The same very useful and important law holds as to slips, with this difference, that when the slip forms an angle of 90° with the pavement, there is a great uncer tainty whether the strata on the other side are thrown up or down ; and this can only be proved by mining and boring ; only it must be remarked, that if, in perfora ting the slip, the coal is found neither thrown up or down, the occurrence is not a slip, but simply a fissure of the strata.
Dikes and slips are denominated upthrow or down throw, according to the position in which they are met in working the mine. Thus, in Fig. 10. Plate CCCLXXXVIII. if the miner is advancing to the rise, the dike AB neither throws up nor down. CD is a downthrow dike to the rise of a eel-tail: number of fa thoms, and El: is an upthrow dike also to the rise. On the contrary, if the dikes are met with in working from the rise to the dip, the names of the dikes, as to being and downthrow, are exactly reversed; viz. -what was an upthrow in the first instance, is a down in the second ; and, in meeting the dikes or slips this reverse way, the same law of course holds as to the angle formed by the pavement, and the intersecting line of the dike or slip.
In the same manner, dikes and slips, which run from the dip to the rise direction, are denominated upthrows or downthrow s, to the east, to the west, to the south, or to the north, according to the line of bearing of the coal-field, and the direction in which the dikes or slips are met with in the mine; so that a dike or slip which is an upthrow to the east, when pursuing the coal mine eastward, is a downthrow dike or slip, when pursuing the coal mine westwatd.
Hitches are slips of the strata of a small size, and are so denominated when the slip or dislocation does not exceed the height of the coal ; they are frequently very small, even the least part of an inch, yet though of the smallest size, their effects are distinctly visible affecting the coal. llitchcs are frequently denominat ed, very appropriately, steps by the miner. Plate CCCLXXXIX. Fig. 2. shews the effect of hitches upon the strata, where the hitches A, B. C, D. E, F, G, II, dislocate the coal and accompanying strata. At A the hitch is the height of the coal, at B it is half the height up, at C and D the effect is less, at E it is nearly the height of the coal, at F the effect is scarcely visible, at G it is down, and at H the coal is again thrown up to the extent of its height. Hitches do not extend gene
rally very far, either in a longitudinal or in a perpen dicular direction ; for though they may be found in one or two coals in a coal-field, they may not be seen in the the upper or under coals, so that their effects in the coal-field are commonly very limited, when compared with the effects of dikes or of slips.
Troubles —These affect the stratum or bed of coal, and are of various kinds, as before mentioned, viz.
1st, Some in irregular beds.—This trouble sometimes begins as a horizontal division in the coal scarcely visi ble, gradually increasing in thickness, and that fre quently to such a degree as to render the coal altoge ther unworkable ; in a thick bed of coal, a stone of this kind sometimes increases from a line even to several fathoms in thickness, in which case one or other of the portions of the coal thus divided, is either abandoned as unworkable, or each of the two divisions is wrought as a separate and distinct coal. It is to be remarked, that the two portions of coal are together generally equal in thickness to the bed of coal before the stone made its appearance ; if however bands of stone or of argillaceous clay are found lying regularly through coals in a coal-field, these are not reckoned troubles.
2d. .Nips.—This is a very remarkable trouble in coal, and is not very common ; it is produced by the roof and pavement of the coal gradually approaching each other, until not a vestage of coal is to be seem. The extent of this trouble is from an area of a few feet to that of many acres. The horizontal form of a nip is very irregular, and when found, no idea can be formed of its extent ; it is frequently the case, that if the coal has an argillaceous roof and pavement, these disappear with the coal, and the upper and under harder strata of sandstone are in contact, divided only by a very thin sooty line. Plate CCCLXXXIX. Fig. 3. repre sents the vertical, and Fig. 4. the horizontal section of a nip.