Geological Society. 4t$7 



the place of an equivalent thickness of normal coal-stuff, they 

 survive as swellings in the coal-seam. 



The infilling of the erosion-channels, usually of muds and sands, 

 which often show current-bedding, sometimes includes masses of 

 conglomerate with, in exceptional cases, boulders measuring up to 

 3 feet in length. The pebbles are almost invariably of clay- 

 ironstone, never much rounded, and presumably the product of the 

 erosion of the measures through which the stream has cut its way. 



Other disturbances of the coal-seams, commonly miscalled 

 ' wash-outs,' the author believes to to be due to earthquakes, 

 and he holds that in Coal-Measure times earthquakes had an 

 importance which has never hitherto been suspected. 



The area in which our Coal Measures accumulated he supposes 

 to have resembled generally such alluvial tracts as . were the 

 scene of the great earthquakes of Assam and New Madrid de- 

 scribed by Mr. R. D. Oldham and Mr. Myron Fuller, save that 

 in the Coal Measures peat-beds were piled in a much more 

 numerous suite, and were on a vaster scale both of thickness and 

 of area than in any part of the modern world where earthquake 

 phenomena have been studied. Some of the effects of earth- 

 quakes in Coal-Measure times might be expected consequently to 

 be of a magnitude greater than the effects of recent earthquakes, 

 but the types of phenomena are similar. 



The formation of permanent and transient ridges, troughs and 

 fissures, the lurching out of place of belts of the superficial strata, 

 great displacements by the subterranean flow of quicksand, traces 

 of ' sandblows ' and of the caving-in of river-banks have all been 

 recognized by the author in coal-seams. 



Disturbances of this character are frequent along the margins of 

 erosion-channels, just as earthquake-formed fissures and ridges are 

 often marked beside recent rivers in alluvial tracts. 



A striking abnormality in coal-seams consists in the intrusion 

 into the coal of sedimentary material, or the encroachment of 

 masses of amorphous sandstone as ' rock-rolls.' The author 

 attributes these to the invasion of sands rendered mobile by 

 excess of water, and perhaps of gas, and moving under the impulse 

 of waves of elastic compression produced by earthquakes. 



An earthquake-wave would tend to push forward the water 

 contained in a peat-bed enclosed beneath a cover of laminated 

 clay or mud. Where this cover was impenetrable the effect 

 would be merely transient; where the tenacrty of the cover could 

 be overcome, or where it came to an edge through erosion or 

 failure of deposition, water would be ejected from the peat. If 

 this passed into a sand-bed a quite small excess of water, whether 

 accompanied or not by the gases generated in the peat by decom- 

 position, would be sufficient to convert the sand into quicksand ; 

 and, in turn, wherever the sand-bed itself was not confined within 

 impenetrable laminated muds there would under the elastic strains 

 of the earthquake be an extravasation of quicksand into adjacent 



Phil. Mag. S, 6. Vol. 38. No. 226. Oct, 1919. 2 M 



