26 STEVENSON— FORAIATION OF COAL BEDS. [April 21. 



the low-lying horizontal sea-strand. Changes of level, elevation and 

 subsidence, led to burial of the plants under the ocean ; sand and 

 clay were deposited on the plant covered surface ; dunes were 

 formed, on which plants grew to run the same course. Through 

 repetition of this process, the different beds were formed, separated 

 by sand and clay. The conditions were like those of the present 

 day, for submerged bogs and forests have been observed at many 

 places along the coasts of Europe and America. 



Well preserved stems are wanting because the plants lacked a 

 dense interior structure. Filled stems are rare in Tertiary deposits 

 because the trees were dicotyledonous ; whereas they abound in the 

 Coal Measures because the loose interior structure decayed quickly. 

 Plants grew in these hollowed stumps ; Goeppert found Lepido- 

 dendron, Calaniitcs and ferns in decayed SigiUaria; in the stump 

 of Lepidodcndroii he found the stem of a new genus, two feet long 

 and vertical. 



If the coal had become compact or if the quiet were undis- 

 turbed, the boundary between coal and the succeeding deposit is 

 sharply defined ; at most one finds only impressions of stems lying 

 upon the upper surface. This latter condition occurs frequently in 

 Upper Silesia, where the coal is composed chiefly of Sigillaria. It 

 is cjuite true that filled stems occur even within the coal itself; 

 Goeppert found them. He explains their presence by supposing 

 that clay and sand were brought down by floods before con- 

 solidation of the coal, before the spaces between the stems had 

 been obliterated by compression. In the same way he accounts 

 for Brandschiefer or bituminous shale ; the influx of muddy waters 

 caused the alternation of laminae of bright coal, containing 2 per- 

 cent of ash, with dull layers, containing much mineral charcoal and 

 20 to 25 percent of ash. 



The overlying beds were deposited after complete formation of 

 the coal bed and the time-interval between the two d^osits is as 

 variable as the intervals interrupting the formation of a coal bed 

 itself. Partings in coal beds show how the time required for dif- 

 ferent types of deposits may vary. A parting, ten inches thick, may 

 be equivalent in time to a sandstone deposit elsewhere, many fathoms 



26 



