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



If the alluvium covering the peat bogs came gradually it would 

 be mingled with a greater or less quantity of vegetables, which had 

 to undergo the same changes as the underlying mass in order to 

 become coal. One ought to find in the alluvium the same propor- 

 tion of carbon and hydrogen as in the coal itself, or at least nearly 

 so. If this relation do not exist, evidently some external influence 

 has been exerted. And the relation does not exist; the variation 

 increases as one recedes from the coal ; this irregularity must be 

 due to some slow action becoming appreciable through lapse of 

 time. Everything seems to indicate that slow oxidation went on in 

 the shales, acting chiefly on the hydrogen, for which oxygen has 

 the greater affinity, so that it has converted the vegetable matter into 

 anthracite in the more distant part of the shales. 



According to diis conception, coal with abundant gas could have 

 been formed only when the material was protected against atmos- 

 pheric agencies. The many varieties of coal owe their origin 

 rather to unequal degrees of protection ; the fattest coals give off the 

 most abundant grisou — evidence that the enclosing rocks are im- 

 permeable. 



Wild®^ in describing the Lancashire coal-field, referred to the 

 " bullions " which are characteristic of the Mountain-Four-foot coal 

 bed. These, embedded in the coal, are ferro-calcareous " concre- 

 tions " more or less pyritous, frequently enclosing mineralized wood, 

 " showing the woody and cellular structure of the plants which 

 have produced the seams of coal from which the concretions are 

 extracted." Shells are absent, the nodules being for the most part 

 fossil wood in varying degrees of preservation. The coal bed is 

 persistent and its roof shale contains concretions, known as " baum- 

 pots," which at times are embedded partly in the coal. These are 

 ironstone or calcareous, sometimes weigh 40 pounds and contain 

 marine shells but rarely any wood. 



After a review of all the coal beds he considered the question of 

 their formation. The generally accepted theory that coal comes 

 from growth in situ seems to be a natural conclusion, for the roots 

 in the underclay pass through several layers. It is true that under- 



*' G. Wild, "Lower Coal Measures of Lancashire," Trans. Manchester 

 Geol. Soc, Vol. XXL, 1892, pp. 364 et seq. 



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