Sandstone Dykes in the Cumberland Coalfield. 499 



water have been expelled, an invariable accompaniment of earth- 

 quakes in alluvial tracts. The shape of these drops and their 

 grouping negative the idea that they are infill ings of orifices 

 occasioned by escapes of gas arising from the decomposition of 

 the peat. 



Fissures filled with sand or other materials, the ' sandstone 

 dykes ' of American writers, are not so common in the Midland 

 •Coalfield as in some other coalfields, as, for example, Whitehaven ; 

 but a number exist. They show contortion where passing through 

 the seam — proving that the coal-substance had not undergone its 

 full compression at the time when the fissure was produced. 



Trough-shaped hollows, called ' swilleys ' or ' swamps,' to which 

 some coal-seams are particularly prone, the author attributes to 

 earthquake effects, such as the subterranean movements of sand, 

 as quicksand. They are not tectonic, for exceedingly rarely, if 

 ever, is more than one seam on the same vertical affected. Some- 

 times the formation of the swilley was coincident with the forma- 

 tion of the seam, as is proved by changes within the trough in the 

 nature of the seam — particularly the occurrence of cannel. 



All the phenomena here described were produced prior to the 

 production of the larger faults of this coalfield ; but minor faults, 

 some affecting upper seams and not lower, others lower and not 

 upper, are probably to be attributed to earthquake action. 



A large number of examples of each type of phenomenon, 

 drawn from the examination of over thirty mines in the coalfield, 

 are discussed. 



2. ' On Sandstone Dykes or Rock-Riders in the Cumberland 

 Coalfield.' By Albert Gilligan, D.Sc, B.Sc, F.G.S. 



The occurrence of these sandstone dykes was brought to the 

 notice of the author when engaged in investigations into the 

 interruptions in the coal-seams of this area. They have been 

 encountered at various times in pits distributed all over the Coal- 

 field ; but those more particularly examined were met with in the 

 workings of the Bannock Band and Main Band Seams at Lady- 

 smith Pit, one and three-quarter miles south of Wellington Pit, 

 Whitehaven. 



The pit-shaft is 1080 feet deep, and has been sunk through tire 

 St. Bees Sandstone, Gypsiferous Marls, Permian, and Whitehaven 

 Sandstone to the productive Lower Coal Measures. Splendid 

 cliff-sections of the Whitehaven Sandstone and succeeding beds, 

 which dip southwards, can be seen in a traverse of the shore from 

 Whitehaven southwards round Saltoni Bay. The coal- workings 

 have been opened up south of the shaft, and therefore pass under 

 St. Bees Head. 



The dykes certainly pass through the Bannock Band and Main 

 Band Seams and the intervening measures, which are about 5 I tret 

 thick ; but their full vertical extent has not been determined. 

 Their horizontal extent is variable: the longest has been traced 

 for more than a mile. They all run practically parallel one 



