102 STEVENSON— FOR^IATION OF COAL BEDS. [April 21. 



in huge depressions. It is clear that many channels fed the coal 

 basin, but all worked after the same fashion. The process through- 

 out was that which Ochsenius terms " Barrenwirkungen " or barri- 

 cade action. 



The memoir discusses many details, rock fragments in coal, 

 stumps filled with sandstone, the occurrence of gypsum, the presence 

 of land shells, all of which are explained very readily by the theory. 

 The conditions at Senftenberg, described by Potonie, are clearly due 

 to this barricade-action. 



The numerous coal beds of the Carboniferous were deposited 

 quietly, but they are rarely more than 15 meters thick; whereas the 

 brown coal beds are comparatively few in number, show irregular 

 deposit and at times attain a thickness of 50 meters. The explanation 

 is simple. The soft plants of the Carboniferous had, at most, a 

 diameter of one meter and a height of 40 meters, so that they floated 

 easily in a few meters of water over the "barricade"; whereas the 

 Tertiary and Quaternary giant trees had a diameter of 10 meters and 

 a height of 170 meters, so that they needed a depth of, say, 15 meters 

 to float them over the "barricade." Clearly a depth of one meter 

 would sink more quickly to some centimeters so as to permit only 

 " Spulgut " to pass than would a depth of 15 meters— whence the 

 more frequent interruption of coal deposit in the Carboniferous and 

 the great constancy of formation in Neozoic time. 



Almost all our mighty coal deposits are freshwater formations, 

 which came into existence through the factor of " Barrenwirkungen." 

 Autochthonv holds in their formation an exceedingl}- limited place 

 in comparison with that of allochthony. 



Schmitz^'" has contributed a series of important papers to the 

 literature of the subject. 



In 1894, he regarded the ;'// .s-/7// doctrine as merely a hypothesis. 

 The presence of transported pebbles in the coal itself rather favors 

 the doctrine that the coal is comjiosed of transported materials. 



"" G. Schmitz. " A pr<)])os des cailloux roules dn liouillcr," ./«;;. Soc. Geol. 

 dc Bclgiquc, XXT., i8(;4, \^\^. Ixxi-lxxv; "La signification geogenique des 

 Stigniaria au mur des conches d'liouille," Ann. Soc. Sicnt. dc BntxcUes, XXL, 

 i<^07, 6 pp.; " Formation snr place de la lionillc," Kcv. des. Quest. Scicntifiqucs, 

 Avril, 1906, 35 ])|),. I) pi. 



102 



