Reports & Proceedings — Geological Society of London. 135 



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 tenacity 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 decomposition, 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 beds, or its expulsion as "sand-blows" 

 at the surface of the ground. When impenetrable mud-beds occurred 

 in a sufficiently yielding condition, such extravasation of sands might 

 carry these beds with them in a more or less stretched condition, and 

 so be perpetuated as solid rolls enveloped in a wrapping of stratified 

 shales. 



Lurching of the superficial strata took place on a considerable 

 scale. The evidence is found in the gaps (often miscalled " wash- 

 outs") of a type usually narrow and not sinuous, in respect of which 

 the loss of coal is compensated for by swellings or folds of the seam, 

 or by the overriding of-the seam by great flakes of coal still retaining 

 the characteristics of the seam. These flakes always show torn and 

 ragged edges, which are sometimes splayed out and interpenetrated by 

 tongues of sandstone or of amorphous "clunch", and the fine laminae 

 of the coal preserve their parallel arrangement to the extremities of 

 the projections without contortion. In some cases the flake has 

 been thrown in complicated folds, and in one instance completely 

 inverted. 



The inference is that the flake of coal was not moved (" over- 

 thrust") by any tectonic stress, but that under the impulse of an 

 earthquake a mass of unconsolidated, or but partly consolidated, peat- 

 fituff or lignite was projected forward by its own inertia in a medium, 

 usually of sand, which, through excess of water and gases, had only 

 such resisting power as belongs to a fluid. 



Such disturbances are (with some doubtful exceptions) always 

 limited to single seams and their contiguous measures, and there is 

 cumulative evidence that usually the coal-stuff, and always the 

 measures, were unconsolidated at the time of the movement. In the 

 overriding flakes the coal retains undistorted vegetable structures in 

 its excessively tender " mothei'-of-coal " layers. The "cleat" in the 

 overriding flakes follows the orientation general to the locality. 



