THE PENNSYLVANIAN PERIOD 631 



combustion becomes ash, is present in all plants. 1 Along with the 

 large amount of coal which is pure, or nearly so, there is much 

 which contains some admixture of earthy matter. Where the 

 admixture of earthy matter is small, the coal may still be used; but 

 from poor coal of this sort, there are all gradations into carbona- 

 ceous shale. Black shales are commonly associated with coal-beds. 



The purity of some coal-beds over great areas warrants the 

 conclusion that they were made of vegetation which grew where 

 the coal is. The character of the vegetation of the coal shows that 

 it grew on land or in swamps. Had it been washed down from its 

 place of growth to the situations where the coal is, it should have 

 been mixed with earthy sediment, and the product, after the neces- 

 sary changes in the vegetable matter, would have been very unlike 

 the purer coal-beds. Furthermore, the nearly uniform thickness of 

 many of the coal-beds over great areas, sometimes many thousand 

 square miles, constitutes a strong objection to the hypothesis that 

 it was drifted together by any process whatsoever. 



Some of the facts which support the theory that the vegetation 

 grew where the coal-beds are, may be noted. Thus (1) beneath 

 each coal-bed there is, as a rule, a layer of clay with roots (or root 

 marks) in the position of growth. The clay seems to have been 

 the soil in which the coal vegetation was rooted. (2) In association 

 with the coal-beds, the stumps of trees are sometimes found still 

 standing as they grew (Fig. 445). (3) In the coal-beds, or in the 

 associated layers of shale, imprints of the fronds of ferns or fern-like 

 plants are found. They are often so numerous and so perfect as 

 to indicate that they were buried where they fell, without being 

 drifted by moving waters from one place to another. (4) The 

 layer of rock next overlying a coal-bed often contains abundant 

 remains of vegetation, especially in its lower part, as if the conditions 

 which brought about its deposition resulted in the destruction of 

 the forest growth which had preceded. In such situations, trunks 

 of trees 50 and 60 feet long, and 2 or 3 feet in diameter, are some- 

 times found. (5) The vegetable matter in and about coal-beds is 

 made up of the trunks, small stems, leaves, and fruits of the various 



1 Many of the modern allies of the coal-plants contain as much as five per 

 cent of ash, and a few much more. 



