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The Rosroe Formation comprises a series of Lower Ordovician (Llanvirn)conglomerates and sandstones, that lies on the southern limb of the South Mayo Trough, withinthe Iapetus Suture Zone of western Ireland. Trace element chemistry of granite boulders within theformation indicates a continental, rather than a volcanic arc character that can be correlated tolatest Precambrian granites within the Dalradian Metamorphic Block, part of the deformedLaurentian margin. A minority of the clasts may correlate with syn-collisional granites, similar to,but older than, the Oughterard Granite of Connemara. Pb isotope compositions of K-feldspargrains within the sandstones, measured by both ion microprobe and conventional massspectrometry, show a clear Laurentian a† nity, albeit with greater source variability in the sandgrains compared to a limited range in the proximal boulders. Palaeo-current indicatorsdemonstrate dominant derivation from the NE, with a signi® cant axial E± W ¯ ow. We proposethat the Rosroe Formation records unroo® ng of a rapidly exhuming Dalradian metamorphic beltin North Mayo, following extensional collapse of the Grampian Orogen starting at»468Ma,withminor input from a southerly arc source. The lack of metamorphic input from the S untildeposition of the Derryeeny Conglomerate argues that the Connemara terrane was not positionedS of South Mayo Trough through strike-slip faulting until after the end of Rosroe sedimentation(460± 443Ma).KEY WORDS: Caledonides, Connemara, Dalradian, geochemistry, isotopesThe early Llanvirn Rosroe Formation is a prominent, coarse-grained sequence of sandstones and conglomerates exposedon the southern limb of the South Mayo Trough of westernIreland (Fig. 1). The South Mayo Trough is unusual in beinglocated in the suture zone, but without having su• ered strongdeformation or metamorphism. Although it has been variouslyinterpreted in the past, a consensus now agrees that the troughrepresents the forarc to an intra-oceanic Lough Nafooey Arc(Dewey & Shackleton 1984). Deposited within the suturezone, the Rosroe Formation is ideally located to constrainthe timing and erosional response to docking of the apparentlyoceanic Lough Nafooey island arc and its forearc basin (SouthMayo Trough) against the passive margin of Laurentia. TheEarly Ordovician arc-continent collision event is consideredto be the cause of subsequent Grampian Orogeny (Dewey S Dewey & Ryan 1990; van Staal

Acknowledgments We are grateful to Dalradian Gold Ltd. for providing the sections for petrographic analysis, geochemical data, and general support. We would also like to thank the following: John Still, Alison Sandison, and Jenny Johnston of the School of Geosciences, University of Aberdeen, for assistance with the SEM studies (JS) and with preparing figures (AS and JJ); NERC for ongoing funding of the Argon Isotope facility at SUERC; Jim Imlach and Ross Dymock at SUERC for technical assistance; and Martin Lee at the School of Geographical and Earth Sciences at the University of Glasgow for use of the SEM/CL equipment. The paper has benefitted significantly from comments by the official reviewers and unofficial reviews by Garth Earls, Jamie Wilkinson, Mark Cooper, and Adrian Boyce, and detailed conversations with Ian Alsop (structural geology of the Sperrins) and Nyree Hill and Gawen Jenkin (gold mineralization in the Caledonides). The authors are entirely responsible for the conclusions expressed.

Quantitative description and prediction of vein hosted deposits using fractal statistics have been tested using data collected from the Curraghinalt gold deposit, Sperrin Mountains, County Tyrone, N. Ireland. Goldbearing quartz veins are hosted by Neo-Proterozoic Dalradian (Southern Highland Group) age psammites, pelitic schists and graphitic schists. Veins trend E to ESE and cross-cut regional structures. Vein thickness (z) distributions closely approximate a power-law distribution of the form N(⩾ z) = cz -D ,with an exponent (D) of 0.76 over three orders of magnitude. Vein spacing data approximate a log normal distribution with a mean of 2.92m. Numerical analysis indicates that the veins are more clustered than expected from a random distribution. Regions of high percentage extension and high vein density define vein clusters. The vein clustering relationship may be combined with the power-law thickness distribution and used in a predictive manner. The data may be used to constrain the probability of intersecting veins of potentially mineable dimension.

SIRS—Anderton's (1985) review of Dalradian sedimentation and tectonics must represent a milestone in the history of Dalradian research, in that, for the first time the ourcrop pattern of Dalradian units and their sedimentological features have been fitted into a regional model which, in my opinion, forms a basis for speculation. Since Anderton draws from my work at Loch Creran (Litherland 1980) as one of the key areas where Dalradian units can be mapped across the strike for considerable distances, I raise the following points with respect to his reinterpretation of that area and the Dalradian as a whole.

French1 and Eugster and Skippen2 have demonstrated the considerable variation in proportions of gas species which may be produced in the system C-O-H when oxygen fugacity is buffered by both graphite and by differing iron redox assemblages. Some Dalradian schists of the sillimanite zone in NE Scotland show mineral associations which may be interpreted in terms of such variation. The rocks which outcrop over an area about 1 km square at the Buck of the Cabrach, Banffshire, contain the essential assemblage musco-vite-biotite-quartz-plagioclase-cordierite-andalusite (sillimanite). The presence of two aluminium silicate polymorphs indicates disequilibrium and has been interpreted as a record of temperature migration across the andalusite/sillimanite univariant boundary during recrystallisation3,4.

WE have dated samples of schistose granite from Shetland using Rb–Sr methods. This provides further evidence for the early phase of Caledonian high grade metamorphism and migmatisation (the Grampian event) which has been detected all the way from western Ireland to northern Norway.

THE occurrence of the blue amphiboles glaucophane and crossite in metamorphosed basic rocks is characteristic of terrains metamorphosed at high pressures. Most examples are from relatively recent fold belts. We record here a Caledonian blueschist from the Dalradian Supergroup in South Achill Island, County Mayo (Fig. 1). It is the first ‘Old’ blueschist from Britain to occur in a regularly bedded and folded sequence and may imply an early high pressure metamorphic event in this part of the Dalradian, evidence for which was generally destroyed by either erosion or later metamorphism.

For reference to original paper, see this Bibliography Vol. 34, No. 11, 17 E70-32334 (see author9s reply below)

The material on which this paper is based was collected by Dr. W. J. McCallien, formerly of the Geological Department of Glasgow University, and has been made available to the writer by Dr. G. W. Tyrrell, under whose direction the present investigation has been carried out.1 The counties of Donegal and Tyrone in northern Ireland are built up for the most part of rocks belonging to a metamorphic series which represents an extension of the metamorphic series of the south-west and central Grampians of Scotland. They consist of crystalline schists, quartzites, limestones and dolomites of Dalradian age. Intrusions into this series comprise: ( a ) an Epidiorite group (designated on the geological maps ‘hornblendic igneous rocks’) forming part of the Dalradian; ( b ) a Spilite-Keratophyre group of Ordovician age; ( c ) a Lamprophyre-Porphyrite group (the felstones and felstone porphyries of the geological maps) of Caledonian age; ( d ) a Camptonite-Monchiquite group of Permian age; and ( e ) a group of Crinanites, Olivine-basalts and Tholeiites (referred to on the geological maps as ‘basalts and dolerites of Tertiary age’). Much of the region, especially in the west, is occupied by granite which is later than the epidiorites but older than the other dykes. Previous research on this region is very scanty. The present paper deals only with the Caledonian and Tertiary dykes; the former have a NE-SW trend while the latter strike NW-SE or NNW-SSE veering at times to E-W. They occur mostly as minor stringers of only a few feet in width (2 to 12 feet), with This 250-word extract was created in the absence of an abstract

SEMI-CONCORDANT quartz veins occur in garnet grade amphibolite facies pelites, semipelites and siliceous dolomites between Spean Bridge and Roybridge, 15 miles east of Fort William, Invernesshire. Fluid inclusion studies by one of us (F. W. S.) on samples from one such vein provide pressure and temperature information compatible with the results of geothermometric work on carbonates combined with thermodynamic calculations on the carbonate bearing assemblages (R. P.).

POTASSIUM–ARGON age determinations have been made on specimens of granites and gabbros representative of the ‘younger’ Caledonian plutonic intrusions of north-east Scotland. Unlike the ‘older’ igneous rocks these ‘younger’ intrusions post-date the earlier fold movements and acme of regional metamorphism in the Dalradian schists1. The exact relation of the gabbro intrusions to the established sequence of movements and metamorphisms in the Dalradian rocks has received considerable attention2–6. It is evident that their emplacement post-dated the large recumbent (F1) fold structures of north-east Scotland (Banff Nappe), the second phase (F2) structures and the static (post-F2) climax of regional metamorphism; also the evidence strongly favours deformation of the gabbros by the Boyndie–Buchan group of folds (F3) and is suggestive of their intrusion as a single large sheet2.

gently to the southwest (the "mappable folds"), movements. It is also likely that there is not a which are responsible for the generally steep sharp separation in time and that some measplunge of the minor cross-folds, ure of contemporaneity of 1 and 2 and of 2 and 3 Although we should describe only 1 and 3 as is to be supposed. Thus, in summary, we may make comparisons with the conclusions of BaiTABLE 1 ley and of Weiss and McIntyre (Table 1). King and Weiss and Finally, let us briefly clarify our employment Rast Bailey McIntyre of the terms "Caledonoid" and "cross-fold." IniF. Caledonoid, Recumbent tially these were used merely as convenient desmajor refolds ignations of approximate directions. Certainly cumbent folds they were not intended to imply age of moveF2 Cross-folds, B, recumbent; ment. Despite adverse comment on the suitamainly NW plunge bility of these terms by Weiss and McIntyre

Brick-red sandstones and conglomerates of Upper Old Red Sandstone age unconformably overlie chocolate-brown to purple-brown sandstones and conglomerates of Lower Old Red Sandstone age on Ardmore Peninsula, Dunbartonshire. The unconformity is well shown on the western side of the peninsula in the tidal area of the River Clyde (Nat. Grid: NS 788313) and a conglomerate from each formation has been analysed (Table 1). The specimens crushed for analysis were collected as close as possible to the plane of unconformity by Mr. Edward Scorgie during a study of the peninsula ( Trans. geol. Soc. Glasg., 22, 295). Fragments greater than one inch across were separated and not included in the crushed samples. The Lower Old Red Sandstone conglomerate is made up essentially of fragments of quartzite set in a siliceous matrix with a red iron oxide cement. The fragments in the Upper Old Red Sandstone conglomerate are predominantly low-grade Dalradian schists containing variable amounts of quartz, chlorite and muscovite. The matrix is dominantly siliceous and is cemented by calcite and red iron oxide. These petrographic characters are reflected in the chemical analyses: the relatively greater amount of SiO2 in the Lower Old Red Sandstone conglomerate (Table 1—A) is the result of the abundance of quartzite fragments while the relatively greater proportions of TiO2, Al2O3, FeO, MgO and K2O in the Upper Old Red Sandstone conglomerate (Table 1—B) result from the presence of schist fragments. The CaO and CO2 proportions reflect the carbonate fraction of the matrix, which is more abundant in the This 250-word extract was created in the absence of an abstract

In September 1910, while on holiday in Strathdon, I discovered an outcrop of a peculiar rock which I think worthy of a short description. It was found in contact with the beautiful foliated olivine gabbro at Pooldhulie Bridge on the Don, which has been described and figured by Dr Teall in the Geological Survey's Memoir on Sheet 75, p. 22. The outcrop extends from immediately under the bridge for about 15 yards westwards, and being actually in the bed of the river, would probably be inaccessible in ordinary states of the water. At the date of my visit the river was very low, and one could easily step from the foliated gabbro on to the exposed ledge. It really rests against the gabbro, and appears to dip away from it southwards, at an angle of about 45°. As regards its macroscopic characters, the rock on a fresh fracture is of a somewhat mottled, greenish-grey colour, of flinty character, breaking with a hackly fracture, and shows evident colour banding in darker and lighter colours. Along and parallel to the banding are to be seen numerous porphyritic crystals, many of them more or less rounded. These are easily diagnosed as felspars. On the whole, the rock presents to the naked eye the characters of a rhyolite, and this was the first opinion entertained as to its nature. Its specific gravity was found to be 2·84. As regards its microscopic characters, two sections were cut, one as far as possible parallel to the

IT can no longer be stated categorically that metamorphism of the Dalradian Series in Connemara occurred before the Didymograptus extensus zone of the Arenig Series, because those sediments generally accepted as the earliest post-tectonic sediments in the area have provided a Didymograptus bifidus zone (Lower Llanvirn) graptolite fauna. This means that the only stratigraphical younger age limit which can be placed on the Dalradian metamorphism in Connemara is some time plane at or near the base of the bifidus zone. This upward extension of the younger, minimum stratigraphical limit of the Dalradian metamorphism necessarily extends considerably that interval in Late Cambrian–Early Ordovician times during which an important phase of orogeny could have taken place. Furthermore, the statement by Leggo et al.1 that “the base of the Arenig would seem to be younger than 510 ± 35 m.y.”, on the evidence of the rubidium/strontium age given by the post-metamorphic and (hitherto) pre-Arenig Oughterard granite, should be amended to refer to the base of the bifidus zone.

RECENT developments in structural mapping in the Dalradian rocks of Scotland have led to the re-interpretation of regional metamorphism and recognition of its phase-by-phase development1–5. In this respect, textural studies have proved particularly useful in so far as relict textures are preserved as inclusions within porphyroblasts, thus permitting a recognition of fabrics6, which in the rock as a whole, owing to subsequent recrystallization, have disappeared. In the middle-grade zones of regional metamorphism of Dalradian rocks, garnet porphyroblasts are particularly abundant, and their inclusions can be grouped into the pre-porphyroblast, the syn-porphyroblast and the post-porphyroblast.

The Lower Fine-grained Quartzite (Middle Dalradian) of Islay, Scotland is subdivided into two facies. Facies 1 consists of massive-bedded, cross-stratified and rippled orthoquartzites. The basal portion is massive, planar bedded, with asymmetrical wavy surfaces on the tops of each bed. The middle and upper portions of Facies I are characterized by sharp-based and sharp-topped trough and planar sets of cross-stratification whose orientation is bimodal-hipolar. Upper sets of cross-stratification are rounded; some sets are drapped with mudstone beds. Both straight-crested and ladder ripples are common. Micro-cross-laminated sets are separated by wavy and bifurcated flaser bedding. Facies 2 consists of siltstone and mudstone, with accessory fine-grained orthoquartzite. Ubiquitous mudracks, isolated flat and thick lenticular bedding, tidal bedding and burrowing structures are common to this facies. The two facies are organized into a sharp-based, fining-upward sequence with Faces 1 at the base and Facies 2 at the top. The base of the sequence consists of a sharp contact overlain with massive sandstones (Interval A), which grades into cross-stratified sandstone (Interval B). Next above is a thin set of microcross-laminated orthoquartzite (Interval C) whose orientation is 90° or 180° to the underlying crossstratified set. Some of the cross-bedded and micro-cross-laminated orthoquartzites are draped with thin mudstone (Interval D). The top of the fining-upward sequence consists of Facies 2 mudstones and siltstones (Interval E). The depositional environments of both Facies 1 and 2 are interpreted to be tide-dominated. The sandstones of Facies 1 were deposited in both a tide-dominated shallow, sub-tidal and intertidal low tidal flat or low tidal sand bar environment, whereas Facies 2 siltstones and mudstones were deposited in a high tidal flat environment. The sharp-based, fining-upward sequences are regressive sequences consisting of basal shallow subtidal sandstones (interval A), intertidal sand flat or sand bar sandstones (Intervals B, C, D) and high tidal flat mudstones (Interval E). Comparison of the sedimentary features of the Lower Fine-grained Quartzite with sedimentary features from both Recent tidal areas of the North Sea and of the Minas Basin of the Bay of Fundy, and 45 documented intertidal and tide-dominated clastic sedimentary rocks, confirms the tidal origin of these beds. The depositional environments of many orthoquartzites may be similar to the tidal environments of the lower Fine-grained Quartzite.

STILPNOMELANE has been found as a constituent of the schists of many low-grade areas throughout the world1–3. In the Dalradian it has been described from the South-west Highland epidiorites4–6 but not from the greywackes and pelites of the Upper Dalradian, although it is mentioned as occurring in “the quartzo-feldspathic green schists of the Southern Dalradian” by Voll7.

For reference to paper under discussion, see this Bibliography Vol. 34, No. 11, 17 E70-32334

In the area between Loch Katrine and Loch Venachar in the southern Scottish Highlands, the Ben Ledi grits occur as part of the Dalradian graywackes. In a northwestern belt the grits show strongly folded fracture cleavage, and in a southeastern belt unfolded fracture cleavage. The implications are discussed, and resemblance to a similar relation of structures in the Moine schists of Morar is noted.

REGIONAL deformation associated with the emplacement of Tertiary igneous rocks is well known in the British Isles. There appears to be some evidence in mid-Argyll, recorded on the one-inch maps of the Geological Survey of Scotland1, which suggests that crustal distortion (broadly similar to that described from east Greenland by Wager and Deer2) may have occurred in this area during the Tertiary. This distortion is now reflected by a change in the trend of the Old Red Sandstone minor dyke swarm, the north-north-east–south-south-west tear-faults and, also, by variations in the strike of the Dalradian country-rocks; the district over which this change takes place coincides with the path of the Tertiary dyke swarm centred on Mull.

(1) The characteristic soils and plant communities developed on Dalradian limestone at particular locations in central Perthshire were studied. Representative sites were selected from field data collected in the course of a soil and vegetation survey. (2) For the free-draining soils there is a succession of development corresponding to a progressive increase in leaching. Soil formation is determined by the degree to which the local influence of parent material has countered the regional influence of climate. (3) Soils developed on limestone rock in situ are assigned to the brown rendzina sub-group. Their characteristic properties include an A/C or AR profile sequence, coarse texture and a well-developed crumb structure. Although surface horizons are slightly leached, the soils have high base saturation. Shallow brown calcareous soils, which have an incipient B horizon, comprise another subgroup. Both rendzinas and brown calcareous soils are very rare in central Perthshire. (4) Deeper soils developed on the mixed schist-limestone glacial drift are brown forest soils with free drainage. They display a wide range of base status; high (>50%) and low (

The Ballantrae ophiolite complex was probably an island-arc-marginal–basin assemblage that formed and was thrust onto a pre-Caledonian continent between 490 and 470 m.y. ago; only a brief time separated the birth and death of this marginal basin. Middle Arenig is dated at about 480 m.y. B.P., and therefore the idea of a pre-Arenig Dalradian structural evolution can be extended from Ireland to Scotland. The time of opening and closure of this marginal basin is marked in the Dalradian rocks to the north by mafic igneous intrusions, D 3 deformation, and a change in the nature of the plutonic activity.

Descriptive techniques (lithofacies codes) and vertical profile analysis emphasizing lithofacies contacts and geometry allow objective characterization and comparison of diamict(ite) sequences irrespective of geologic age. Upper Precambrian diamictites (?650−600 m.y.) of the Middle Dalradian Port Askaig Formation in Scotland are traditionally interpreted as products of repeated ice-sheet advances over an emergent shallow marine shelf and deposition by in situ basal melt-out. The Dalradian sequences are, however, fundamentally different from modern and Pleistocene diamicts deposited by either lodgment or basal melt-out and fail to show evidence of tractional processes associated with basal ice movement. Scottish diamictite sequences are similar to assemblages of massive and stratified diamict lithofacies, of late Pleistocene age, deposited subaqueously below floating ice and recently described from the Lake Ontario Basin of North America. A glaciomarine depositional model may have implications for the origin and paleogeographical setting of other upper Precambrian diamictites of similar age and stratigraphic context in East Greenland, Spitsbergen, and Scandinavia.

From detailed radiometric and pressure-temperature estimates of the metamorphic history, uplift, and erosion of the Dalradian rocks of the British Isles, we find that 15–25 km of erosion took place mainly in Ordovician time. None of this vast volume of material is apparently present in the Ordovician sedimentary basins that now flank the Dalradian block. We conclude that the Ordovician basins to the south of the Dalradian block are exotic terranes, and we document their emplacement into their present position during Late Ordovician to Early Silurian time (i.e., Taconic) by using evidence mainly from western Ireland.

Rb-Sr whole-rock isochrons are presented for four late Caledonian granites from northeast Scotland. These isochrons are believed to define a single 460–m.y. B.P. magmatic event, which caused isotopic homogenization of strontium on a whole-rock scale in slate of the Macduff Group, which is part of the Upper Dalradian Series intruded by the granites. The geochronology of the region supports the suggestion that all Caledonian deformation and metamorphism took place during a relatively short episode between the Early Ordovician sedimentation of the Upper Dalradian Series and the Middle to Late Ordovician granitic magmatism. A premetamorphic granite at Portsoy, which intrudes strata of the Middle Dalradian Series, yields a Precambrian isochron age of 669 ± 17 m.y. This is probably a late or postorogenic granite related to a pre-Caledonian metamorphism already recognized within the Moine Series. This Precambrian age cannot be used to support a recent hypothesis in which the Older basic igneous rocks of northeast Scotland are related to a Cambrian(?) subduction zone. All the granitic rocks of northeast Scotland have initial Sr 87 /Sr 86 ratios of 0.714 to 0.717, irrespective of age. This indicates that the granites are derived from a deep, underlying crustal layer, possibly Lewisian (Precambrian) gneiss, and not from the mantle or subducted oceanic lithosphere.

The development of knowledge of the Highland Schists of north-east Scotland is reviewed and is used to present a general synthesis of the distribution and metamorphic condition of the rocks of some 2000 square miles of country. The schistgeology is interpreted as essentially controlled by the great Banff Nappe. The rocks of the Banff Division, probably the equivalents of the Upper Dalradian of Perthshire, form a tectonic sheet on the Lower Dalradian, the separation plane cropping out as the Boyne Line. Beneath the Banff Division, the Lower Dalradian is arranged in a recumbent anticline, closing south-eastwards and sliced in its upper limb by the Boyne dislocation, a lag. The core of the recumbent anticline is made of gneisses, often migmatitic in origin, above which, in the Banff Division itself, is the wide development of andalusite-cordierite-schists typical of this province and interpreted as a regional thermal zone closely connected in origin with the gneisses. The metamorphism and migmatisation are broadly later than the orogeny but it may be suggested that the gigantic movement of the gneiss core was in some way facilitated by the migmatisation of its components

New evidence of red weathering and superficial tectonic fissures enable a Caledonian unconformity surface, known in north Arran as Hutton's unconformity, to be traced almost continuously around the north and west coast of Arran via Loch Ranza, Catacol, Thundergay and Pirnmill to Dougrie. Typically the unconformity surface can be inferred to be a seaward dipping surface, evident near cliff-tops and near low-water mark—the dip due to Tertiary granitic uplift. If there is a major Highland Boundary fault it is located precisely at Dougrie. The Caledonian unconformity can thus be traced round most of the outcrop of the Dalradian and pillow lava suite and differs markedly north and south of the Highland Boundary Zone.

Bordering the gabbro area of the Cabrach on the south, a belt of serpentine extends for nearly five miles (8 km.) from west to east, and is faulted against Dalradian schists at its southern margin. The ultrabasic rocks are extensively sheared, on predominantly vertical surfaces which lie between 075 ° and 095 ° , and are crossed by north-east faults. Structures within the serpentine belt indicate that it was subjected to sinistral wrench movements, after the emplacement of the serpentines. Many intrusive acid vein-rocks occur in the serpentines. The ultrabasic masses are faulted against hypersthene-gabbos which adjoin on the north; transitional types between gabbro and serpentine have not been recorded. The serpentines occupy a fault-zone, which is considered to be a continuation of the dislocation boundary of the Insch Igneous Complex, as seen to the east of the RhynieBogie Old Red Sandstone outlier.

Rb-Sr and K-Ar isotopic measurements from Dalradian metasediments and associated intrusions of the Caledonian belt of Scotland indicate a magmatic and metamorphic history extending over 130 m.y. Magmatic events took place approximately 530, 440, and 400 m.y. ago. The mid-Cambrian activity included the formation of minor granitic bodies (previously referred to as the Older Granites), as well as nonfoliated granites and pegmatites. Microcline granites and pegmatites were emplaced 440 m.y. ago during Ordovician times. The last phase of Caledonian magmatic activity took the form of forcefully and passively emplaced postorogenic granites 400 m.y. ago. Two regional thermal events at 440 to 460 m.y. and 400 to 420 m.y. ago are recorded. The former event is discussed in terms of either a widespread retrograde metamorphism (M 3 ), or uplift and cooling of a high-grade, static, pre-Arenig (M 2 ) event. The 400 to 420 m.y. event was restricted to the westerly margins of the present Dalradian outcrop and is thought to be associated with a younger (M 4 ) metamorphism. Whole rock analyses of metasediments from widely differing structural and stratigraphic levels show that the majority of sediments had a relatively uniform Sr 87 /Sr 86 ratio of about 0.719 at time of deposition. A whole rock isochron of 670 ± 30 m.y. is interpreted as either an approximate or maximum depositional age for the bulk of the Dalradian sedimentary rocks. Unusually high initial Sr 87 /Sr 86 ratios in most of the granites reflect remobilization of crustal material enriched in radiogenic Sr. A close grouping of initial Sr 87 /Sr 86 ratios around 0.717 from most of the granites, regardless of age, is interpreted as indicating a common source region. High initial Sr 87 /Sr 86 ratios of approximately 0.730 from two granitic masses may reflect remobilization of the Dalradian sedimentary rocks during regional metamorphic activity. Although the 400 m.y. passively and forcefully emplaced granites cannot be distinguished chronologically from one another, distinct differences in their initial Sr 87 /Sr 86 ratios of 0.717 and 0.707, respectively, may indicate distinct petrogenetic differences. Because of the variable distribution of radiogenic Sr between the Dalradian sediments and lower crustal levels, the initial Sr 87 /Sr 86 ratios of the magmatic intrusions can be used as an indicator of their relative depth of formation. A working hypothesis is outlined for the region including a series of magmatic and metamorphic events 530, 440, and 400 m.y. ago.

The detrital mineralogy and pebble content of the Middle Jurassic Great Estuarine Series of the Inner Hebrides are described. Pebbles are rare, but are valuable indicators of provenance, and some are described in detail. They include vein quartz, Moine granulite, ?Cambrian quartzite and chert. The light minerals of the sands, which vary considerably in feldspar content, are described with the aid of twenty-three modal analyses. The description of the heavy minerals is based on sixty-five grain-counts of bromoform separations, including some from beds above and below the Great Estuarine Series and from equivalent beds in Sutherland. Removal of hornblende by source weathering, and of kyanite and sphene by interstratal solution, is demonstrated. The evidence for the provenance of the sediment is discussed, and a palaeogeographical map presented. It is concluded that the Scottish mainland, including Moine, Dalradian, Torridonian, Cambrian and Old Red Sandstone, was a major source; a minor Lewisian source is located in the north-west. The history of the Minch Basin is briefly reviewed; there is no evidence of a ‘North Atlantis’.

WE would like to congratulate the authors on their final diagnosis of these “coral-like” structures from the Dalradian Series. We are sorry to detect a certain severity in their criticism of our treatment of information given at an earlier stage, however. We were misled to believe, quite unambiguously, that the material concerned was of fossil origin, and an Ordovician age seemed to be the inevitable implication. Nowhere in the text of our paper did we take this as proof of Ordovician sedimentation in the Dalradian Series, but only as one aspect of the arguments for it which we considered “reasonably justifiable” (p. 373). The authors themselves are guilty of misrepresentation by omitting the initial “If …” from the sentence they quote from p. 376.

Sedimentation structures in Upper Dalradian rocks belonging to the Banff Division of north-east Scotland suggest that the rocks were deposited in deep water by the action of turbidity currents. These structures, including grading of grits, greywackes and flags, and slumping, are described in detail and compared with the structures of sediments in other series of greywacke-facies. It is suggested that the change of sedimentary facies at the beginning of Upper Dalradian times may have been due to the emergence of a new land-mass on the north-western side of the basin of deposition, providing a rapid supply of coarse detritus. The uplifting of this land-mass might have been associated with an early phase of Caledonian folding and metamorphism.

RECENTLY a hypothesis on the age of the main orogenic deformation of the Dalradian rocks of Britain proposed by Fitch, Miller and Brown1 was criticized on stratigraphic grounds by T. G. Miller2. I find myself in agreement with the essentials (though not the whole) of Miller's argument against a Pre-Cambrian age for the main orogenic deformation of the Dalradian rocks, and feel that some further points should be made on this aspect of the ‘working hypothesis’ of Fitch, Miller and Brown. A fuller account of my views on the geographical and time relationships of the ‘early Caledonian’ and ‘late Caledonian’ orogenies of Britain is being prepared to amplify the conclusions briefly indicated in the introduction of an earlier work3, but it would appear pertinent to raise briefly some of the relevant points now.

IN a recent communication1 an outline is given of the sequence of metamorphic and structural events which affected the Lewisian rocks of the Glenelg inlier. In this outline a period of folding, about 740 million years old, and certainly older than 560 million years, is considered to form part of the Caledonian orogeny. Further can be mentioned the intrusion of the Ben Vuroch granite into Dalradian sediments between 700 and 500 million years ago2.

TWO muscovite-rich red phyllites, one of chemical composition (per cent): SiO2 59.6, TiO2 0.9, Al2O3 18.1, Fe2O3 3.4, FeO 4.6, MnO 0.1, MgO 3.4, CaO 0.2, Na2O 0.9, K2O 4.7, H2O 3.4, P2O5 0.2, CO2 nil, total 99.5, carry a quartz–paragonite–muscovite–chlorite–haematite–rutile–apatite mineral assemblage. They come from the Dunoon phyllite, a low-grade horizon of the Dalradian, ½ and ¾ mile north along the coast from Dunoon, Argyllshire. This is the first recorded appearance of paragonite as a rock-forming mineral from the Scottish metamorphic rocks, and its appearance elsewhere in low-grade rocks without albite is considered likely.