SLIGO GEOLOGY
The county forms the north-western extremity of the great central floetz limestone field of Ireland, interrupted in two places by the mica slate formation, one to the south of Lough Gill, the other along the western - mountain range, which in its utmost extent stretches from Foxford in Mayo, by Collooney, to Manor-Hamilton in Leitrim. This range is very narrow, seldom exceeding three miles, and at Collooney being less than a quarter of a mile in breadth. It is generally succeeded by beds of red or yellowish sandstone or by limestone. The sandstone formation is of very unequal thickness and irregular in its arrangement, in some places rising into mountains, in others not exceeding 20 or 30 feet in height: it is sometimes, though rarely, interstratified with red or grey sandstone slate, in which case its resemblance to that of the coal formation has led to expensive and illusory attempts to obtain this valuable mineral.
Iron-ore is abundant in many places, particularly at Ballintogher and at the base of the Ox mountains. Near Screevenamuck are extensive excavations whence the ore was raised as long as timber could be procured to make charcoal for smelting it: the last furnace was extinguished in 1768. Lead-ore has been found in several parts of the limestone district, and worked for some time feebly and unprofitably. A silver mine, which produced some specimens of very pure metal, was worked near Ballysadere. Iron pyrites and sulphate of copper are often found in small detached pieces, and some pure specimens of the latter metal were found in the Awenmore and Collooney rivers; black oxyde of manganese is often seen on the surface and very large pieces of the ore have been found in several parts. At the foot of some of the mountains, and in the beds of some rivers, carbonate of copper and various kinds of ochre, all indicative of extensive mineral deposits, have been discovered; as also very large and beautiful amethysts in the neighbourhood of Ballymote.
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