a less practised eye there seems no appreciable difference between them.
Changes, however, do occur both in the grouping and the quality of beds of coal. Changes in the grouping may occur either in the thickening or thinning of the measures between the coals, separating coals that were together, or bringing together those that were separate, or sometimes, but less frequently, even in the thickness of the coal-seams themselves, the very same becoming thicker in one place than another, or thinning out so as to occasionally disappear altogether. When, indeed, it becomes possible to trace a bed of coal over an indefinitely wide area, we must necessarily reach its termination by gradual thinning out in some direction or other.
Changes also occur in the quality and character of coals, so that sometimes, the very same bed or part of a bed, may within a comparatively short space become so altered in quality as to form quite a different variety of coal, and, of course, be no longer recognizable as the same, unless it be actually traceable by continuous working from one locality to the other, or can be referred to continuously traceable beds above or below it.
The South Staffordshire coal-field is distinguished from all others in the British Islands, and from most of those in the rest of the world, by the fact, that over a considerable part of its area a number of coals come together, resting one upon the other, with little or no interstratified shale or "parting" till they form a great seam of coal 30 feet in thickness. This has long been celebrated under the name of the Thick or Ten-yard Coal. The number of beds composing this remarkable compound seam are reckoned at from 10 to 14 in different places, according to the presence or absence of some of the beds, or of the separations between them. The "partings" of shale between the beds vary also occasionally in number and thickness, so that in some places the aggregate thickness of the coals is not more than 20 feet, with 10 feet of interstratified shales, &c.; while in others, the coals alone attain the dimensions of 36 feet, and with 3 feet of partings, make a total thickness of 39feet.
The Thick coal retains this structure, more or less completely, over all the district around Dudley as far as Bilston, Wednesbury, Oldbury, Halesowen, Brierly Hill, and Kingswinford. In two of these directions, however, namely, westwards towards Kingswinford, and northwards, towards Bilston, the remarkable phenomenon known as the "Flying reed" seems to point out the commencement of a great alteration in the grouping of its beds. The two upper beds of the Thick coal are separated from the rest gradually, but rapidly, by interposed shales and sandstones, so that in the space of a mile or thereabouts they form a distinct seam, called the "Flying reed," which, towards the west, becomes as much as 100 feet above the Thick coal, and more than 200, towards the north.
Towards the north, also, between Bilston and Wolverhampton, a separation occurs in the middle of the Thick coal, a bed of shale
