We have now to describe the general effect produced upon the coals and accompanying strata by dikes and slips, in order to shew how coals wave, or are spread over a great district of country ; and this is essentially necessary before attempting to describe the modes which have been adopted in searching and boring for coal.
Although, as before mentioned, there are instances of coal-fields of a circular bason form, and more com monly of a long elliptical shape, yet there are few in stances, comparatively, of this entire form, either in the right or inverted position ; yet we reckon that the en tire bason form is the true and complete form of a coal field, and that the general form of coal-fields are por tions of this bason shape, produced by the natural effects of dikes and slips, of which the following detail, with the corresponding figures, Plate CCCLXXXIX. will elucidate. Fig. 6. represents a horizontal section or plan of a coal-field, as found immediately under the allu vial cover. Only two beds of coal are represented in this example, that the subject may be rendered as little complex as possible, and, therefore, whatever changes in position, and whatever dislocations take place in the coals, take place also in all the accompanying strata, at least this may be admitted in a general point of view. The area in the figure is supposed to contain a space of 64 square miles, viz. a square of 8 miles on the side. The coal district is bounded by transition rocks on the north, more elevated than the coal district. A, is an elliptical bason-shaped coalfield, where the coals a, b, dip inwardly, as represented by the points of the arrows, hut they dip much quicker next the transition rocks, as they then conform with the dip of these ; and though the coals are the same distance from each other in every part of the bason, when measured at right angles to the pavement, they appear in the horizontal section much closer upon the north than on the south side of the bason. This apparent difference depends entirely upon the angle of inclination which the coals make in particular posi tions with the horizon, as will be shewn distinctly in the vertical section. Upon the south of the coal-field A, the slip B is found dislocating the-strata, throwing the coals down from the surface 140 yards to the south, and forming the coal-field C, where the same coals of the coal-field A are found again, having the same parallelism of dip. The slip D is found upon the south of the coal field C, dislocating the strata, and throwing the coals down from the surface 100 yards to the south, and form ing the coal-field E, where the coals of the coal-field C are also found. The slip F is found dislocating the strata, and throws the coals down 210 yards, and forms the coal-field G in the same manner as the former coal fields, as represented by the coals a, b. The coal-field G, in place of a slip to the south of it, takes, as it is termed, a counter dip, occasioned by a ridge in the strata of a saddle shape, beyond which ridge the same coals form with a contrary dip, as represented by the arrows, and form the coal-field H. In this manner coal-fields
are found stretching over a district of country in a crop and dip direction, somewhat similar to what is represent ed in the Figure, but with many varieties arising from the obliquity of the slips to each other. This spreading out of the coal-fields in the crop and dip direction is pro duced by slips generally lying, not parallel, but some what in the direction of the line of bearing, and by ridges and counter dips lying in the same direction. Be sides the coals a, b, inserted in the Figure, as an exam ple, all the coals, above and below these, with their ac companying strata, suffer the same alteration as before mentioned.
While these slips and dislocations lying nearly in the line of the bearing of the coal have the effect of throw ing the coals over a district of country in the line of dip and rise, the dikes, slips, and dislocations which lie in an oblique direction to the former, such as the slips marked K in the Figure, have the effect of stretching or extending the coal-field over the country in the line of bearing. The lines marked L, are dikes which only se parate the strata the thickness of the dike, without throw ing the coals and the accompanying strata either up or down. It will be seen from the Figure that the oblique dislocations, when they throw the strata down, and the same parallelism of dip remains unaltered, that the crops extend to a greater distance before they meet the allu vial cover ; and, on the contrary, when the slip is an upthrow, the crops of the coals and accompanying stra ta are diminished in breadth by coming sooner in contact with the alluvial cover. K c, is a slip down to the east, forming the coal-field NI. K d is a slip down to the west, forming the coal-field N. K e is a slip up to the west, which cuts off the coal-field N in that direction. K f is an upthrow slip to the east, which has the effect of throwing back the crops of the strata. K g is a down throw slip to the west, which has the effect of throwing forward the crops of the coals and strata. K h and K i being downthrow slips to the west, have the same ef fect as in the last instance. K k being an upthrow slip to the west, has the effect of throwing back the coals and accompanying strata. In this manner coal-fields are extended over a district of country in the line of bearing, in every variety of change, according to the line of obliquity in which the slips occur with regard to the line of bearing, and the extent of the dislocation, as to the magnitude of the slip. These of course fre quently change the direction of the line of level at times to the extent of 90°, as must be obvious from inspecting the Figures ; because, generally, the line of level is a line parallel to the crop, that is, parallel to the line of the coals where they iotersect the plane of the alluvial cover, which will be more particularly exemplified when the working of coal is treated of.