ferous strata, there are sometimes found outliers obscurely attached to the main formation, or detached portions forming small basins, not more than a mile in diameter, and called in the West Riding of Yorkshire, Swilleys. Seams of coal, inferior in thickness to those belonging to the Newcastle coal formation, and interstratified with the encrinital limestone, as well as with sandstone and shale, are spread over most parts of Northumberland; but owing to these "landsale collieries" being generally inconsiderable in point of depth and extent of workings, the continuity of the beds of coal has never been accurately ascertained. Sections of coal mines in this formation are to be found in the fourth volume of the Geological Transactions, where an account of Shilbottle colliery, which supplies Alnwick with fuel, is given: and in the Transactions of the Natural History Society of Newcastle, sections of the more important mines in the vicinity of Berwick-upon-Tweed are inserted. Mr. Wynch, from whose interesting papers these particulars are derived, gives a section of the colliery close to the old castle of Blenkinsop, 33 miles west of Newcastle. The deep pit at this place was 56 fathoms, and the viewer considered the position of the coal to be below the four fathom limestone, and above the great limestone of the Alston Moor mining field, and that the bed of coal was the same as that worked in the more extensive mines on Tynedale Fell. From these collieries Carlisle derives its coal.*

The unexplored coal beds enveloped in the limestone, become important in considering the unresolved problem of the extent and consequent duration of the northern mines.

* Lond. and Edin, Phil, Mag. and Journal, Oct. 1833, p 274.

CHAPTER VII.

COAL MEASURES.

Meaning of the terms "Coal Measures"-Arrangement, contortions, and dislocations of strata—Vertical section of a deep pit near Newcastle-Tabular view of substances passed through-Gosforth colliery Depth of the High Main Seam at Jarron-Sections of Mines at Dudley and Bilston—Inequality in the thickness of matter occurring between certain Coal seams-Tabular view of strata at WhitehavenSynopsis of Coal measures at Ashby-de-la-ZouchStaffordshire, Lancashire, Derbyshire, and Yorkshire coal fields-Sheffield-Section at HalifaxNotices of the coal strata in South Wales, Scotland and Ireland-Occurrence of iron ores in the coal formation.

By the term "measures," it is merely intended to designate the stratification of any particular coal district, comprising what belongs to the dip, thickness, and depth, and composition of the several solid matters exposed and raised in the progress of mining. It will be obvious that, on sinking a pit in any coal-field, consisting of cavities bounded in the manner described in the preceding chapter, the concentric beds, of

K

whatever composition, might be expected to be met with more highly inclined, as the working takes place further from the centre of the mineral basin, supposing its form to be tolerably complete. The realization of this probability, however, will always depend very much upon the precise character of the local formation; for, as the strata by upheaving or dislocation, may be thrown into a variety of positions, from the horizontal to the vertical, so the difficulty of judging from any data short of actual inspection, is proportionately enhanced.

Mr. Conybeare, in speaking of the coal measures says, "the strata are generally inclined, and frequently at a very high angle, being entirely unconformable to those more horizontal beds which overlie them; they frequently also exhibit contortions as rapid and singular as those which occur in the transition slate rocks below: appearances of this kind are displayed in a manner peculiarly striking on the coasts of Bride's-bay, Pembrokeshire, near Littlehaven. It may be observed, that where the associated solid masses of limestone and sandstone are elevated in high angles, but still disposed in nearly regular planes, the more tender argillaceous beds are generally twisted, and as it were crumpled together. The Mendip Hills and adjacent collieries in Somersetshire, afford an excellent illustration of this fact, which strongly suggests the idea of a mechanical force which has elevated the more solid rocks en masse; while

* It is very uncommon to find the carboniferous strata thus highly inclined; yet at a place called the Bank, betwixt Edmonstone and Niddry, no very far from Edinburgh, about two miles south west from the sea, the strata are said to be in a perfectly vertical position; pits of a considerable depth having some years ago, been sunk in a seam of coal from top to bottom, without going into the stone on either side, which, had the working been horizontal or nearly so, would have been the roof and the floor of the coal.

DISLOCATIONS OF STRATA.

131

the more yielding materials, giving way to its lateral pressure, have become irregularly contorted. These phenomena cannot be attributed to any internal power like crystallization; for they appear to be common to all rocks, even those most decidedly mechanical in their structure. The faults, or as they may be most appropriately termed, dislocations of the coal fieldsand of which we shall treat in the next chapter-are still more irresistible evidences of their having been affected by violent mechanical convulsions subsequently to their original formation. These faults consist of fissures traversing the strata, extending often for several miles, and penetrating to a depth in very few instances ascertained; they are accompanied by a subsidence of the strata on one side of their line, or (which amounts to the same thing) an elevation of them on the other; so that it appears that the same force which has rent the rocks thus asunder, has caused one side of the fractured mass to rise, or the other to sink; it being difficult, if not impossible to say (since in either case the disjointed masses would be the same) in which direction the absolute motion has taken place. Thus the same strata are found at different levels, on opposite sides of these faults, which appear to derive their name from their baffling for a time the pursuits of the miner; they are also called traps, probably from a northern word signifying a step, and the elevation or subsidence of the strata is described as their trap up or down. The change of level occasioned by these dislocations sometimes exceeds 500 feet; whence we may infer the immense violence of the convulsion, which had power to produce motions of such vast masses to such an extent. The fissures are usually filled by clay, which has sub

sequently filtered in, and often includes fragments disrupted from the contiguous strata; their direction usually approaches to vertical."*

1

It will be at once apparent, from the preceding remarks, that the same beds of coal in sections of the same basin may be found at very different depths indeed, should the field in which they occur, besides possessing great sphericity of internal structure happen to be intersected by faults or fissures; and, on the same account, it will be clear, that the details of one pit can rarely be taken as indicative of the depth to which it may be necessary to sink for coal in any particular district. It may indeed, exhibit generally the nature of the measures in any proximate portion of the same field; and also, if the dip of the strata has been ascertained, it may afford in connexion with other circumstances, good criteria for determining the probable success of an adjacent shaft. To shew the various sorts of substances through which the northern miner has to pass before he comes at the object of his efforts, the following section of the strata, south of the main dike in Montagu Main colliery, 3 miles above Newcastle, is taken from a well written article on the subject.† The numbers in the first column on the left hand form an index, from which it will be immediately perceived, where the same strata occur; the second column contains the number of the strata; the third the names of each; and the fourth or numeral columns, express the thickness of each stratum in fathoms, yards, feet, and inches. It may also be premised concerning the five or six different classes of substances named, that whinstone is the hardest-so hard indeed, that

Introd. Geol. Part I. p. 348. + Rees's Cyclopædia, art. Coal.