a note on dalradian pillow lavas, strabane, co. tyrone

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  • A Note on Dalradian Pillow Lavas, Strabane, Co. TyroneAuthor(s): William J. McCallienSource: Proceedings of the Royal Irish Academy. Section B: Biological, Geological, andChemical Science, Vol. 43 (1936/1937), pp. 13-22Published by: Royal Irish AcademyStable URL: http://www.jstor.org/stable/20490421 .Accessed: 16/06/2014 20:48

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  • 1I 13 ]

    II.

    A NOTE ON DALRADIAN PILLOW LAVAS, STRABANE;, CO. TYRONE.

    By WILLIAM J. McCALLIEN, D.Sc., F.R.S.E.,

    Glasgow University.

    (PLATE I.)

    [Read 27 JANUARY. Published 19 MARCH, 1936.]

    Introduction.

    THE present note is intended to record the discovery for the first time

    of pillow lavas in the Dalradian rocks of Ireland. For convenience it is

    suggested at the outset that these rocks should be called the Strabane

    Pillow Lavas, because it seems that Strabane, 14 miles south of

    Londonderry, is likely to remain the type locality for the rocks (Fig. 1).

    Since their discovery in 1932 there has been reason to believe that the

    Strabane Pillow Lavas have a wide distribution, for similar rocks have

    been found in recent years in the Sperrin Mountains by J. J. Hartley of

    Belfast University. Professor E. B. Bailey and the writer have seen the

    continuation of the lavas on the western side of the River Foyle (River

    Mourne) in the Lifford district.

    There are also outcrops of igneous schists at localities to the north of

    Strabane, between -it and Burn Dennet, notably at Artigarvan (Fig. 1),

    but it is not yet known whether these represent different stratigraphical horizons or repetitions by folding of the Strabane lavas. Unfortunately the district is thickly covered with drift and exposures are poor.

    The Strabane district is included in sheet 17 of the Geological Survey map of Ireland, and the rocks are described in the accompanying memoir

    (1889). Portlock in his great work "Report on the Geology of Londonderry">

    (1843) recorded the discovery of "greenstone slate" containing epidote and albitw in the neighbourhood of Strabane. Unfortunately this was not followed up by the Survey officers. The present writer suspected that Portlock's greenstone slate might be the continuation of the Green Beds of Antrim, and so visited Strabane at Easter in 1932. Hp was not long

    there when he wrote to his chief, Professor E. B. Bailey, that the Antrim

    Green Beds had been found. In June of the same year Professor Bailey

    PROC. R.I.A., VOL. XLIn, SECT. B. [C]

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  • 14 Proceedings of the Royal Irish Academy.

    acoompanied the writer to Strabane, and on this visit they discovered pillows in the green schists of the quarry on the north-eastern outskirts of the town. The rocks of Patten's Glen (Fig. 3) which the writer had taken to be the Green Beds were then seen to be merely sedimentary layers of the same nature interbedded with the lavas.

    L04ONDOt4> p , st

    u~~~~~~~fue wono-srt Utea

    FIG. 1.

    Sketch-map of the district north-east of Strabane.

    Stratigraphy.

    It is not yet possible to indicate the position of the Strabane Pillow

    Lavas in the Dalradilan sequence. Tdhis will probably follow from

    Hartley's work in the, Sperrns to the east and north-east of the Straeba;ne

    district. Onue or two general remaks may, however, be made at this istage.

    The Strabane lavas lie a eonsiderable distance below the grits and

    slates whichl, running south-west through southern Inishowen, bend east

    ward between Ljondonderry and ILetterkenny, and cross the River Fsoyle in

    an easterly and north-easterly direction some mles south of Ljondonderry.

    Further, the most northerly lavas recorded here underlie an important

    limestone which runs approximately east and west a short diistance north

    of Artigarvan, and which in all probability connects with the great

    Dungiven Limestone to the eat.

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  • MCOALLIE -A Note on lrad4an Pillow Lavas, Strabane. 15

    In the Strabane district the best outcrop of this limestone occurs on the northern bank of a glacial spillway falling eastward from Ballylaw School, a little over a mile from Artigarvan. A hornblende-schist also outcrops in this spillway south of the limestone. South of the epidiorite, sehistose grits with biotite are exposed in many places. They are often associated with mica-schists and black schists, and in Glenmornan River these pass into calcareous schists and limestone below the Smithy at

    Artigarvan. From the Smithy southward to beyond Green Lodge-400 yards-the river is occupied by pillow lavas, with limestone between the pillows.

    CA

    Arlt'iarvamL.9~ iC

    "D PILLoW LAvAs.

    | 3S Scnsrzrs. Catherine.

    FIG. 2.

    Sketch-map of the Artigarvan district.

    Under the Artigarvan pillow lavas, mica-schists and pebbly grits are exposed for 200 yards in Glenmornan River, and the same belt of grits and schists (with biotite) outcrops in Owenreagh Burn (2 miles north-east of Knockavoe), in Strabane Glen (a glacial spillway runnimg north and south, one mile east of Strabane, Plate I, fig. 2), and in Knoekavoe and

    Craigore.

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  • 18 Proceedings of the Royal Irish Acadenvy.

    The Strabane Pillow Lavas structurally underlie this spread of grits and, as has already been said, they are best exposed in the neighbourhood of Strabane itself. Quarries have been opened in them in several places, and a good natural section occurs in Patten's Glen.

    Underlying the lavas in the Curly Hill district of the town a good coarse crystalline limestone, associated with biotite-schist, has been exten sively quarried for burning purposes.

    l_A house r0 0 0 0

    -- ~0 000 --

    ~0 0 0

    0 0 0

    000

    2~~~~~~~~~~~

    JclwXst ___________, _

    FmG. 3.

    Sketch-map showing the distribution of the rocks in the immediate neighbourhood of Strabane.

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  • MCCALIJEN-A Note on Dilraddan Pillow Lavas, Strabane. 17

    In addition to thie isolated outcrops of the different rocks which have

    been mentioned above, reference may be made to good quartzite exposures which occur a mile north of Silverbrook Bridge, 4 miles east of Artigarvan,

    bnd which overlie an important limestone exposed in the Burn Dennet

    half a mile east of Silverbrook. This limestone in its turn rests on siliceous

    and micaceous schists containing poor exposures of tremolite-schist. Another outcrop of green sehist, probably of extrusive igneous origin, occurs at Dunnamanagh.

    Strabane Pillow Lavcas.

    In hand-specimen the typical rocks of this group consist of massive pale ,green schists in which fibrous amphibole and felspar can be seen with

    a lens. The felspar often forms knotty projections from broken surfaces. Many specimens effervesce with dilute acid. Quartz veins, aggregates of large epidote crystals, and veins of asbestos also occur, and although the rocks are free from any appearance of bedding -such as occurs in green

    schists of sedimentary origin, there are often streaks and irregular patches of dark slaty material in them. Sediments do oceur between the lavas and

    are indistinguishable from Green Beds but they are subordinate to the other varieties. Where pillow structure is best developed the interspaces between the pillows are filled with dark coarsely crystalline limestone and dark slate (Plate I, fig. 1). Some of the lavas are very porphyritic, with undeformed phenocrysts over half an inch in length. The coarsest rock which was seen in the Strabane district occurs in a prominent knoll on

    the western side of the high road a short distance north of Strabane

    Water Works. In some outcrops the phenocrysts are irregularly distributed, sometimes sparse, sometimes crowded together in one specimen, and often drawn out into augen.

    There are in nearly all exposures amphibole-schists in which all ma