s:^ COAL 



of the middle area dip regularly northward till interrupted by the great north slip y, 

 which dislocates the strata, and throws them up 700 feet ; that is to say, a lino pro- 

 longed in the direction of any one well-known seam, will run 700 feet above the lino 

 of the same seam as it emerges after the middle slip. Immediately adjoining the 

 north slip, the coals and coal-field resume their course, and dip regularly northward, 

 running through a longer range than either of the other two members of the basin, 

 till they arrive at the valley of the Devon, at the foot of the Ochill mountains, where 

 they form a concave curvature, or trough a, and thence rise rapidly in an almost 

 vertical direction at b. Here the coals, with all their associated strata, assume con- 

 formity and parallelism with the face of the syenitic-greenstone strata of the Ochill 

 mountains c, being raised to the high angle of 73 degrees with the horizon. The 

 coal-seams thus upheaved, are called edge-metals by the miners. 



In this remarkable coal-field, which has been accurately explored by pitting and 

 boring to the depth of 703 feet, there are no fewer than 142 beds, or distinct strata of 

 coal, shale, and sandstone, &c., variously alternating, an idea of which may 

 be had by inspecting Jig. 478. Among these are 24 beds of coal, which 

 would constitute an aggregate thickness of 59 feet 4 inches : the thinnest 

 seam of coal being 2 inches and the thickest 9 feet. The strata of this 

 section contain numerous varieties of sandstone, slate-clay, bituminous shale, 

 indurated clay, or fire-clay, and clay ironstone. Neither trap-rock nor lime- 

 stone is found in connection with the workable coals ; but an immense bed 

 of greenstone, named Abbey Craig, occurs in the western boundary of 

 Clackmannanshire, under which lie regular strata of slate-clay, sandstone, 

 thin beds of limestone, and large spheroidal masses of clay ironstone, with a 

 mixture of lime. 



' With regard to slips in coal-fields,' says Mr. Bald, ' we find that there is 

 a general law connected with them as to the position of the dislocated strata, 

 which is this : When a slip is met with in the course of working the mines 

 if when looking to it, the vertical line ot the slip or fissure, it forms an 

 acute angle with the line of the pavement upon which the observer stands, 

 we are certain that the strata are dislocated downwards upon the other side 

 of the fissure. On the contrary, if the angle formed by the two lines above 

 mentioned is obtuse, we are certain that the strata are dislocated or thrown 

 upwards upon the other side of the fissure. When the angle is 90, or a right 

 ^ Q angle, it is altogether uncertain whether the dislocation throws up or 

 down on the opposite side of the slip. When dikes intercept the 

 strata, they generally only separate the strata the width of the dike, 

 without any dislocation, either up or down ; so that if a coal is inter- 

 cepted by a dike, it is found again by running a mine directly 

 forward, corresponding to the angle or inclination of the coal 

 with the horizon.' Wernerian Society's Memoirs, vol. iii. p. 133. 



The Johnstone coal-field, in Renfrewshire, is both singular and 

 interesting. The upper stratum of rock is a mass of compact 

 greenstone or trap, above 100 feet in thickness, not at all in a 

 conformable position with the coal strata, but overlying ; next 

 there are a few fathoms of soft sandstone and slate-clay, alternat- 

 ing, and uncommonly soft. Beneath these beds, there are no 

 fewer than 10 seams of coal, lying on each other, with a few 

 divisions of dark indurated clay. These coal-seams have an 

 aggregate thickness of no less than 100 feet; a mass of com- 

 bustible matter, in the form of coal, unparalleled for its accumu- 

 lation in so narrow a space. The greater part of this field contains 

 only 5 beds of coal ; but at the place where the section shown 



in fig. 479 is taken, these 5 coals seem to have been overlapped or made to slide over 

 each other by violence. This structure is represented in Jiff, 480, which is a section 

 of the Quarrelton coil in the Johnstone field, showing the overlapped coal and the 

 double coal, with the thick bed of greenstone, overlying the coal-field. 



Fig. 481 is intended to represent an extensive district of country, containing a 

 great coal-basin, divided into numerous subordinate coal-fields by dislocations. The 

 lines marked b are slips, or faults ; the broad lines marked c denote dikes : the former 

 dislocate the strata, and change their level, while dikes disjoin the strata with a wall, 

 but do not in general affect their elevation. The two parallel lines marked a, represent 

 two seams of coal, variously heaved up and down by the faults ; whereas the dikes are 

 seen to pass through the strata without altering their relative position. In this 

 manner partial coal-fields are distributed over a wide area of country in every direction. 



Fig. 482 is an instance of a convex coal-field exhibited in Staffordshire, at the 

 Castle-hill, close to the town of Dudley. 1, 1, are limestone strata ; 2, 2, are coal. 



