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A HISTORY OF LANCASHIRE Gannister and Upper Foot seams, which takes place along an irregular north-west and south-east line a little to the north of the Rossendale anticlinal. Although up to the point of union the individual thicknesses of the two mines are but two feet six inches and eight inches respectively, yet at their junction the united seams swell out to a thickness of nearly eight feet, and the average over a great area is four feet.

The special features of the union of these two seams were dealt with by J. Aitken, and his explanation is probably the correct one, viz., that a part of the Gannister area was one of subsidence, the submersion going on until a sufficient depth beneath water was obtained to allow of the deposition of sufficient detritus to form the rock mass overlying that mine and separating it from the thin coal above. He goes on to say in his paper: 'It would further appear that the surface over which the four-foot coal was then in process of formation remained stationary and undisturbed, and that the operations of nature were not in any way interrupted.'

This view is supported by the fact that the coal of the Four-Feet Mine is considerably thicker than the aggregate of the two mines while separate, the growth of vegetation over the area being evidently continuous during the period in which the submerged portion was being silted up. When the latter had taken place, the coal forest grew out over the shallows, giving rise to the thin 'Upper Foot' coal, after which the whole area occupied by the Four-Feet Mine and the Upper Foot coals was submerged, and a uniform deposit of mud took place.

The Upper Foot Mine is worthy of note, not on account of its thickness, which is almost invariably 12 inches, but because of the occurrence of great quantities of 'bullions' or coal balls within it, each bullion ball containing portions of coal plants in which the structure has been so well preserved as to allow of the closest microscopical investigation. From these bullions were obtained the stems, etc., of coal plants described by Binney, Carruthers, Williamson, Hick, Scott, and others.

Upon the coal are found numerous flattened limestone nodules called 'bawn-pots,' each with a thin crust of iron pyrites and containing well-preserved examples of Goniatites, Orthoceras, Pterinopecten (Aviculopecten), and Posidoniella.

The coals are all bituminous and caking. Iron pyrites occurs as nodules in some of the coals, and also as a thin film upon joint planes, in some cases (the upper seams) so abundantly as to seriously injure the usefulness of the coal. The demand for these coals is entirely local, and their use as fuel is restricted to engine boilers and the open fireplaces of the people.

The fireclays under the Gannister seams have been worked at times in conjunction with the coal, as they make excellent firebricks, drain pipes, etc. Works of this description can be seen at Colne, Townley near Burnley, Sharneyford, north-east of Bacup, Littleborough, and other places.

MIDDLE COAL MEASURES

We have already alluded to the broken-up character of these measures, whereby small isolated portions have been dignified with the name of coalfields. The most southerly patch is the Manchester coalfield, which is about four miles long from north-north-west to south-south-east and a mile and a half broad across its greatest diameter. This coalfield, whilst relatively insignificant and now little used, is of considerable geological importance in that the upper coal measures are well developed. The Middle Coal Measures are deep seated and scarcely touched, owing to the great thickening of the barren measures below the Four-Feet coal of Bradford and Clayton.

This latter seam was formerly supposed to be the equivalent of the Worsley four-feet seam which marks the upper limit of the Middle Coal Measures in other parts of Lancashire, but more recent researches seem to render this correlation doubtful. Attempts to reach the thick coals of the Middle Coal Measures have hitherto failed, the unproductive beds lying below the Bradford Four-Feet having been penetrated by Mr. Livsey to a depth greater than should have been necessary had the Crumbourke and Rams Mines occupied the same position relatively to the Bradford Four-Feet as they do to the Worsley Four-Feet.

Professor Hull is of opinion that at least 616 yards of barren measures will have to be penetrated below the Bradford and Clayton Four-Feet seam before a workable coal is reached.

In this present state of our knowledge it is best to regard the presence of the Middle Measures as certain, and the upper limit as undetermined.

The Upper Coal Measures which have been worked in this coalfield will be dealt with elsewhere.

SOUTH LANCASHIRE COALFIELD

This coalfield which is extremely irregular and much cut up by faults, can be best dealt with by a division into districts.