Geologically the county is composed of two great districts, divided into two nearly equal portions by the course of the Roe. The western is the extensive mountain tract reaching from that river to Strabane, in which mica slate predominates in such proportions as to compose nine-tenths of the whole; it is accompanied by primitive limestone in the lower districts, especially in those bordering on the vale of the Roe. On the eastern bank of the same river this system of mountains is succeeded by a range of secondary heights, reposing on and concealing the mica slate, which dips under them eastward. On these is piled a vast area of basalt, forming the basis of almost the entire country between the Roe and the Bann. These basaltic strata dip with the fall of the hills towards the north-east, to meet the opposite dip of the strata on the other side of the Bann, forming the other half of this great basaltic tract.

The covering of basalt appears to acquire its greatest thickness on the north, where, as in the cap of Benyevenagh, it is more than 900 feet thick. Between the basalt and the subjacent mica slate are found in close succession many of the most important formations which occupy a great part of the southern and eastern counties of England. Next to the basalt (descending westward towards Lough Foyle and the vale of the Roe, and to the rich lands in the vale of Moyola and its vicinity) is found chalk, in beds of an aggregate thickness of about 200 feet, analogous to the lower beds of the English chalk formation, and therefore approaching in character to white limestone, being used and commonly designated as such. Even in its fossils and organic remains, this chalk is perfectly identified with that of England.

Next is seen mulatto, precisely analogous to the green sandstone formations of England: the mulatto rests immediately on a lias limestone, blue and argillaceous, disposed in small beds alternating with slate clay, and distinguished by ammonites, gryphites, and other fossil remains. The lias, in turn, reposes, as in England, on beds of red and variegated marl, containing gypsum, and even distinguished by numerous salt springs; and this marl is underlaid by a thick deposit of red and variegated sandstone, containing clay galls, and in its turn incumbent on the mica slate formation.

Sometimes, however, the mulatto and lias are entirely wanting, and the chalk may be seen immediately resting on the sandstone, both of which are constant and continuous. The deep valleys separating the detached eminences of the basalt region afford abundant evidence of their formation in excavations of part of the solid strata by some vast convulsions or operations of nature.

North-east of the source of the Roe is a small detached district of mica slate, nearly surrounded by the basaltic ridges of Benbradagh and Cragnashoack, and forming the entire mass of the mountain of Coolcoscrahan. The mountain limestone, which is micaceous and granular, occurs to the most remarkable extent on the north-west side of Carntogher mountain, in Bennady glen, near the old church at Dungiven, at Banagher, near Clady, near Newtown-Limavady, and on Slieve Gallion mountain, where it contains crystallised hornblende in abundance. Hornblende slate occurs in Bennady glen, Aglish glen, and the bed of the Roe river near Dungiven, where it is contiguous to the primitive limestone. Porphyry is the fundamental rock on the east side of Slieve Gallion, and one variety resembles sienite, with which it is in connection. Transition trap also occurs on Slieve Gallion.

The transition limestone, intervening in a few places between the primitive formations and the sandstone, is of the same kind as that which occupies so great a portion of the central counties: it is of a smoke grey colour, contains two sorts of terebratulites, and nodules of glassy quartz, which render it dangerous to blast; but being, nevertheless, the best species in the county for manure and all ordinary purposes, it is most extensively quarried. The sandstone extends the entire length of the county, from its northern extremity near Down hill up the eastern side of the Roe, and surrounding Cragnashoack and Carntogher mountains, whence it stretches by the eastern declivity of Slieve Gallion into the county of Tyrone.

The upper strata of chalk are characterised by parallel beds of flinty nodules; and, at their junction with the basalt, these flints are found imbedded in the lowest member of the trap deposit: it is curiously affected by intersecting dykes filled with basalt. The only great geological phenomenon exhibited on the sea-coast is the gradual emergence of the chalk from under the trap beds. The basalt is chiefly tabular, with the varieties called greenstone, amygdaloidal wacké, &c. A laminated schist of the mica slate formation is quarried between Derry and Newtown; there is a good quarry of lamellated schist between Bond's glen and Gossaden; gneiss occurs in the quarries of the mica slate near the Faughan river; granite on the northern summit of Slieve Gallion; the finest rock crystals are found in Finglen, Dungiven, Banagher, and in the primitive mountains near Learmount; and steatite is found in the basaltic region. Iron is found disseminated through many of the strata of the county, and in the basalt is sometimes so abundant as to affect the needle.

Ironstone, found in great abundance in Slieve Gallion, was formerly worked, but the undertaking was abandoned on the failure of fuel. The metal is found in a mixed state with manganese; and in the mountain streams mounds of it are observed in the character of yellow ochre. To the abundance of this metal in the peat moss are owing the red colour and weight of the ashes. Coal, copper, and lead have been found in very small quantities.

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