The geology of the county presents many remarkable features. The lowest strata are those of the primary mountain range entering from the county of Sligo and extending from south-west to north-east: this range is mostly composed of mica slate; a green steatite, thickly studded with valuable garnets, has been found at the foot of a mountain near Lurganboy. On the western base of Benbo a clay of a blueish white hue has been found and used for fullers' earth: the western side of the same mountain is traversed by a metallic vein containing copper pyrites, which was formerly wrought.

Veins of the sulphuret of lead have also been largely wrought in several places between Benbo and Lurganboy. This primary range is generally bordered on both sides by beds of variously coloured freestone, to which limestone succeeds in every direction, occupying the remainder of the northern portion of the county and forming part of the great limestone field of Ireland. All the central portion of the county forms part of the great Connaught coal field, constituting a vast basin of which Lough Allen is the centre.

The principal vein of coal is about 3 feet thick, of very great extent, and of excellent quality both for domestic purposes and for smelting; but the beds are often interrupted by faults, by which portions of the strata are, broken and thrown upwards from 20 to 40 yards. It was originally discovered in the Munterkenny mountains, and such was the importance attached to the discovery that a parliamentary grant was made for the formation of roads to it, but the workings were soon discontinued in consequence of a fault, by which the stratum was considerably elevated, which induced the workmen, who were ignorant of the cause of the interruption and of the means of remedying it, to relinquish any further operations. In the northern part of the coal district the beds are found only in the higher parts of Lugnacuillagh and Lacka.

Extensive quarries of very fine-grained yellowish white sandstone are worked near the summit of Glanfarn mountain for window seats and various ornamental purposes. Lacka mountain contains a great bed of sandstone, the strata of which form a succession of abrupt precipices with considerable flat intervals between them: above the sandstone are beds of slate clay succeeded by layers of coal from 4 to 6 inches thick, alternating with beds of sandstone: this field has been but little worked.

The stratification of Lugnacuillagh mountain, on the borders of Cavan, much resembles that of Lacka. The remainder of the coal district to the east of Lough Allen is composed of the great mountain group of Slieve-an-irin, or Slieve-an-Jaroin, "the Iron Mountain;" its stratification is extremely irregular. Three layers of coal have been discovered in it, one of good quality, 18 inches thick. Rich clay ironstone abounds also at various elevations, and was worked so long as timber could be procured to feed the furnaces: those of Drumshambo, the last in operation, were abandoned in 1765. The ore of this mountain is said to be far richer than that on the Roscommon side of the lake. In the channels of many of the streams descending from it are found beds of pipe clay and yellow ochre. Manganese is also found in great abundance.

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