106 STEVENSON— FORMATION OF COAL BEDS. [April 21. 



water-courses swelled, a great quantity of material, inorganic detri- 

 tus, was brought down to form the intervening bed, on which, when 

 (|uiet was restored, tlie j)lant material was deposited anew. Period- 

 ical changes, slight crustal movements, variation in fall of rivers, 

 lead to deposit of a great mass of rock over the coal bed ; the thick- 

 ness of this intervening rock depending on the extent and contin- 

 uance of those changes. When (juiet returns, the forested swamp 

 again expands. Many localities with particular species of plants 

 had been destroyed wholly and those forms do not reappear in 

 later beds — an explanation of the irregular occurrence of plant- 

 forms in the series. 



The lake was comparatively deep, for the Zwickau measures are 

 about 400 meters thick. By accepting this hypothesis of a lake, one 

 finds explanation also of the origin of the great salt-content char- 

 acterizing the Zwickau deposits — in 1854, 400,000 kilos of sodium 

 chloride and 15,000 kilos of calcium chloride were obtained from 

 mine waters of the Tufen Planitzer beds. 



In 1903, Sterzel (jualified a statement made on p. 90 of the pre- 

 ceding paper, which refers to the value of erect stems as evidence. 

 The only stems of that sort, observed by him, were " Sargdeckel," 

 the " coal-pipes " of English miners. One Sigillaria stump, exam- 

 ined by him, was completely cut off at the base, with no trace of 

 Stigniaria. It had been torn from its place by running water, robbed 

 of basal branches and then deposited in the roof of the bed, where 

 its softened bottom was flattened under pressure. He notes that 

 the overlying rock is sharply defined, that there is no passage of 

 plants from the coal, such as would be the case if the place of plant- 

 growth were flooded by masses of rock material. 



Lemiere^"* presented a memoir to the Geological Congress of 

 1900, which discussed the conversion of vegetable matter into coal. 

 In 1904, he returned to the subject and considered in addition the 

 manner in which coal beds accumulated. The discussion is based 

 largely on the assumption and conclusions of Fayol that the coal 



'"^ L. Lcmicre, " Sur la transformation ties vegetaux en conil)ustibIe fos- 

 .siles," C. R. Congrcs Geo!, liilcni., Paris, 1901, pp. 500-520; "Formation et 

 recherches comparics des divers combustibles fossiles," Bull. Soc. de I'lnd. 

 Mill., 4""'. scr., IV., V. Published separately, 1905. Citations from pp. 70-142. 



106 



