can be traced above this NE gully and small exposures yield thermally metamorphosed basalt lavas.
The endogenous zone: The marginal dolerite was extensively modified by assimilation of the carbonate country rock; access of lime precipitated pyroxene to form pyroxenites of progressively more alkaline compositions. Again the best occurrence is in the SE gully, where veins of pyroxenite penetrate the exogenous zone rocks. The normal olivine dolerite merges within a metre of the contact into pyroxene-rich dolerite which carries vesicles lined chiefly with thomsonite associated with analcime, stilbite, natrolite, scawtite, and calcite. Zeolite minerals along with hydronephelite may replace the plagioclase. The pyroxene is enriched in diopside molecules as a result of lime assimilation. Nearer the contact and occasionally as veins or segregations occur titanaugite rocks of coarser grain size and of a lustrous black colour in hand specimen. The mineralogy varies with the degree of lime assimilation, and titanaugite-plagioclase, titanaugite- plagioclase-nepheline, and titanaugite-nepheline rocks occur. Graphic intergrowths of the pyroxene and nepheline characterise the more undersaturated rocks. The nepheline is altered to hydronephelite, and an olivine of iron-rich composition, fayalite, may be present. One thin vein of pyroxenite contains anorthoclase, fayalite, magnetite, apatite, thomsonite, and alkali-amphibole. The coarsest-grained members of this endogenous zone are those bearing melilite and occuring at the immediate contact with the limestone. Titanaugite-melilite rocks with plagioclase or nepheline are further modifications of the titanaugite pyroxenites but occur more closely associated with the metasomatised limestone as sharply defined segregations and veins of melilite-rock. Melilite is the predominant mineral associated with alkali pyroxene and grossular garnet.
Scawt Hill achieved international recognition through the researches and publications of C.E. Tilley. As a mineral site it yielded not only new minerals, notably larnite, hydrocalumite, rankinite, portlandite, and scawtite, but many rare ones in unique parageneses. Similar occurrences were identified at other localities worldwide and many of the research workers visited the Co. Antrim site.
The high temperature formation of the silicate minerals and their subsequent hydration, mirrored the industrial manufacture and use of cement. Again industrial mineralogists and chemists visited this natural counterpart to their own work.
The origin of alkali and alkali basic igneous rocks is still a highly contentious issue; Tilley was able to prove, most convincingly, that limestone assimilation could produce a differentiation series of basic igneous rocks markedly undersaturated and enriched in alkalis but on a very restricted scale. This contribution to petrogenesis was appreciated worldwide.
With the development of more sophisticated methods of mineral analysis it is inevitable that many more new and rare minerals will be found in the Scawt Hill rocks.
The remarkable development of mineral assemblages in both the exogenous and endogenous zones of the Scawt Hill dolerite plug provided evidence not only of a high temperature metamorphism and metasomatism of a pure limestone rock, but also of the effect of lime assimilation on basic magma.
High temperature or pyrometamorphism is demonstrated by the unique mineralogy and the unstable equilibrium preserved by rapid cooling. The existence of a high temperature form of calcium orthosilicate, bredigite, and its apparent inversion to a lower and less dense polymorph by dusting under the shock of a hammer blow, is just one of the mineral geothermometers.
The chemical reaction between basaltic magma and the carbonate wall-rock with the assimilation of lime and the release of carbon dioxide not only used up the available silica in the magma, producing more and more desilicated rocks but altered the viscosity of the modified melt and promoted a coarser grain size. The genesis of alkali basic igneous rocks in a limited quantity, demonstrated very clearly the limitations of this kind of wall-rock reaction in the petrogenesis of alkali rocks.
This site offers good sections and material for the progressive high temperature and low pressure metamorphism of a pure calcium carbonate rock with only silica, in discreet bodies, as an impurity. It also demonstrates the progressive desilication of a basic magma, by lime assimilation, to the end point of undersaturation when melilite bearing rocks were formed.

Tilley, C.E. has published many papers on the geology and mineralogy of Scawt Hill. These can be found in the Mineralogical Magazine, 1928 - 1947, and the Geological Magazine, 1927-28-51. The most notable contributions are included in the reference list.
McConnell, J.D.C. in the mid fifties worked on comparable rocks at Ballycraigy, near Larne, in Co. Antrim. His publications appear in the Mineralogical Magazine for 1954 and in the American Mineralogist for 1955 and make reference to Scawt Hill material.
This site is also important for Mineralogy and Metallogenesis. See Key Site 536 - Scawt Hill.

Minerals:Afwillite, Analcime, Bredigite, Calcite, Cebollite, Ettringite, Gehlinite, Grossular, Gyrolite, Hydrocalumite, Hydronephelite, Larnite, Merwinite, Natrolite, Okenite, Pectolite, Perovskite, Plombierite, Portlandite, Rankinite, Scawtite, Spinel, Spurrite,
Rocks:Basalt, Dolerite, Limestone
FossilGroups:No data
Fossil List:
Relations:Contact, Intrusion
NonGeol:The site was previously declared as part of a much larger ASI which extended past Robin Young's Hill to Sallagh Braes and included a wide range of upland grassland habitats and rare species.
Length:No dataWidth:No dataHeight:378 m O.D.
Depth:No dataArea:No data  
Approach:Access can be made by ascending the farm lanes from the Carncastle road, D352093, or, for a pleasanter walk, along the Ulster Way from the car park on the Carncastle- Glenarm road, D334074.
Restrictions:Not entered
Management:The site is somewhat remote from the nearest surfaced roads and this has protected the outcrop from too intensive a sampling. Exposures of contact rocks in the SE gully are massive and difficult to sample; a heavy hammer is required and in thoughtless hands could disfigure the rock surfaces and spoil them as a field demonstration piece. Sampling should be limited to genuine research workers. The scree below still offers loose material of most rock types but it has been well collected over during the past 6 decades and melilite-bearing rocks and metasomatised flint nodules are difficult to find. The NE gully is more fragile and the volume of rock remaining is limited. Again sampling should be left to the research worker.
Threats:Further sampling
Uses:The site is just above the intake of enclosed pasture and is used for grazing sheep.
Potential:Not entered
Educ. Level:Undergraduate / Postgraduate


See O.S. Sheet 9 (Ballymena, Larne) and the ASI map of 19.11.69.

Map No:None entered
Rec Type ESCR report Recorder:  
Enterer: E M Porter
Updates: 3 May 2003 / 27 JAN 01 / 03 FEB 97 / 27 JAN 97
Previous SiteNext Site