Geological Society. S85 



loses its cohesion and becomes sootv, and the same change was no- 

 ticed elsewhere near dislocations, though no trap was visible. 



Dislocations. — The author says that there is probably no coal-field 

 of equal size in the kingdom so greatly shattered as that of Coalbrook 

 Dale. The faults are most numerous and complicated where the 

 measures are thinnest, the miner in those parts rarely proceeding 20 

 yards without interruption, and frequently not more than two or three j 

 but when so close together the dislocations are small in effect and ex- 

 tent, and are connected with others of greater magnitude. 



The larger faults tilt the strata in various directions, but have ge- 

 nerally a parallelism of strike, and deviate but very slightly from a 

 straight line. Sometimes the sides of the disjointed strata are in conr 

 tact, when the edges of the beds of coal and shale have a shining stri- 

 ated surface, but at others the sides are separated several yards, the 

 interval being filled with the debris of the strata. The inclination of 

 the principal faults as well as of the minor, obeys no general law, and 

 even in the same fault it occasionally varies from 45° to !)0°. The 

 difference of level on the opposite sides of the principal dislocations 

 also varies considerably ; thus the Lightmoor fault, at Malmslee and 

 Old Park produces a difference of level of 600 or 700 feet, but at 

 Sned's Hill of only 300, and a branch of it does not affect the strata 

 more tnan 50 or GO feet. In some instances the change of level is by 

 steps or hitches, owing probably to unequal resistance, or a series of 

 small dislocations. Another character of the large faults is their sub- 

 dividing, more especially at the extremities — the subdivisions occa- 

 sionally taking a direction at right angles to the main fault, but when 

 they are numerous they diverge from it only a few degrees and extend 

 but a short distance. 



The author then describes minutely the chief faults ; the two prin- 

 cipal of which, bounding the field on the east and partly on the west, 

 bring the disjointed edges of the coal measures in a level with those 

 of the new red sandstone ; and he afterwards gives a table of the minor 

 faults, containing the name of each fault with its direction, extent, 

 average angle of inclination, breadth, fall, the greatest difference of 

 level produced by it, and the localities at which the difference of level 

 varies ; and from the phenomena presented by the faults, and the 

 fact that the field is a platform raised above the level of the sur- 

 rounding country, the author infers that the coal-field has been ele- 

 vated above its original position ; he also adds that the contortions of 

 the beds are not of any great magnitude. 



Superficial detritus or drift. — Thick beds of gravel and sand cover a 

 large portion of the surfiice, and are considered by the author as con- 

 sisting of two distinct deposits. The lower, which is of local occur- 

 rence, though from 20 to 50 feet thick, consists of fine-grained red 

 sand, containing beds of small angular pebbles of the adjacent rocks, 

 and thin, distinct seams of marl or clay. Imbedded in the sand are 

 frequently found masses of coal, some of them six feet in diameter. 

 The upper deposit is composed of rolled pebbles of rocks, composing 

 the coal-field and its boundaries, imbedded in coarse reddish sand. 

 Its distribution is more regular than that of the lower division ; apd 



Third Series. Vol. 9, No. 55. Nov. 1836. 2 Z 



