It is from this basin shape that all the other coal-fields are formed, which are segments of a basin produced by slips, dikes, or dislocations of the strata. If the coals were dislocated by two slips— one slip throwing the strata down to the east, and the other slip throwing them as much up in the same direction, the out crops of the coals would be found at some distance around.
The chief difficulty in exploring a coun try in search of coal, or one where coal fields are known to exist, arises from the great thickness of alluvial and other cover, which completely hides the out crop or basset edge of the strata, called by miners the rock-head i as also the fissures, dikes, and dislocations of the strata, which so entirely change the structure and bearings of coal-fields, and cause often great loss to the mining adventurer. The alluvial cover, on the other hand, is beneficial, by protecting the seams of the strata from the superficial waters and rains, which would be apt to drown them, if they were naked.
The accompanying cut is a good IlIns tral ion of a coal-field which has under gone alterations since its deposition:— Here we see it broken into three subor dinate coalfields, 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 alterna tions, and preserve nearly unchanged their angle of inclination to the horizon. The section shows a south coal-field dipping northerly, till it is cut across by an extensive slope, which dislocates the coal and the parallel strata to the enormous extent of 1230 feet, by which all the coals have been thrown up, not simply to the day, but are not found again till we advance a mile north ward on the line of the dip, where the identical seams of coal, shale, &c. are observed once more with their regular inclination. TheSe coals of the middle area, 'dip regularly northward till inter rupted by a slip running opposite, which dislocates the strata, and throws them up 700 feet ; that is to say, a line pro longed in the direction of any one well known seam, will run 700 feet above the line of the same seam as it emerges after the middle slip. Immediately adjoining
the last slip, the coals and coal-field resume their course, and dip regularly northward, running through a longer range than either of the other two mem bers of the basin.
With regard to slips in coal-fields, there is a general law connected with them as to the position of the, dislocated strata, which is this :--l-When a slip is met with ,in the course of working the mines—if when looking to it, the vertical line of the slip or fissure, it forms an acute angle with the line of the pave ment upon which the observer stands, we are certain that the strata are dislo cated 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 up wards upon the other side of the fissure. When the angle is or a right angle, it is' altogether uncertain whether the dishication 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 intercepted 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.
The following is the description of the several varieties of coal as given by Dr. Ure :— 1., Cubical coal.—It is black, shining, compact, moderately hard, but easily frangible. When extracted in it conies out in rectangular masses, which the smaller fragments arc cubical, The lamella; (reed of the coal) are always parallel to the bed or plane on which the coal rests ; a 'fact which holds generally with this substance.` There are two va cubical coal : the apen-burnling and the caking. The latter, hoWever small its fragments may be, is quite available for fuel, in consequence of its agglutinating into a mass at a moderate heat, by the: abtuidance of its bitumen.. This kind the' true smithy or forge coal, because it readily forths itself into a vault round the blast of the bellows, which serves for a cupola in concentrat ing the heat on objects thrust into the cavity.