26 ANCIENT PLANTS 



evidence for this belief. The grounds for recognizing 

 coal as consisting of practically pure plant remains are 

 many and various, so that only the more important of 

 them will be considered now. The most direct sugges- 

 tion lies in the impressions of leaves and stems which 

 are found between its layers; this, however, is con- 

 fronted by the parallel case of plant impressions found 

 in shales and limestones which are not of vegetable 

 origin, so that it might be argued that those plants in 

 the coal drifted in as did those in the limestone. But 

 when we examine the black impressions on limestone 

 or sandstone, an item of value is noticeable; it is often 

 possible to peel off a film, lying between the upper and 

 lower impression, of black coaly substance, sometimes an 

 eighth of an inch thick, and hard and shining like coal. 

 This follows the outline of the plant form of the impres- 

 sion, and it is certain that this minute " coal seam " was 

 formed from the plant tissues. It is, in fact, a coal seam 

 bearing the clearest possible evidence of its plant nature. 

 We have only to imagine this multiplied by many plants 

 lying tightly packed together, with no mineral impurities 

 between, to see that it would yield a coal seam like those 

 we find actually existing. 



In some cases in the coal itself a certain amount of 

 the structure of the plants which formed it remains, 

 though usually, in the process of their decay the tissues 

 have entirely decomposed, and left only their carbonized 

 elements. Chemical analysis reveals that, beyond the 

 percentage of mineral ash which is found in living plants, 

 there is little in a pure sample of coal that is not car- 

 bonaceous. All the deposits of carbon found in any 

 form in nature can be traced to some animal or vege- 

 table remains, so that it is logical to assume that coal 

 also arose from either animal or plant debris. But were 

 coal of an animal origin, the amount of mineral matter 

 in it would be much larger as well as being of a different 

 nature; for almost all animals have skeletons, even the 

 simplest single -celled protozoa often own calcareous 



