110 



DISLOCATIONS. 



The western side of this coal-field, is most marked by the protrusion of trap. This 

 rock juts out in bosses of irregular shape at and about the village of Little Wenlock, 

 between seven and eight hundred feet above the sea, surrounded by broken layers 

 of carboniferous sandstone and limestone. The most prevalent variety of this trap is 

 a hard crystalline, dark-coloured rock, which may be called basaltic greenstone, though 

 in some cases the grains of felspar are with great difficulty distinguishable from those 

 of hornblende, and then the mass must be considered basalt. Olivine is here and 

 there an ingredient, and in one of the hummocks at the Horse Hays, nearer the centre 

 of the field, the rock contains prehnite, while in the western limit of the tract, there 

 are greenstones of a coarse grain which pass into tufaceous amygdaloids. 



Wherever these basaltic rocks appear, the contiguous strata of the coal-field are 

 much dismembered. This is seen not only on and near the surface, but has been 

 proved by numerous underground workings, the full details of which will be found in 

 Mr. Prestwich's memoir. It is sufficient for the present to state, that the trap of this 

 field is very similar to that which in subsequent chapters will be described at greater 

 length, as bursting through and dislocating the coal measures of other tracts, particu- 

 larly in the Clee Hills, where the phenomena of this class being much more striking 

 will be explained at greater length. This brief notice of the characteristic and preva- 

 lent trap rocks of Coal Brook Dale is, however, a natural prelude to a notice of the dis- 

 locations of the strata produced by such eruptions. 



Faults. — The striking dislocations to which the carboniferous strata of this tract 

 have been subjected, are indeed well accounted for by the protrusion of the above-men- 

 tioned volcanic rocks. Referring my readers who desire further details to the valuable 

 memoir of Mr, Prestwich, I shall here do little more than direct their attention to the 

 principal lines of fault as laid down upon the map, and to the diagram, PI. 29. fig. 16., 

 which explains some of the principal fractures. Mr. Prestwich has well observed, that 

 there is probably no coal-field of equal size in the kingdom which has been so greatly 

 shattered as this. It has, in fact, been powerfully elevated from beneath the sur- 

 rounding Lower New Red Sandstone in separate wedge-shaped masses, the apices of 

 which terminate at Lilleshall and Newport. The longitudinal lines of fault by which 

 the field is thus cut up, run nearly from south-west to north-east, and when viewed on 

 the map, resemble the sticks of a slightly expanded fan, the handle of which is the 

 trappean hill of Lilleshall (to be described in the sequel) . These lines of greatest frac- 

 ture are also traversed by cross faults of minor importance, trending from east to west, 

 and north-west to south-east, &c. The most powerful of the longitudinal faults is that 

 which bounds the coal-field, on the east, and separates it from the Lower New Red 

 Sandstone. The coal-measures along this line are not less than one thousand feet thick, 

 and as some of the lower seams of coal are thrown up to the level of the overlying 

 strata of the New Red Sandstone, this upcast (the main fault of Mr. Prestwich) is thus 

 shown to have exceeded one thousand feet, though to what further extent has not yet 



