LONDONDERRY, a northern county of Ireland in the province of Ulster, bounded north by the Atlantic, west by Lough Foyle and Donegal, east by Antrim and Lough Neagh, and south by Tyrone. The area is 522,315 acres, or about 816 sq.m. Pop. (1926) exclusive of co. bor. is 94,54o. Old Red Sandstone and Lower Carboniferous Sandstone overlie old rocks in the south and east, meeting the igneous "green rocks" of Tyrone, and the granite intrusive in them, at the north end of Slieve, Gallion. Triassic sandstone covers the lower slope of Slieve Gallion on the south-east towards Moneymore, and rises above the Carbon iferous Sandstone from Dungiven northward. At Moneymore the western scarp of the White Limestone (Chalk) and the over lying basalt of the great plateaux dip down eastward under Lough Neagh. The basalt scarp, protecting chalk and patches of Liassic and Rhaetic strata, rises to 1,26o ft. in Benevenagh north of Limavady. A raised shelf with post-glacial marine clays forms the flat land west of Limavady. Haematite has been mined on the south flank of Slieve Gallion. The principal river is the Roe, which flows northward from the borders of Tyrone into Lough Foyle below Newton-Limavady. Farther west the Faughan also falls into Lough Foyle, and the river Foyle passes through a small portion of the county near its north-western boundary. In the south-east the Moyola falls into Lough Neagh, and the Lower Bann from Lough Neagh forms for some distance its eastern boundary with Antrim. The only lake in the county is Lough Finn on the borders of Tyrone, but Lough Neagh forms about 6 m. of its south-eastern boundary. Castlerock, Downhill, Magilligan and Portstewart are seaside resorts.
Early inhabitants were the O'Cathans or O'Catrans, who were tributary to the O'Neills. Towards the close of the reign of Eliza beth the county was seized and received the name of Coleraine, having that town for its capital. In 1609, after the confiscation of the estates of the O'Neills, the citizens of London obtained possession of the towns of Londonderry and Coleraine and ad joining lands, 6o acres out of every i,000 being assigned for church lands. The common council of London elected a body
of twenty-six for its management, who in 1613 were incorporated as the Irish Society, and retained possession of the towns of Londonderry and Coleraine. Their estates were sequestrated by James I., and in 1637 the charter of the Irish Society was can celled. Cromwell restored the society to its former position, and Charles II. granted it a new charter. In the insurrection of 1641 many towns and villages were burned. There are several stone circles, and a large number of artificial caves. The most ancient castle of Irish origin is that of Carrickreagh; and of the castles erected by the English those of Dungiven and Muff are in good preservation. The ruins of Dungiven Abbey, founded in 1109, stand on a rock about zoo ft. above the river Roe.
The excessive rainfall and the cold and uncertain climate are un favourable for agriculture. Along the sea-coast there is a dis trict of red clay formed by the decomposition of sandstone, and near the mouth of the Roe is a tract of marl. The staple manu facture of the county is linen. The manufacture of coarse earthen ware is also carried on, and there are large distilleries and brew eries and some salt-works. Coleraine is the headquarters for salmon and eel fisheries on the Bann. The deep-sea and coast fisheries are centred at Moville in Co. Donegal. The city of Lon donderry is an important railway centre. The Northern Counties (Midland) main line reaches it by way of Coleraine and the north coast of the county, and the same railway serves the eastern part of the county, with branches from Antrim to Magherafelt, and Magherafelt to Cookstown (Co. Tyrone), to Draperstown and to Coleraine, and from Limavady to Dungiven. The Great North ern railway reaches Londonderry from the south, and the city is also the starting-point of the Londonderry and Lough Swilly rail way. Londonderry (including the Borough of Londonderry) re turns 5 members to the Parliament of Northern Ireland and member to the Parliament of Great Britain and Northern Ireland.