The whole of the mountains in this county are of primary formation; but rocks of secondary formation are frequently found overlaying the primary at the base of the mountains. The secondary formation on the northern range is red sandstone covered with slate clay, which alternates with floetz sandstone; as also, limestone alternating with slate clay: to the south is the old red sandstone covered with floetz limestone. A line drawn from the southern shore of Achill and continued along the southern base of Coraan mountain, would very nearly pass along the junction of the primary and secondary formations. The primary rocks pass by a narrow isthmus between the secondary rocks at Castlebar, to the shore of Clew bay, and join the primary range, which commences at Croagh Patrick and proceeds thence into Joyces' country. No secondary rocks are met with between Achill head, on the west coast, and Balderric on the north.

Near this latter point the secondary formation commences, and continues without interruption along the coast by Downpatrick, Killala, and Sligo to Ballyshannon. From the junction at the sea shore near Balderric, the edge of the primary country takes a direction nearly south to Croghan mountain, at the base of which, in the bed of the Owenmore, the old red sandstone appears overlaying mica slate. From Croghan the junction of the rocks may be traced to the northern base of Nephin, and thence, crossing Lough Conn, it proceeds along the northern base of the range of primary mountains from Foxford to Ballysadare.

The great mountain chain, commencing at Nephin and extending in a north-western direction by the valley of the Owenmore to Coraan mountain, is entirely composed, except at Nephin Beg, of mica slate and granular quartz rock. Nephin mountain, the strata of which are fully disclosed by the fissures already noticed, is formed of mica slate, of very variable proportions. The stratification is for the most part nearly vertical, but very irregular. The composition of Nephin Beg is very similar to that of the larger mountain. The western part of the county between Achill head and the north coast is for the most part a low plain of bog on a bottom of mica slate, backed by mountain ranges of quartz rock. In the island of Achill the rocks are chiefly quartz; in the adjoining peninsula of Coraan, conglomerate sandstone.

In the peninsula of the Mullet the country is mostly covered with silicious sand. On the western shore, where the rocks are visible, they are more crystalline than the mountains on the east. Upon the whole, this part of Erris much resembles the islands of Col and Tirey in the Hebrides. At Dunfeeny bay, the alternating beds of sandstone and clay-slate are covered by almost innumerable alternations of thin beds of black shale and black limestone. Many of the beds of black shale contain balls of clay iron-stone; some so carbonaceous as to soil the fingers very much, and to emit a dense black smoke when thrown on the fire.

The old red sandstone at the base of Croghan mountain is covered by a series of rocks of slate clay and floetz sandstone alternating; and though no beds of coal have been discovered, there is reason to think that those series belong to that formation; the existence of this mineral is also shewn by a variety of indications in the mountains of Derinkee, near Westport. The iron-ore, the sandstone, the ochre (which is found in abundance and of good quality) and the ferruginous scum on the lakes and rivers, all seem to point out its existence there: in Slieve Carne, in Clanmorris barony, there are also many indications of coal.

The character of the limestone which alternates with the clay-slate along the north-eastern coast, from Balderric to Ballyshannon, is very peculiar: the beds rarely exceed two feet in thickness; some are of a dull black colour and contain no marine remains; others are almost wholly composed of muscle shells. Iron-ore is found in abundance on the lands of Cross, in Erris, and at Tallagha in the same barony: it is also met with in abundance in the bed of Clonoure river, and in the Deel river, where it is found in small lumps of a beautiful red colour. Iron-works formerly erected by Sir George Shaen, near the Mullet, and more lately on the Deel by Mr. Rutledge, were discontinued from the want of fuel.

Manganese is abundant in several parts, but the richest deposits are in the neighbourhood of Westport and in the vale of the Owenmore; clay fit for porcelain and every other description of earthenware is found in inexhaustible beds, and cargoes of the finer sorts are shipped for England. Fullers' earth and pipe clay are also abundant and very good; clay for bricks abounds in every barony. Slate quarries on an extensive scale have been worked near Westport; freestone of a good colour, very compact and easily wrought, is abundant in several places; and grey marble, beautifully marked and susceptible of a fine polish, is plentiful in the barony of Murrisk: excellent hones are procured in the hill of Bocca.

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