Three granitoid types are recognised in the Famatinian magmatic belt of NW Argentina, based on lithology and new geochemical data: (a) a minor trondhjemite–tonalite–granodiorite (TTG) group, (b) a metaluminous I-type gabbro-monzogranite suite, and (c) S-type granites. The latter occur as small cordieritic intrusions associated with 1-type granodiorites and as abundant cordierite-bearing facies in large batholithic masses. Twelve new SHRIMP U-Pb zircon ages establish the contemporaneity of all three types in Early Ordovician times (mainly 470-490 Ma ago). Sr- and Nd-isotopic data suggest that, apart from some TTG plutons of asthenospheric origin, the remaining magmas were derived from a Proterozoic crust-lithospheric mantle section. Trace element modelling suggests that the TTG originated by variable melting of a depleted gabbroid source at 10-12kbar, and the I-type tonalite-granodiorite suite by melting of a more enriched lithospheric source at c. 5 kbar. The voluminous intermediate and acidic I-types involved hybridisation with lower and middle crustal melts. The highly peraluminous S-type granites have isotopic and inherited zircon patterns similar to those of Cambrian supracrustal metasedimentary rocks deposited in the Pampean cycle, and were derived from them by local anatexis. Other major components of the S-type batholiths involved melting of deep crust and mixing with the I-type magmas, leading to an isotopic and geochemical continuum.

Most granitic batholiths contain plutons which are composed of low-variance mineral assemblages amenable to quantification of the P– conditions that characterise emplacement. Some mineral thermometers, such as those based on two feldspars or two Fe–Ti oxides, commonly undergo subsolidus re-equilibration. Others are more robust, including hornblende–plagioclase, hornblende–clinopyroxene, pyroxene–ilmenite, pyroxene–biotite, garnet–hornblende, muscovite-biotite and garnet–biotite. The quality of their calibration is variable and a major challenge resides in the large range of liquidus to solidus crystallisation temperatures that are incompletely preserved in mineral profiles. Further, the addition of components that affect Kd relations between non-ideal solutions remains inadequately understood. Estimation of solidus and near-solidus conditions derived from exchange thermometry often yield results >700°C and above that expected for crystallisation in the presence of an H2O-rich volatile phase. These results suggest that the assumption of crystallisation on an H2O-saturated solidus may not be an accurate characterisation of some granitic rocks.

Vapour undersaturation and volatile phase composition dramatically affect solidus temperatures. Equilibria including hypersthene–biotite–sanidine–quartz, fayalite–sanidine–biotite, and annite–sanidine–magnetite (ASM) allow estimation of Estimates by the latter assemblage, however, are highly dependent on . Oxygen fugacity varies widely (from two or more log units below the QFM buffer to a few log units below the HM buffer) and can have a strong affect on mafic phase composition. Ilmenite–magnetite, quartz–ulvospinel–ilmenite–fayalite (QUILF), annite–sanidine–magnetite, biotite–almandine–muscovite–magnetite (BAMM), and titanite–magnetite–quartz (TMQ) are equilibria providing a basis for the calculation of .

Granite barometry plays a critical part in constraining tectonic history. Metaluminous granites offer a range of barometers including ferrosilite–fayalite–quartz, garnet–plagioclase–hornblende–quartz and Al-in-hornblende. The latter barometer remains at the developmental stage, but has potential when the effects of temperature are considered. Likewise, peraluminous granites often contain mineral assemblages that enable pressure determinations, including garnet–biotite–muscovite–plagioclase and muscovite–biotite–alkali–feldspar–quartz. Limiting pressures can be obtained from the presence of magmatic epidote and, for low-Ca pegmatites or aplites, the presence of subsolvus versus hypersolvus alkali feldspars.

As with all barometers, the influence of temperature, , and choice of activity model are critical factors. Foremost is the fact that batholiths are not static features. Mineral compositions imperfectly record conditions acquired during ascent and over a range of temperature and pressure and great care must be taken in properly quantifying intensive parameters.

Asteroxylon Mackiei was a plant of more complicated organisation and larger size than either Rhynia or Hornea, which have been described from the silicified peat-bed at Rhynie in the two preceding papers of this series. The generic name refers to the stellate outline of the xylem of the stem as seen in cross section, while the specific name commemorates the original discovery of the plant remains by Dr Mackie.

A specific type of granitoid, referred to as sanukitoid (Shirey & Hanson 1984), was emplaced mainly across the Archaean–Proterozoic transition. The major and trace element composition of sanukitoids is intermediate between typical Archaean TTG and modern arc granitoids. However, among sanukitoids, two groups can be distinguished on the basis of the Ti content of the less differentiated rocks of the suite: high- and low-Ti sanukitoids. Melting experiments and petrogenetic modelling show that they may have formed by either (1) melting of mantle peridotite previously metasomatised by felsic melts of TTG composition, or (2) by reaction between TTG melts and mantle peridotite (assimilation). Rocks of the sanukitoid suite were emplaced at the Archaean–Proterozoic boundary, possibly marking the time when TTG-dominated granitoid magmatism changed to a more modern-style, arc-dominated magmatism. Consequently, the intermediate character of sanukitoids is not only compositional but chronological. The succession of granitoid magmatism with time is integrated in a plate tectonic model where it is linked to the thermal evolution of subduction zones, reflecting the progressive cooling of Earth: (1) the Archaean Earth’s heat production was high enough to allow the production of large amounts of TTG granitoids formed by partial melting of recycled basaltic crust (‘slab melting’); (2) at the end of the Archaean, due to the progressive cooling of the Earth, the extent of slab melting was reduced, resulting in lower melt:rock ratios. In such conditions the slab melts can be strongly contaminated by assimilation of mantle peridotite, thus giving rise to low-Ti sanukitoids. It is also possible that the slab melts were totally consumed in reactions with mantle peridotite, subsequent melting of this ‘melt-metasomatised mantle’ producing the high-Ti sanukitoid magmas; (3) after 2·5 Ga, Earth heat production was too low to allow slab melting, except in relatively rare geodynamic circumstances, and most modern arc magmas are produced by melting of the mantle wedge peridotite metasomatised by fluids from dehydration of the subducted slab. Of course, such changes did not take place exactly at the same time all over the world. The Archaean mechanisms coexisted with new processes over a relatively long time period, even if they were subordinate to the more modern processes.

In situ, microscale, U-Pb isotopic analyses of zircon using the SHRIMP ion microprobe demonstrate both the potential and the limitations of zircon U-Pb geochronology. Most zircons, whether from igneous or metamorphic rocks, need to be considered as mixed isotopic systems. In simple, young igneous rocks the mixing is principally between isotopically disturbed and undisturbed zircon. In polymetamorphic rocks, several generations of zircon growth can coexist, each with a different pattern of discordance. A similar situation exists for igneous rocks rich in inherited zircon, as these contain both melt-precipitated zircon and inherited components of several different ages. Microscale analysis by ion probe makes it possible to sample the record of provenance, age and metamorphic history commonly preserved within a single zircon population. It also indicates how the interpretation of conventionallymeasured bulk zircon isotopic compositions might be improved.

The extent of 64 glaciers that existed in the Lake District during the Loch Lomond Stadial is inferred from end, hummocky and fluted moraines and illustrated by detailed maps. Glacier surfaces are contoured and the equilibrium firn lines of the glaciers are calculated. Firn line altitudes were strongly influenced by snowfall, glacier aspect and the transfer of snow by wind from adjacent high ground. Snowfall was associated mainly with south to southeast winds and the mean direction of winds responsible for snow transfer was probably slightly west of south. It is inferred that mean July temperature at sea-level was slightly below 8°C. Protalus ramparts and other snowbed deposits indicate the former presence of numerous perennial snow beds down to altitudes of 200-300 m although the widespread occurrence of fossil gelifluction sheets and lobes points to large snow-free areas above these altitudes in summer. Fossil valley-wall rock glaciers indicate a mean annual temperature at sea-level no higher than 1°C. During deglaciation the great majority of the small glaciers retreated actively for 100-150 m and then probably decayed in situ, but some, at least, of the larger glaciers were active during much of the time their margins retreated.

From the abounding moraines, drainage features (fig. 21) and other marginal indications an attempt has been made to reconstruct the successive phases of the ice in its retreat into the corries of the Highlands and Islands (Pl. I). Two late-glacial stages are recognised. During the first, the Highland Glaciation, an ice-margin ran from the Orkney Islands across the mouth of the Moray Firth to the Buchan and out to sea north of Aberdeen. Twelve substages (A–L) of retreat, arbitrarily selected, have been traced through the country, except in the Moraineless West where they are unrepresented.

At the maximum of the second stage, the Moraine Glaciation, the ice readvanced to the line stage M (beaded line in Pl. I). The retreat from this line, the inner boundary of the Moraineless West and of the 100-foot raised beach (Pl. I), is divided into nine substages (N–V), based upon a consideration of snowlines. Substage N corresponds to the 50-foot raised beach, substage P to an important readvance.

The snowline throughout the late-glacial period ran in the west parallel with the meridians and rose eastwards. The disposition of the snowlines for stage M is given in fig. 22, p. 900.

The distribution of the ice in the British Isles during the North British, Highland and Moraine Glaciations is represented in fig. 23, p. 923.

Three main stages are recognized in the evolution of the Scottish Caledonian orogen. Firstly, the development of the depositional framework from late Pre-Cambrian to early Ordovician times is outlined, The thick Moine and Dalradian sediments, accumulating on a continental rise, are shown to be equivalent to the shelf sequence of the foreland, and likely correlations are suggested. Secondly, the events comprising the Caledonian deformation and metamorphism of these sediments are reviewed and related to the development of a Benioff zone and coupled oceanic trench along the southern margin of the orogen through the Ballantrae complex. Comparison of stratigraphic and isotopic evidence for the age of these events leads to the conclusion that all major deformation and metamorphism occurred during a relatively short climactic episode 480–510 m.y. ago, within the Arenigian stage of the Ordovician. The third factor in the evolution of the orogen is post-climactic uplift and erosion continuing throughout Upper Ordovician, Silurian and Lower Devonian times. Contours of K–Ar mica ages are presented and related to the geological evidence for this prolonged period of isostatic recovery and thermal adjustment. The style and timing of granitic plutonism, which is closely associated with this third stage, may be indicative of crustal behaviour during uplift rather than continued metamorphism at depth.

Musculature of the pectoral and pelvic appendages and girdles of adult and nestling Maiasaura peeblesorum (Dinosauria: Ornithischia: Hadrosauridae) from the Late Cretaceous of Montana is restored according to a phylogenetically based methodology. This methodology uses an explicit, independently derived phylogenetic hypothesis of the fossil taxon and related extant taxa to generate a series of inferences regarding the presence of a muscle, its number of components, and the origin(s) and insertion(s) of these components. Corroborative osteological evidence is sought on the fossil in the form of scars and processes that fulfill the criteria for muscular attachment according to generalisations based upon extant vertebrates. A total of 46 muscles are restored, although separate attachment sites for numerous muscles cannot be discerned on the fossils. Osteological evidence for several muscles can be found in nestlings of Maiasaura despite their skeletal immaturity. Results of the phylogenetically based approach and new hypotheses for homologies of deep dorsal thigh muscles suggest that it is more parsimonious to restore the femoral insertions of M. iliofemoralis on the greater trochanter and M. puboischiofemoralis internus on the anterior (lesser) trochanter, a reversal of the traditional interpretation. The often-cited osteological specialisations of birds for flight are not accompanied in all instances by profound myological transformations, and birds must be included in any attempt to restore the myology of extinct dinosaurs.

The pre-Carboniferous Midland Valley of Scotland comprises three tectonic elements: an arc, a proximal fore-arc basin and a marginal basin. These tectonic elements have been juxtaposed by strike-slip and thrust faulting, both of which have effected a 300% reduction in the width of the orogenic belt.

Rocks which span Arenig to Late Devonian or Early Carboniferous times and which are found S of the Highland Boundary fault have no clasts of certain Dalradian provenance despite substantial uplift of the Dalradian block at this time. This, combined with other evidence, suggests the Midland Valley to have been remote from this rapidly uplifting terrane. The Dalradian block, eroded down by c. 410 Ma was thrust southeastwards in Late Devonian–Early Carboniferous times. However, this thrust movement was minor, yielding little sediment, but it caused Dalradian rocks to cover the northern margin of the Midland Valley where (1) the source for part of the Old Red Sandstone rocks existed and (2) the faults along which the Midland Valley block was transported to dock against the Dalradian block are thought to be present. The existing Highland Boundary fault is therefore seen as a late Old Red Sandstone reverse fault which covered more significant older structures.

Until the last few years, diapirism reigned supreme among granitoid ascent mechanisms. Granitoid masses in a variety of material states, from pure melt through semi-molten crystal mushes to solid rock, were believed to have risen forcefully through the continental crust to their final emplacement levels in a way analogous to salt domes. The structural analogy between granite plutons and salt diapirs, which gained acceptance in the 1930s, has clearly been attractive despite the pessimistic outcomes of thermal models and, at best, ambiguous field evidence.

In contrast with traditional diapiric ascent, dyke transport of granitoid magmas has a number of important implications for the emplacement and geochemistry of granites that have yet to be fully explored. Rapid ascent rates of ≍ 10 2m/s predicted for granite melts in dykes (cf. m/a for diapirs) mean that felsic magmas can be transported through the continental crust in months rather than thousands (or even millions) of years, and that large plutons can in principle be filled in <104 a. Granitic melts are likely to rise adiabatically from their source regions, leading to the resorption of any entrained restitic material. Ascending melts in dykes close to their critical minimum widths may have little opportunity to assimilate significant amounts of country rock, and if source extraction is sufficiently rapid, most crustal contamination will be restricted to the site of emplacement. Rates of pluton and batholith inflation will be determined by the amount and rate of melt extraction at source.

The construction of large plutons and batholiths piecemeal from a number of magma pulses separated by periods of relative quiescence provides a means of reconciling rapid ascent rates with times for batholith construction based on average rates. Field and seismic evidence that shows batholiths as large, sheet-like structures with flat roofs and floors is consistent with a general model for plutons and batholiths as laccolith-type structures, fed from depth by dykes. The overall geometry of this type of structure helps ameliorate the space problem, which developed as a consequence of the unrealistic volumes of upwelling granite associated with the classical diapir model.

Study of satellite images from most of the area of the Canadian mainland once covered by the Laurentide ice sheet reveals a complex pattern of superimposed drift lineations. They are believed to have formed subglacially and parallel to ice flow. Aerial photographs reveal patterns of superimposition which permit the sequence of lineation patterns to be identified. The sequential lineation patterns are interpreted as evidence of shifting patterns of flow in an evolving ice sheet. Flow stages are recognised which reflect roughly synchronous integrated patterns of ice sheet flow. Comparison with stratigraphic sections in the Hudson Bay Lowlands suggests that all the principal stages may have formed during the last, Wisconsinan, glacial cycle. Analogy between Flow stage lineation patterns and the form and flow patterns of modern ice sheets permits reconstruction of patterns of ice divides and centres of mass which moved by 1000–2000 km during the glacial period. There is evidence that during the early Wisconsinan, ice sheet formation in Keewatin may have been independent of that in Labrador–Quebec, and that these two ice masses joined to form a major early Wisconsinan ice sheet. Subsequently the western dome decayed whilst the eastern dome remained relatively stable. A western dome then re-formed, and fused with the eastern dome to form the late Wisconsinan ice sheet before final decay.

Because of strong coupling between three-dimensional ice sheet geometry and atmospheric circulation, it is suggested that the major changes of geometry must have been associated with large scale atmospheric circulation changes.

Lineation patterns suggest very little erosional/depositional activity in ice divide regions, and can be used to reconstruct large scale patterns of erosion/deposition.

The sequence of flow stages through time provides an integrative framework allowing sparse stratigraphic data to be used most efficiently in reconstructing ice sheet history in time and space.

This part of the account of our examination of the plants preserved in the silicified peat-bed of early Old Red Sandstone age found at Rhynie will be devoted to the consideration of a number of general questions concerning the four Vascular Cryptogams which it has yielded.

1. In the first place the morphological characters of Rhynia Gwynne-Vaughani, R. major, Hornea, and Asteroxylon will be reviewed and the attempt made to reconstruct the external appearance of these plants as they grew. In relation to this a few additional features of the plants will be described.

2. This will lead naturally to a consideration of the general bearings upon plant-morphology of the facts mentioned in this series of papers.

In Part I a general account was given of the silicified peat-bed found at Rhynie, and one vascular plant was described in detail under the name of Rhynia Gwynne-Vaughani. Further study has shown that there are two species of Rhynia which we now distinguish as Rhynia Gwynne-Vaughani and Rhynia major. The account in Part I applies to both these species. Along with them there occurs a plant of similar grade of organisation to Rhynia, though quite distinct from that genus: this we name Hornea Lignieri. Asteroxylon Machiei, on the other hand, which will be described in the next part, was a plant of larger size and much more complex morphology.

We are thus now able to establish the existence and main features of four archaic Vascular Cryptogams from the Rhynie bed. In the present paper additional notes on R. Gwynne-Vaughani are given, and R. major and Hornea Lignieri described in detail. In conclusion, the morphological bearings of the facts will be briefly considered.

This paper reviews the petrogenesis of Himalayan leucogranites (HHγ) on the basis of field, petrological and geochemical data collected over the last fifteen years. HHγ are intruded at the top of the 2 to 8km-thick High Himalayan Crystallines (HHC). These are metamorphosed (Ky to Sill) and present much evidence of partial melting. During the MCT thrusting, the already metamorphosed HHC were thrust on top of the weakly metamorphosed Midland Formations, inducing the main phase of Himalayan metamorphism. The genesis of HHγ and North Himalaya leucogranites (NHγ) associates thrusting along the MCT, propagation of inverted metamorphism, liberation of large quantities of fluid in the Midlands, and partial melting of the HHC.

The restricted compositions of the granites are close to minimum melt compositions; variations in the alkali ratio probably relate to the variable amount of B, F and H2O. The HHγ were issued from the migmatitic zone around 700°C and 800 MPa., and still emplaced some 10 to 15 km below the surface. This syn- to late-tectonic emplacement of the leucogranites lasted for more than 10 Ma according to isotopic ages (25 to 14 Ma).

O, Rb–Sr, Nd–Sm and Pb isotope studies corroborate the unambiguous filiation between the HHC and the leucogranites in central Nepal. They also imply that the plutons are generated as numerous poorly mixed batches of magma produced preferentially in specific zones of the source rock. δD values may be explained by infiltration of water from the Midlands in the melting zone, and/or by water degassing during crystallisation. The positive covariations between Sr-, Nd- and O-isotope ratios relate to the variations in the original sediment composition of the source gneisses. Whereas trace element characteristics often date back to the anatectic process, limited magmatic differentiation is recorded by the biotite. These granites are typical crustal products, keeping track of some of the pre-Himalayan evolution together with that of their own origin.

Clavate (club-shaped) structures rimming mid-Cretaceous Burmese amber from Myanmar, previously misdiagnosed as fungal sporocarps, are shown to be domichnia (crypts) of martesiine bivalves (Pholadidae: Martesiinae). They are similar in form to Teredolites clavatus Leymerie, 1842 and Gastrochaenolites lapidicus Kelly & Bromley, 1984; however, the former identification is preferable, given that they are martesiine crypts in amber as opposed to a lithic substrate. Cross-cutting relationships between the clavate features and inclusions in the amber demonstrate that the features post-date hardening of the resin. The fills of the crypts are variable, including sand grade sediment of very fine to coarse sand grainsize, and sparry calcite cements. In some cases, the articulated valves of the pholadid bivalve responsible are visible inside the borings. However, one remarkable specimen contains two pairs of articulated shells ‘floating’ in amber, not associated with crypts; an observation that suggests that the resin was still liquid or soft when the bivalves were trapped in the resin. One individual is associated with an irregular sediment-filled feature and shows shell breakage. Formation of a solid rim around a liquid central volume has been documented in subaqueous bodies of resin in modern swamp forests, and argues for a close proximity between the amber-producing trees and a brackish water habitat for the bivalves. The presence of pyrite as thin films and crystal groups within Burmese amber is further consistent with such a depositional environment. Comparison of the size of pholadid body fossils with growth rates of modern equivalents allows the duration of boring activities to be estimated and suggests that small fossil pholadids in Burmese amber became trapped and died within 1–2 weeks of having settled on the resin. Larger examples present within well-formed domichnia formed in hardened resin. Since ‘hardground’ describes early lithified sediment as a substrate and ‘woodground’ describes wood as a substrate, the term ‘amberground’ is used here to described borings in an amber substrate.

Using a combination of optical microscopy and X-ray texture goniometry, an integrated microstructural and crystallographic fabric study has been made of quartz mylonites from thrust sheets located beneath, but immediately adjacent to, the Moine thrust in the Assynt and Eriboll regions of NW Scotland. A correlation is established between shape fabric symmetry and pattern of crystallographic preferred orientation, a particularly clear relationship being observed between shape fabric variation and quartz a-axis fabrics.

Coaxial strain paths dominate the internal parts of the thrust sheets and are indicated by quartz c- and a-axis fabrics which are symmetrical with respect to foliation and lineation. Non-coaxial strain paths are indicated within the more intensely deformed quartzites located near the boundaries of the sheets by asymmetrical c- and a-axis fabrics. These kinematic interpretations are supported by microstructural studies. At the Stack of Glencoul in the northern part of the Assynt region, the transition zone between these kinematic (strain path) domains is located at approximately 20 cm beneath the Moine thrust and is marked by a progression from symmetrical cross-girdle c-axis fabrics (30cm beneath the thrust), through asymmetrical cross-girdle c-axis fabrics to asymmetrical single girdle c-axis fabrics (0·5 cm beneath the thrust).

Tectonic models (incorporating processes such as extensional flow, gravity spreading and tectonic loading) which may account for the presence of strain path domains within the thrust sheets are considered, and their compatibility with local thrust sheet geometries assessed.

The lower jaw of the Devonian tetrapod Acanthostega is described for the first time. Redescriptions are provided for the lower jaws of the elpistostegid Panderichthys, the Devonian tetrapods Elginerpeton, Obruchevichthys, Metaxygnathus, Ventastega and Ichthyostega, and the Carboniferous tetrapods Crassigyrinus, Megalocephalus and Gephyrostegus. The character distri- butions thus revealed differ considerably from previous accounts, particularly in the wide distribution of certain primitive characters. Meckelian ossification in the middle part of the jaw is widespread among Devonian tetrapods, being demonstrably absent only in Acanthostega. Among Carboniferous tetrapods, a tooth-bearing parasymphysial plate is shown to be present in Crassigyrinus and Megalocephalus (having already been demonstrated by other authors in Whatcheeria and Greererpeton). A phylogenetic analysis of 26 early tetrapods including all the aforementioned genera, scored for 51 lower jaw characters, produces at least 2,500 equally parsimonious trees. However, the lack of resolution lies largely in a big top end polychotomy containing anthracosaurs, temnospondyls, seymouriamorphs, microsaurs and a nectridean-amniote clade. Below this polycho- tomy, which may correspond approximately to the tetrapod crown group, there is a well-resolved stem-group containing, in descending order, Megalocephalus, Greererpeton, Crassigyrinus, (jaws associated with) Tulerpeton, Whatcheeria, Acanthostega, Metaxygnathus, Ichthyostega, Ventastega and Metaxygnathus (unresolved), an Elginerpeton-Obruchevichthys clade, and Panderichthys. This conflicts with recently published phylogenies by Coates and Lebedev & Coates, which place Tulerpeton and all post-Devonian tetrapods in the amphibian or amniote branches of the tetrapod crown group.

The Indus-Tsangpo suture and its adjoining tectonic zones are well displayed in the Ladakh Himalayas where four tectonic zones have been distinguished, viz. the Zanskar, Indus suture, Shyok suture and Karakoram zones. The Zanskar zone is made up of Precambrian basement of the Zanskar crystalline complex and overlying Phanerozic sediments including Upper Palaeozoic volcanic rocks of the Zanskar Supergroup; they form the northern margin of the Indian plate. The Indus suture zone consists of a remnant of tectonised oceanic lithosphere represented by the Shergol melange and the Nidar complex with a former volcanic arc indicated by the volcanogenic Dras and Khardung formations and the Ladakh plutonic complex. The Shyok suture zone does not represent a tectonic repetition of the Indus suture; it is interpreted as a relic of a back-arc basin. The Karakoram plutonic complex appears to be genetically related to the Ladakh plutonic complex; both were generated from the subducting Indian oceanic plate. It is proposed that the boundary between the Indian and Eurasian plates does not lie along the Indus and Shyok sutures, but is located further N at the junction of Central Pamir (Alpine-Himalayan) and North Pamir (Hercynian).

The Midland Valley of Scotland was an arc–interarc region during most of Ordovician—Devonian time. This arc terrane extends beneath the allochthonous Southern Uplands and probably beneath the southern Highlands. Models of Caledonian plate tectonism which regard the Midland Valley as a fore-arc basin are rejected principally on the grounds that (i) the Ordovician sequence at Girvan, in the very SW of the Midland Valley, was generated in a proximal fore-arc basin to the immediate S of a contemporaneous plutonic–volcanic arc, and (ii) the source for Silurian sediments in the southern part of the Midland Valley could not have been a rising trench-slope-break, but igneous basement and conglomerates with clasts of metamorphic basement, i.e. the southward extension of the Midland Valley.

The Midland Valley arc first comprised mainly plutonic rocks, some of which may have been basic but most of which were certainly granitic. Little is known of the ages of volcanic clasts in Silurian conglomerates (this time may have been a period of relative volcanic quiescence), but the Silurian–Devonian sequence is considered to have formed in an interarc basin which, like many other basins of this kind, began as marine (Early Silurian) and ended as fluvial (Devonian). At this final Silurian–Devonian stage, the Midland Valley arc was dominated by effusive rocks which made a substantial contribution to the sediments.

In this interpretation, the present Old Red Sandstone volcanic rocks are seen as the final stage of a volcanic arc which occupied the position of the present Midland Valley from at least Llanvirn to Early Devonian time.