delicate apparatus of foliage, are all spread forth before him, little impaired by the lapse of countless ages, and bearing faithful records of extinct systems of vegetation, which began and terminated in times of which these relics are the infallible historians.” [Illustration: Fig. 9.--Pecopteris Adiantoides. Fig. 10.--Sphenopteris Affinis. Fossil Ferns found in the Coal Measures of Europe and America.] The next important fact that points to the vegetable origin of Coal is, that wood and Coal are both composed of the same ultimate elements--carbon, hydrogen, and oxygen. This analogy is the more remarkable when we are told that no other rock except Coal exhibits anything approaching to this composition. It is true that the elements just enumerated do not exist in the same proportions in wood and in Coal. But the difference, when rightly understood, rather tends to confirm our theory that the one is derived from the other. There is more Carbon in Coal than in wood; while there is less oxygen and less hydrogen. To explain how this may have come to pass during the process of transition, we must call in the assistance of the chemist. It appears from the researches of Liebig that, when vegetable matter is buried in the earth, exposed to moisture, and partially or entirely excluded from the air, the process of decomposition sets in, and that under this process carbonic acid gas and carburetted hydrogen gas are slowly evolved. At the same time a portion of the oxygen when set free would naturally enter into a new combination with a portion of the hydrogen, and form water. The result of these several changes would necessarily be, that the accumulation of vegetable matter buried in the earth would part, in course of time, with no small share of its carbon, its hydrogen, and its oxygen, but not with all in the same proportions: for the new combinations would use up more of the oxygen than of the hydrogen, and more of the hydrogen than of the carbon.[58] In other words, if the process should have gone on for a sufficient lapse of ages, these elements would no longer exist together in the proportions which are necessary to constitute wood, but would rather exist in the proportions which are found to constitute coal.[59]