812 



NA TURE 



[December i 6, 1922 



course across it ; the channel we will say was, like 

 most channels, deeper in the middle than at the sides, 

 and in the middle actually cut through to the seat- 

 earth. Then the channel silted up completely, so that 

 a cast of its meandering course in sands or mud reach- 

 ing 40 feet in thickness at the maximum, hut much 

 thinner at the margins, was formed; then the upper 

 bed of peat formed to a further depth of 40 feet. The 

 conversion of the peat into coal would reduce it to 

 two beds, each, let us say, 2 feet in thickness at tin 

 maximum, enclosing the sediment with a proportionately 

 smaller thickness in the eroded peat on either margin 

 of the channel. The sedimentary mass would have 

 the transverse section of a plano-convex lens, the 

 convexity being downward, but when the peat under 

 the edges of the sediment is condensed to one-twentieth 

 of its original bulk the base becomes almost flat, and 

 the unconsolidated mass of sediments adjusts itself 

 thereto. Thus the curve, originally at the base of the 

 mass, reproduces itself in the top of the mass, which 

 was originally quite flat and now is curved. The lens 

 of infilling has reversed its curvature. 



When a seam is deeply eroded the only too familiar 

 phenomenon of a " wash-out " is formed. 



The most common abnormality is the occurrence 

 of belts or patches of " proud coal " in which the seam 

 swells up to twice or thrice its normal thickness — 

 sometimes, though not always, by repetition of the 

 whole seam or of the upper part, either by shearing 

 or by overfolding. 



It has been suggested that all the violent displacement 

 and over-ridings are brought about by tectonic agency, 

 and that they are thrust-planes. The localisation to 

 a single stratigraphical plane should suffice to discredit 

 this explanation. An amplification of the same 

 explanation ascribes the displacements to a thrust 

 with a movement from S.E. to N.W. and a common 

 cause to the cleat or cleavage of the coal which is 

 normally directed to the N.W. It suffices to refute 

 this to remark that the wash-outs I have explored 

 in the Yorkshire coalfield are aligned in four principal 

 directions, so that if superposed they would give what 

 may be called the Union Jack pattern, i.e. N.E. — S.W., 

 N.W.— S.E., N— S., and E.— W. 



Moreover, if these so-called " wash-outs " are not 

 due to the erosive effects of contemporaneous or sub- 

 contemporaneous streams, but to flat-hading faults, 

 anv coal displaced should be presently found again 

 without any loss whateven That swellings and 

 duplications of the seam occur we have already noticed, 

 and such phenomena have been pointed to as evidence 

 that there is " no loss " of coal in connexion with 

 the so-called wash-outs. But losses and the gains by 

 duplication do not, in fact, balance. A simple and 

 convincing case is a wash-out in a thin seam, in which, 

 "by taking measurements of the thickness of coal 

 present and the breadth of the barren area, I have 

 been able to show that a gap with no coal for 210 feet 

 is compensated for by only 35 feet of excess on the 

 margin. 



Seismic Phenomena in the Seams. 



While the displacements and duplications are totally 

 unlike those produced by faults, there are cases in 

 which the seam appears to have been subjected to 



NO. 2772, VOL. I 10] 



.1 stretching tension and to have broken under the 

 strain. Along the zone of such a stretch great confusion 

 is commonly found. Masses of sedimentary materials, 

 of the coal seam, and slabs and seams of cannel 

 commonly occur, besides a curious argillaceous sub- 

 stance unlike anv natural rock with which I am 

 acquainted. In its unstratified structurelessness it 

 suggests a kind of consolidated sludge such as might 

 be produced by violently stirring or shaking a quantity 

 of not too liquid mud. Where the seam abuts against 

 this stuff it presents usually a nearly vertical ragged 

 edge, its bright and dull layers preserving their 

 characteristics quite up to the contact. 



The explanation I have offered is that all these 

 disturbances which complicate the already complex 

 features of wash-outs are the effect of the lurching 

 of the soft alluvial materials by earthquake agency. 

 Every predicable subterranean consequence of earth- 

 quake action upon unconsolidated alluvial deposits, 

 such as the Coal Measures were, can be seen in the 

 Yorkshire Coalfield. The lurchings, the rolling and 

 heaving of sand-beds, the shaking to pulp of the 

 muddy deposits, the rending and heaving of the peat, 

 cracks in the peat, and cracks infilled with extraneous 

 material passing through the strata; and lastly, 

 though actually the first clue to the explanation, 

 masses of sandstone in the form of inverted cones 

 (" dog's-teeth," " paps," or " drops "), descending on 

 to coal-seams, which I interpret as the deep-seated 

 expression of the sand-blows that are the invariable 

 accompaniments of earthquakes in alluvial tracts. 



An earthquake sweeping across an alluvial plain 

 beneath which lay a thick bed of water-charged peat 

 overlain by laminated clay, and that in turn by sand 

 and an upper layer of mud or clay, would throw the 

 peat and its watery contents into a state of severe 

 compression which would result in the bursting of 

 the immediate cover of clay and the injection of water 

 into the sand, and, probably, a large quantity of gas, 

 converting it thus into quicksand. This in turn would 

 eject water in the form of fountains through the upper 

 muddy or silty stratum, producing sand-blows and 

 craters on the surface. When the disturbance subsided 

 sand would run back down the orifice into the funnel 

 above the peat. These are the " drops." They are 

 commonly flanged down the sides, showing that they 

 were formed upon a line of crack. An earthquake 

 not infrequently gives rise to permanent deformations 

 of soft deposits either by the lurching of the surface 

 and the production of permanent wrinkles, or by 

 subterranean migration of quicksand so as to produce, 

 here a sag or hollow, there a ridge or bombement. 

 Mr. Myron Fuller's admirable account of the effects 

 of the New Madrid earthquake of 1816 as observed 

 one hundred years after the event, is full of the most 

 interesting and suggestive observations, not the least 

 so those upon the sand-blows and sand-filled fissures 

 containing lignite — the sand having come up from 

 a bed lying at a depth of not less than 80 feet — the 

 elevated tracts, and the new lakes produced by sub- 

 sidence. 



The " Cleat " or " Slynes " of Coal. 

 One feature of coal-seams I must discuss before 

 I conclude, though it will not at first appear clear 



