The county exhibits some very striking geological features. The red sandstone formation embraces a considerable portion of its southern and eastern parts, while the greatest part of the north and west belongs to the clay-slate formation. In both districts there are considerable exceptions. The clay-slate is intersected by a vein of micaceous limestone, which first appears in the bed of the river Poe; thence passing near Newtown-Stewart and crossing the Munterlowney mountains, it terminates near the village of Dunamanagh, in the northern extremity of the county. Detached portions of limestone, similar to that of the great central field of Ireland, are to be met with in many parts: white limestone, containing numerous nodules of flint, similar to that of Antrim, is found near Coagh.

Near Cookstown is a species dissimilar to all the others, and containing a great variety of organic remains: the vein extends southwards to Stewartstown and is disposed in strata varying from five inches to four feet in thickness. But the most remarkable geological feature of the county is its coal formation, in which, though the field is of small extent as compared with those in the south and west, it surpasses them in the thickness of the seams and quality of the mineral. The district around this coal field contains rocks of every class, from the more ancient of the primary to the latest of the secondary or alluvial formations.

In the Coal Island works the coal rests on fireclay, in Drumglass on soft porous sandstone, and in Annahoe on blue clunch; but as the country in which the collieries are situated is covered with alluvial soil to the depth of from 20 to 30 feet, it is often difficult to trace the various beds. In its external aspect it is in general similar to that composed of sandstone; the surface exhibiting an assemblage of low hills with steep acclivities and flattened summits, rarely exceeding 100 feet in height: when higher, their upper part is generally composed either of new red sandstone or of trap. The Coal Island district is 8 miles long by an average breadth of 2 ½ miles, and therefore comprehends an area of about 1140 acres; the Annahoe district is little more than a mile long by half a mile in breadth, and may therefore contain about 500 acres. Both districts contain sandstone, sandstone slate, shale, argillaceous ironstone, and fire-clay. The composition and external character both of the coal and of its accompanying strata are nearly similar in the two divisions: it burns rapidly, giving out a bright blaze and intense heat, like that of Ayrshire. The shale, called by the miners metal, varies in colour from light blueish white to black, is extremely soft, and decomposes rapidly on exposure to the atmosphere: it sometimes contains impressions of ferns, myrtle, and gigantic reeds.

An uncommon species of clay-stone, extremely compact and difficult to break, occurs interstratified with the shale. Argillaceous iron-stone is not abundant; when found, impressions of a large species of fern are frequently detected in the interior. The fire-clay, which lies immediately beneath the bed of coal, is so soft as to form a pulpy mass on the admixture of the slightest moisture, and by allowing the pillars of coal which support the roof to sink into it, immediately swells and would close the workings were not great precautions adopted. This clay makes fire-bricks equal to those of Stourbridge.

Great irregularity prevails in the direction and inclination of the coal strata: the main dip in the southern extremity is north-east; in the northern, south-west; but it is frequently altered by wavings or undulations, which are generally north and south. Besides these undulations, which throw the strata into confusion, the continuity of the beds is often broken by slips or faults. The average angle of the strata with the horizon is about 11° 30', or one foot of fall for five of length, but in many places it increases to 50°: the difficulty of clearing off the water is much increased by this increase of angle.

The quantity of coal capable of being produced from the Coal Island district may be estimated from the fact that, in the immediate vicinity of the village, there are seven workable beds of coal, amounting, in the aggregate, to 34 feet of coal in a depth of 244 yards: no instance occurs in the great mining districts of England of an equal number of beds so near each other. From the sulphureous and ferruginous appearance of the water in many places, it is evident that large quantities of iron ore are deposited here.

Clay, of various colours, for making bricks, may be procured in all parts of the county. Good flooring and ridge tiles, garden pots, and coarse earthenware are made in the neighbourhood of Moy and Killyman. Excellent pottery is manufactured near Coal Island: the clay, which is of a muddy white before it is baked, is made up into small oblong wedges of about a pound each, and sold as a substitute for fullers' earth, for which purpose it is sent to all parts and brings back a profitable return. A line of escars proceeds from Killyman, by Dungannon, Ballygawley, and Clogher, to Five-mile-town, where it enters the county of Fermanagh. Those in this county are formed of nodules of basalt, greenstone, porphyry, limestone, chalcedony, jasper, and agate: a branch of them near Fintona is almost exclusively formed of chalcedony, jasper, agates, and quartz.

At Killeshill and Newtown-Saville the formation of the escars is as regular as if they had been artificially arranged. In the sandstone formation in Killyman, fossil fishes of several species are found, among which the trout and pike can be distinctly recognised: on raising the stone from the quarry, the fish is found imbedded in it, one side of it being raised in high relief, and the concave impress of it in the lower stone exhibits the marks of the gills, eyes, and scales with the utmost accuracy.

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