million gallons constructed in 1898-1906 on the Cray, a
tributary of the Usk. A large industrial area around Neath is supplied
from Ystradfellte. Merthyr Tydfil draws its supply from the lesser Taff,
while Cardiff's main supply comes from the Great Taff valley, where,
under acts of 1884 and 1894, two reservoirs with a capacity of 668
million gallons have been constructed and a third authorized.

In the east of the county, at the foot of the Black Forest Mountains, is
Llyn Safaddan, or Brecknock Mere, now more generally known as Llangorse
Lake (from being partly situated in the parish of that name). It is
about 3 m. long by 1 m. broad, being the largest lake in South Wales.
Upon an artificial island in the lake traces of lake-dwellings were
discovered in 1869, together with the bones of red deer, wild boar and
_Bos longifrons_.

_Geology._--The oldest rocks in Brecknockshire are the Llandeilo
shales and intrusive diabases of pre-Llandovery age which near Builth
extend across the Wye from Radnorshire; another patch with volcanic
outflows comes up at Llanwrtyd, and at both places they give rise to
mineral springs. Next follow the Bala Beds, which, with the succeeding
Lower and Upper Llandovery shales, sandstones and conglomerates, form
the sparsely populated sheepwalks and valleys which occupy most of the
north-western part of the county. These rocks are much folded and the
shales are locally cleaved into slates, while the sandstones and
conglomerates form scarps and ridges. To the south-east of this region
a narrow outcrop of Upper Llandovery, Wenlock and Ludlow sandstones
and mudstones follows, uncomformably overlying the Llandeilo and Bala
rocks, and dipping conformably under the Old Red Sandstone; they
extend from Newbridge-on-Wye and Builth through Llangammarch (where
there are mineral springs) towards Llandovery, while a tongue of
Ludlow rocks brought up by faulting extends from Erwood on the Wye for
8 m. south-westwards into the Old Red Sandstone. The remainder and
greater part of the county is occupied chiefly by the gently inclined
Old Red Sandstone; in the dissected plateau of the Black Mountains
north of Crickhowell the lower marls and cornstones are laid open,
while south of Brecon the conglomeratic upper beds form the escarpment
and plateaus of the Beacons. The southern edge of the county is formed
by the scarps and moorlands of the Carboniferous Limestone and
M