The Durness Quartzite-limestone.—This succession begins at or below the Lower Cambrian and, according to American palaeontologists, extends into the Lower Ordovician (Durness lith ology and fauna are of American facies). The base passes smoothly and unconformably across Torridonian east-south-east wards on to Lewisian exposed in a pre-Cambrian anticline. The stratigraphy is persistent for ioo miles along the strike: basal quartzite, 32o ft.; quartzite with vertical worm "pipes," 270 ft.; dolomitic shales, mudstones and dolomites, with Olenellus, etc., 5o ft.; grit, with Salterella and also Olenellus, 3o ft.; limestones and dolomites in seven stages, some fossiliferous, 1,5oo feet. Plutonic (possibly early Devonian) intrusions of alkali-syenite, borolanite, etc., accompanied by sills, cut the Durness sediments. Contact effects include dedolomitisation.
The Durness fossiliferous succession with its intrusions passes east-south-eastwards in disturbed fashion under the Moine Thrust overlain by crystalline schists. This thrust has been traced from Durness to Skye, and may continue through Islay. Near Durness, a thrust-outlier (Klippe) of the Moine Thrust-mass (nappe) has been preserved by down-faulting io miles in advance of the main outcrop. In Assynt and Glencoul, there are particu larly good exposures of the disturbed belt with subsidiary thrusts bringing 1,5oo ft. slices of Lewisian for miles over Cambrian. Recumbent folding occurs in and near Skye. Isoclinal packing, schuppen Struktur, crushing (mylonisation), slaty cleavage, etc., are diagrammatically exposed at many localities.
Pre-Devonian of the Highlands South-east of Moine Thrust.—Sedimentary quartzo-felspathic schist or gneiss, with sufficient mica to give a flaggy structure, is prevalent north-west of the Caledonian canal. Micaceous gneiss representing shales is also common. Such rocks are grouped together as Moine Schists. A limestone with particular associates occurs a little above the Moine Thrust in Eireboll. Several minor outcrops of Lewisian igneous and sedimentary types are generally interpreted as Lewisian inliers interfolded with unconformable Moines. The Inchbae porphyritic granite, now largely augen-gneiss, was in truded into the Moines before these had suffered regional meta morphism. Much of the resultant indurated hornf els, escaping subsequent deformation, retains minutiae of sedimentary struc ture, such as grains, ripple-marks and sun-cracks. The Moine Schists seem to decrease in metamorphism, near the Moine Thrust, but evidence is blurred by crush-phenomena. In Skye
there is a thrust-mass under the Moine Thrust, the rocks of which are variously interpreted as Moines or Torridonian. Some think all Moines are metamorphosed Torridonian ; most regard the Moine Metamorphism as pre-Torridonian.
Moine-like rocks cross the Caledonian canal fault, but half of this south-eastern district consists of a varied assemblage of undated sedimentary schists known as Dalradian. Quartzites, limestones and graphitic schists are characteristic members; the Schichallion conglomerate may be glacial; pillow lavas occur at Loch Awe ; basic intrusions are wide-spread; granites, that have shared to some extent in the movements, are common in the east. The Dalradians overlie Moine-like rocks, pitching off them towards Loch Awe and Banffshire. Large-scale recumbent folds with fold-faults (slides), the whole refolded, are typically shown at Ballachulish and elsewhere.
Zonal mapping of regional metamorphism was first carried out in the Dalradian belt. The grade, varying from sillimanite gneiss to roofing slate, is low at Loch Awe, in Banffshire and along the south-eastern Highland border.
Probable Upper Cambrian pillow lavas, radiolarian cherts and fossiliferous shales outcrop discontinuously along the Highland border fault. In Arran they overlie Dalradians. Farther north east they are associated with a grit and conglomerate group (Margie Beds) and appear overthrust by Dalradians. At Stone haven they are unconformably overlain by uppermost Silurian (Downtonian) sandstones with mudstones, tuffs and conglom erates (2,76o ft.), that yield fish and Dictyocaris.
Silurian of Lesmahagow and Pentland Anticlines in the Central Valley.—Lesmahagow exposures of Silurian show : Wenlock greywacke and shale (base not seen), 1,300 ft.; Ludlow mudstone, greywacke and shale 1,480 ft.; Downtonian yellow, red and chocolate sandstone with two conglomerates and some mudstone, 2,700 feet. The Wenlock-Ludlow rocks have yielded many brachiopods and molluscs, especially in the Pentlands, also scorpion, phyllocarids, eurypterids and fish. The three last appear again in the Downtonian, accompanying a transition from marine to Old Red Sandstone conditions.
Ordovician and Silurian of the Southern Uplands.- Closely packed isoclinal folding (seldom accompanied by cleav age) and rapid cross-strike change of facies are characteristic. Nothing older than Arenig has been recognised. The succession is marine and ends with Wenlock near Girvan and with Ludlow on the English Borders.