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



dental rather than essential constituents, more likely to be found in 

 cannel and shales, deposited in ponds near lycopod forests, than 

 in the swampy or peaty deposits, whence the coal beds proceed. 

 While giving credit to Huxley and his predecessors for calling atten- 

 tion to the importance of spores in coal, he is compelled to maintain 

 that they have generalized on insufficient basis, that sporangitic beds 

 are exceptional among coals and that cortical and woody matters 

 are most abundant. The purest layers of coal are composed of 

 flattened trunks; other coals are made up of finely comminuted par- 

 ticles, mostly epidermal tissues — not only from fruits and spore 

 cases but also from leaves and stems. 



Mietzsch^''"^ attempted to answer the question, how did the vege- 

 table material accumulate in great beds? Was it brought down by 

 rivers from forest covered areas or did the plants grow where the 

 coal is now found? The mode of occurrence can be explained 

 measurably by either supposition ; at times one pr(Kess may act alone, 

 at time it may be j^ermissible to regard both as contributing. 



He describes the heaping up of driftwood along streams as well 

 as on coasts, whither it has been carried by currents ; and he thinks 

 that in this way may have originated some tertiar\- deposits of 

 lignite, composed almost wholly of stems stripped of their bark. 

 But many deposits of lignite and brown coal contain stems with 

 bark, twigs, leaves and fruit preserved. The Suterbrander lignite 

 of Iceland was formerly sujjposed to be driftwood, because of the 

 present conditions in that land ; but Heer discovered well-preserved 

 buds, leaves and twigs of the plants, represented by the stems, which 

 still retain their bark. The same criterion must be applied to the 

 black coals. Many deposits of these and the greater number of 

 brown coals have numerous tokens, rendering improbable, in part 

 impossible, the supposition that tliey were made of transported plant 

 masses. It is difficult to understand tlie regularity and vast extent 

 of coal beds on the theory of transport, for driftwood accumulations 

 are irregular and of small superficial extent. 



The composition of coal tells against the theory of transport, for 

 in most beds the ash is very small — surprisingly small, for in the 

 process of coalification no ])art of the mineral content of the nlants 



"^H. Mictzscb, "Geologic (kr KoliIciilaRcr." Leii)zis, 1875, iip. ^44-2^7. 



48 



