134 Reports & Proceedings — Geological Society of London. 



miles ; others being of great width «and of irregular form, and due 

 possibly to shifting and meandering streams. 



Split seams of the type in which the seam rejoins are kindred 

 phenomena, but in these cases the erosion was always contemporary, 

 and, after a channel was filled up with sediments, peat-producing 

 plants spread completely across the infilling. 



Great diversity in the phenomena of splits and wash-outs arises 

 from the differences in the ratios of shrinkage during consolidation 

 of the various constituents, coal undergoing a shrinkage variously 

 estimated from -^ to ■£§■ of the peat from which it is formed ; mud 

 undergoing, as Sorby showed, a considerable though lesser degree of 

 reduction ; and sand undergoing almost no reduction at all. Thus 

 the hog-back section of split seams is due to the shrinkage of the 

 enclosing coal-substance letting down a relatively incompressible 

 infilling of a channel deeper in the middle than at the sides. In the 

 process the lower surface of the sedimentary mass would flatten to 

 adjust itself to the floor, and the top would consequently assume a 

 curve corresponding generally with the original lower curve, but 

 reversed. The upper element of the seam has some species of seat- 

 earth which arches over the hog-backed inclusion. 



Cannel, which the author considers to be due to a kind of vegetable 

 pulp that underwent most of its decomposition and chemical change 

 coincidently with deposition, acts as a substance of little com- 

 pressibility ; and, whenever pools of cannel-pulp took 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 

 three 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 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 described 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 earthquakes 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 bv the author in coul-seams. 



