COAL 817 



These drifts of plants now forming streaks of coally matter in the sandstones or 

 shales including them, are sufficient to show that though numerous coal-beds may bo 

 the result of the growth of a peculiar vegetation in places, the roots of which required 

 and penetrated a suitable soil beneath, it might so happen that extensive and deep 

 accumulations of drifted plants may wholly form coal-beds under favourable circum- 

 stances." De la Beche. 



True coal is so altered from its original vegetable condition as to have left scarcely 

 any trace of its history. It is generally, however, associated with sands and 

 clays, exhibiting numerous fragments of the ancient vegetation that obtained at the 

 time of its formation ; but these fragments are so far removed in every respect from 

 the existing form of vegetation as to afford little clue to the condition of the Earth 

 in this respect. In coal all trace of true woody fibre has disappeared ; the water 

 originally present, and so injurious in the less altered forms of vegetable fuel, is en- 

 tirely absent, or if present at all, is so rather mechanically than chemically, while the 

 water originally in the plant appears to have undergone decomposition, the hydrogen 

 uniting with some part of the carbon, to form carburetted hydrogen gas, often exist- 

 ing in the cells, and between the plates of the coal under considerable pressure, and 

 the oxygen being almost entirely removed. The former vegetable has now become a 

 mineral substance, and lies in vast beds of variable thickness, and overlaying each 

 other to the extent sometimes of more than a hundred in a single district ; such beds 

 being regularly interstratified with deposits of sand and clay, and occupying a distinct 

 geological position, being, with only a few exceptions, confined to rocks belonging to 

 the newer part of the Palaeozoic series. 



The changes undergone by vegetable matter when buried in the earth, and accumu- 

 lated in large quantities, and the length of time needed to produce any marked altera- 

 tion, are subjects rather more interesting, it may seem, to the chemist than to the 

 practical man, who looks only for fuel that he may employ economically. But in- 

 asmuch as the real condition of coal varies considerably, it is desirable that the whole 

 history of coal and lignite beds should be generally understood by any one using these 

 substances. 



Vegetable matter consists of carbon in combination with oxygen and hydrogen, 

 as its principal constituents ; nitrogen forming but a small, although an important, 

 part in its economy. A large quantity of water is also present ; and so long as the 

 vegetable lives, there is a constant change and circulation of material particles kept 

 up, replacing and renewing the different portions. When death takes place, there is 

 a tendency to decomposition, or the separation of the whole into minute atoms having 

 no further relation to each other. But this is frequently checked by various condi- 

 tions, such as the presence of some substances derived from plants themselves, or the 

 absence of sufficient oxygen gas to allow the change to take place by combining with 

 the carbon to form carbonic acid gas, the first step in the process of destruction. 

 These causes act constantly but partially, and thus a large quantity of vegetable 

 matter is always in the course of decomposition, while in particular spots a large 

 quantity is constantly being accumulated. The latter condition is seen in our climate 

 in the gradual but steady increase of peat bogs. 



That coal is derived from the vegetable kingdom no longer admits of a doubt, but 

 the class of plants to which more especially we are to look for the origin of coal, is still 

 a matter of much uncertainty. The idea generally entertained is that which supposes 

 a natural basin in which vegetable matter is deposited, the layers varying in thickness, 

 becoming covered with mud or sand. 



Some microscopic observers assure us that they are enabled to detect ligneous 

 structure in bituminous coal. Mr. Quekett has given a great number of drawings in 

 proof of this, and he refers the coal to the woody matter of an extinct class of the 

 Conifera. Botanists of eminence, however, assure us that there is no evidence of ligne- 

 ous structure in any of the examples brought forward in proof of that hypothesis. 

 Others maintain that such structure, though observable in the charcoal-like layers 

 called ' mother of coal,' cannot be detected in the bituminous parts, and that by far 

 the greater portion of the coal is composed of the inacrospores and microspores of 

 Lepidodendra, and other lycopodiaceous plants. The spores of these, or of allied 

 plants, are found more or less abundantly in all true coal ; indeed, in some cases 

 they appear to make up the mass of the mineral, whilst in others they seem to have 

 been crushed together, thus forming the brown streaks commonly seen in microscopic 

 sections of coal. 



Sir Charles Lyell, in his excellent ' Manual of Elementary Geology,' enters largely 

 and with his usual lucid manner, into the consideration of the carboniferous plants. 

 There can be no doubt of the existence of the remarkable flora described by him during 

 the period when our beds of fossil fuel were forming. Keferring to Sir William 

 Logan as his authority, Sir Charles says, ' It was observed that while in the overlying 



VOL. I. 3 G 



