19II-] STEVENSON— FORMATIOX OF COAL BEDS. 65 



ing material might tioat away to deep water within the swamp, so 

 as to be heaped into pecuHar massive layers like cannel. Flooding of 

 the plain and deposit of mineral matter checked formation of coal; 

 but the swamp would be re-established and a second formation be 

 made ; or possibly for a long period only rock material might be 

 deposited. 



How far variation in the water niveau may atiect the question 

 is considered only so far by v. Giimbel as to let him warn against 

 the conception that basins, now filled by a thick series of coal 

 bearing deposits, were filled with water in like manner at the be- 

 ginning. These bowls were filled very gradually ; they must be 

 thought of as filled temporarily by a relatively shallow pond of 

 water, which little by little reached a higher level. 



At times, marine remains occur in strata between the coal beds, 

 a condition which seems opposed to the explanation offered. But 

 this occurrence is due to the fact that the low swamp land was 

 spread out near the sea and was exposed to invasions, so that remains 

 of marine animals might be enclosed in materials originating on the 

 land. ^Marine or brackish water forms might be enclosed in the coal 

 deposit itself, if it were formed alongside an arm of the sea. 



In general, coal beds are an autochthonous product of dead, 

 broken and disintegrated plant fragments with only local and petty 

 contribution of transported material of the same character. 



A\'ethered*'^ called attention to the fact that coal seams are not 

 single beds, but are separated by partings into benches which may 

 dift'er in quality as well as structure. Sometimes Stigmaria are 

 present in the partings. 



The Cannock Chase or Shallow seam, near Edinburgh, has in its 

 upper bench, i foot lo inches thick, the brownish layers composed 

 of macrospores and microspores, while the bright layers, containing 

 some woodv tissue, are composed mostly of a structureless material 

 which he terms " hydrocarbon " in preference to " bitumen." 

 Whence this comes he does not know, but wood tissue may con- 

 tribute to it. The middle division of the bed is very different, 

 consisting almost whollv of *' hydrocarbon " with very few spores, 



* E. Wethered. " On the Structure and Formation of Coal," Q. J. G. S., 

 Vol. XL, 1884, Proceed., pp. 59, 60. 



PROG. AMER. PHIL. SOC, L. I98 E, PRINTED APRIL 25 I9II. 



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