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 carbonaceous. All the deposits of carbon found in any form in nature can be traced to some animal or vegetable remains, so that it is logical to assume that coal also arose from either animal or plant débris. 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 shells, sponges have siliceous spicules, molluscs hard shells, and the higher animals bones and teeth. These things are of a very permanent nature, and would certainly be found in quantities in the coal had animals formed it. Further, the peat of to-day, which collects in thick compact masses of vegetable, shows how plants may form a material consisting of carbonized remains. By certain experiments in which peat was subjected to pressure and heat, practically normal coal was made from it.
[Illustration: Fig. 15.--Part of a Coal Ball, showing the concentric bandings in it which are characteristic of concretions]
[Illustration: Fig. 16.--Mass of Coal with Coal Balls, A and B both enclosing part of the same stem L]
Still a further witness may be found in the structure of the "coal balls" described in the last chapter. These stony masses, lying in the pure coal, might well be considered as apart from it and bearing no relation to its structure; but recent work has shown that they were actually formed at the same time as the coal, developing in its mass as mineral concretions round some of the plants in the soft, saturated, peaty mass which was to be hardened into coal later on.[3] All "coal balls" do not show their concretionary structure so clearly, but sometimes it can be seen that they are made with concentric bands or markings like those characteristic of ordinary mineral concretions (see fig. 15). Concretions are formed by the crystallization of minerals round some centre, and it must have happened that in the coal seams in which the coal-ball concretions are found that this process took place in the soft plant mass before it hardened. Recent research has found that there is good evidence that those seams[4] resulted from the slow accumulation of plant débris under the salt or brackish water in whose swamps the plants were growing, and that as they were collecting the ground slowly sank till they were quite below the level of the sea and were covered by marine silt. At the same time some of the minerals present in the sea water, which must have saturated the mass, crystallized partly and deposited themselves round centres in the plant tissues, and by enclosing them and penetrating them preserved them from decay till the mineral structure entirely replaced the cells, molecule by molecule. Evidence is not wanting that this process went on without disturbance, for in fig. 16 is shown a mass of coal in which lie several coal balls, two of which enclose parts of the same plant. This means that round different centres in the same stem two of the concretions were forming and preserving the tissues; the two stone masses, however, did not enlarge enough to unite, but left a part of the tissue unmineralized, which is now seen as a streak of coal. We have here the most important proof that the coal balls are actually formed in the coal and of the plants making the coal, for had those coal balls come in as pebbles, or in any way from the outside into the coal, they could not have remained in such a position as to lie side by side enclosing part of the same stem. There are many other details which may be used in this proof, but this one illustration serves to show the importance of coal balls when dealing with the theories of the origin of coal, for they are perfectly preserved samples of what the whole coal mass was at one time.