All the plain district is based on limestone, varying in appearance and quality. The upper beds are commonly of a grey colour and of secondary formation, abounding with petrifactions, principally madrepores. The lower beds are more commonly of a blackish hue, and the stone contains large portions of argillaceous and silicious earths, which frequently render it unfit for burning: this impure limestone, called calp, is often accompanied by thin layers of Lydian stone, which are sometimes so numerous and minute as to give the rock a striped appearance. The calp beds are commonly succeeded by strata of black limestone of a crystalline structure, susceptible of a high polish; but in the northern parts of the county, the limestone of the lower beds, even where they come in contact with the sandstone, are of a light grey colour, and of a crystalline texture and susceptible of polish.

Silicious sandstone appears in several parts of the county rising up from beneath the limestone bed and forming isolated hills, and likewise composing the long ridge of Slievebawn, where it appears on the summit in large broken masses. Of similar composition is the hill of Ballyfermoile, and at Belanagare the sandstone appears at the surface in very thin flags, which are used in the vicinity for roofing houses. In the more western part of the county, beyond Castlerea, sandstone appears in various places, and limestone is comparatively rare. But by far the most interesting part of Roscommon, in reference to its geological formation, is the northern mountainous district on the confines of Lough Allen, forming the celebrated coal and iron district of Arigna.

This coal district forms a portion of that of the county of Leitrim, but of its two most important divisions, one is wholly and the other chiefly in the northern end of this county. The strata are arranged with great regularity, rising immediately into the high flat-topped mountains of Bracklieve and Slieve Curkagh. They dip conformably with the subjacent limestone, and in opposition to the southern declivity of the mountains; but the continuity of the different beds is frequently broken by faults, where the strata of one part of a hill have slipped down to a lower level, producing a variation of level of from 20 to 40 yards.

In the series of strata the lowest and first above the limestone base is blank slate clay, about 600 feet in thickness, in the upper part of which are shale and thin beds of sandstone; it likewise contains numerous beds of clay iron-stone, from half an inch to two feet in thickness. Resting on it there is from thirty to sixty feet of greyish white rock, called the first or great sandstone. Above this succeeds black slate clay, from nine to twenty feet thick, covered by grey sandstone from six to ten feet thick, on which rests sandstone from one to three feet in thickness, with fossil impressions, known by the name of "seat rock," incumbent on which is fire-clay of a similar thickness.

This forms the seat of a stratum of coal intermixed with thin laminae of shale, from one to three feet thick, above which is greyish white sandstone, from four to twenty feet; next, black slate clay from six to fifteen feet; and then sandstone from twelve to fifteen feet. This forms the seat of the second coal stratum, which is of good quality, and the only one yet discovered that will repay the labour of the miner: it varies from one foot four inches to two feet six inches, and appears to promise an abundant produce: the stratum is thicker, and the coal better, than any before known. Its roof is grey, soft slate clay, from ten to fifteen feet thick, above which is white sandstone, from twenty-four to forty-five feet, on which rests the third and uppermost seam of coal, from eight to nine inches only in thickness.

Above it is slate clay in beds varying in thickness, generally soft and black, and containing innumerable thin layers of clay iron-stone: these beds are unitedly from 100 to 200 feet thick, and are succeeded by blackish grey sandstone slate in thin layers, from 30 to 60 feet thick, capped by sandstone flag, from 30 to 50 feet, which forms the summits of the coal mountains, and is the highest stratum in the county in geological and in actual elevation: the chief workings now in operation vary from 260 to 270 feet below the surface.

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