70 SECTIONAL ADDRESSES. 



the connection between great normal faults and the occurrence of wash- 

 outs was too close to be merely fortuitous. But what the cause might 

 be I was quite unable to suggest, and it was not until many years had 

 elapsed that enlightenment came from a wholly unexpected quarter. 



In brief, the explanation I have offered in a communication to the 

 Geological Society of London, in a paper that has not yet been placed 

 in full before that body, 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. The present is 

 not the occasion for amplifying the preliminary account of my evidence 

 and argument published in the Proceedings of the Geological Society 

 (No. 1,031, Jan. 17, 1919), but I may say this, that every predicable 

 subterranean consequence of earthquake action upon unconsolidated 

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

 Yorkshire Coalfields. 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. 



Let us imagine 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, the impulse 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 la.rge quantity of gas, converting it thus into quicksand. 

 This in turn under the stress of the earthquake 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 fomied 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 subsidence. His photographic illustration 

 of Eeelfoot Lake with its broken and hollowed trunks of drowned trees 

 must appeal to the imagination of every Coal Measure geologist. 



Displacements or undulations of the surface of the Coal Swamps 

 are readily traceable in many, perhaps in most, of the seams in this 

 coalfield, but it is not always possible to prove their contemporaneity. 



