h-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.

_Geology._--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