498 Geological Society: — 



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 con- 

 tortion. 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 ') b}^ any tectonic stress, but that under the impulse of an 

 earthquake a mass of unconsolidated, or but partly consolidated, 

 peat- stuff 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 struc- 

 tures in its excessively tender ' moth er-of- coal ' layers. The ' cleat ' 

 in the overriding flakes follows the orientation general to the 

 locality. 



The gap left by the projection forward of the belt of seam is 

 filled with an un stratified sludge-like substance, commonly con- 

 taining angular masses of stratified argillaceous or arenaceous 

 material. A very finely-contorted specimen of sandstone from a 

 true 'wash-out' shows quite plainly that the disturbance took place 

 before the material was indurated. 



In harmony with the contention that the overriding masses are 

 not due to tectonic ' overthrusts ' is the fact that reversed faults 

 are almost unknown in the coalfield, are never of considerable throw, 

 and many collieries have never seen one. 



In the roofs of many coal-seams and projecting slightly into the 

 coal are very curious roughly-conical masses of sandstone, familiar 

 to the miners as ' drops ' (or by other names) ; but these have, so 

 far as the author knows, hitherto escaped notice by any geological 

 writer. They are commonly wrinkled on the surface, as though 

 partly telescoped, and often have a flange on two sides, showing that 

 they were produced on the site of a crack. They are commonly 

 ranged in long rows. These the author interprets as casts of the 

 funnel-shaped orifices through which the sands surcharged with 



