A Correlation of Silurian Rocks in the British Isles

A correlation of Silurian rocks in the British Isles

LEONARD ROBERT MORRISON COCKS CHARLES HEPWORTH HOLLAND RICHARD BARRIE RICKARDS & ISLES STRACHAN CONTENTS x Silurian stratigraphical classification Io3 2 Wales and the Welsh Borderland IO6 3 North-West England. II5 4 Scotland. 5 Ireland .... I18 6 Concealed Silurian rocks of Eastern and Southern England I24 7 Cornwall. I26 8 Correlation with the Silurian successions of Bohemia, Thurin~ ia, Podolia, Estonia, and Gotland I27 9 References ~29 SUMMARY A discussion of Silurian stratigraphical classifi- Midland Valley of Scotland. A separate chart cation is followed by detailed comments upon is provided for all the Irish Silurian. The con- the correlation of Llandovery, Wenlock, cealed Silurian rocks of Eastern and Southern Ludlow, and post-Ludlow pre-Gedinnian rocks England are considered relatively briefly, as throughout the British Isles. A main correlation are the enigmatic records from Cornwall. chart shows selected successions in Wales and Finally a chart and short commentary serve to the Welsh Borderland, North-West England, correlate the Silurian successions of Bohemia, and Scotland. Subsidiary charts relate respec- Thuringia, Podolia, Estonia and Gotland with tively to the Silurian inliers south-east of the the standard employed here for the British main Welsh outcrops and to those of the Isles. There is an extensive bibliography.

i. Silurian stratigraphical classification

THE HI ST O R'¢ of the establishment of the Silurian System and of its primary sub- division was reviewed by Whittard (I 96 I) in the Lexique Stratigraphique International, which work also provides the historical background to much of the nomenclature referred to in our correlation. Whittard's final introductory comment was that Gothlandian (as an alternative to the modern meaning of Silurian) 'is a term which should be allowed to fall into disuse.' 'Silurian' was in fact adopted by the Inter- national Geological Congress at its Council meeting on 23rd August 196o (Sorgen- frei I964). The terms Lower, Middle, and Upper Silurian (again reviewed by Whittard i96i ) are ambiguous and in the British Isles the tendency has been to allow these to pass out of use. Regrettably they are far from exorcised from international literature. We here follow the Report of the Stratigraphical Code Sub-Committee of the Geological Society of London (George et al. I967) in regard to the prefixes lower, middle, and upper as appropriate only to informal usage. The terms Llandovery (Murchison 1859 ) and Valentian (Lapworth 1876 ) have come to be regarded as synonymous for the lowest series of the Silurian System. We

Jl geol. Soc. vol. x27, I97I, pp. Io3-x36, Figs. (folders) x-9. Printed in Northern Ireland.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/2/103/4884364/gsjgs.127.2.0103.pdf by guest on 01 October 2021 L. R. M. Cocks, C. H. Holland, R. B. Rickards & L Strachan abandon the term 'Valentian Series' in favour of the earlier 'Llandovery Series.' For a review of the whole question see Toghill (i969). Lapworth's (188o) term Salopian was eventually extended upwards by O. T. Jones (in Evans and Stubblefield 1929) to include the whole of the Ludlow as well as the Wenlock. It is thus still employed from time to time, though Whittard ( 196 I) noted that 'The tendency nowadays is to allow the term to fall into disuse'. It finds no place in our classification. Four stages have recently been erected within the Llandovery Series (Cocks, Toghill and Ziegler I97O), using the type area in Wales. However, since the base of the Llandovery Series is not exposed near Llandovery and since correlation there is difficult on account of facies changes and lack of fossils, these authors have also moved the location for the definition of the base of the basal Llandovery stage, the Rhuddanian, away from Wales to Dobb's Linn, near Moffat, Scotland. Since it was Lapworth who originally defined the Ordovician System, it is fitting that the Ordovician-Silurian boundary should be drawn at his most classical locality. Here there is no hint of unconformity, and a complete sequence is exposed representing all of the upper Ordovician and lower Silurian graptolite zones. The boundary at Dobb's Linn is drawn at the base of the persculptus Zone. Cocks, Toghill & Ziegler also recommend the elimination of the term 'Middle Llandovery', and divide the Llandovery Series into the two approximately equal parts ('Lower' and 'Upper'), as originally defined by Murchison. The dividing line is between the Idwian and Fronian Stages. This recent redefinition of a cate- gory between the stage and the series is in contradiction to the recommendation referred to on page lO3 and we can only suggest an informal usage thus: 'lower Llandovery' and 'upper Llandovery'. The equivalence of various levels in the shelly and graptolitic facies has been the subject of much recent work; briefly, the firm correlations in the Llandovery Series are between Upper Rhuddanian (A4) and the cyphus Zone, upper Idwian (Bs) and the convolutus Zone, lower Fronian (C1) and the sedgwickii Zone, and mid-Telychian (C5) and the griestoniensis Zone. The remaining zones are correlated mainly by interpolation between these fixed points, although the equivalents shown on the left-hand side of the correlation charts are also the result of taking into account a large number of smaller items of information. Since the Second World War, the Wenlock Series has received less attention than the Llandovery and Ludlow. Thus the time is not yet ripe for the formal erection of stages within the Wenlock Series; chiefly because correlation of the predominantly shelly type area with the graptolite sequence has not yet been perfected. However, in recent years, far more attention has been paid to the Wenlock, although the results are as yet largely unpublished, and names will be given to stages in due course. The basis for the Wenlock correlations suggested in this paper has been almost entirely graptolitic, linked to the zonal scheme originally established in Central Wales. As for the boundaries of the Series, its base (the bottom of the Buildwas Beds in the River Severn, Shropshire) probably approximately coincides with the base of the centrifugus Zone (Cocks & Rickards 1969); its top is defined by the base of the Ludlow Series.

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This latter was defined by Holland, Lawson and Walmsley (I 963) at a standard section in the Ludlow district, where the lowest (Eltonian) stage of the Ludlow Series was taken to follow directly upon the local representative of the Wenlock Limestone. Unfortunately, the position of this selected level in terms of the graptolite sequence remained ambiguous. Holland, Rickards and Warren (1969) have now attempted to resolve this problem by closer study of the graptolite sequence across the Wenlock/Ludlow boundary. An element of ambiguity still remains but, pending an international decision on the standard section for the base of the Lud- low Series, these latter authors conclude that an equation of the present boundary, as defined in the Ludlow district, with the base of the nilssoni Zone in the graptolitic facies is the nearest approximation to the truth which is at present possible. Holland et al. (i963) also divided the Ludlow Series into four stages, all of them upon the basis of accurately documented standard localities. Justification for the setting up of these stages in the kind of facies present at Ludlow itself lies in the status of the district as the type area for the series. There are difficulties in extend- ing the correlation of the stage boundaries beyond the immediate area of the Welsh Borderland, particularly at the level of the Eltonian/Bringewoodian boundary where there is no coincidence with a graptolitic biostratigraphical division (Hol- land in the press). A standard locality for the base of the Downtonian 'Stage' was also described by Holland et al. (1963) in the 'Ludford Lane' section at Ludlow, this providing for the boundary between the Ludlow Series and the Old Red Sandstone. Discussion of the thorny question of the Silurian/Devonian boundary, itself" providing a veritable case history of stratigraphical procedure, has finally resulted in a proposal from the Committee on the Silurian/Devonian Boundary and Stratig- raphy to its parent body, the International Commission on Stratigraphy, that this boundary be taken at the base of the Monograptus uniformis Zone (McLaren 1969). The consequence of this in terms of the Old Red Sandstone magnafacies in the British Isles is that most, if not all, of the Downtonian must now be included within the Silurian. An internationally acceptable name for this interval of the standard stratigraphical column must await further procedure (already initiated) involving the selection of a standard locality for the base of the uniformis Zone. Our use of 'post. Ludlow pre-Gedinnian' on the left-hand side of the correlation chart is clear if non-committal. Allen and Tarlo (1963) would redefine the Downtonian in a way which has subsequently (e.g. Jaeger 1965) been taken to imply a correlation with this interval. The status of the term 'Downtonian' may best be decided along with the associated (but Devonian) divisions of the Old Red Sandstone such as 'Ditto- nian' and 'Breconian'. These younger divisions, at least, will never be regarded as standard stratigraphical terms in the sense of the Geological Society's Provisional Code (George et al. 1967). Nor can they reasonably be regarded as biostratigraphical terms. If anything they may perhaps be best absorbed into a lithostratigraphical nomenclature. However, Martinsson (i 969) has recently advocated that 'Down- tonian' be used for the top series of the Silurian on the understanding that its upper boundary be made to coincide with the upper boundary of the P~idoli of Bohemia. Holland (in the press), on the other hand, suggests that the series term for the Io5

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/2/103/4884364/gsjgs.127.2.0103.pdf by guest on 01 October 2021 L. R. M. Cocks, C. H. Holland, R. B. Rickards & L Strachan post-Ludlow pre-Gedinnian interval should be chosen in relation to the internationally decided location for the standard section across the Silurian/Devonian boundary. There is very little evidence within the British Isles ofa graptolite sequence above the leintwardinensis Zone of the higher middle part of the Ludlow Series. Below this we include a complete set of graptolite zones, and in a few cases subzones, which provide a widely applicable basis for biostratigraphy. The middle Wenlock rigidus Zone is the equivalent of the Cyrtograptus symmetricus Zone of older publications. Of the Silurian shelly fossils of the British Isles, the brachiopods have so far proved most useful, as indeed they have elsewhere in the world. The stages of the Llan- dovery and Ludlow Series depend very much upon them and though the present informal divisions of the Wenlock used in the correlation charts are related to graptolite zones, it is possible to go some way towards their recognition in terms of the brachiopod sequence. Of other groups of shelly fossils, the trilobites (see, for example, Shergold 1967) and the ostracodes have proved of use particularly in the Ludlow, and the latter promise to be of value in Downtonian correlation (Shaw I969). Vertebrates are also significant for Downtonian correlation. Micropalaeontology is only recently beginning to be exploited seriously. The conodonts have yet to be shown to be more precise than other already available biostratigraphical tools. Spores become useful in the higher part of the succession where the change from marine Ludlow rocks to quasi-marine Downtonian provides its own problems (Richardson & Lister I969). Other groups of fossils such as the nautiloids may yet be shown to have stratigraphical value. When the standard classification of the correlation charts is considered in relationship to local successions the Silurian of the British Isles is found to be something of a monument to stratigraphical chaos. We commenced this work with the intention of attempting to translate local successions into modern (and now widely acceptable) lithostratigraphical terms. To have done so would have been to take the responsibility of altering almost every stratigraphical sequence previously described. We conclude that we should be defeating our own primary purpose, which is to provide a clear statement of the degree of correlation at present possible between successions already described and recognisable in the literature and our standard classification. Only in a few cases have we felt it reasonable to alter previous local nomenclature. However, we hope that this correlation may at least provide a healthy root-stock on to which future work in Silurian stratigraphy may be grafted. Such stratigraphical codes as that of the Geological Society of London (George et al. 1967) present a nomenclatorial hierarchy which it is surely simple to maintain. 2. Wales and the Welsh Borderland Correlation of the Silurian rocks of Wales and the Welsh Borderland is expressed in ten representative columns (A to J) of the main correlation chart (Fig. 2). Lo- cations of the lettered columns of the main chart are given in the related sketch- map (Fig. I). An additional chart (Fig. 4) and corresponding map (Fig. 3) are xo6

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/2/103/4884364/gsjgs.127.2.0103.pdf by guest on 01 October 2021 A correlation of Silurian rocks in the British Isles provided for the eastern and south-eastern Silurian inliers of Central England, the Welsh Borderland, and South Wales, which have been the subject of so much published work. The Welsh Borderland is the type area for the Silurian System (Murchison I835 ) and for the Wenlock and Ludlow Series (see Lexique, op. cit.); that for the Llandovery Series is in South Wales. Modern stage terminologies for both Llan- dovery (Cocks et al. 197 o) and Ludlow (Holland et al. 1963) have been set up in the type areas. As mentioned in the introduction, Wenlock classification has not yet proceeded so far, though there is valuable work as yet unpublished, notably that of M. G. Bassett on the Wenlock brachiopods of Wales and the Welsh Borderland, We thank Dr. Bassett for information and advice.

(A) MAIN CORRELATION CHART (Columns A to J) (A) West Pembrokeshire. This column is based upon the excellent section in Marloes Bay but there are smaller (and different) areas of Silurian rocks in south- west Pembrokeshire at Freshwater West and Freshwater East. All these areas require further investigation in modern terms. Earlier accounts are available in Cantrell et al. (I916) and Dixon (I92I). Recent work by Ziegler, McKerrow, Burne & Baker (i 969) has shown that the Skomer Volcanic 'Series' is probably of Llandovery, not Arenig, age. The lower unfossiliferous parts may possibly be late Ordovician. These authors introduce the term 'Skomer Volcanic Group' to include also the succeeding 'Conglomerate Series'. The unconformity shown between the Conglomerate 'Series' and Corallif- erous 'Series' is closely dated by Llandovery brachiopods. At the top of the Coral- liferous 'Series' the beds pass conformably into the Sandstone 'Series'. The faunas here are very poor but Dr. Bassett tentatively suggests that the base of the Ludlow is somewhere in the bottom 3o metres of the Sandstone 'Series'. Walmsley (i962) has discussed the age of the incoming of red beds in Pem- brokeshire and reasonably concludes that this may be at a level within, rather than at the top of, the Ludlow. Holland (I969 a, I969 b) compares the West Pembrokeshire situation with the known early onset of Old Red Sandstone conditions within the Ludlow succession of S.W. Ireland. The succession of Dittonian rocks in Pembrokeshire has been reviewed by Allen (I963) who notes the occurrence of Traquairaspis symondsi recorded by E.I. White from beds 'mapped at an intermediate to high position in the Lower Marl Group' and indicating the lowest zone of the Dittonian as redefined by Allen and Tarlo (I963). (B) Haverfordwest. This area is in need of revision, though much local detail is available in Strahan et al. (I914). The basal Cartlett Beds have yielded Tretaspis sp. (Cocks I968), indicating that they span the Ashgill-Llandovery boundary. There is no accurate age for the Uzmaston Beds, apart from general upper Llandovery. They lie below the Canaston Beds and also yield Atrypa reticularis, indicating a late Fronian or early Telychian age. The Canaston Beds yield Costi- stricklandia lirata at Blackpool, west of Narberth, directly beneath their unconformity with the Old Red Sandstone. The Coralliferous Series of the Winsle area is poorly exposed and completely surrounded by much younger beds, but middle

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Wenlock graptolites, associated with a shelly fauna, have been found. The Rose- market Beds, in particular, need revision. They rest unconformably upon the Pre-Cambrian Johnston Series (not clear from chart) and yield both middle and upper (Fronian) Llandovery brachiopods. However, in at least one locality at present mapped as Rosemarket Beds (an old quarry south of Great Nash, Grid Ref. SM/975o o949) , Ashgill fossils in a lithology similar to that of some of the Slade Beds are found. The Lower Old Red Sandstone of the Templeton outcrop, to the east of Haver- fordwest, contains pteraspids in its upper beds near Templeton itself (Strahan et al. 19 I4) ; but the lower part of the succession may well extend below the Ditto- nian into the Downtonian. (C) Llandovery. Stages have recently been erected in the type Llandovery area (Cocks et al. 197o), but Jones's (1925) divisions are still the lithological basis of the succession. Whether or not there is an unconformity under the basal A~ sandstone is not known, the scanty evidence is equivocal. In contrast with the findings of Jones, the only unconformity within the type succession is that at the base of C~, and this break appears to span less than one graptolite zone. The Wenlock of the area has never been fully described. The best section is that in the River Sawdde, where lower Wenlock sandstones rest on the uppermost Llandovery with no sign of unconformity. The higher beds are problematical; Williams ( 1952, p. 198) records an unconformity in which the 'time interval may be equivalent to the zones of Cyrtograptus symmetricus, C. linnarssoni and C. rigidus' (now termed the rigidus, linnarssoni and eUesae Zones) ; however, J. H. Price (unpublished Ph.D. thesis) mapped a 'Middle Wenlock' and 'Upper Wenlock' siltstones. In fact the extent and duration of any breaks are unknown; they may be quite small and local--the Wenlock sequence is about I2OO metres thick. Upper Wenlock Rocks, yielding a prolific shelly fauna with Meristina obtusa and other distinctive upper Wenlock brachiopods, are definitely proved in the Golden Grove area a short distance to the south. Potter & Price (i 965) have described the Ludlow and Downtonian rocks of the district, though as yet only in particular reference to four comparative sections: the Clawdd British and Cwm Dwr sections to the north-east of Llandovery, together with the Sawdde and Cenen Valley sections to the south-east. In spite of some strong facies developments, such as the deltaic Trichr~g Beds, the largely shelly faunas are correlateable with the succession at Ludlow. 'Roman C. Beds' on the correlation chart refers to the Roman Camp Beds. The overstepping Downtonian is unusual in the richness of its marine or quasi-marine faunas. Potter and Price relate those of the Long Camp Beds to the lowest part of the Shropshire Downto- nian but further investigations are likely to add to the refinement and possible extent of correlation. They (Potter & Price I965, p. 392) record a single specimen of Pteraspis leathensis from a loose boulder of typical red marls in the Sawdde gorge. Thus the uppermost part of the Red Marls extends above the Downtonian into the Dittonian and the Silurian/Devonian boundary lies somewhere within the Red Marl succession. (D) Builth. The Trecoed Beds (Ziegler et al. 1968 ) are a thin Llandovery strip io8

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/2/103/4884364/gsjgs.127.2.0103.pdf by guest on 01 October 2021 A correlation of Silurian rocks in the British Isles sandwiched between the underlying Ordovician and the overlapping Wenlock. The 'Wenlock Shales' are those in which Elles (I 9oo) established the sequence of Wenlock graptolite zones; it is doubtful, however, whether the basal centrifugus Zone is present. The area was revised by Jones (1947). Straw's (1937) comprehen- sive study of the faunal sequence in the higher Ludlow rocks of the Builth district was the inspiration for many later investigations of the Ludlow rocks of Britain. A subsequent account (Straw i953) provides useful additional information on the very similar sequence at Cwm Graig Ddu some 9 kilometres to the south-west of Builth. Here the Downtonian begins with the Tilestones which correspond approximately to the Long Quarry Beds of Column C. These in turn are followed by green marl, etc. under red beds. Straw (I93o) described the Downtonian sequence in both these and other South Wales areas. (E) Presteigne and Knighton. At Old Radnor, the Dolyhir Limestone, lateral equivalent of the Nash Scar Limestone, rests directly upon the Pre-Cambrian (Garwood & Goodyear i9io), but at Presteigne the limestone is unconformable upon the upper Llandovery Folly Sandstone (Ziegler et al. i968, p. 749), whose base is not seen. Whether or not there is an unconformity above the limestone is unknown, but it is overlain by rigidus Zone graptolites (Kirk I95Ia ), so that any break would not be large. The Olive Mudstones (Kirk I95Ib ) are of lundgreni Zone age and younger. The Ludlow rocks of the area to the south of Presteigne belong in terms of facies to the Buihh development (Holland and Lawson I963). The succession here has been summarised by Kirk (1951 b). To the north and north-west of Presteigne are the various Clun Forest districts described by Stamp (I 919), Earp (1938, I94o), and Holland (1959)- Their Ludlow successions are similar to one another though extensive developments of slump beds (Earp 194 o) are present in certain areas. The Downtonian, which begins with the characteristic Platyschisma helicites Beds (abbreviated in chart) is also similar throughout. At Bishop's Castle Allender (in Allender et al. I96O) has so far provided only a summary of the successsion. Its lowest part appears, in fact, to resemble that of the Long Mountain syncline rather than those of the other Clun Forest districts. The Knighton succession is used in the column above the Olive Mudstones (Holland I959). Holland has attempted a correlation between these various areas and subsequently (Holland i962 ; Holland et al. I963) a generalised succession for the Ludlow rocks of the basin facies has been provided to take account of developments of both the Builth and Clun Forest types. The higher part of the Red Downtonian of the Clun Forest areas has so far proved unfossiliferous. In terms of thickness it is certainly possible that the succession is indeed confined to the Downtonian rather than reaching up into the Dittonian. (F) Long Mountain. Although Wade (I9II) divided the Llandovery into the Cefn Group and Buttington Shales, Whittard (I932) extended the terminology of Wenlock Edge to cover the area. Ziegler, Cocks & McKerrow (1968, p. 766) have dated the beds. Above them is the Trewern Brook Mudstone Formation in which Palmer (i97o) and Cocks & Rickards (i969) have noted the centrifugus, murchisoni, and riccartonensis Zones thinly developed at the base. This contrasts with the earlier xo9

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views ofdas Gupta (1932) who believed the first two of these to be absent. Palmer has revised the stratigraphy of the Wenlock, Ludlow, and Downtonian rocks of the Long Mountain syncline (Das Gupta 1932, Austin I925) by rigorous study of both graptolitic and shelly faunas. A revised terminology for the whole Silurian succession is provided in a preliminary communication (Palmer 197o ). There is a very limited outcrop of the lowest part of the Red Downtonian. (G) Llangollen. No work has been published on the Llangollen area since that of Wills and Smith (i922), apart from the sedimentological studies of Cummins (I 957, 1959 a, 1959b), these latter including brief summaries of the stratigraphy of the Wenlock and Ludlow rocks. Wills and Smith were troubled about the age of the Dinas-Br~.n 'Group', its fauna having an 'Upper Ludlow' aspect and lying above the Zone of Monograptus leintwardinensis, yet containing Dayia navicula as the commonest fossil. This last was regarded as precluding an 'Upper Ludlow' age. It is now known that Dayia navicula is commonly found in the lower part of the Whit- cliffian, particularly in certain facies, and thus the Dinas BrAn 'Group' falls into place as low Whitcliffian in age. (H) Denbighshire and Conway. This area is undergoing revision by the Institute of Geological Sciences and Dr. P. T. Warren has kindly supplied nomenclature and notes for the Wenlock and Ludlow rocks. These actually relate to the area covered by I" geological (N.S.) Sheets lO 7 (Denbigh) and 95 (Rhyl). In the Isle of Anglesey the Silurian is represented only by the graptolitic Llandovery sequence of Parys Mountain (Bates I968). Elles (19o9) described the area around Conway, in which the Conway Castle Grits lie conformably upon the anceps Zone Deganwy Mudstones. The correlation of the Grits is debateable, with conflicting faunal reports; they may be partly, or even wholly, of Ashgill age. Above them the Gyffin Shales yield diagnostic graptolites at several horizons (Elles 19o9, pp. I85-6 ), including a proved crenulata Zone fauna. The Shales pass up conformably into the Benarth Grits, the lateral equivalent of the Denbigh Grits. However, south of Llanrwst the thickness of the Llan- dovery is at least 650 m. (P. T. Warren, personal communication.) The Denbigh Grits have been described, particularly in sedimentological terms by Cummins, whose researches, as mentioned in the notes on Column G, also extend into the higher Wenlock and Ludlow rocks above (Cummins 1957, 1959 a, 1959b). However, at the base of the Wenlock our terminology on the correlation chart switches to that of the Institute of Geological Sciences. The Zones of C. centrifugus, C. murchisoni, and M. riccartonensis are known from the Conway area but, except for an occurrence of the first south of Llanrwst, have not yet been mapped and confirmed on graptolites from Sheets lO 7 and 95. The 'nassa[dubius Interregnum' of Jaeger (1959) is recognised and is restricted to a few metres of strata. The two 'Mottled Mudstone' horizons can be mapped throughout the area and, indeed, into the Lake District, Central Wales, the Long Mountain, etc. Other formations are locally developed and will be referred to in the published memoir. Dr. Warren also points out that the Upper Nantglyn Flags/Elwy Group boundary in the middle of the nilssoni Zone is an important facies change which is more or

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Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/2/103/4884364/gsjgs.127.2.0103.pdf by guest on 01 October 2021 A correlation of Silurian rocks in the British Isles less coeval throughout North and Central Wales and possibly also North-West England (and see Bailey 1964). In Wales it separates a graptolitic, muddy siltstone facies below from a shelly facies with slumped beds, sandstones and siltstones, and subordinate graptolitic strata above. There is no well defined tumescens Zone in N.W. Denbighshire. Graptolites referable to the type material range throughout the Elwy Group; and monotypic assemblages are found interbedded with scanicus Zone faunas which include the zone-fossil. Although there is no tumescens Zone as such, an incipiens Zone is recognised at this horizon, which on the presence of M. clunensis probably correlates with the uppermost Elton Beds/Lower Bringewood Beds at Aymestry. Earp (i 944) pointed out that in Clun Forest the tumescens Zone is replaced by a zone with spinose graptolites of M. chimaera type. Holland (I959) recorded the same graptolite sequence in the Knighton district. The Zone of M. leintwardinensis has not been proved within the confines of Sheets IO 7 and 95, but may occur in the extreme south-east of the area and is present in the Clwydian Range (Woods and Crosfield I925). The Institute of Geological Sciences proposes in any case to restrict the term Elwy Group to strata lying below the leintwadirnensis Zone. (I) North Cardiganshire. The lower part of the column is based on the Plynlimon area (Jones 19o9). We have replaced Jones's use of the term 'Stage' by 'Group," and his term 'Group' by 'Formation', so as to preclude confusion with the term 'Stage' as now used in stratigraphy. The sedimentology of the Aberystwyth Grits has been described by Wood & Smith (i959). The higher beds shown are those of the Trannon area (spelt 'Tarannon' by Wood, x9o6 ). Bassett (i955) has described the Llandovery and Wenlock succession in the Talerddig district, Montgomeryshire, and discusses correlation with successions at Abbey Cwmhir and Llanidloes, as well as Tarannon. (J) lfenlock Edge and Ludlow. The Llandovery of Shropshire, well mapped by Whittard (I928 , 1932), has recently been revised by Ziegler, Cocks & McKerrow (1968). The divisions of the Wenlock in the type area were originally described by Davidson & Maw (I88i), and modified by Pocock et al. (I938). The age of the Buildwas Beds is discussed by Cocks & Rickards (I969) who argue that the beds may represent the murchisoni Zone, and even perhaps the centrifugus Zone but positive evidence is still lacking. The Coalbrookdale Beds contain graptolites near their base indicative of the upper half of the riccartonensis Zone. The geology of the area around Church Stretton, Craven Arms, Wenlock Edge and Brown Clee Hill is described in a recent memoir of the Institute of Geological Sciences (Greig et al. I968 ). At the base of the Wenlock Limestone the column begins to relate to the Ludlow district rather than to Wenlock Edge. Holland, Rickards, and Warren (1969) have established that the Wenlock Shale outcropping in the core of the Ludlow anticline ranges from lundgreni Zone to ludensis Zone, the base of the latter occurring some Ioom. below the base of the Wenlock Limestone. Some at least of the Wenlock Limestone is also within the ludensis Zone and such evidence as is available suggests that the Lower Elton Beds, which follow directly above the limestone, may be of nilssoni Zone age. This last point is certainly not entirely proven and it may be that

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the ludensis Zone will eventually be shown to extend some or all of the way through the Lower Elton Beds at Ludlow, themselves only 30 to 45 m. thick. Problems of correlation about the Wenlock/Ludlow boundary are referred to on pages 105 and 127- The use of the term 'Beds' for the local stratigraphical divisions within the Ludlow District was introduced by Holland, Lawson & Walmsley (1959) and later these units were fully defined (Holland et al. I963). Their use has been criticised by Phipps (1962) and defended by Holland, Lawson & Walmsley (1962) on the grounds that they are essentially 'combined units' based upon both lithostratigraphical and biostratigraphical characters. Certainly the publication of a new triple nomenclature--lithostratigraphical, biostratigraphical, and chrono- stratigraphical--all at once for the same (albeit type) area--would not have found acceptance in Britain a decade ago. Whitaker (I962) applied the same nomenclature to the Leintwardine district to the west of Ludlow, though here he found it necessary to distinguish basal from higher Lower Leintwardine Beds. Shergold & Shirley (i 968) have recently described the Ludlow rocks between Craven Arms and Much Wenlock (Wenlock Edge) and here suggest a refinement of the correlation of the Leintwardine Beds of the Welsh Borderland. A commentary from Lawson & Whitaker (I969) includes a detailed table of distribution of selected fossils in the Lower and Upper Leintwardine Beds and Lower Whitcliffe Beds. Along Wen- lock Edge, as elsewhere in the Welsh Borderland, there are breaks in the succession at the base of the Leintwardine Beds and within the Upper Leintwardine Beds. Thus, Shergold and Shirley refer to 'Basal' and 'Higher' divisions of both Lower and Upper Leintwardine Beds. The Downtonian rocks of the Ludlow district were described in great detail by Elles & Slater (I9O6). Basing their work upon a modern study of vertebrate faunas by E. I. White, H. A. Toombs, and themselves, Ball & Dineley (1952) have presented a general Old Red Sandstone stratigraphy for the Clee Hills district to the east of Ludlow and a more detailed study of the succession in the Brown Clee Hill (Ball & Dineley I96i ). Allen & Tarlo (I963) have subsequently suggested a redefinition of the Downtonian/Dittonian boundary with the latter beginning at the base of the "'Psammosteus' Limestones Group". The Downtonian, thus restricted, has been taken to equate approximately with the post-Ludlow, pre-Gedinnian interval (see pages IO 5 and 128).

(B) SUBSIDIARY CORRELATION CHART (FIGURE 4) FOR THE SILURIAN INLIERS OF CENTRAL ENGLAND, THE WELSH BORDERLAND, AND SOUTH WALES The Ludlow rocks of the Usk, Woolhope, Gorsley and May Hill inliers were included in the correlation table provided by Holland, Lawson & Walmsley (I963, Table II) and these authors commented also upon the Ludlow rocks of the Mal- verns and Central England. Holland & Lawson (1963) brought all these and the Cardiff inlier into correlation with other Welsh areas, in order to achieve a com- prehensive picture of facies patterns in the Ludlow. Since the publication of these works Phipps and Reeve (I967) have given an account of the Ludlow rocks of the Malvern Hills. Ziegler, Cocks & McKerrow (1968) have used brachiopod lineages,

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linked to some extent with the graptolite sequence, as a basis for correlation of the Llandovery rocks of the Welsh Borderland involved in their reconstruction of the Llandovery transgression. This work again refers to the various inliers treated in the following pages. Dr. M. G. Bassett has in preparation a palaeontological contribution which will include a generalised stratigraphical table for the Wenlock Series of the Welsh Borderland and South Wales. Cardiff. The area has not been fully revised since the original work by Sollas (I879), but preliminary results by Bassett (I969) indicate that no beds older than late Wenlock are exposed--the Llandovery brachiopods quoted by Sollas are mis- identifications. We are grateful for Dr. Bassett's comments upon the whole of this column. There is a gap with no exposure between the youngest Ludlow beds (with their Bringewoodian fauna) and the 'red marls'. Dr. Bassett hopes to excavate here in the near future. Usk. Walmsley (1959) has described the Usk inlier. The centre of the anticline consists of poorly exposed Wenlock Shale, which beds yield Meristina obtusa, indicating an upper Wenlock age. Walmsley identified the Ludlow Bone Bed and referred to the rocks above as 'Speckled Grit'. This he compared lithologically with the Downton Castle Sand- stone of Shropshire. Above came the 'Red Marls'. The recent memoir of the Institute of Geological Sciences on the country around Newport, which includes the southern half of the Usk inlier, (Squirrell & Downing I969) presents a classification for the Old Red Sandstone which is applicable not only to the Newport (249) Sheet but also follows that adopted in the Chepstow (250) and Monmouth (233) Sheets. In it the Ludlow Bone Bed (not included on chart) is followed by the Downton Castle Sandstone and this by the Raglan Marl Group. The last contains the 'Psammosteus' Limestone in its top 3 ° m and, following Allen & Tarlo (1963), this takes the beds into the basal Dittonian. Tortworth. Curtis (1955) has described the Llandovery and Wenlock strata of the Tortworth inlier. The Lower Trap, which is an extrusive basalt, rests unconformably upon Tremadoc Micklewood Beds. The correlation and animal communities of the upper Llandovery are described by Ziegler et al. (I 968, p. 76o), and the unconformity with the Devonian by Curtis & Cave (1964). Dr. Curtis kindly informs us that he and the Institute of Geological Sciences are in agreement on the use of the term 'Brinkmarsh Beds' for the 'Wenlock Shale' of the Tortworth inlier. About 4 kilometres to the north of the main Silurian outcrop is a narrow strip of Ludlow rocks which reaches the Bristol Channel at Tites Point. These beds again were briefly described by Curtis (1955) but Holland et al. (1963) have commented more fully upon their age. White (I 967) finds that at Tites Point Whitcliffian beds appear to pass conformably into those equivalent to the Downton Castle Sand- stone. A bone bed is present which is at or very near the horizon of the Ludlow Bone Bed. Cave and White have in preparation a paper on 'Exposures of Ludlow rocks and associated beds at Tites Point and near Newnham, Gloucestershire'. The succession is to be compared with that at May Hill and reference made to the marked southward thickening recorded in the Brookend Borehole (Cave & White, I968; and see page I24).

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The Wenlock and Downtonian rocks of the very small inliers near Wickwar, Gloucestershire, were described by Whittard & Smith (1944) Wenlock sediments and volcanics in the eastern Mendips are referred to by Ziegler et al. (I968, p. 765). May Hill. The column is largely taken from the work of Lawson (I955), with minor revisions by Ziegler et al. (I968, p. 757). New Red Sandstone lies unconformably on Ludlow to Downtonian rocks in the north-eastern part of the inlier but to the west the Downtonian outcrop is continuous with the main outcrop of Old Red Sandstone of South Wales and the Welsh Borderland. Gorsley. The several very small 'inliers' of Wenlock and Ludlow rocks in the Gorsley area of Herefordshire between the Silurian inliers of May Hill and Wool- hope were described by Lawson (I954). Three of them are quarries cutting down through the Downtonian cover. Another very small inlier west of Newham, situated between the Old Red Sandstone and the Trias, in this case to the south of May Hill, has been found by Dr. H. C. Squirrel to show Elton or Lower Bringewood Beds, Lower Leintwardine Beds, Upper Leintwardine Beds, and possibly Lower Whit- cliffe Beds. Woolhope. The upper Llandovery is seen only in the core of the Woolhope dome and was recently dated by Ziegler et al. (I968, p. 757). The rest of the column is taken from Squirrel & Tucker (i96o). Minor breaks appear in the succession in the extreme south of the inlier, i.e. towards the 'Gorsley axis' (Holland & Lawson i963) and there is a detailed treatment of these in Squirrel and Tucker (op. cit.). The problem of the Downtonian/Dittonian boundary in Herefordshire, as in Shropshire, is one of definition and the synthesis by Allen and Tarlo (I963) is again relevant. Malverns. Phipps & Reeve (I967) have brought up to date the classic stratigraphy of Phillips (1848 ) and Groom (I 91 o). The basal Wych Beds overlap the Cambrian to rest unconformably on the Pre-Cambrian (although Phipps and Reeve think the latter junction to be faulted). Ziegler et al. (I968, p. 753) describe and date the Llandovery in detail. Phipps and Reeve felt unable to employ the so called 'combined units' defined by Holland et al. (I959, I963) for the Ludlow of the type area, though they were able to correlate with the four stages. In particular they find the Rilbury Siltstone and Aymestry Limestone Members to be diachro- nous, and the latter in the Ledbury Hills to cross upwards into the Leintwardinian. Abberleys. The Silurian outcrop of the Abberley Hills is essentially a continuation of that of the Malverns and, again, the stratigraphy has been revised by Phipps & Reeve (i967). The Llandovery succession shown on the chart is that of the Old Storridge Common area (Ziegler et al. I968, p. 75o). A general account of the area is available in the Droitwich memoir (Mitchell et al. 196 I). South Staffordshire. The Llandovery-Cambrian unconformity is exposed at Rubery (Ziegler et al. I968, p. 764). The section above this is best known from the nearby Walsall borehole (Butler x937) , which showed more than 2o metres of shales under the Barr Limestone. The poorly exposed Wenlock Shale of the district was described by Cantrill (in Barrow et al. I919) and the Dudley Limestone by Butler (1939). II4

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The highest beds exposed at Wren's Nest, Dudley, are the Lower Ludlow Shales which are covered unconformably by Coal Measures. However, at Turner's Hill some 3 kilometres to the north-west of Dudley, and at Gornal, Ball (I95I) has described an area of Ludlow and Downtonian rocks. Holland et al. (1963, p. I5o ) have also considered the correlation of the Ludlow succession at Lye. Their con- clusions are incorporated in the higher Ludlow portion of the column. The Ludlow and Downtonian rocks of all these small inliers in South Staffordshire were originally described by King and Lewis (I9I 2), who provided a sketch map of their relative positions. Ball (I95I) uses the term 'Downton Castle Sandstone Group' to include the Ludlow Bone Bed, Turner's Hill Bed and Gornal Sandstone. Until Wills (i947) recorded a specimen ofHemicyclaspis murchisoni in the Gornal Sandstone it had been regarded as of Coal Measure age. Lithological terms for the Downtonian rocks and in particular the lower 'Grey' as distinct from 'Red' Downtonian are legion and the correlation shown across the whole chart is based largely upon lithological similarities throughout a relatively thin sequence. 3- North-West England The most recent summary of Silurian stratigraphy in North-West England is that of Furness, Llewellyn, Norman, and Rickards (I967). Whilst including much recent work by these authors this deals only with the Wenlock and Ludlow strata. It has proved necessary to include in the present correlation chart a considerable amount of further (unpublished) information obtained by Rickards, Strachan, and others, which is mentioned in the notes below. The successions are shown in columns K to N of the main correlation chart (Fig. 2) and the areas in question are indicated on the related sketch-map (Fig. I). (K) Windermere. The Basal Beds, or Atrypaflexuosa Bed of Marr and Nicholson (1888), immediately underlie atavus Zone strata in Skelgill and may be equivalent, in part, to this zone or to earlier Silurian zones. In the Howgill Fells (adjacent column) similar, hard calcareous Basal Beds underlie strata yielding an acuminatus Zone assemblage, and thus are probably largely ofpersculptus Zone age. It is certain that the Brathay Flags, typically developed in the Windermere district, encompass almost the full suite ofWenlock graptolite zones. They represent continuous deposition of fine grained muds from centrifugus to lundgreni zones (Rickards i969) , when the instability typical of Ludlow times first manifests itself. Recent work by Rickards (1969) has shown that the lowest two-thirds of the Middle Coldwell Beds yield a ludensis Zone fauna, whilst the age of the highest beds remains in question. The lower Coldwell Beds are entirely of lundgreni Zone age (Rickards op. cit.). The Lower Coldwell Beds thin out completely on the east side of Windermere; their greatest thickness is towards the western end of the main Lake District outcrop. The Kirkby Moor Flags, strictly speaking, do not crop out in the Windermere district. They are included in column K for completeness and the time interval II5

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indicated (late Leintwardinian to early Downtonian) is taken from recent unpublished information kindly supplied by Dr. R. Shaw from an area nearer Kendal. Faunas at this level are strongly facies controlled and the first Kirkby Moor Flags lithology (and fauna) appears in alternation with a typical late Leintwar- dinian assemblage. Similarly, at the upper limit, Kirkby Moor Flag lithologies (and fauna) are seen to alternate with more characteristic Downtonian rocks. Dr. Shaw refers to the latter as 'Scout Hill Flags'. These yield the diagnostic Downtonian ostracode, Frostiella groenvalliana. (L) Howgill Fells. In :954 Wilson (unpublished Ph.D. Thesis, University of Birmingham) identified approximately 8 metres of distinctive grey mudstones immediately below the Brathay Flags in the Howgill Fells. Since this lithostratigraphical division can be recognized and mapped across the main Lake District outcrop--maintaining its thickness and horizonua more formal recognition of the unit is proposed in the correlation chart. The discontinuous, underlying Red Beds, are similarly stratigraphically useful. The Brathay Flags (type locality at Brathay quarry near Windermere) are recognizable with certainty in the Howgill Fells and it has not been thought necessary to erect a new formational name for this unit. The true thickness is unknown since much of the lundgreni Zone mudstones may be cut out by faulting. In the nearby Barbon and Middleton Fells (Furness et al. 1967) the thickness of the lundgreni Zone mudstones and siltstones exceeds the total thickness of the Wenlock as seen in the Howgill Fells. The higher strata in the Basal Ludlow Limestones are of nilssoni-scanicus Zone age (Rickards 1967) , whilst the lowest beds, overlying lundgreni Zone mudstones, are presumably of approximately ludensis Zone age, that is equivalent to the lower part of the Middle Coldwell Beds (see above). The basal Ludlow Limestones are overlain by up to 15 metres of graptolitic mudstones which are the exact lithological equivalent of the much thicker Upper Coldwell Beds of the main Lake District outcrop. (M) Cross Fell. Discontinuous Llandovery exposures (Burgess et al. 197o ) strongly suggest that the Cross Fell sequence is closely comparable with that of the Howgill Fells. A well developed rnaximus Sub-zone and Red Beds horizon both suggest close affinity with the Howgills succession, rather than with that of the main Lake District outcrop. Exposures of the Brathay Flags in the Cross Fell inlier are discontinuous and probably faulted. Recent work by the Institute of Geological Sciences (Burgess et al. 197o ) has established a possible riccartonensis Zone assemblage and a definite linnarssoni Zone (middle Wenlock) fauna. (N) Horton-in-Ribblesdale. A detailed account of the area was given by King and Wilcockson (i 934)-

4. Scotland Columns for the Southern Uplands of Scotland and Girvan are provided in the main correlation chart (Fig. 2). A subsidiary chart (Fig. 6) and related sketch-map (Fig. 5) show the Silurian inliers of the Midland Valley of Scotland. (0) Southern Uplands. In this area the Birkhill Shales near Moffat provide the II6

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standard zones for the lower part of the graptolitic succession in the Silurian (Toghill i968b ). Above this level, however, considerable thicknesses of unfossiliferous sediments are generally present and since the area is tectonically disturbed the successions are based on correlations with other areas. The rocks of the Gala Group are largely greywackes and shales with occasional graptolitic beds which have yielded fossils indicative of the turriculatus and crispus Zones in a number of places. The griestoniensis fauna has so far been recorded only from the type locality of the species itself. Grieston Quarry lies to the west of the most graptolitic beds of the Gala Group and its structural position is not clear. To the south-east of the Galashiels area, the greywackes are supposed to be different lithologically and they are generally unfossiliferous (the Hawick Rocks) until, continuing south-eastwards, the graptolitic basal Wenlock beds are reached. There is no evidence in the Southern Uplands for the presence of a crenulata Zone and the determination of M. crenulata itself from the Galashiels area (Elles & Wood I9I I, pl. 41 fig. 4 d) is a misidentification of a poorly preserved graptolite. On the east coast, the Gala Group can be recognised from Siccar Point to St. Abb's Head and has yielded graptolites of the turriculatus and crispus Zones. Between Coldingham and Eyemouth, Shiells & Dearman (I966) have claimed on structural grounds that the beds are much older than those to the north-west but there is no fossil evidence to support this. In the area further to the south-east, from Eye- mouth to Burnmouth, the beds include red and purple shales similar to those of the Hawick Rocks and, although recent collecting by the Institute of Geological Sciences has added many new graptolite localities in the St. Abb's-Siccar Point area, no fossils have been found to the south-east. The stratigraphical position of the Hawick Rocks is in doubt since they have not yielded any good faunas (or floras). Peach and Horne (I899), following Lap- worth, put them immediately below the Wenlock Riccarton Beds which occur to the south-east along the southern margin of the Southern Uplands. Craig & Walton (I959) , on the basis of way-up criteria, claimed that the Hawick Rocks younged fairly consistently to the north-west and that they were therefore younger than the basal Wenlock murchisoni Zone proven to the south-east. Warren (I 964), in the area south of Hawick, also found evidence for younging to the north-west in the Hawick Rocks, but in addition he had a succession ofgraptolite zones in the Wen- lock which became younger to the south-east. He therefore considered the Hawick Rocks as unfossiliferous lateral equivalents of the graptolitic Wenlock. Tulloch (1967) found sedimentary evidence in the Langholm area that both Hawick and R.iccarton Beds were younging to the north-west. The graptolite faunas, however, clearly come in belts indicating a general younging to the south-east and the structure has been interpreted to allow for this overall pattern while leaving local areas younging to the north-west. Since there is no reason for supposing that the Riccarton and Hawick Rocks have had different structural histories, it appears possible that the generally unfossiliferous Hawick Rocks could regionally be younging to the south-east while locally displaying the younging to the north-west observed by Craig & Walton and by Warren. This leaves the broad picture much as Peach and Home described it (Gala Beds followed to the south-east by Hawick

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and then by Riccarton Beds) but, of course, does not necessarily imply that their structural interpretation was correct. Rust (i965) has recorded a small graptolite fauna from the Hawick Rocks which suggested a pre-Wenlock age but it is not sufficiently diagnostic to be relied on. (P) Girvan. The succession at Girvan is important as providing links between the shelly and graptolitic facies in the Llandovery. A modern study is in progress and some of the results are incorporated into the present correlations. Lapworth (I882) admitted that his stratigraphical sequence was compiled from different parts of the Girvan area but, unfortunately, some of the best fossil collecting localities are stratigraphically uncertain which has led to later misinterpretation. Re-examination of the critical areas should establish better correlations. In particular, the Rhynchonella Grits, which Lapworth placed below the Camregan Limestone and R. maximus Shales, have at their most fossiliferous locality provided faunas which have been correlated in Wales with post-maximus Zone horizons. Inliers in the Midland Valley of Scotland. The group of inliers just to the north of the Southern Uplands Fault has been re-examined in recent years (see for example Rolfe i96x), although much of the work remains unpublished. A good account of the Pentlands--Lesmahagow inliers is given by Walton (i965). The general conclusion is that much of what was called Downtonian by Peach & Horne (I899) is now regarded as no younger than Wenlock, or perhaps early Ludlow, and that considerable thicknesses of Llandovery are present. In the Stonehaven area, however, close to the Highland Boundary Fault, the succession of some 84o metres of sandstones, siltstones, and volcanic conglomerates and tufts has yielded fish and arthropod faunas which can be correlated with the lower part of the Downtonian of the Welsh Borderland (Westoll 195 x). The Portencross inlier (Patterson I949) has recently yielded a spore assemblage of Downtonian-Dittonian type (Downie & Lister 1969). The subsidiary correlation chart (Fig. 6) does not include graptolite zones as correlations with the standard zonal classification are unknown, except that the beds below the Igneous Conglomerate and its equivalents are approximately on the upper Llandovery/Wenlock boundary.

5. Ireland

The Silurian rocks of Ireland (Fig. 7) are considerably less well known than those of Britain. Harper reviewed the state of knowledge in 1948. Since then various workers referred to below have contributed to a relatively detailed understanding of the Lower Palaeozoic rocks of Mayo and Galway in the north-west of Ireland. Within the southern tract of Silurian inliers, Cope (1959) has described the Silurian rocks of the Devilsbit Mountain district and Weir those of Slieve Bernagh and the Cratloe Hills. Holland (1969 b) has given a preliminary account of the Silurian of the western (Clogher Head) inlier of the Dingle Peninsula, and in this and a complementary paper (Holland 1969 a) has attempted to set the Irish Silurian into ix8

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(A) MAYo AND o ,w Y (A) Glare Island and Louisburgh. The stratigraphy in this column is taken from Dr. W. E. A. Phillips' Ph.D. Thesis on the geology of Clare Island (Trinity College, Dublin, 1966) and we are grateful for his personal comments on the applicability of this succession also to the Louisburgh district on the nearby mainland. Correlation of the Louisburgh sequence depends upon lithology, thickness, and the general setting of the area in relation to both Clare Island and the adjacent Croagh Patrick district to the east. The Knocknaveen Pebbly Arkose Formation is probably of Ludlow age but it is possible that it extends into the Downtonian. It does not resemble the Dingle Group of County Kerry. Phillips and Rickards have an account and detailed map of the Louisburgh district in preparation and intend to use the nomenclature shown in the column. A radiometric (potassium-argon) date of 412 4- 8 m.y. for the base of the Knock- naveen Pebbly Arkose Formation on Clare Island has been obtained from potash feldspar in a welded tuff (J. A. Miller personal communication). (B) Croagh Patrick. Anderson (196o) described the Wenlock strata of south Mayo and referred here to shelly faunal evidence for the underlying upper Llandovery and for the lower part of the Wenlock itself. Dewey (I 963) has revised the stratigraphy and his succession is shown in the column. (C) Galway. Separated from the Silurian outcrops of Louisburgh and Croagh Patrick by a tract of Ordovician, the north Galway occurrences lie to the south of KiUary Harbour and eastwards towards Lough Mask. They have been the subject of various recent investigations (McKerrow and Campbell 196o, etc.). The revised stratigraphy shown is that of Piper (i 967) and M. G. Laird (personal communication). The former has dealt with the Llandovery sequence and the latter with the Wenlock. Dr. Laird has kindly provided their agreed stratigraphy for both series. The upper Llandovery is well dated by brachiopods. The Kilbride Formation yields a C5 fauna in its lower part (Annelid Grit of earlier nomenclature) and a C6 fauna in its upper part (Finny School Beds of earlier nomenclature). The overlying Tonalee Formation provides no diagnostic faunal evidence but Piper (i 967) notes that such red and purple sediments are widespread in Britain at the level of the crenulata Zone. Rickards and Smyth (1968) have identified graptolites from both murchisoni and riccartonensis Zones, as well as from the 'middle Wenlock' (employed loosely in the sense of rigidus to ellesae Zones). The Lough Muck Formation of

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(B) NORTHERN INLIERS (E) Lisbellaw. The Lisbellaw Conglomerate (abbreviated on chart) and its underlying and overlying mudstones were described by Harper & Hartley (i938) and referred to again by Harper (I 948) in his general review. There are a few shelly fossils present and a substantial graptolite fauna (below, above, and in sediments interbedded with the conglomerate) indicates the gregarius Zone. (F) Pomeroy. The folded Ordovician and Silurian rocks around Pomeroy were described in some detail by Fearnsides, Elles, and Smith (I9O7). The relatively thin Llandovery sequence is entirely in graptolitic facies and plentiful collections were made throughout the 'Little River Group' which comprises the first five divisions shown in the column above the Ashgill. There are limited difficulties in correlation with a modern zonal scheme such as that provided by Toghill (1968b). The top 'Corrycroar Group' yielded no diagnostic graptolites though Fearnsides et al. made comparison with the Gala or Tarannon. The overlying rocks of the district are the so called Fintona Beds of Lower Old Red Sandstone (but post-Downtonian) age (Charlesworth I963, p. I83). The same situation holds at Lisbellaw, and possibly Charlestown, but elsewhere in Ireland the unconformable post-Silurian cover is always of Upper Old Red Sandstone or Lower Carboniferous age.

(C) ~SOUTHERN UPLANDS' The most extensive area of Silurian rocks in Ireland clearly rcpresents a south- casterly continuation of the Southern Uplands of Scotland. The Scottish outcrops are not without their problems (pages 1 i7-I i8), but the equivalent Irish region remains little known in modern terms. Howcver several separated areas are now under active investigation. (G) Lough Acanon. The Ordovician and Silurian rocks of the few square kilometres around Lough Acanon have recently been described by Phillips & Skeving- ton (I968). There is probably a conformable sequence from the Climacograptus wilsoni Zone of the Caradoc to the acuminatus Zone of the Llandovery. An erosional surface is followed by a turbidite sequence whose base is believed to be still within the lower Llandovery. (H) Coalpit Bay. The highly graptolitic sections along the north-east coast of the 'Irish Southern Uplands' were described both stratigraphically and palaeonto- logically in Swanston and Lapworth's important contribution on the Silurian rocks I20

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of County Down (Swanston & Lapworth x876 ). According to Lapworth 'The Coalpit Bay Division of the County Down Silurians has yielded all the Graptolites of the Birkhill shales, with the exception of one special group, viz., that of the Rastrites maximus zone, which lies at the very summit of the Moffat series'. Modern work on this graptolite sequence will undoubtedly prove rewarding. Toghill (I968a) has already described one new species from the lower part of the cyphus Zone at Coalpit Bay. He lists the associated species. (I) Tieveshilly. Swanston & Lapworth (I876) recognised that the graptolitic beds to be seen at Tieveshilly were younger than any others then recognised in north-eastern Ireland. Monograptus riccartonensis was recorded as common. The full fossil list suggests that lower zones may also be present.

D) SOUTHERN INLIERS

(j) Balbriggan. The graptolitic Silurian rocks of Balbriggan are at present under investigation by Rickards and the column shows the state of knowledge to date. Faulting cuts out the persculptus Zone at the base of the succession and there is another fault gap above this zone. Graptolitic evidence then takes the sequence through to the turriculatus Zone, where greywackes appear which persist for the remainder of the Llandovery. The graptolitic mudstones of the Wenlock have been followed into the riccartonensis Zone. Above this are greywackes with mudstones in which the linnarssoni and lundgreni Zones have been identified. (K) Portrane. A small area ofgreywackes, siltstones, and mudstones on the coast at Portrane appears to represent a thrust mass overlying the Ordovician. Although these rocks are structurally less complex than the underlying Ordovician, questions of the competence of the various beds are probably involved. In the absence as yet of graptolitic or other fossil evidence these probably Silurian sediments can be assigned only on the flimsiest lithological basis to the higher part of the Llandovery. A summary of the Lower Palaeozoic succession and references to earlier investigations are provided by Harper (1948). (L) Kildare. Wright (I967) has recently shown that the olive and red mudstones of Dunmurry Hill in the small Kildare inlier are facies equivalents of the Ashgill Chair of Kildare limestones and their associated beds. The succeeding 'Green Grits' which occur at both Dunmurry Hill and Chair Hill are accepted as Silurian in age and show lithological similarities to the beds referred to above at Portrane. Wright notes that 'it would be highly speculative in the light of present evidence to suggest that they were of Llandovery (Valentian) age; they may equally well be Salopian'. (M) Slieve Bloom. The same unsatisfactory situation still obtains in the scattered Silurian inliers of the Slieve Bloom Mountains where, to quote Harper (z948), 'the lithology is of Silurian type'. The beds are described in the old Geological Survey of Ireland memoir (O'Kelly i866). 'Orthoceras' is the only fossil recorded. (N) Knockshigowna Hill. The very small inlier of Knockshigowna Hill is referred to in the Irish Geological Survey memoir (Jukes i862). Baily (I869) in his brief 121

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/2/103/4884364/gsjgs.127.2.0103.pdf by guest on 01 October 2021 L. R. M. Cocks, C. H. Holland, R. B. Rickards & L Strachan survey of graptolites in Ireland notes here that 'Graptolithus priodon is remarkably abundant in close proximity to fossils of an Upper-Llandovery type'. A puzzling feature of this latter shelly fauna, as noted by Harper (I948), was the supposed occurrence of Trinucleus. Dr. Harper has kindly commented to the effect that the beds are likely to be Silurian and that the record of Trinucleus is probably incorrect. He has submitted some fossils from the collection of the Geological Survey of Ireland to Professor Alwyn Williams who regards them as not older than 'Upper Llandovery' and suggested an age of C,----C15. Dr. J. Shirley regarded a Calymene from the collection as 'Ashgill to Llandovery'. These fossils were from the mudstones associated with the conglomerates of the inlier. Dr. Harper's present view is that these particular beds are probably 'Upper Llandovery'. Subsequently Rickards has identified Monograptus riccartonensis from Dr. Harper's specimens, thus indicating the additional presence of lower Wenlock beds. (0) Slieve Aughty. There is nothing to add to Harper's (1948) comment that to the south of the small inliers of dated Ordovician rocks here is a larger area of 'grits' and shales, which have not yielded fossils but which lithologically suggest a Silurian age. The hint at a Wenlock to low Ludlow age in the column is inspired only by the known ages of similar successions in other inliers. (P) Arra Mountains, (Q) Slieve Bernagh, and (R) Cratloe Hills. The inliers of Slieve Bernagh and the Cratloe Hills have been described by Weir (1962) but there are difficulties in reconciling his succession with his fossil records. The columns represent our understanding of the facts. There are upper Llandovery brachiopods and the presence of middle to upper Wenlock strata is confirmed by the presence ofMonograptusflemingii. Rickards & Archer (x969) have recently described a hither- to unknown small inlier of Ordovician and associated Llandovery rocks both isolated by faulting. The Llandovery represents the first record of dark graptolitic Llandovery shales in Ireland south of a Balbriggan-Westport line. The acuminatus, atavus and turriculatus Zones have all been identified and Rastrites maximus, indicating the maximus Subzone, is recorded for the first time in the Irish Republic. The succession in the Arra Mountains is still to be described in modern terms. (S) Slieve Felim, Silvermines, Devilsbit Mountain. Cope (1959) has described the north-eastern part of this relatively large inlier and the column relates to his succession. Wenlock rocks dated by graptolites (Monograptusflemingii, Cyrtograptus lundgreni, etc.) are separated by faulting from a smaller area of Ludlow, also with graptolites. Cope provided records of Wenlock graptolites from elsewhere in the inlier and in a separate contribution (Cope 1954) described some cyrtograptids and retiolitids. Current work by Mr. R. J. P. Doran has so far confirmed the presence of middle to upper Wenlock in the southern part of the inlier. (T) Galty Mountains. The Silurian rocks of the Galty Mountains are at present under investigation by Miss A. A. Jackson and finds of Monograptusflexilis and M. flemingii already indicate the presence of middle Wenlock, apparently relatively high in the local succession which is lithologically varied. (U) Slievenaman. Harper's (I 948) brief comment on this inlier alone adds to that of the Irish Geological Survey memoir (Jukes and du Noyer I858), where the

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Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/2/103/4884364/gsjgs.127.2.0103.pdf by guest on 01 October 2021 A correlation of Silurian rocks in the British Isles inlier is referred to as the Ninemilehouse tableland. There is at present no evidence beyond some lithological similarity (e.g. with the succession in the Leinster massif) that these Lower Palaeozoic rocks are Silurian rather than older. New work in the area has now been initiated. (V) Comeragh Mountains. A similar situation holds in the again ill-exposed supposedly Silurian tract to the east of the Comeragh Mountains though we do now have one specimen of an Orthograptus from here, which at least indicates the presence of beds older than upper Llandovery. Capewell (I957) described the Old Red Sandstone of the Comeragh Mountains and separated off as the so- called Croughaun Beds, several hundred metres of green quartzitic sandstones and conglomerates with tufts, agglomerates, and purple siltstones. Unlike the local Old Red Sandstone these beds are 'intensely folded'. They are said to differ from the normal Lower Palaeozoic sediments of the area. It is possible as implied by Charlesworth (i963, p. 19 o) that they are in fact of late Ludlow or Downtonian age. (W) Leinster Massif. The Lower Palaeozoic rocks to the west of the Leinster Granite are shown on the Irish Geological Survey maps as being of Ordovician age. Mr. P. Briick (personal communication) has investigated the central tract in considerable detail and believes the lithological succession to be more appropriate to an Ordovician followed by Silurian sequence. His stratigraphical units, not previously published, are given in the column. He suggests that the same situation obtains to the north and south of his area of investigation. We indicate this on the sketch map (Fig. 7). Preliminary palynological investigation by Mr. R. J. P. Doran has proved as yet inconclusive as to Ordovician or Silurian age.

E) DINGLE PENINSULA

The two areas of fossiliferous Silurian rocks in the Dingle Peninsula are faunally (and probably palaeogeographically) distinct from the other Irish Silurian inliers (Holland i969a , I969b ). Both were described in the Geological Survey memoir by Jukes & du Noyer (1863). (X) Anascaul. Shackleton (i94o), in the course of a general structural discussion of the Dingle Peninsula, refers to minor evidence of Silurian vulcanicity in the south-eastern (Anascaul) inlier in comparison with the Clogher Head occurrences to the west. He mentions monograptids from Caherconree, the mountain at the north-eastern end of the Silurian strip. There are certainly shelly faunas, some at least of which appear to resemble those of the Croaghmarhin Formation of the West Dingle area. Relationships with the overlying Dingle Group require elucidation. Mr. T. R. Walsgrove has established the presence in part of the area of a graptolite fauna of incipiens Zone age. (Y) West Dingle (Clogher Head). The fossiliferous Silurian rocks of the western end of the Dingle Peninsula with their substantial evidence of vulcanicity were described in detail by Gardiner and Reynolds (19o2). Shackleton (194o) argued I23

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on structural grounds that the 'Dingle Beds' were Downtonian in age, resting unconformably upon the fossiliferous Silurian. Holland (I969b) has published a summary of the succession introducing the terminology shown in the column. A full account is in preparation. Contrary to the views of Gardiner and Reynolds there is no evidence of Llandovery faunas. A richly fossiliferous Wenlock shelly sequence passes into the Ludlow, where the higher (Leintwardinian) fauna is very similar to that of the Welsh Borderland. These highest fossiliferous beds pass up conformably into the overlying Dingle Group, which must therefore be partly of Ludlow age. The great thickness of the purple sediments suggests that they continue into the Downtonian, and possibly beyond this into the Dittonian.

6. Concealed Silurian rocks of Eastern and Southern England

During the first part of the Silurian Period, there were two geographically distinctive sea areas: one over what is now Wales and the Welsh Borderland and the other over south-eastern England. There were palaeogeographical changes towards the end of Ludlow times and the two areas may subsequently have become merged during the Downtonian. The now concealed strata laid down under the western sea of earlier Silurian times are essentially a simple extension of the well-known surface outcrop. Since the Llandovery was a time of transgression to east and south over the Welsh Border- land, the only borehole showing it is that of Lower Lemington, near Moreton-in- Marsh, which carries a C5 Eocoelia Community (see map at end of Ziegler, Cocks & McKerrow i968 ). There may be a continuous sub-surface Wenlock outcrop between Wenlock Edge and the inliers at Dudley and Walsall under the Upper Palaeozoic, but the area east of the Malverns is unknown. At Lower Lemington, for example, Coal Measures lie directly upon the Llandovery. There is no good evidence that the Wenlock was ever widespread in the Cotswold area, and the same is true for the Ludlow. South of the Malverns, however, a considerable thickness of Ludlow rocks of shelly facies occurs in the Brookend Borehole, just to the south of the Severn Estuary (Cave & White i968 ). In the south of England and the east Midlands, Silurian deposits are unknown (for example the Trias is unconformable upon the Pre-Cambrian in Charnwood Forest), and it seems reasonable to postulate these areas as land during most, if not all, of Silurian time. In the centre of southern England, in the Hampshire Basin area, Mesozoic and Upper Palaeozoic rocks are too thick to permit observation as to whether the Silurian is present or not. However, under the south-east of England a fair number of boreholes, chiefly put down for coal or water, have encountered the Silurian. Since their palaeontology was in the main scantily described in the original papers, they are reviewed here from the original material, most of which is in the Museum of the Institute of Geological Sciences in London: (i) Brabourne, Kent (Lamplugh et al. I923). The only identifiable macrofossils

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are an indeterminable trilobite fragment, a bryozoan colony, and some broken fragments of rhynchonellid ribbing; but chitinoza indicate a probable upper Llandovery age. (ii) Chilham, Kent (Lamplugh et al. I923). crispus Zone graptolites were identified by Miss Elles. (iii) Cliffe, Kent (Bullard et al. I94o ). The material includes a small Eoplecto- donta, a crinoid fragment, and an atrypoid; and is most probably of Wen- lock age. (iv) Lowestoft, Suffolk (Bullard et al. I94o ). There are no macrofossils, but a spore assemblage indicates an upper Silurian age. (v) Shalford, Surrey (Falcon & Kent i96o ). The macrofossils include Strick- landia lens progressa and Eocoelia hemisphaerica indicating a C1 age. (vi) Stutton, Suffolk (Bullard et al. I94O). The macrofossils consist only of an orthocone and a fragment of what may be a eurypterid; but the microfossils indicate an age of lower Wenlock, or perhaps upper Llandovery. (vii) Ware, Hertfordshire (Whitaker & Jukes Brown i894 ). The macrofossils in- elude large dalmanitids and Meristina obtusa, indicating an upper Wenlock age. (viii) Weeley, Essex. There are no macrofossils, but the microfossils indicate a Llandovery age. We are indebted to Dr. T. R. Lister for advance notice of the microfossil information for the Brabourne, Lowestoft, Stutton, and Weeley, boreholes. In addition, there were boreholes at Culford (Suffolk), Harwich (Essex), The Isle of Grain (Polly Adam's Corner), and Sheerness (Kent), which penetrated rocks which may be of Silurian age but which did not yield fossils. The borehole at Bobbing, Kent (Lamplugh et al. i923) , recorded in the literature as yielding Silurian, is in fact in upper Ordovician rocks (Lister, Cocks & Rushton Kent i97o ) . To summarise for the south-east of England: during the Llandovery there were both shelly (Shalford, Brabourne) and graptolitic (Chilham) deposits. During the Wenlock the same held true, with Cliffe and Ware in the shelly facies and Stutton perhaps further off-shore. Later the situation is not clear; there are no proved Ludlow or Downtonian (but see below) shelly or graptolitic rocks in the south- east, and the rocks of the Lowestoft borehole, which could represent non-marine conditions, are of uncertain age. The Downtonian, as a deltaic deposit (Allen & Tarlo i963) , covers the Welsh Borderland as far east as Birmingham, and south to the Severn Estuary, and concealed strata of this age are proved in many local boreholes. In the north of Eng- land and in the east Midlands, the Downtonian is absent. In south-east England, various boreholes end in 'Old Red Sandstone' but the presence of Downtonian is represented only by the Streatham Common Borehole (Ball & Dineley i96i , p. 217), which yielded fish of a high Downtonian or low Dittonian horizon in an Old Red Sandstone-like facies. Of great interest, however, is the Little Missenden borehole in Buckinghamshire (Straw x933) , which included fully marine Down- tonian beds with a varied shelly fauna. The regional significance of this is indicated on page I26.

x25

The most recent summary of the enigmatic Silurian rocks of Cornwall is that by House (in House and Selwood 1966 ). In the area of Sheet 353 (Mevagissey) of the Institute of Geological Sciences, fossils in limestone lenticles along the Veryan section of the Crush Zone have been referred to the Silurian. Hendriks (1937) reviewed early records and added others. Various nautiloids were considered to be Bohemian forms and to indicate a Ludlow age. In particular Hendriks quoted a report from Dr. J. Koliha to the effect that two named species of 'Cardiola' in association with Scyphocrinus 'prove that your black Orthoceras limestone belongs to the Ludlow and corresponds to Bohemian Stage Eft'. Reid (19o7) also discussed Cornish records of supposed Silurian fossils, including the Scyphocrinites determined by Bather (19o7) from Catasuent Cove. Green (19o4) and Green and Sherborn (I 9o6) respectively have also listed supposed Ludlow and Wenlock faunas. Crick's determinations of Ludlow nautiloids from Porthluney were later doubted by Spath (Hendriks 1937, p. 333)- Thus the idea of the presence of a Bohemian facies of higher Silurian rocks in Cornwall has become established in the literature. The association of 'Cardiola' with graptolites and nautiloids (and with nautiloids alone in areas where the graptolites have disappeared) is in fact widespread in the Silurian of the world (Berry and Boucot 1967; Holland 1959, P. 453). However, the association of these forms with Scyphocrinites is certainly reminiscent of the Central European Silurian and 'banks' of this crinoid are particularly characteristic of the top of the P~idoli 2). At the time of the various contributions referred to above, this in turn implied an equivalence with the British Ludlow succession. Now the P~idoli, though still Silurian, is to be regarded as approximately the same age as the (restricted) Down- tonian of the British Isles. This, of course, alters our whole view of the spatial and temporal situation of these Cornish rocks. There is now a hint of their falling into place as a marine equivalent of the classical Downtonian, fitting reasonably into a southward sequence from the quasi-marine Downtonian of the Welsh Borderland to the more truly marine Long Camp Beds of the Llandovery district (Potter and Price 1965) and the Little Missenden Borehole (page 125), and thence to the facies displayed in the P~idoli of Central Europe. Holland (I969b) has already made this tentative suggestion and a personal communication from Professor Scott Simpson suggests that he has been thinking of it as a reasonable hypothesis that sedimentation in this Cornish area was more or less continuous from Ordovician to late Devonian times. Professor M. R. House, who also has been kind enough to write to us about this problem, suggests that discoveries of conodonts may eventually provide an answer. He has already noted the identification of Lower Devonian conodonts from some localities supposedly yielding Silurian fossils (House I965). Clearly more work is required but this is one of these exceedingly difficult areas in which conclusive results may well not be forthcoming. In the area of Sheet 359 of the Institute of Geological Sciences, Stubblefield ,26

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/2/103/4884364/gsjgs.127.2.0103.pdf by guest on 01 October 2021 A correlation of Silurian rocks in the British Isles (1939) has already eliminated any present 'palaeontological justification for claiming that Silurian rocks exist in the Meneage Crush Zone'.

8. Correlation with the Silurian successions of Bohemia, Thuringia, Podolia, Estonia, and Gotland

North American use of the British series terms Llandovery, Wenlock, and Ludlow, followed by the Czechoslovakian P~idoli (or some variant upon it), is becoming increasingly common. In Central Europe the classical Silurian stratigraphy of Barrande has naturally remained in use. Perhaps the greatest benefit of work during the last decade on the resolution of the Silurian/Devonian boundary problem (Bou~zek, Horn~ & Chlup/t~ 1966; Holland 1965; McLaren 1969) has been the bringing into phase of these two previously incorrectly correlated stratigraphies. Our chart (Fig. 9) shows the Bohemian stages in correlation with our standard classification and also includes columns for the well known and important Silurian areas of Thuringia, Podolia, Estonia, and Gotland. Bohemia. Described in numerous publications including the classical palaeontological monographs of Barrande, the Silurian rocks of the so-called Barrandian area of Czechoslovakia, running south-westwards from Prague, are rich in fossils, display lateral and vertical facies changes of great interest, but in detail appear to be structurally complex. A general account is available in Hornet (1962). Though there are differences in some zonal indices the Llandovery and Wenlock successions can be closely correlated on the basis of their graptolites. This is particularly the case now that Bohemian graptolites are becoming increasingly recognised in the British Isles. The base of the Budnax~ium has traditionally been taken at the base of the vulgaris [-- ludensis] Zone and Holland et al. (I969) have commented upon this difficulty. As Martinsson (I 969) has pointed out it precludes the use of a mixed scale as by Bou~ek et al. (i 966) of Llandoverian-Wenlockian- Budnafiium. Horn~ lists a Gothograptus nassa Zone immediately beneath the ludensis Zone and this is the position of the 'nassa/dubius Interregnum' of Jaeger (I 959) which is now widely recognised from Southern Poland through Thuringia to Wales and the Welsh Borderland. The equation of the base of the PHdoli with the base of the Ludlow Bone Bed (i.e. the base of the Downtonian) is becoming increasingly accepted. The key to the correlation as explained by Martinsson (1967) and Shaw (1969) is the link between the ostracode sequence and the graptolites as seen for example in the Leba Number i borehole in Pomerania. However, trilobite evidence is also relevant and further investigations of such groups as the condonts and spores may increase the refinement of the correlation. The equation shown is evidently acceptable to such specialists as Jaeger (i 962, 1965) and Bou~ek et al. (I 966), though the latter authors have implied the difficulties therein. Martinsson's correlation table (1967, p. 372) shows eflI graptolites in the Leba Number I borehole at a level below that of the top of the Ludlow Bone Bed. Certainly, as Holland (in thepress) has commented, x27

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the correlation of the base of the Ludlow Bone Bed with that of the Bohemian PHdoll 'should not be allowed (as yet) to assume the significance of a dictum'. The correlation of the base of the Bohemian Lochkovian with the British sequence depends upon the acceptance or otherwise of Allen and Tarlo's (I963) redefinition of the Downtonian/Dittonian boundary. The correlation table of Bou~ek et al. (i 966) shows the alternative effects of using this redefined boundary or the previous higher level for the base of the Dittonian. The correlation of the P~tdoll with Allen and Tarlo's concept of the Downtonian involves a web of information which must take into account, for instance, the Podolian sections, where it is possible to make some connection between vertebrate and marine in- vertebrate evidence (Boucot and Pankiwskyj 1962 ). Thuringa. Some of the Lower Palaeozoic black shales of Thuringia, thin and exceedingly rich in graptolites, are ideal examples of their kind. The Silurian rocks and graptolite sequence have been meticulously studied by Jaeger (1959, 1964) to whom all Silurian workers are in debt for his elucidation of the highest Silurian to Devonian graptolite sequence. Llandovery and Wenlock successions may be readily correlated with those in the British Isles and Jaeger's original recognition of the important 'nassa/dubius Interegnum' has already been referred to above. Podolia. The Dnestr River and its tributaries in Podolia provide a unique display of Silurian rocks with the almost undisturbed sequence exposed in kilometre after kilometre of splendid river cliffs. Basing their work upon previously published palaeontological information from Koztowski, Nikiforova and other workers, Boucot & Pankiwskyj (1962) produced an account in English (and also a map) of Llandovery to Gedinnian stratigraphy without (at that time) actually having visited the area. In recent years Nikiforova and her associates from the All Union Geological Scientific Research Institute in Leningrad have pursued a com- prehensive research programme here. A short summary of the stratigraphy is given in Nikiforova, Predtechenskij and Abushik (i967). In the Summer of 1969 the sections were demonstrated to a group of Silurian and Devonian specialists from all over the world at the Third International Symposium on the Silurian/Devo- nian boundary and Lower and Middle Devonian Stratigraphy (Holland I969c). A detailed guide book (Nikiforova and Predtechesnskij 1968 ) was prepared for the occasion. There are still problems of correlation here which can be solved only by careful comparison of fossil material from Podolia with that from other parts of the world. Thus material collected from the Kitaigorod Horizon suggests that the beds are Wenlock rather than Llandovery in age. There is also still some difficulty in the reconciliation of various previously published accounts. The column represents an attempt at such reconciliation. The position of the Silurian/Devonian boundary is now clear because of important discoveries of Monograptus uniforms in Podolia (Koren 1968 ). Stratigraphically critical conodonts are also present above and below this boundary (Drygant 1967, Mashkova 1968 ) allowing correlation with Walliser's (I 964) conodont zonal scheme. :Estonia. Correlation between Britain and Estonia has improved greatly with the

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graptolite finds by Kaljo (i968), particularly from the Llandovery and Wenlock. The sequence seems fairly complete, although whether the lowest Llandovery persculptus and acuminatus Zones are present is as yet doubtful. The lower part of the succession has been described by Nestor & Kala (i 968) who found that the Tamsal Stage (previously placed between the Juuru and Raikkiila Stages) was a lateral facies equivalent of part of the Juuru Stage. The boundary between the Adavere and Jaani Stages is not yet correlated in detail, but turriculatus and griestoniensis Zone graptolites are now known from the former, and murchisoni and riccartonensis Zone graptolites from the latter. At the top of the succession, the Kaugatuma and Ohesaare horizons are probably of Downtonian age (Kaljo & Viira I968 ). The lower part of the Kaugatuma has been separated off as the Kuressare Stage (see Mark-Kurik (1969)). Gotland. In the last ten years there has been much stratigraphical revision on Gotland, particularly from the ostracode studies by Martinsson, which he has recently summarised (Martinsson I967). The lowest formation seen in surface outcrop is the Lower Visby Marl, which is of high Telychian (C6) age, bearing Palaeocyclus and Costistricklandia. Lower horizons within the Silurian are known from boreholes, but whether all the Llandovery is present under Gotland is as yet unknown. Hede (I96O) provides a general summary of the stratigraphy.

ACKNOWLEDGEMENTS.We have tried, so far as is possible, to acknowledge personal communications at appropriate places in the text. Many other individuals have, by ready discussion, helped to clarify our ideas towards an attempt at Silurian correlation in the British Isles. We thank Mrs. Barbara Prendergast of Trinity College, Dublin, for her substantial assistance during preparation of the paper.

9. References ALLEN, J. R. L. 1963. Depositional Features of Dittonian Rocks: Pembrokeshire compared with the Welsh Borderland. Geol. Mag. xoo, 385-4oo. & TARLO, L. B. 1963. The Downtonian and Dittonian Facies of the Welsh Borderland. Geol. Mag. xoo, 129-55. ALLENDER, R., HOLLAND, C. H., LAWSON, J. D., WALMSLEY, V. G. • WHITAKER, J. H. McD. I96O. Summer Field Meeting at Ludlow. Proc. Geol. Ass., Lond. 7I, 2o9-32. ANDERSON,J. G. C. 1960. The Wenlock Strata of South Mayo. Geol. Mag. 97, 265-75. AUSTIN, J. E. I925. Notes on the Highest Silurian Rocks of the Long Mountain. Proc. Geol. Ass., Lond. 36, 38i-2. BAILEY, R. J. 1964. A Ludlovian facies boundary in south Central Wales. Geol. Jr. 4, I-2O. BAILY, W.H. 1869. Notes on Graptolites and allied Fossils occurring in Ireland. Quart. J. geol. Soc. Lond. 25, x58-62. BALL, H. W. 195 I. The Silurian and Devonian rocks of Turner's Hill and Gornal, South Stafford- shire. Proc. Geol. Ass., Lond. 62, 225-36. & DINELEY, D. L. I952. Notes on the Old Red Sandstone of the Clee Hills. Proc. Geol. Ass., Lond. 63, 2o7-I4. & ~ I96I. The Old Red Sandstone of Brown Clee Hill and the adjacent area. I. Stratig- raphy. Bull. Brit. Mus. (Nat. Hist.) Geol. 5, t75-242. BARROW, G., GIBSON, W., CANTRILL, T. C., DIXON, E. E. L., & CtmmNOTON, C. H. x9I 9. The Geology of the country around Lichfield, including the northern parts of the South Stafford- shire and Warwickshire Coalfields. Mere. geol. Surv. Engld and Wales (Sheet I54), I-3O2.

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BASSETT, D. A. x955. The Silurian rocks of the Talerddig, district Montgomeryshire. Quart. J. geol. Soc. Lond. xxx, 239-64. BASSETT, M. G. x969. The age of the oldest Silurian beds of the Rumney (Cardiff) inlier. Geol. Mag. xo6, 90-2. BATES, D. E. B. x968. The Lower Palaeozoic brachiopod and trilobite faunas of Anglesey. Bull. Brit. Mus. (nat. Hist.) Geol. x6, x25-99. BATHER, F. A. x9o7 . The discovery in West Cornwall of a Silurian crinoid characteristic of Bohemia. Trans. R. geol. Soc. Cornwall x3, I92 BERRY, W. B. N. & BOUCOT, A. J. I967. Pelecypod-Graptolite Association in the Old World Silurian. Geol. Soc. Am. Bull. 78, x5x5-22. BOU~EK, B., HORN?, R. & CHLUPA~, I. X966. Silurian versus Devonian. Acta Mus. nat. Prag. 2~B, 49-66. BoucoT, A. J. & PA_~KrWSKYJ, K. I962. Llandoverian to Gedinnian stratigraphy of Podolia and adjacent Moldavia. Symposiums-band der 2. internationalen Arbeitstagung iiber die Silur/Devon- Grenze und die stratigraphie yon Silur und Devon. Bonn-Bruxelles .,96o. Stuttgart, x-I x. BuLta~, E. C., GASKELL, T. F., HARr.A_~, W. B. & KEm~GRANT, C. I94o. Seismic investigations of the Palaeozoic floor of east England. Phil. Trans. Ser. A. 239, 29-94. BtrRGEss, I. C., RICK.mU~s, R. B. & STRACH~, I. (X970). The Silurian strata of the Cross Fell area. Bull. geol. Surv. G.B. 32, x67-82. BUTI.a~R, A. J. i937. On Silurian and Cambrian rocks encountered in a deep boring at Walsall, South Staffordshire. Geol. Mag. 74, 241-57. I939. The stratigraphy of the Wenlock Limestone of Dudley. Quart. J. geol. Soc. Lond. 95, 37-74. ~RILL, T. C., DIXON, E. E. L., THOMAS, H. H. & JON~S, O. T. x916. The geology of the South Wales Coalfield. Part XII. The country around Milford. Mem. geol. Surv. Engld and Wales (Sheet 227), x-x85. CUU'~.WELL, J. G. i957. The stratigraphy, structure and sedimentation of the Old Red Sandstone of the Comeragh Mountains and adjacent areas, County Waterford. Ireland. Quart. J. geol. Soe. Lond. xx2, 393-4x2. CAVE, R. & WHITE, D. E. I968. in Summ. progr, geol. Surv. G.B. [for I967], 75-6. ~L~SWORTH, H. A. K. x96o The Lower Palaeozoic Inlier of the Curlew Mountains Anticline. Proc. R. Irish Acad. See. B. 6x, 37-5 o. C~L~SWORTH, J. K. x963 . Historical Geology of Ireland. Oliver and Boyd, Edinburgh and London, x-565 . COCKS, L. R. M. x968. Some strophomenacean brachiopods from the British lower Silurian. Bull. Brit. Mus. (nat. Hist.) Geol. x5, 283-324. & Rxcx~,a~.DS, R. B. x969. Five boreholes in Shropshire and the relationships of shelly and graptolitic facies in the Lower Silurian. Quart. J. geol. Soc. Lond. x24, [for x968], 2x3-38. , TOGHILL, P. & ZIEOLER, A. M. x97o. Stage names within the Llandovery Series. Geol. Mag. xo7, 79-87. CoP~, R. N. i954. Cyrtograptids and Retiolitids from County Tipperary. Geol. Mag. 9 x, 3x9-24 . x959. The Silurian rocks of the Devilsbit Mountain District, County Tipperary. Proc. R. Irish. Acad. See. B. 6o, 217-42. CR~aG, G. Y. & WALTON, E. K. Sequence and structure in the Silurian Rocks of Kirkcudbrlght- shire. Geol. Mag. 96, 2o9-2o. Ctr~r~ms, W. A. x957. The Denbigh Grits; Wenlock Greywackes in Wales. Geol. Mag. 94~ 433-5x- x959a. The Nantglyn Flags; Mid-Salopian basin facies in Wales. Lpool Manchr geol. J. 2, x59-67. I959b. The Lower Ludlow Grits in Wales. Lpool Manchr geol. J. 2~ x68-79. CURTIS, M. L. K. I955. Lower Palaeozoic in MacInnes, C. M. and Whittard, W. F. (Eds). Bristol and its adjoining counties. Bristol, 3-7- & C~v~, R. x964. The Silurian~Old Red Sandstone unconformity at Buckover, near Tort- worth, Gloucestershire. Proc. Bristol Nat. Soc. 3 o, 427-42. I3o

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]:)AS GUt'TA, T. I932. The Salopian graptolite shales of the Long Mountain and similar rocks of Wenlock Edge. Proc. geol. Ass. Lond. 't3, 325-63 • DAV1DSON, T. & MAW, G. 188 I. Notes on the Physical Character and Thickness of the Upper Silurian Rocks of Shropshire, with the Brachiopoda they contain grouped in Geological Horizons. Geol. Mag. 28, I oo- 9. DEWEY, J. F. x963 . The Lower Palaeozoic stratigraphy of central Murrisk, County Mayo, Ireland, and the evolution of the south Mayo trough. Quart. J. geol. Soc. Lond. xxg, 313-44 . DIXON, E. E. L. 192 I. The geology of the south Wales Coalfield. Part XIII. The country around Pembroke and Tenby. Mem. geol. Surv. Engld and Wales (Sheets 244 and 245), 1-22o. DOWNIE, C. & LISTER, T. R. I969. The Sandy's Creek Beds (Devonian) of Farland Head, Ayrshire. Scot. Jl. geol. 5, 193--206- DRYGANT, D. M. I967. Some data on the Conodont zones and the age of the passage beds from Silurian to Devonian in Podolia. Palaeontol. Sb. No. 4, Part 2, 56-9 . [In Russian; English summary.] EAm~, J. R. 1938. The higher Silurian rocks of the Kerry district, Montgomeryshire. Quart. J. geol. Soc. Lond. 94, 125-60. -- I94O. The geology of the south-western part of Clun Forest. Quart. J. geol Soe. Lond. 96, x-x x. I944 Observation on Upper Silurian graptolites. Geol. Mag. 8x, I8X-5. ELLES, G. L. 19oo. The Zonal Classification of the Wenlock Shales of the Welsh Borderland. Q uart. J. geol. Soc. Lond. 56, 37o-4I 4. I9o 9. The Relation of the Ordovician and Silurian Rocks of Conway (North Wales). Quart. J. geol. Soc. Lond. 65, 169-94. & SLATFR, I. L. I9O6. The Highest Silurian Rocks of the Ludlow District. Quart. J. geol. Soc. Lond. lie, I95-222. & WOOD, E. M. R. I9oi-I8. Monograph of British Graptolites. Palaeontogr. Soe. [Mongr.] London, 1-539. EVANS, J. W. & STUBBLFFIEIa), C.J. i929. Handbook of the Geology of Great Britain. London, 1-556. FALCON, N. L. & KV.NT, P. E. I96O. Geological results of petroleum exploration in Britain I945- I957. Mem. geol. Soe. Lond. 2, 1-56. FFARNSm~S, W. G., ELLIS, G. L. & SMITH, B. I9O 7. The Lower Palaeozoic rocks ofPomeroy. Proc. R. Irish Acad. See. B. 26, 97-I28. Fum~ss, R. R., LL~.WELLYN, P. G., NORMAN, T. N. & RICKAm)S, R. B. I967. A review of Wenlock and Ludlow Stratigraphy and Sedimentation in N.W. England. Geol. Mag. xo4, 132-47. GAIn)IN-R, C. I. & R~'ZNOLDS, S. H. 19o2. The fossiliferous Silurian beds and the associated igneous rocks of the Clogher Head district (Co. Kerry). Quart. J. geol. Soe. Lond. 58, 226--66. CTARWOOD, E. J. & GOODYEAR, E. I918. On the geology of the Old Radnor District, with special reference to an algal development in the Woolhope Limestone. Quart. J. geol. Soc. Lond. 7'1, I-3O. GEOROE, T. N., et al. I967. Report of the Stratigraphical Code Sub-Committee. Proc. geol. Soc. Lond. xli3$, 75-87. GREIG, D. C., WRIGHT, J. E., HAINS, B. A., MITCHELL, G. H., et al. x968. Geology of the Country around Church Stretton, Craven Arms, Wenlock Edge and Brown Clee. Mem. geol. Surv. Engld and Wales (Sheet 166). 1-379. Gm~N, U. 19o4. On the Discovery of Silurian Fossils of Ludlow age in Cornwall. Geol. Mag. 5 x, 289-90. & SHEFa30~, C. D. 19o6. Lists of Wenlockian Fossils from Porthluney, Cornwall; Ludlowian Fossils from Porthalla; and Taunusian Fossils from Polyne Quarry, near Looe, Cornwall. Geol. Mag. 53, 33-5. GROOM, T. 191 o. The geology of the Malvern and Abberley Hills, and the Ledbury District. Geol. Assoc. Jubilee Vol., 698-738. I-Ina~ER, J. C. 1948. The Ordovician and Silurian rocks of Ireland. Proc. Liverpool Geol. Soc. 2o, 48-67 . & HARTLEV,J. J 1938. The Silurian Inlier of Lisbellaw, Co. Fermanagh, with a note on the age of the Fintona Beds. Proc. R. Irish Acid. See. B. 45, 73-87 •

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HEDE, J. E. i96o. The Silurian of Gotland. Int. Geol. Congr., XXI Sess., Norden, I96o. Swedish geological guide-books, Excursion CI 7, Stockholm, 44-87. HE~RmS, E. M. L. I937. Rock Succession and Structure in South Cornwall: A Revision. With notes on the Central European Facies and Variscan Folding there present. Quart. J. geol. Soc. Lond. 93, 322-67. Ho~ LA~D, C. H. x959. The Ludlovian and Downtonian rocks of the Knighton district, Radnor- shire. Quart. J. geol. Soc. Lond. xx4, [for x958], 449-82. I962. The Ludlovian-Downtonian succession in Central Wales and the Central Welsh Borderland. Symposiums-band der 2. internationalen Arbeitstagung iiber die Silur/Devon----Grenze und die Stratigraphie von Silur und Devon. Bonn-BruxeUes I96o. Stuttgart, 87--94. x965. The Siluro-Devonian Boundary. Geol. Mag. xo2, 213-2 I. I969a. The Welsh Silurian Geosyncline in its regional context in Wood, A. (Ed.) The Pre- Cambrian and Lower Palaeozoic Rocks of Wales. Cardiff, 2o3-x 7. x969b. The Irish counterpart of the Silurian of Newfoundland. Gander Symposium t967. Am. Ass. Petrol. Geol. Mem. x2, 298-3o8. -- I969c. Third International Symposium on the Silurian-Devonian Boundary and the Stratig- raphy of the Lower and Middle Devonian. Leningrad, USSR, x8 July-- 5 August, x968. Geological Newsletter, x969, No. I, 2o- 4. (in press). Problems of classification and correlation in the Wenlockian, Ludlovian, and post- Ludlovian pre-Gedinnian stratigraphy of the British Isles. Symposium volume of Third Inter- national Symposium on the Silurian-Devonian Boundary, and the Stratigraphy of the Lower and Middle .Devonian, Leningrad, USSR, x968. & LAwsoN, J. D. x963. Facies patterns in the Ludlovian of Wales and the Welsh Borderland. Lpool Manchr geol. J. 3, 269-88. , LAwsoN, J. D. & WALMSLEV,V. G. x959. A revised classification of the Ludlovian succession at Ludlow. Nature, Lond. x84, xo37- 9. --, & ~ I962. Ludlovian Classification--A reply. Geol. Mag. 99, 393-8. -- & ~ 1963. The Silurian rocks of the Ludlow district, Shropshire. Bull. Brit. Mus. (Nat. Hist.) Geol. 8, 93-I7I. , RI~s, R. B. & WARREN, P. T. 1969. The Wenlock graptolites of the Ludlow district, Shropshire, and their stratigraphical significance. Palaeontology, Lond. x2, 663-83. HORN'2, R. J. I962. Das Mittelb6hmische Silur. Geologic, Berl., xx, (8), 873-916. HousE, M. R. i965. in discussion of paper: The unstratified sedimentary rocks of the Meneage aroa, Cornwall. J. L. M. Lambert. Proc. geol. Soc. Lond. x6x 9, x7-2o. & SELWOOD, E. B. I966. Palaeozoic Palaeontology in Devon and Cornwall, in Hosking, K. F. G. and Shrimpton, G. J. (Eds.) Present Views on some aspects of the Geology of CornwaU and .Devon. Blackford, Truro, 45-86. JAEGER, H. I959. Graptolithen und Stratigraphie des jiingsten Thtiringer Silurs. Abh. deutsch. Akad. Wiss. Bed., K1. Chem. Geol. Biol. x959, (2), I-I97. t962. Das Silur (Gotlandium) in Thiiringen und am Ostrand des Rheinlschen Schieferge- birges (Kellerwald, Marburg, Giessen). Symposiums-band der :,. Internationalen Arbeitstagung iiber die Silur]Devon-Greene und die Stratigraphie yon Silur und Devon. Bonn-Bruxelles t96o, Stuttgart, Io8-x 35. I964. Der gegenw~irtige Stand der stratigraphischen Erforschung des Thiiringer Silurs. Abh. dtsch. Akad. Wiss. Berl. KI. Bergb. Huttenw. Montangeol. x964 (a), 27-5 x. 1965 . Symposiums-Band der 2. internationalen Arbeitstagung iiber die Silur/Devon-Grenze und die Stratigraphie yon Silur und Devon, Bonn-Bruxelles I96o. Geologie. x4, 348-64 • JONES, O. T. x9o 9. The Hartfell-Valentian succession in the district around Plynlimon and Pont Erwyd (North Cardiganshire). Quart. J. geol. Soc. Lond. 65, 463-537 • I925. The Geology of the Llandovery district: Part I. The Southern Area. Quart. J. geol. Soc. Lond. 81, 344-88. ~. I947. The geology of the Silurian rocks west and south of the Cameddau Range, Radnor- shire. Quart. J. geol. Soc. Lond. xo3, 1-36. Jtrt:~s, J. B. I862. Explanation of sheet x26, etc. Mere. geol. Surv. Ireland, x-4o.

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Jtr~s, J. B. & Du NOYER, G. V. I858. Data and descriptions to accompany Quarter Sheet 46 N.W. Mem. geol. Surv. Ireland, x-24. & Du NOYER, G. V. x863. Explanation of sheets I6O, I6I, I7x, etc. Mem. geol. Surv. Ireland, 1-57. I~LjO, D. L. I968. Some problems in the correlation of the Silurian in the East Baltic area and other regions of Europe. Dokl. Sov. Geol. [for 23rd Int. geol. Congr. Section 9], I28--x33. [In Russian; English summary.] & VH~, V. I968. Note on the age of the Ohesaare Stage of Estonia. Eesti NSV Tead. Akad. Toim. xT, 43o-I. KINo, W. B. R. & WILCOCKSON,W. H. 1934- The Lower Palaeozoic rocks of Austwick and Itorton- in-Ribblesdale, Yorkshire. Quart. J. geol. Soc. Lond. 9o, 7-3 x. KrNo, W. W. & LEwis, W. J. 1912. The Uppermost Silurian and Old Red Sandstone of South Staffordshire. Geol. Mag. xg, 437-43, 484-9 I, KinK, N. H. 1951 a. The Upper Llandovery and Lower Wenlock rocks of the area between Dolyhir and Presteigne, Radnorshire. Proc. geol. Soc. Lond. x47x , 56-8. x95xb. The Silurian and Downtonian rocks of the anticlinal disturbance of Breconshire and Radnorshire: Pont Faen to Presteigne. Proc. geol. Soc. Lond. x474, 72-4. KOREN, T. N. x968. Early Devonian monograptids found for the first time in the Borshchov Hori- zon, Podolia. Dokl. Acad. Sci. U.S.S.R. x82, 938-4 o. [English translation, American Geological Institute, Washington.] L~adPLUGH, G. W., KITCHen, F. L. & PRrNOLE, J. i923. The concealed Mesozoic rocks in Kent. Mem. geol. Surv. Engld and Wales, I-~49. L~a'WORTH, C. x876. The Silurian system in the south of Scotland in J. Armstrong et al. (Eels), Catalogue of the Western Scottish fossils. Glasgow, 1-28. x88o. On the geological distribution of the Rhabdophora. Ann. Mag. nat. Hist. (5) 5, 45 -6~- I882. The Girvan Succession. Quart. J. geol. Soc. Lond. 38, 537-666. LAWSON, J. D. x954. The Silurian Succession at Gorsley (Herefordshire). Geol. Mag. 9 x, ~27-37. x955. The Geology of the May Hill Inlier. Quart. J. geol. Soc. Lond. xxx, 84-I x6. & WHrrA~a~R, J. H. McD. i969 . Correlation of the Leintwardine Beds. Geol. J. 6, 329-32. LISTER, T. R., CocKs, L. R. M. & Rusm'oN, A. W. A. x97o. The basement beds in the Bobbing Borehole, Kent. Geol. Mag. x~, 6o I-3. ~-KuRxK, ELGA. 1969. Distribution of vertebrates in the Silurian of Estonia. Lethaia. ~,, 145-5 ~. MASnXOVA, T. V. x968. Conodonts of the genus Icriodus Branson et Mehl., x938, from the Borschov and Chortkov Horizons, Podolia. Dokl. Acad. Sci. U.S.S.R. x8z, 94I-4 . [English translation, American Geological Institute, Washington.] M~au~, J. E. & NICHOLSON,H. A. I888. The Stockdale Shales. Quart. J. geol. Soe. Lond. 44, 654-732. ~TmSSON, A. x967 . The succession and correlation ofostracode faunas in the Silurian of Gotland. Geol. Frren. Stockh. Frrh. 89, 35o-86. I969. The series of the redefined Silurian System. Lethaia, ~,, I53-6I. McI~Patow, W. S. & C.~PB~LL, C. J. x96o. The Stratigraphy and Structure of the Lower Palaeozoic rocks of North-West Galway. Sd. Proc. R. Dublin Soc. Ser. A. x, No. 3, 27-5 I. Mel_au~N, D. J. I969 . Report from the Committee on the Silurian-Devonian Boundary and Stratigraphy to the President of the Commission on Stratigraphy. Prague, August 9, x968" Geological Newsletter, xg~, No. I, 24-34. MITCX-mLL, G. H., POCOCK, R. W., TAYLOR, J. H., et al. I96I. Geology of the Country around Droitwich, Abberley and Kidderminster. Mem. geol. Surv. Engld and Wales (Sheet x82), I-I37. MURCHISON, R. I. 1835. On the Silurian System of rocks. London and Edin. Phil. Mag. 7, 46-52. 1859. Siluria (3rd edition). London. N~STOR, H. & KArat, E. 1968. A stratigraphic revision of the lowermost Silurian in the Northern East-Baltic area in Stratigraphy of the Baltic Lower Palaeozoic and its correlation with other areas. Vilnius, Latvia. I-318. NIKIFOItOVA, O. I. & PREDTECH~NSKU, N. N. 1968. A guide to the geological excursion on Silurian and Lower Devonian deposits of Podolia (Middle Dnestr River). Third International Symposium on Silurian[Devonian boundary and Lower and Middle Devonian stratigraphy. Leningrad.

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----, PREDTECHENSKIJ, N. N. & ABUSHIK, A. F. x967. The importance of the Silurian section of Podolia for the definition of the Siluro-Devonian boundary in Europe. International Symposium on the Devonian System, Calgary, Alberta, I967. Alberta Soc. Petrol. Geol., 867-73. O'KELLV, J. x866. Explanation of sheets I x7 and I i8. Mem. geol. Surv. Ireland, I-3I. PALUeR, D. C. I97O. A stratigraphical synopsis of the Long Mountain, Montgomeryshire. Proc. geol. Soc. Lond. x66o, 34I-6. PATTERSON, E. M. I949. The Old Red Sandstone Rocks of the West Kilbride--Largs District, Ayrshire. Trans. geol. Soc. Glasg. 2x, 2o7-36. PEACH, B. N. & HOm'~E, J. 1899. The Silurian Rocks of Britain. Vol. I. Scotland..MIem. Geol. Surv. U.K., 1-749. PmLLIPS, J. x848. The Malvern Hills, compared with the Palaeozoic Districts of Abberley, Wool- hope, May Hill, Tortworth, and Usk. Mem. geol. Surv. U.K., 2 (I), 1-33 o. PHILLIPS, W. E. A. & SKEVINGTON, D. I968. The Lower Palaeozoic rocks of the Lough Acanon area, Co. Cavan, Ireland. Sci. Proc. R. Dublin Soc. Ser. A. 3, No. 14, x4I-8. PHIPPS, C. B. 1962. The Revised Ludlovian Stratigraphy of the Type Area--A Discussion. Geol. Mag. 99, 385-39 ~. & RF.EVF., F. A. E. I967. Stratigraphy and geological history of the Malvern, Abberley and Ledbury Hills. Geol. J. 5, 339-68. PIPER, D. J. W. 1967. A New interpretation of the Llandovery Sequence of North Connemara, Eire. Geol. Mag. xo4, 253-67. PoCOCK, R. W., WHITEHEAD, T. H., WEDD, C. B., ROBERTSON, T., et al. x938. Shrewsbury District Including the Hanwood Coalfield. Mem. geol. Surv. Engld and Wales (Sheet I52), 1-297. POTTER, J. F. & PRICe, J. H. 1965. Comparative sections through rocks of Ludlovian-Downtonian age in the Llandovery and Llandeilo districts. Proc. Geol. Ass. Lond. 76, 379-4o2. REIn, C. x9o 7. The Geology of the Country around Mevagissey. Mem. geol. Surv. Engld and Wales (Sheet 353), 1-73. RICHARDSON, J. B. & LISTER, T. R. I969. Upper Silurian and Lower Devonian spore assemblages from the Welsh Borderland and South Wales. Palaeontology, Lond. x2, 2oI-52. RICKARDS, R. B. I967. The Wenlock and Ludlow succession in the HowgiU Fells (north-west Yorkshire and Westmorland). Quart. J. geol. Soc. Lond. x23, 215-5 I.

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-- & ARCHER, J. B. 1969 . The Lower Palaeozoic rocks near Tomgraney, Co. Clare. S¢i. Proc. R. Dublin Soc. Set. A. 3, No. 21,219-3o. -- & SMYa'H, W. R. 1968. The Silurian Graptolites of Mayo and Galway. Sci. Proc. R. Dublin Soc. Set. A. 3, No. 12, x29-34. ROLFe, W. D. I. I96r. The Geology of the Hagshaw Hills Silurian Inlier, Lanarkshire. Trans. Edinb. geol. Soc. x8, 24o-69. RusT, B. R. 1965 . The stratigraphy and structure of the Whithorn area of Wigtownshire, Scotland. Scot. Jl geol. x, Ioi-33. SHACKLETON, R. M. I94o. The succession of rocks in the Dingle Peninsula, Co. Kerry. Proc. R. Irish Acad. See. B. ,16, I-I2. SHAW, R. W. L. 1969. Beyrichiacean ostracodes from the Downtonian of Shropshire. Geol. Fiiren. Stockh. F6rh., 9 x, 52-72. SHERGOLD, J. H. 1967. A revision of Acastella spinosa (Salter I864) with notes on related trilobites. Palaeontology, Lond. xo, 175-88. -- & SmRr~-G J. 1968. The faunal-stratigraphy of the Ludlovian rocks between Craven Arms and Bourton, near Much Wenlock, Shropshire. Geol. J. 6, I 19-38. SMELLS, K. A. G. & DEARMAN, W. R. 1966. On the possible occurrence of Dalradian rocks in the Southern Uplands of Scotland. Scot. Jl geol. % 231-42. SOLLAS, W.J. i879. On the Silurian District of Rhymney and Pen-y-lan, Cardiff. Quart. 3". geol. Soe. Lond. 35, 475-5o7 • SO~G~NFR~I, T. (Editor). I964. Rept. Int. Geol. Congr. XXI Sess., Norden, I96o. XXVIII, 254 and 277.

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Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/127/2/103/4884364/gsjgs.127.2.0103.pdf by guest on 01 October 2021 L. R. M. Cocks, C. H. Holland, R. B. Rickards & L Strachan

Xt~'HITTAR.D, W. F. I932. The stratigraphy of the Valentian rocks of Shropshire. The Longmynd- Shelve and Breidden Outcrops. Quart. J. geol. Soc. Lond. 88, 859-9o2. x96I. Lexique Stratiqraphique International. Vol. 1. Europe. Fasc. 3 a, Angleterre, Pays de GaUes, l~cosse: V Silurien, 1-273. & SMITH, S. I944. Unrecorded inliers of Silurian rocks, near Wickwar, Gloucestershlre, with notes on the occurrence of a stromatolite. Geol. Mag. 8I, 65-76. WILLIAMS, A. I952. The geology of the Llandeilo district, Carmarthenshire. Quart. J. geol. Soc. Lond. xoS, x77-2o 7. WILLS, L.J. x947. Midland Group Field Meeting Report, Ellowes Park and Turner's Hill. Pro¢. Geol. Ass. Lond. 58, 7 I-2. •- & SMITH, B. I922. The Lower Palaeozoic Rocks of the LlangoUen Disrtict, with special reference to the Tectonics. Quart. J. geol. Soc. Lond. 78, 176-226. WooD, A. & SmTH, A. J. 1959. The sedimentation and sedimentary history of the Aberystwyth Grits (Upper Llandoverian). Quart. J. geol. Soc. Lond. Xl 4, I63-95. WOOD, E. M. R. 19o6. The Tarannon Series of Tarannon. Quart. J. geol. Soc. Lond. 62, 644-7 ° x. WOODS, E. G. & CROSVlFLD, M. C. x925 . The Silurian Rocks of the Central Part of the Clwydlan Range. Quart. J. geol. Soc. Lond. 8x, 17o-94 . WRIGHT, A. D. x967. A note on the stratigraphy of the Kildare Inlier. Irish Nat. J. x5, No. I2, 34o-43 • ZIZGLZR, A. M., COCKS, L. R. M. & MCI~RROW, W. S. I968. The Llandovery transgression ofthe Welsh Borderland. Palaeontology, Lond. xx, 736-82. , MCKExROW, W. S., BURNF, R. V. & BAKER, P. E. x969. Correlation and Environmental Setting of the Skomer Volcanic Group, Pembrokeshire. Proc. Geol. Ass., Lond. ~, 4o9-439 . Manuscript received I8th February i97o; read x8th February t97o.

L. R. M. Cocks, M.A., D.PHIL, F.O.$. Department of Palaeontology, British Museum (Natural History), Cromwell Road, London, SW 7.

Prof. C. H. HOLLAND, M.A., B.SC., PH.D., F.G.S., Department of Geology, Trinity College, Dublin, Ireland.

R. B. RICKARDS, PH.D., F.O.S. Sedgwick Museum, University of Cambridge.

I. STRACHAN, PH.D., F.G.S., Department of Geology, The University, Birmingham, 15.

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