DISLOCATIONS OF THE OSWESTRY COAL-FIELD. 



147 



Faults. 



This tract is affected throughout by a great number of faults, many of which can be 

 studied in the ravines and natural escarpments of limestone, which rise from beneath 

 the coal-field. Thus, great transverse breaks can be observed between the ends of each 

 ridge of limestone, and splendid examples of dislocated masses are exposed in the pic- 

 turesque cliffs of Craig-y-rhiw, where this rock is in contact with Silurian schists and 

 near to a point of eruptive trap. It is, however, only from the miner that we can ob- 

 tain a correct knowledge of those faults which have affected the productive coal-field. 

 The Lwynymain or big fault runs north and by east, and south and by west. On the hilly 

 or western side of this fault the coal was worked in the Dog pit, at a depth of two hun- 

 dred and twenty yards, while on the lower or eastern side, the same seams of coal are 

 within forty yards of the surface. There is one other principal fault, nearly parallel to 

 the above, which occurs further on the dip, by which the coal strata descending towards 

 the plain, are again heaved up. This is another upcast on the dip, and to an extent of 

 fifty yards. By inspection of the map it will be seen that both these faults are only 

 slightly divergent from the principal strike, or line of elevation of the adjacent moun- 

 tains on which the coal-field reposes. A third fault, equally an upcast to the east, but 

 diverging considerably from the two above mentioned, occurs close to the western side 

 of the new works on the Drillt : this is a throw of twenty yards. There are many minor 

 faults, but few of these preserve any distinct parallelism to the main faults, or to each 

 other. The phenomena on the sides of these faults, where the ends of the shattered strata 

 have been laid bare, are similar to those observed in other coal-fields, the edges of the 

 strata frequently presenting that polish, known to miners under the name of ' 1 slickensides " 

 It is unnecessary to give further details respecting these dislocations, and I only solicit 

 attention to the fact, that the main faults run in a direction nearly parallel to the 

 elevated range of sandstone and limestone forming the hills at the back of the coal-field ; 

 for as we see that the oldest strata on which these masses repose, have been penetrated 

 by rocks of igneous origin, we may confidently conclude that the forces which elevated 

 the hills, also gave rise to the principal parallel fractures of the adjacent coal measures; 

 and further we observe that the fault of greatest magnitude is nearest to the mountain 

 range, and that as the strata recede from it, the dislocations are of minor extent. Again, 

 the persistence of these north and south faults into Denbighshire is a full confirmation 

 of their dependence on the principal lines of elevation of the adjacent hills, which form 

 the margin of the North Welsh coal-field. 



The average dip of the coal measures is about one in six, and this inclination varies 

 little, either on the west or east side of the great faults. The small portion of this coal- 

 field which is now productive, is a trough in the millstone grit between Oswestry and 

 Sweeny, to the north and south of which the coal is fairly thrown out by the rise of that 

 rock. Nor are there just grounds for hoping, that any great mass of coal is prolonged 



