Showing papers in "Scottish Journal of Geology in 1979"
TL;DR: In this article, igneous zircons from the Glen Dessary syenite complex give concordant U-Pb ages averaging 456 ± 5 Ma which is taken to date crystallization.
Abstract: Synopsis Apparently igneous zircons from the Glen Dessary syenite complex give concordant U-Pb ages averaging 456 ± 5 Ma which is taken to date crystallization. The complex was deformed and metamorphosed to amphibolite facies subsequent to emplacement so that this age is a maximum for these orogenic events in the western Moine Series. Other age data indicate that amphibolite facies conditions associated with pegmatite emplacement continued until ca. 430 Ma contrasting with the north-east Highlands where tectonism and metamorphism was completed by 460 Ma ago. The disparity between hornblende and mica K-Ar ages suggests that cooling from 500 to 300°C took about 20 Ma. Higher blocking temperatures are indicated for U-Pb in monazite and sphene than for U-Pb in apatite.
84 citations
TL;DR: In this paper, a series of lateral moraines and other ice-marginal features in the Wester Ross area of the NW Highlands are described and interpreted as marking the limit of a glacial readvance that took place in this area sometime between the Late Devensian maximum of 17 000-18 000 B.P.
Abstract: Synopsis A series of lateral moraines and other ice-marginal features in the Wester Ross area of the NW Highlands is described. These total nearly 30 km in length, and indicate the former existence of a lobe of ice nearly 25 km in width across Lochs Gairloch and Ewe and the surrounding low ground. The ice-marginal features are interpreted as marking the limit of a glacial readvance that took place in this area sometime between the Late Devensian maximum of 17 000–18 000 B.P. and the onset of the Lateglacial Interstadial. The name Wester Ross Readvance is suggested.
74 citations
TL;DR: The limit of the Loch Lomond Advance in an area extending from the Great Glen to the uplands around Loch Laggan is mapped and the evidence for the limit is described in this paper.
Abstract: Synopsis The limit of the Loch Lomond Advance in an area extending from the Great Glen to the uplands around Loch Laggan is mapped and the evidence for the limit is described. In the north-eastern part of the area, despite high ground, only small glaciers were nourished. On part of the northern slope of the Ben Nevis range some corries lacked glaciers. In contrast major glaciers, up to 700 m thick, developed in uplands to the west and south-west and flowed into the area under consideration. Such differences imply pronounced contrasts in the distribution of snowfall and were a major factor in the development of ice-dammed lakes in Glen Roy and adjacent glens.
61 citations
TL;DR: In this article, common Pb isotopic ratios for 11 feldspar separates from Caledonian granite intrusions (400 Ma old) show a linear alignment on a 207 Pb/ 204 Pb vs. 206 Pb / 204 pb diagram, the best-fit regression line suggesting a lower intercept age of 2700 Ma, using conventional growth-curve models.
Abstract: Synopsis Common Pb isotopic ratios for 11 feldspar separates from Caledonian granite intrusions (400 Ma old) show a linear alignment on a 207 Pb/ 204 Pb vs 206 Pb/ 204 Pb diagram, the best-fit regression line suggesting a lower intercept age of 2700 Ma, using conventional growth-curve models. This accords well with previously published Pb ages for the Lewisian basement in north-west Scotland (Chapman and Moorbath, 1977), and strongly suggests a Lewisian Pb component in the Caledonian granites. Furthermore, the feldspars from the Southern Uplands granites are significantly more radiogenic than those from the Highlands and Midland Valley; this relationship is consistent with the presence of an older crustal layer beneath the more northerly regions which does not extend southwards into the Southern Uplands.
56 citations
TL;DR: In this article, it was shown that the andalusite/kyanite isograd runs as far south as Glen Callater before turning north east to pass along Deeside.
Abstract: Synopsis Separation of ‘regional’ from ‘contact’ andalusite in the vicinity of the newer granite intrusions of Aberdeenshire suggests that the andalusite/kyanite isograd runs as far south as Glen Callater before turning north east to pass along Deeside. The nature of the critical conjunction between this ‘solid’ isograd and the ‘dehydration’ kyanite isograd is still obscure but if we assume the Battock intrusion to have wedged the two apart, then both would dip to the north, the kyanite isograd surface therefore being inverted.
28 citations
TL;DR: In this paper, the cross-lamination of aeolian sandstones of Lower Permian age is investigated in the centres of fault-bounded intermontane basins in the southern United Kingdom.
Abstract: Synopsis Aeolian sandstones of Lower Permian age occur in the centres of fault-bounded intermontane basins in the southern Uplands of Scotland. At Locharbriggs, Dumfriesshire, three quarries expose vertical faces up to 20 metres high and 150 metres long in which the aeolian cross-lamination is cut by three different bounding surfaces of different hierarchical order. These surfaces indicate that the aeolian sandstones can be interpreted in terms of dunes climbing over larger bedforms called draas. Analysis of the cross-lamination and grain sizes show that the bedforms responsible for the strata were of transverse type with longitudinal elements.
28 citations
TL;DR: This article showed that the Torridonian rocks of the NW Caledonian foreland were folded and eroded in Precambrian time, and this folding was coeval with emplacement of the Morarian pegmatites, supporting the view that the morarian was a tectonothermal event of regional importance.
Abstract: Synopsis Recent isotopic studies have shown that parts of the Moine sedimentary sequence underwent orogenic metamorphism before 1000 Ma and this raises doubts as to the significance of the c. 730 Ma ‘Morarian’ ages yielded by pegmatites from the Moine tract. From evidence readily available from Geological Survey publications we show that the Torridonian rocks of the NW Caledonian foreland were folded and eroded in Precambrian time. This folding was coeval with emplacement of the Morarian pegmatites, supporting the view that the Morarian was a tectonothermal event of regional importance, and permitting (but not establishing) the stratigraphic equivalence of the Upper Torridonian with the Moine rocks in western Sutherland.
27 citations
TL;DR: The Southern Uplands Fault and the Ettrick Valley Fault define three tectonic domains exhibiting structural contrasts significant in elucidating the Caledonian Tectonic history as discussed by the authors.
Abstract: Synopsis The Southern Uplands Fault and the Ettrick Valley Fault define three tectonic domains exhibiting structural contrasts significant in elucidating the Caledonian tectonic history of the Southern Uplands. The Girvan area constitutes the northern domain, and is bounded to the SE by the Southern Uplands Fault. NW-verging asymmetrical folds and two generations of NW-translating thrusts are the dominant structures. The middle domain comprises the Northern Belt and the Central Belt southwards as far as the Ettrick Valley Fault. This domain is characterized by a series of listric faults, the Main thrust faults, associated with outcrops of Moffat Shales. These Main faults apparently cut NW-facing monoforms ( F 1 ), and appear to be associated with SE-verging F 2 monoforms. The faults display south-eastward translation, and culminations and depressions of plunge which may relate to the emplacement of igneous plutons . The southern domain includes the Southern Belt, and the outcrop of the Hawick Group immediately to the N. F 1 folds prevail in this tract, and consist of NW-facing monoforms alternating with tracts of buckle folds, associated with steep strike faults considered to pre-date the Main thrust faults. The significance of the contrasts rests firstly, in the dominance of listric faults in the middle domain, and secondly, in the confinement of these faults to a geographically restricted zone SE of the Southern Uplands Fault. The F 1 structures are considered to have been formed initially by the buckling of a flysch cover in response to its decollement on an oceanic basement which was subducting north-westwards underneath the Laurentian continental plate. Continuing subduction generated an accretionary prism in D 2 , defined by the successive south-eastward propagation of the listric faults, with complementary obduction of the Girvan foundation and its sedimentary cover. Later structures ( D 3 - D 4 ) are of lesser significance, and reflect a progressive deceleration of crustal shortening after the orogenic climax.
25 citations
TL;DR: In this article, a lithostratigraphy is proposed within which rocks are attributed to four distinct groups corresponding to those recognized in the NE Midland Valley, and sedimentation time-trends suggest that along the northern margin deposition was controlled by faulting which, west of Loch Lomond, probably occurred along small normal faults shifting their location with time.
Abstract: Synopsis Lower Old Red Sandstone rocks in the western Midland Valley form a red-bed sequence of probable alluvial origin. They accumulated in a NE-SW elongated basin linked in its early stages of development with similar depositional conditions in North Argyll. The basin, whilst changing its morphology with time, maintained a regional south-westerly dipping palaeoslope with deposition in a longitudinally draining floodplain environment replaced by laterally draining alluvial fans close to the inferred northern margins of the basin. A lithostratigraphy is proposed within which rocks are attributed to four distinct groups corresponding to those recognized in the NE Midland Valley. Within this lithostrarigraphical framework changes in sedimentation, palaeogeography and tectonic controls can be studied through time. The sedimentation time-trends suggest that along the northern margin deposition was controlled by faulting which, west of Loch Lomond, probably occurred along small normal faults shifting their location with time. Any extension of the Highland Boundary Fault into this western area was ineffective as a major feature either controlling deposition or defining the basin margins.
22 citations
TL;DR: Using underwater television, eleven recurrent bottom facies have been identified and mapped over an area of about 250 km 2, where Bare rock areas are rare: Turritella -rich fine sands with trails, and crustacean-burrowed muds widespread as discussed by the authors.
Abstract: Synopsis Using underwater television, eleven recurrent bottom facies have been identified and mapped over an area of about 250 km 2 . Bare rock areas are rare: Turritella -rich fine sands with trails, and crustacean-burrowed muds widespread. Coarse sands, fining northwards from megaripple fields in the Sound of Islay and the Passage of Oronsay are dominantly of biogenic origin. The kelp Laminaria saccharina was seen rooted in gravelly parts of the megaripple field in the former area (one of strong tidal currents but reduced wave action) where the calcareous alga Lithothamnium is an important contributor. Barnacles and molluscs dominate in the latter region, which is more exposed to Atlantic swell. Extensive spreads of Modiolus seem to rest on a tough clay surface, possibly of eroded late-Glacial material which also probably underlies Tarbert Bank, since it crops out along its eastern margin. The rippled sand body located on the Bank could represent residual out-wash. Facies analysis by combined underwater television and grab/dredge/box coring is optimal for shelf waters of intermediate depth (10–100 m): it reveals particularly well vagile epifauna, normally missed by conventional grabbing. In addition, it permits sediment/faunal samples to be studied in a truer perspective. Differentiation of facies by television scores over bottom sampling in higher energy, coarser sediments; but in lower energy muds and fine sands of the level bottom, where infauna dominate, biofacies analysis by faunal clustering and resin impregnation of internal structures will probably achieve greater refinement
22 citations
TL;DR: In this paper, electron microscopy has revealed both mechanically and chemically produced surface textures on heavy mineral grains from Palaeocene sands of the central North Sea basin, including breakage blocks, V-shaped indentations, grooves, upturned cleavage plates and conchoidal fractures, mostly originated in a pre-Palaeocene phase of sedimentation.
Abstract: Synopsis Scanning electron microscopy has revealed both mechanically and chemically produced surface textures on heavy mineral grains from Palaeocene sands of the central North Sea basin. Chemical activity initially modified irregular surfaces, producing mamillae. Further solution produced regular facets, the dimensions of which increase with intensity of etching. Variations in composition of the connate waters are thought responsible for slight variations in etching style. Mechanically-produced surface features similar to those seen on quartz grains, including breakage blocks, V-shaped indentations, grooves, upturned cleavage plates and conchoidal fractures, mostly originated in a pre-Palaeocene phase of sedimentation. The possibility of retention of pre-existing surface textures on grains during the sedimentation process must therefore be considered when using such textures in palaeoenvironmental interpretation.
TL;DR: In this article, the first in a series of studies of sediment transport into the Tay Estuary, the directly measured and indirectly estimated sediment loads passing two gauging stations are compared.
Abstract: Synopsis In the first in a series of studies of sediment transport into the Tay Estuary, the directly measured and indirectly estimated sediment loads passing two gauging stations are compared. Discharges of 154 000 tonnes per year pass the lowests station, and 98% of this is in suspension. None of the used indirect methods of calculating the load yields the measured discharge figures for either gauging station. Erosion rates of 187–238 t.km −2 p.a. are established for the 782 km 2 drainage system.
TL;DR: A 3.5 km linear suite of parallel boulder belts formed by the Devensian moraine is described in this paper, where they are interpreted as a large medial moraine that formed during the waning of the ice sheet in western Scotland.
Abstract: Synopsis On the western side of Jura, there exists a remarkable 3.5 km linear suite of parallel boulder belts. The feature originates at 450 m O.D. at the western foot of Beinn an Oir, one of the Paps of Jura, and extends seaward to 30 m O.D. where it ends at a shallow lochan that lies at the junction between the feature and a high late-glacial marine shoreline. The boulder complex is composed in places of up to 4 parallel lines of dominantly angular Dalradian quartzite blocks (the local bedrock), each line rarely exceeding 27 m in width and 2.5 m in vertical thickness. The boulders, including several erratics, range from 0.2 to 1.3 m in length and are here interpreted as a large medial moraine that formed during the waning of the Devensian ice sheet in western Scotland.
TL;DR: In this article, the existence of a dense cylindrical body of approximately 14 km diameter centred at about 61°50′N, 00°30′W, north of the Shetland Islands was found.
Abstract: Synopsis Marine gravity and magnetic data show the existence of a dense cylindrical body of approximately 14 km diameter centred at about 61°50′N, 00°30′W, north of the Shetland Islands. The density and remanent magnetization values derived for this body show it to be similar to the Tertiary basic intrusive bodies along the west coast of Scotland.
TL;DR: In this article, Radiocarbon assay of a sample of the Burn of Benholm peat gave an age of >42 000 B.P. This age and the stratigraphical position of the peat within the basal part of the till of the Strathmore glaciation places the PEAT in the Early or Middle Devensian stage.
Abstract: Synopsis Radiocarbon assay of a sample of the Burn of Benholm peat gave an age of >42 000 B.P. This age and the stratigraphical position of the peat within the basal part of the till of the Strathmore glaciation places the peat in the Early or Middle Devensian stage. Pollen spectra show that the peat was formed in a tundra environment but do not give a more detailed dating of the peat.
TL;DR: A limestone boulder from the Benan Conglomerate (low Caradoc Series?) of the Girvan district yielded a few fossils of North American aspect, including Clelandia reliqua sp. nov, which are indicative of early Canadian (Tremadoc) age as discussed by the authors.
Abstract: Synopsis A limestone boulder from the Benan Conglomerate (low Caradoc Series?) of the Girvan district yielded a few fossils of North American aspect, including Clelandia reliqua sp. nov., which are indicative of an early Canadian (Tremadoc) age. It is suggested that the boulder originated from a shallow-water limestone deposited on a land-mass on the SE margin of the Dalradian trough.
TL;DR: In this paper, a shallow borehole drill into the Canna Basin, Sea of the Hebrides, has demonstrated that they are Upper Oligocene (Chattian) non-marine deposits.
Abstract: Synopsis Analysis of the sediments recovered from a shallow borehole drilled into the Canna Basin, Sea of the Hebrides, has demonstrated that they are Upper Oligocene (Chattian) non-marine deposits The basin is the most northerly of the presently known terrestrial Oligocene basins found on the western side of Britain
TL;DR: In this paper, the authors describe the lower Palaeozoic rocks around the Cairnsmore of Fleet pluton as representative of the Moffat Shale Group, ranging from the Glenkiln or Lower Hartfell Formation (Llandeilo or Lower Caradoc) to the Upper Birkhill Formation (Fronian), conformably though diachronously overlain by greywackes.
Abstract: Synopsis The Lower Palaeozoic rocks around the Cairnsmore of Fleet pluton comprise representatives of the Moffat Shale Group, ranging from the Glenkiln or Lower Hartfell Formation (Llandeilo or Lower Caradoc) to the Upper Birkhill Formation (Fronian), conformably though diachronously overlain by greywackes of later Llandovery age. The Main folds ( f 1 ) display two contrasting styles. Two tracts of northwestward dips, mainly steep and locally inverted, are separated by a tract of concentric folds, upright or overturned to the NW, with wavelengths mostly less than 500 m. The first phase concluded with the generation of complementary wrench fault sets, certain of which were subsequently reactivated. An episode of compressional faulting supervened, generating the southeastward-translating and strike-parallel listric faults of the Central Belt, of which four lie within the area. The tracts of steep NW dips were simultaneously modified by SE-verging open folds having axial planes with moderate SE dips ( f 2 ). Later Caledonian movements, producing crossfolds associated with oblique-slip faulting ( f 3 ) and kink bands ( f 4 ), are of minor significance.
TL;DR: In this paper, the authors used the criteria first employed by Kennedy to identify the line of the Great Glen fault, and extended the course of the structure along the Firth of Lorne to reveal the presence of heterogeneous basement and a constricted yet complex sedimentary basin.
Abstract: Synopsis Marine studies in the Firth of Lorne have revealed the presence of heterogeneous basement and a constricted yet complex sedimentary basin. A total of forty three dive, dredge and borehole samples has allowed both the basement, which consists largely of Moinian, Torridonian and Old Red Sandstone, and the basin itself to be mapped. Using the criteria first employed by Kennedy to identify the line of the Great Glen fault, the course of the structure is extended along the Firth. Divergent fault splays with normal throw probably emanate from the principal Great Glen structure and in turn control the size and nature of the basin. The fill includes proven Triassic and Jurassic together with probable Cretaceous and Tertiary and a complex Quaternary sequence. Permo-Triassic syndepositional movements and late Kimmerian adjustments, together with mid-Tertiary activity are thought to have been the main periods of movement affecting the region.
TL;DR: In this paper, it was shown that during the period of glacial retreat that followed the Loch Lomond Advance maximum, a sequence of ice-dammed lakes formed in the Loch Tulla area of East Argyll, Scotland.
Abstract: Synopsis During the period of glacial retreat that followed the Loch Lomond Advance maximum, a sequence of ice-dammed lakes formed in the Loch Tulla area of East Argyll, Scotland. The main evidence for this lake sequence consists of abandoned shorelines at altitudes of 332 m, 323 m, 319 m, 315 m and 248 m above sea level. These shorelines are restricted to the eastern part of the Tulla basin, and their altitudes correspond with these of local outlet cols. It is considered that the lakes formed on the eastern margin of a mass of downwasting glacier ice and did not occupy the entire basin, as has previously been suggested. The lowest lake probably drained englacially through the downwasting ice mass.
TL;DR: Two-dimensional computer modelling of aeromagnetic data (this paper) agrees reasonably well with the three-dimensional gravity model of Bott and Tuson (1973) as mentioned in this paper, and both models indicate that the gabbro pluton is a large, slightly cone-shaped mass, with a width of approximately 12 km at the top and 18km at the base, penetrating to a depth of about 14 km, and that the granites are a shallow, near-surface phenomenon (less than 2.5 km deep), comprising about five to ten per cent of the total mass
Abstract: Synopsis On the Isle of Skye, a series of Tertiary igneous rocks, including a central layered gabbro and associated granites occurs in an area of basement gneiss overlain by sediments of Cambrian to Cretaceous age. Two-dimensional computer modelling of aeromagnetic data (this paper) agrees reasonably well with the three-dimensional gravity model of Bott and Tuson (1973). Both models indicate that the gabbro pluton is a large, slightly cone-shaped mass, with a width of approximately 12 km at the top and 18 km at the base, penetrating to a depth of about 14 km, and that the granites are a shallow, near-surface phenomenon (less than 2.5 km deep), comprising about five to ten per cent of the total mass of the igneous assemblage. The Beinn an Dubhaich granite is approximately 0.8 km wide and 3.2 km long in outcrop and contains numerous patches of limestone. Ground magnetic measurements and the relationship between the granite-limestone contact and the topography indicates that the granite is a stock which extends to depth and underlies the limestone. Further, the granite surface beneath the limestone is not smooth but rather consists of many lobes which intrude the limestone, possibly along pre-existing fractures.
TL;DR: In this article, a critical review is made of the evidence for correlating the Lower Carboniferous marine limestones of the upper Calciferous Sandstone Measures and Lower Limestone Group of the Paisley area with those of North Ayrshire.
Abstract: Synopsis A critical review is made of the evidence for correlating the Lower Carboniferous marine limestones of the upper Calciferous Sandstone Measures and Lower Limestone Group of the Paisley area with those of North Ayrshire. Of the three different schemes of correlation made in the past, one is supported by recently acquired palaeontological information from IGS boreholes. The main conclusion reached is that the Hurlet and Blackhall limestones of the Paisley area are respectively equivalent to the Broadstone and Dockra limestones of Ayrshire so that the base of the Lower Limestone Group should be drawn at the base of the Broadstone Limestone.
TL;DR: The chilled margin is of tholeiite basalt composition, and the chondrite-normalized rare-earth element pattern is almost flat with a slight light rare earth depletion as discussed by the authors.
Abstract: Synopsis The chilled margin is of tholeiite basalt composition, and the chondrite-normalized rare-earth element pattern is almost flat with a slight light rare-earth depletion. The composition has affinities with a number of other rocks in the Tertiary Igneous Province including some of the Preshal Mhor type dykes, Skye, and is further evidence for the possibly important role of high-calcium, low-alkali, tholeiite magma in the generation of major intrusions in the Province.
TL;DR: In this article, Graptolites are found preserved within the fine lamination of the D interval; generally associated with coarse silt or very fine sand; both large unbroken stipes and debris of broken stipes are present; some sorting of the debris appears to have occurred during transport.
Abstract: Synopsis Graptolites transported by weak turbidity currents act as tools producing characteristic tool marks with impressions of thecae on bed bases. Graptolites are found preserved within the fine lamination of the D interval; generally associated with coarse silt or very fine sand. Both large unbroken stipes and debris of broken stipes are present; some sorting of the debris appears to have occurred during transport. Monograptus drepanoformis and Climacograptus nebula are recorded in situ from the quarry for the first time.
TL;DR: The Beinn an Dubhaich granite intruded and deformed Cambro-Ordovician limestones and Tertiary dykes in west-central Strath on Skye.
Abstract: Synopsis The Beinn an Dubhaich granite intrudes and deforms Cambro-Ordovician limestones and Tertiary dykes in west-central Strath on Skye. The limestones were intensely deformed by radial compression from a centre at the west end of the present granite outcrop, presumably due to the forceful intrusion of an early phase of the granite. Basaltic dykes associated with the Tertiary igneous centres of Skye were deformed co-axially with the earlier deformation in the limestones. Chilled boudin necks in the dykes suggest that this deformation took place before consolidation of the basalt. This was followed by passive intrusion of the main mass of the granite as a large, possibly funnel-shaped sheet.
TL;DR: Using deformed Skolithos pipes as strain markers, a folded sequence of pipe rock quartzites in the Heilam Nappe at Loch Eriboll was shown to have deformed dominantly by flexural flow.
Abstract: Synopsis Using deformed Skolithos pipes as strain markers, a folded sequence of Pipe Rock quartzites in the Heilam Nappe at Loch Eriboll is shown to have deformed dominantly by flexural flow. However, elliptical pipe sections on bedding evince a pre-folding strain. Microstructures observed in the hinge zone imply deformation dominated by the intercrystalline diffusion mechanism, pressure solution. On the limbs microstructural evidence for dislocation creep has almost obliterated features associated with pressure solution. It is concluded that the quartzite sequence deformed by pressure solution during an initial phase of layer parallel shortening and subsequently, when buckling was initiated and fold amplification occurred, the mechanism switched to dislocation creep. It is argued that this change in mechanism is related to changes in strain rate.
TL;DR: In this paper, K-Ar isotopic age determinations on the Tertiary composite sill indicate it is at least 56.3 ± 1.0 Ma old, and the olivine dolerite sheet is also of a T-Ar age, with a similar age to the composite sill.
Abstract: Synopsis K-Ar isotopic age determinations on the Tertiary composite sill indicate it is at least 56.3 ± 1.0 Ma old. The olivine dolerite sheet is likewise of a Tertiary age, with a similar age to the composite sill, and may be related to the crinanite sills of southern Arran. The whole almost certainly belong to the ‘Major Phase’ of igneous activity identified as occurring at 59 Ma in the British Tertiary igneous province.
TL;DR: In this article, a systematic collection and comparison of heavy minerals from stream sediments, glacial deposits and bedrock in the Glen Artney-Glen Lednock area is presented.
Abstract: Synopsis A systematic collection and comparison of heavy minerals from stream sediments, glacial deposits and bedrock in the Glen Artney–Glen Lednock area shows that, despite glaciation and the influx of foreign material, the present heavy mineral distributions in streams clearly reflect the local geology.
TL;DR: The relationship between the metadolerite dykes and the shear zones in the Gruinard Bay area has been discussed in this article, where Davies has presented a rather confusing discussion of the relationship between dyke orientation and shear zone.
Abstract: Sirs—Davies has presented a rather confusing discussion of the relationships between the metadolerite dykes and of the shear zones in the Gruinard Bay area. He refers to two sets of shear zones (NW-SE and E-W) and one set of metadolerite dykes, the dykes trending 090°-100° and 135°, the 135° trend predominating. On page 193, line 8 he states that the 135° trending dykes are "affected" by the NW-SE trending shear zones and that the same 135° trending dykes lie within the predyke, dextral shear zones. When no other explanation is offered, it is hard to see how a predyke shear zone can affect a 135° trending dyke, unless the shear zone controls the dyke orientation. However, he does not say what sort of "effect" the shear zones have on the dykes. The E-W shear zones are said to be sinistral and that they "appear to have originated prior to dyke emplacement" and that they "offset dykes of the 135° set" (page 193, line 20). Given no other explanation the E-W shear zones cannot be predyke and postdyke at the same time. To explain these facts without contradiction, more facts are necessary; however, because he does not seem to find consistent relationships between dykes and sheer zones, three hypotheses seem feasible: (1) the dykes are of more than one age, (2) shear zones on similar trends did not all form during the same episode, (3) the shear zones and the dykes were all roughly synchronous and the same shear zone may have moved both before and after dyke injection (or during). The first hypothesis, involving multiple episodes of dyke injection, necessitates this sequence of events: