Ayrshire, a south-western county of Scotland, bounded N. by Renfrewshire, E. by Lanarkshire and Dumfriesshire, S.E. by Kirkcudbrightshire, S. by Wigtownshire and W. by the Firth of Clyde. It includes off its coast the conspicuous rock of Ailsa Craig, 10 m. W. of Girvan, Lady Island, 3 m. S.W. of Troon, and Horse Island, off Ardrossan. Its area is 724,523 acres or 1142 sq. m., its coast-line being 70 m. long. In former times the shire was divided into the districts of Cunninghame (N. of the Irvine), Kyle (between the Irvine and the Boon), and Carrick (S. of the Doon), and these terms are still occasionally used. Kyle was further divided by the Ayr into King's Kyle on the north and Kyle Stewart. Robert Bruce was earl of Carrick, a title now borne by the prince of Wales. The county is politically divided into North and South Ayrshire, the former comprising Cunninghame and the latter Kyle and Carrick. The surface is generally undulating with a small mountainous tract in the north and a larger one in the south and south-east. The principal hills are Black Craig (2298 ft.), 5 m. south-east of New Cumnock; Enoch (1865 ft.), 5 m. east of Dalmellington; Polmaddie (1750 ft.) 2 m. south-east of Barr; Stake on the confines of Ayrshire and Renfrewshire, and Corsancone (1547 ft.), 3 m. north-east of New Cumnock. None of the rivers is navigable, but their varied and tranquil beauty has made them better known than many more important streams.
The six most noted are the Stinchar (c soft), Girvan, Doon, Ayr, Irvine and Garnock. Of these the Ayr is the longest. It rises at Glenbuck, on the border of Lanarkshire, and after a course of some 38 m. falls into the Firth of Clyde at the county town which, with the county, is named from it. The scenery along its banks from Sorn downwards - passing Catrine, Ballochmyle, Barskimming, Sundrum, Auchencruive and Craigie - is remarkably picturesque. The lesser streams are numerous, but Burns's verse has given preeminence to the Afton, the Cessnock and the Lugar. There are many lochs, the largest of which is Loch Doon, 5&FRAC12; m. long, the source of the river of the same name. From Loch Finlas, about 20 m. south-east of Ayr, the town derives its water-supply. The Nith rises in Ayrshire and a few miles of its early course belong to the county.
The greater portion of the hilly region in the south of the county forms part of the Silurian tableland of the south of Scotland. Along its north margin there is a belt of elevated ground consisting mainly of Old Red Sandstone strata, while the tract of fertile low ground is chiefly occupied by younger Palaeozoic rocks. The Silurian belt stretching eastwards from the mouth of Loch Ryan to the Merrick range is composed of grits, greywackes and shales with thin leaves of black shales, containing graptolites of Upper Llandeilo age which are repeated by folding and cover a broad area. Near their northern limit Radiolarian cherts, mudstones and lavas of Arenig age rise from underneath the former along anticlines striking north-east and south-west. In the Ballantrae region there is a remarkable development of volcanic rocks - lavas, tuffs and agglomerates - of Arenig age, their horizon being defined by graptolites occurring in cherty mudstones and black shales interleaved in lavas and agglomerates. These volcanic materials are pierced by serpentine, gabbro and granite.
The serpentine forms two belts running inland from near Bennane Head and from Burnfoot, being typically developed on Balhamie Hill near Colmonell. Gabbro appears on the shore north of Lendalfoot, while on the Byne and Grey Hills south of Girvan there are patches of granite and quartz-diorite which seem to pass into more basic varieties. These volcanic and plutonic rocks and Radiolarian cherts are covered unconformably by conglomerates (Bennan Hill near Straiton and Kennedy's Pass) which are associated with limestones of Upper Llandeilo age that have been wrought in the Stinchar valley and at Craighead. South of the river Girvan there is a sequence from Llandeilo - Caradoc to Llandovery - Tarannon strata, excellent sections of which are seen on the shore north of Kennedy's Pass and in Penwhapple Glen near Girvan. Llandovery strata again appear north of the Girvan at Dailly, where they form an inlier surrounded by the Old Red Sandstone and Carboniferous formations. Representatives of Wenlock rocks form a narrow belt near the village of Straiton. Some of the Silurian sediments of the Girvan province are highly fossiliferous, but the order of succession is determined by the graptolites.
Near Muirkirk and in the Douglas Water there are inliers of Wenlock, Ludlow and Downtonian rocks, coming to the surface along anticlines truncated by faults and surrounded by Old Red Sandstone and Carboniferous strata. In the south-east of the county there is a part of the large granite mass that stretches from Loch Doon south to Loch Dee, giving rise to wild scenery and bounded by the high ground near the head of the Girvan Water, boulders of which have been distributed over a wide area during the glacial period. Along the northern margin of the uplands the Lower Old Red Sandstone is usually faulted against the Silurian strata, but on Hadyard Hill south of the Girvan valley they rest on the folded and denuded members of the latter system. The three divisions of this formation are well represented. The lower group of conglomerates and sandstones are well displayed on Hadyard Hill and on the tract near Maybole; the middle volcanic series on the shore south of the Heads of Ayr and from the Stinchar valley along the Old Red belt towards Dalmellington and New Cumnock; while the upper group, comprising conglomerates and sandstones, form a well-marked synclinal ford at Corsancone north-east of New Cumnock. The Upper Old Red Sandstone appears as a fringe round the south-west margin of the Carboniferous rocks of the county, and it rises from beneath them on the shore of the Firth of Clyde south of Wemyss Bay. The Carboniferous strata of the central low ground form a great basin traversed by faults, all the subdivisions of the system being represented save the Millstone Grit. Round the north and north-east margin there is a great development of volcanic rocks - lavas, tuffs and agglomerates - belonging to the Calciferous Sandstone series, and passing upwards into the Carboniferous Limestone. The lower limestones of the latter division are typically represented near Dalry and Beith, where in one instance they reach a thickness of over 100 ft.
They are followed by the coal-bearing group (Edge coals of Midlothian) which have been wrought in the Dalry and Patna districts and at Dailly. The position of the Millstone Grit is occupied by lavas and tuffs, extending almost continually as a narrow fringe round the northern margin of the Coal Measures from Saltcoats by Kilmaurs to the Crawfordland Water. The workable coals of the true Coal Measures have a wide distribution from Kilwinning by Kilmarnock to Galston and again in the districts of Coylton, Dalmellington, Lugar and Cumnock. These members are overlaid by a set of upper barren red sandstones, probably the equivalents of the red beds of Uddingston, Dalkeith and Wemyss in Fife, visible in the ravines of Lugar near Ochiltree and of Ayr at Catrine. In various parts of the Ayrshire coalfield the coal-seams are rendered useless by intrusive sheets of dolerite as near Kilmarnock and Dalmellington. In the central part of the field there is an oval-shaped area of red sandstones now grouped with the Trias, extending from near Tarbolton to Mauchline, where they are largely worked for building stone.
They are underlaid by a volcanic series which forms a continuous belt between the underlying red sandstones of the Coal Measures and the overlying Trias. In the north part of the county, as near Wemyss Bay, the strata are traversed by dykes of dolerite and basalt trending in a north-west direction and probably of Tertiary age.