500 Geological Society. 



to the other in a direction approximately north -north -west and 

 south - south - east. The inclination of the same dyke is not 

 constant, but the greatest deviation from the vertical was 10° 

 south-westwards, and in general the amount was very small. In 

 only one case was a dyke found associated with a small fault, the 

 displacement being 2| feet, and even this died out in a short 

 distance. A noticeable feature was the presence of slickensiding, 

 approximately horizontal, on the sandstone surface. Flutings^ 

 simulating ripple-marks, were present on the sides of the sandstone 

 forming the dyke. 



The average width of the dykes was from 2 to 4 inches, but 

 sometimes they increase to 10 inches or dwindle down to mere 

 films. Occasionally, a lateral displacement was seen when the 

 dyke passed from one type of rock to another. Splitting of the 

 dykes was commonly seen. Veins of calcite and barytes traverse 

 the dykes longitudinally and transversely, while lenticles of shale 

 and coal are also of frequent occurrence in some portions of the 

 dykes. The contact of the coal and dyke-substance was very 

 sharply defined, the coal preserving all its normal features even 

 when adhering to the sandstone. 



In discussing the origin of the fissures and the nature of their 

 infilling, the author draws attention to the fact that the direction 

 of the dykes at Ladysmith is that of the main system of faults in 

 the Cumberland Coalfield. These north-north-west and south- 

 south-east faults profoundly affect the Lower Coal Measures at 

 Ladysmith Pit, but do not pass up into the overlying Permian and 

 Triassic rocks. 



An examination of the cliff- sections of Saltom Bay, where dykes 

 of the same series as those at Ladysmith Pit should emerge, shows 

 that they are not present in the Whitehaven Sandstone and 

 succeeding beds. The inference was, therefore, drawn that the}- 

 were of pre -Whitehaven Sandstone Age. The probable conditions 

 which obtained at the time of the formation of the fissures and 

 their infilling were as follows : — The coal-seams through which the 

 dykes pass had been compressed to their present thickness, while 

 they and the associated measures were sufficiently consolidated to 

 take a more or less clean fracture. The sea in which the deltaic 

 material of the Whitehaven Sandstone was accumulating covered 

 the area. Fractures were produced by earthquake disturbances set 

 up by movement along one of the north-north-west and south- 

 south-east faults, and the sediment on the sea-floor ran in and 

 sealed them up. 



A mineralogical examination of the Whitehaven Sandstone and 

 of the sandstone of the dykes shows that they have much in 

 common, notably in the types of heavy minerals. 



The sequence of events postulated supports the view of an 

 unconformity at the base of the Whitehaven Sandstone. With 

 regard to dykes in other parts of the Coalfield which show con- 

 tortion in passing through the coal-seams, the author argues for 

 their formation before consolidation of the enclosing measures. 



