COAL 



831 



is evident that the dip of the coal-strata belonging to it runs in opposite directions, 

 on the opposite sides, and that all the strata regularly crop out, and meet the alluvial 

 cover in every point of the circumferential space, like the edges of a nest of common 

 basins. The waving line marks the river Devon. 



It is from this basin shape that all the other coal-fields are formed, which are seg- 

 ments of a basin produced by slips, dikes, or dislocations of the strata. If the coal 

 (fig. 467) were dislocated by two slips b c and d e. the slip b c throwing the strata 

 down to the east, and the slip d e throwing them as much wp in the same direction, the 

 outcrops of the coals would be found in the form represented in fig. 470, of which 

 fig. 471 is the section in the line A B, and Jiff. 472 the section in the line c D. 



The absolute shape of the coal-fields in Great Britain has been ascertained with 

 surprising precision. To whatever depth a coal-mine is drained of its water, from 

 that depth it is worked, up to the rise of the water-level line, and each miner con- 

 tinues to advance his room or working-place, till his seam of coal meets the alluvial 

 cover of the outcrop, or is cut off by a dislocation of the strata. In this way the 

 miner travels in succession over every point of his field, and can portray its basin- 

 shape most minutely. 



Fig. 473 represents a horizontal plan of the Clackmannaushire coal-field, as if the 

 strata at the outcrop all around were denuded of the alluvial cover. Only two of tlio 

 concentric beds, or of their edges a, , are 

 represented, to avoid perplexity. It is to be 

 remembered, however, that all the series of 

 attendant strata lie parallel to the above __ 

 line. This plan shows the Ochill mountains, j= 

 with the north coal-fields, of an oblong ellip- | 

 tical shape, the side of the basin next the 

 mountains being precipitous, as if upheaved 

 by the eruptive trap-rocks ; while the south, East 

 the east, and the west edges of the basin 

 shelve out at a great distance from the lower Great 

 part of the concavity or trough, as miners 

 call it. Thus the alternate beds of coal, 

 shale, and sandstone, all nearly concentric in 

 the north coal-field, dip inwards from all 

 sides towards the central area of the trough. 

 The middle coal-field of this district, how- 

 ever, which is formed by the great north 

 slip, is merely the segment of an elliptical 

 basin, where the strata dip in every direction 



473 



West 



to the middle or the axis marked with the letter x ; being the deepest part of the 

 segment. The south coal-field, formed by the great south slip, is likewise the seg- 

 ment of another elliptical basin, similar in all respects to the middle coal-field. 

 Beyond the outcrop of the coals and subordinate strata of the south coal-fields, the 

 counter dip of the strata takes place, producing the mantle-shaped form ; whence the 

 coal-strata in the Dunmore field, in Stirlingshire, lie in a direction contrary to those 

 of the south coal-field of Clackmannanshire. o, are the Ochill mountains. 



Fig, 477 is a very interesting section of the main coal-basin of Clackmannanshiro, 

 as given by Mr. Bald in the Wernerian Society's Memoirs, vol. iii. Here we see it 

 broken into three subordinate coal-fields, formed by two great faults or dislocations of 



the strata ; but independently of these fractures across the whole series, the strata 

 continue quite regular in their respective alternations, and preserve nearly unchanged 

 their angle of inclination to the horizon. The section shows the south coal-field 

 dipping northerly, till it is cut across by the great south slip x, which dislocates the 

 coal and the parallel strata to the enormous extent of 1,230 feet, by which all the 

 coals have been thrown up, not simply to the day, but are not found again till we 

 advance nearly a mile northward, on the line of the dip, where the identical seams of 

 coal, shale, &c., are observed onco more with their regular inclination. These coals 



