LONGFORD GEOLOGY
The northern boundary of the great limestone field of Ireland passes through this county, forming part of the hilly tract which, rising in the north-eastern part of it, proceeds into several of the northern counties. The line of division between the limestone and clay-slate proceeds from the Camlin river, near Longford, by St. Johnstown, between Lough Kinale and Lough Gownagh, to the head of Lough Sheelin. The portion of the county to the south of this line is based on limestone, the general range of which approaches to the east and west, and the dip towards the south.
An isolated mass of sandstone forms within the limestone field the hill of Slieve Goldry near Ardagh, and another at Ballymahon extends on both sides of the Inny: this kind of rock may also be observed to the west of the clay-slate formation, in the north-western extremity of the county, occupying, beyond its limits, also a considerable space on both sides of the Shannon in the counties of Roscommon and Leitrim; and on the hill at Shroid, a little east of Longford, conglomerate of a very compact structure crosses the country in a very extraordinary manner, rising in wavy undulations, frequently submerging, and again presenting itself on the surface.
There are numerous escars in all the level districts, forming a portion of the great chain which passes from the coast of Killala bay, through the centre of the island, to Lough Neagh; and here, as in every other part of their course, they are formed of fragments of primitive and secondary rocks, evidently rounded by attrition, but the greater portion of nodules in the escars of this county are of limestone, and near the base, in almost every instance, are great quantities of fine calcareous sand and marl, which are everywhere used as manure, and, on some kinds of land, are far more beneficial than lime.
Notwithstanding the abundance of limestone, sandstone, and gravel, pure water is rather scarce. At Ledwithstown is a spring of excellent water gushing out of the marble rock in a copious stream, which is very highly esteemed.
The mineral treasures of Longford are few. Lead ore has been found in several of the limestone quarries, but no practical efforts have yet been made to trace out the vein; it has also been found in some of the mountain streams, and even turned up by the plough: ochres of various colours are common.
Near the shores of Lough Gownagh are extensive rocks of iron-stone of a very superior kind, equal to the best Swedish ore, and the rocks appear to be inexhaustible, not being detached, or in thin layers like those of the Arigna district, but regular in formation and of a deep red colour. Coal shale appears in several places around Burlaghy, and near Lough Gownagh; but from the situation and arrangement of the contiguous strata, its continuation is doubtful. Near Ledwithstown, and in some other places, marble of a deep grey colour is very abundant; it takes a high polish, and is worked into chimney-pieces and other domestic ornaments.
An analysis of the blue marl that forms a bed more than ten feet thick under the bog near the Inny, gave, of carbonate of lime 44.4 parts, carbonate of magnesia 1.4, alumine 27.2, and silex 27.0. The white marl of the same district gave, of carbonate of lime 87.3 parts, bog stuff and vegetable matter 10.7, alumine 1.0, silex 0.9, and oxyde of iron 0.1. The blue clay under the bog near the Shannon gave, of carbonate of lime 53.0 parts, alumine 36.0, silex 11.0.
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