The Ordovician commences with Arenig pillow lavas capped by Arenig-Llandeilo radiolarian cherts (7o ft.). Near Girvan 1,500 ft. of Arenig volcanic rocks are cut by serpentine, gabbro and granite which erosion bared before deposition of conglom eratic Upper Llandeilo (83o ft., including 6o ft. interbedded fossiliferous Stinchar limestone). South-east of Stinchar Valley, Upper Llandeilo suddenly becomes conformable to the radio larian cherts and consists of mudstones and grits (5oo ft.) and conglomerate (500 ft.). Farther south-east it passes to grit and greywacke (i,000 ft.) and then near Moffat to 20 ft. of grapto lite shale (Glenkiln). The Caradoc (including Ashgillian) at Gir van is mostly mudstones, grits, flags and shales (2,800 ft.) with interbedded shelly and graptolitic faunas. Near Moffat this reduces to 1 oo ft. of graptolite shale and barren mudstone (Hartfell). Acid lavas occur at Wrae.
A general palaeontological break introduces the Silurian, although there was no upheaval at Moffat. The Llandovery at Girvan consists of conglomerates, grits, flags, shales and thin limestones (i,o5o ft.) with interbedded shelly and graptolitic faunas. Near Moffat there are only 1 oo ft. of graptolite shale (Birkhill). The Tarannon, both at Girvan and Moffat, consists of grits, flags and shales with occasional graptolitic intercalations. This series is thinner and finer-grained at Girvan (2,100 ft.) than at Moffat (3,00o to 4,000 ft.). Wenlock is doubtfully repre sented near Girvan by conglomerates, grits, flags and shales, (500 ft. preserved) with minor graptolitic and shelly layers. Near the English Borders Wenlock conglomerates, grits, grey wackes, shales and mudstones (i,000 to 1,500 ft.) contain sev eral graptolitic bands with occasional eurypterids. Shelly layers also occur. In the same district Ludlow mudstones with lime stone-nodules and grits (5oo-75o ft.) yield shells.
The Llandeilo unconformity at Girvan is probably a partial continuation of the post-Cambrian pre-Downtonian unconformity at Stonehaven. Upper Llandeilo and subsequent Ordovician and Silurian rocks of the southern Uplands tell of derivation of sedi ment from the north-west. Much of their material can be re ferred to denudation of Cambro-Arenig lavas, cherts and plu tonics. Quartzite appears as pebbles in the Caradoc, and mica schist in the Llandovery.
Undated post-Silurian mineral veins occur at Lead Hills.
faunas and floras. Fish have furnished most of the determinable fossils. The Lower Old Red Sandstone or Caledonian is most fully preserved in the Central Valley (19,00o ft., including lavas, Kincardineshire). A widespread type is dull purplish brown sand stone. Upper members in Strathmore are red sandstones and marls. Conglomerates attain great prominence towards the High lands and southern Uplands. North-westwards there is con formity with Downtonian (Lesmahagow and Stonehaven) and violent unconformity with schists (within Highlands). South-east wards there is violent unconformity with Downtonian, etc. (Pentlands, Girvan, Cheviot). A basal conglomerate of Silurian greywacke extends north-westward to Lesmahagow. Volcanic rocks (basalts, andesites, rhyolites) occupy a roughly central posi tion in the Lower Old Red sequence (6,5oo ft. in Ochils, well exposed also Pentlands to Ayrshire). In the Highlands (Oban, Glen Coe, Ben Nevis) and also at Cheviot, lavas greatly exceed sediment. Glen Coe and Ben Nevis are famous for cauldron subsidences. At both localities granites (or granodiorites) with north-eastern dyke-swarms are later than the lavas. Granites cutting Lower Old Red Sandstone also occur at Distinkhorn (Ayr shire) and Cheviot. Others cut folded Silurian (southern Up lands). Many undated Highland granites are probably of Lower Old Red age, but granite pebbles are well known in Lower Old Red Sandstone conglomerates (Glen Coe, Stonehaven).
Middle Old Red Sandstone or Orcadian is widely developed in north-east Scotland (18,000 ft. in Caithness). It largely con sists of flags, often bituminous, calcareous, ripple-marked and sun cracked. The earlier part of the Orcadian is restricted to Caith ness, but later divisions extend into Orkney and round the Moray Firth. The Rhynie Chert with wonderfully preserved plants is generally referred to the Orcadian. The Upper Old Red Sand. stone (some thousands of feet thick) is found in the Central Valley and north-east Scotland. The Lower Old Red Sandstone was locally much faulted, folded and eroded before the deposition of the Upper. The Upper is also unconformable to the Middle, except perhaps in Shetland. Red sandstones with some wind rounded grains are common in the earlier parts of the Upper Old Red Sandstone; while paler sandstones with cornstones occur towards the conformable base of the Carboniferous.