44 8 PENNSYLVANIAN PERIOD 



of rock, and geologists are agreed, in a general way at least, as to its 

 mode of origin. 



Origin. There is no doubt that coal is of vegetable origin. 

 Except by the accumulation of vegetable matter, no way is known 

 by which such beds of carbon could be brought into existence. 

 Furthermore, the coal and its associated shales contain abundant 

 remains of plants, in places even recognizable tree-trunks in the 

 form of coal, and microscopic study has revealed the fact that much 

 coal is but a mass of altered, though still recognizable vegetable 

 tissues. Concerning the exact manner in which the beds of vege- 

 table matter accumulated, and the conditions under which it was 

 converted into coal, there is some difference of opinion. 



Much coal is essentially pure, containing little matter of any 

 sort which was not in the plants which gave origin to it. Purity 

 does not mean freedom from ash, since mineral matter, which on 

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

 large amount of coal which is nearly pure, there is much which con- 

 tains some earthy matter. Where the admixture of earthy matter 

 is small, the coal is still usable; but from poor coal of this sort, there 

 are all gradations into carbonaceous shale. 



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 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 necessary 

 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, some of them many thou- 

 sand square miles, is a strong objection to the hypothesis that its 

 substance was drifted together by any process whatsoever. 



Some other facts which support the theory that the vegetation 

 grew where the coal-beds are, may be noted, (i) Beneath many 

 coal-beds there is 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, stumps of trees are found still standing as they grew (Fig. 

 393)- (3) I n coal-beds, or in the associated layers of shale, imprints 

 of the fronds of ferns or fern-like plants are found. They are in 

 places so numerous and so perfect as to indicate that they were 



