The smaller bogs, excluded from the foregoing computation, are very numerous in some parts. In the single county of Cavan there are above 90 bogs, not one of which exceeds 800 English acres, but which collectively contain about 17,600 English acres, without taking into the account many bogs the extent of which is from five to twenty acres each.
Most of the bogs which lie to the eastward of the Shannon and which occupy a considerable portion of the King's County and the county of Kildare, are generally known by the name of the Bog of Allen. It must not however be supposed that this name is applied to any one great morass; on the contrary, the bogs to which it is applied are perfectly distinct from each other, often separated by high ridges of dry country, and inclining towards different rivers as their natural directions for drainage..
The surface of the land rises very quickly from the Bog of Allen on all sides, particularly to the north-west, where it is composed to a con siderable depth of limestone gravel, forming very abrupt hills. In places where the face of the bills has been opened the mass is found to be composed of rounded limestone, varying in size from two feet in diameter to less than one inch; the largest pieces are not so much rounded as the small, and frequently their sharp angles are merely rubbed off. They are usually penetrated by contemporaneous veins of Lydian stone, varying in colour from black to light gray. The colour of the limestone is usually light smoke-gray, rarely bluish-black ; when it is bluish-black the fracture is large conchoidal; that of the gray is uneven, approaching to earthy. The Lydian stone when unat tached to the limestone has usually a tendency to a rhomboidal form, sometimes cubical ; the edges are more or less rounded; the longitudi nal fracture is even, the cross fracture is conchoidaL The Grand Canal from Dublin to Shannon Harbour passes through a considerable part of the great bog-district of Ireland. In forming this canal it was necessary to make considerable embankments, the surface-water of the canal being generally on a higher level than the surface of the immediately adjoining bogs. Where this was not the case advantage was taken of the circumstance to conduct the drainage of the bogs into trenches for the supply of the canal.
The bogs situated to the south of the great belt in the centre of Ireland occur in Tipperary, Kilkenny, Clare, and Queen's County ; those to the north of that belt occur in Antrim, Down, Armagh, Tyrone, and Londonderry.
It appeared from the examination of the surveyors appointed by parliament in 1810 to investigate the nature and extent of the bogs in Ireland, that they consist of "a mass of the peculiar substance called peat, of the average thickness of twenty-five feet, nowhere less than twelve nor found to exceed forty-two—this substance varying mate rially in its appearances and properties in proportion to the depth at which it lies. The upper surface is covered with moss of various
species, and to the depth of about ten feet is composed of a mass of the fibres of similar vegetables in different stages of decomposition, proportioned to their depth from the surface, generally however too open in their texture to be applied to the purposes of fuel ; below this generally lies a light blackish-brown turf, containing the fibres of moss, still visible though not perfect, and extending to a further depth of perhaps ten feet under this. At a greater depth the fibres of vege table matter cease to be visible, the colour of the turf becomes blacker and the substance much more compact, its properties as fuel more valuable, and gradually increasing in the degree of blackness and corn pactuess proportionate to its depth ; near the bottom of the bog it forms a black mass, which when dry' has a strong resemblance to pitch or bituminous coal, having a conchoidal fracture in every direction, with a black shining lustre, and susceptible of receiving a considerable polish." The surface of Irish bogs is not in general level ; indeed it is most commonly uneven, sometimes swelling into hills and divided by val leys, thus affording great facilities fOr drainage. None of the bogs of Ireland which have been described occur on low ground, a fact which seemed to strengthen the opinion of their having always originated from the decay of forests. This theory of the original formation of bogs was at one time very generally adopted, but the result of more recent investigations shows that it cannot be supported. That some bogs may have been formed in this manner is not denied. It is stated in the Philosophical Transactions,' No. 275, that "the Romans under Ostorius, having slain many Britons, drove the rest into the forest of Hatfield (in Yorkshire), which at that time overspread all the low country ; and the conqueror taking advantage of a strong south-west wind, set fire to the piteh-trees of which the forest was chiefly com posed, and when the greater part of the trees were thus destroyed, the Roman soldiers and captive Britons cut down the remainder, except a fewlarge ones, which were left growing as remembrancers of the destruc tion of the rest. These single trees did not long withstand the action of the winds, but falling into the rivers intercepted their currents, and caused the waters to rise and flood the whole flat country ; hence tho origin of the mosses and moory bogs which were afterwards formed there." This moorland near Hatfield, seven miles north-east from Doncaster, and about Thorne, is now a boggy peat covered with heath, several feet higher than the adjoining land, and very wet ; whence it has been aptly compared to a sponge full of water. The Thorne waste with some adjacent tracts and the Hatfield Moor contain about 12,000 acres.