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



at length form a new surface on which vegetation would begin once 

 more. This would give a smaller number of beds. The, at times, 

 great thickness and the frequent irregularity of coal beds in lim- 

 nisch areas may be explained in part by supposing that they were 

 not formed wholly as peat deposits, luit received masses of uptorn 

 vegetation, swept out by floods, and this leads to the question of 

 the formation of a particular coal bed. 



There are two theories, transport (Anschwemmung) and in situ 

 (an Ort und Stelle). Both may be correct. The great beds, be}ond 

 doubt, are of in situ origin, but there are many deposits which can 

 be explained only by transport of plant masses. 



It is known that streams bring down astonishing quantities of 

 plant material; that ocean currents carry driftwood far and that 

 it accumulates in vast masses on shores. Currents of the olden time 

 must have been similar. If the widespread masses were buried 

 under sediments, they would be transformed into coal beds. Neu- 

 mann thinks that repetition of this process at mouths of streams in 

 lakes or on the sea-coast would give a system of strata like the 

 present series of coal beds with intervening sandstones and shales. 

 Such drift masses are irregular in extent and thickness, often as 

 blocklike masses. Such transported material would give conditions 

 like those observed in coal beds of some limnisch areas, great irregu- 

 larity and variation in thickness, breaking up into separate benches, 

 some of them cxcessvely thick. He thinks that under especially 

 favorable conditions a coal bed might be formed in this way which 

 would resemble one formed in situ. He considers also that this 

 theory of transport explains many regular coal beds, such as those 

 between limestones or other strata distinctly marii;e, as well as beds 

 resting directly on granite, limestone, etc., without an underclay. 

 He. agrees with Murchison that in some cases the transport theory 

 has value. 



But for the greater part of the coal beds, the in situ theory must 

 be accepted ; their material was produced by vegetation an Ort und 

 Stelle. All beds continuous over great areas, with regular and not 

 too great thickness and with a stigmaria-filled underclay are to be 

 explained in this way. But one must not think that there were 

 real forests, which were thrown down in place, compressed by in- 



30 



