COAL PRECLUDED BY GEOLOGICAL INFORMATION. 41 



The departments of Natural History to which that term, 

 popularly, is more particularly applied those of Botany and 



tuminous or flaming coal (and probably also some kinds of coal that do 

 not flame) has been derived from vegetable matter, the chemical ele- 

 ments of which have been caused to assume a new arrangement, while its 

 organic structure has been obliterated, by the slow operation of chemical 

 affinities, after the deposition upon it of the earthy matter, now forming the 

 shales and sandstones, with which the beds of coal are interstratified. 



It might be supposed from this, and the supposition, theoretically, 

 would not be unreasonable, that the occurrence of imperfectly bituminized 

 substances (such as bituminous wood, jet, &c.) must often indicate the 

 proximity, perhaps at a greater depth in the earth, of coal, resulting from 

 the perfect bituminization of similar deposits of vegetable matter. But a 

 more extensive acquaintance with the facts discovered by Geology, would 

 show this supposition to be fallacious ; for it appears, almost universally, 

 that the occurrence of lignite, or bituminized wood, even when the process 

 towards converting it into coal has proceeded so far as to obliterate the 

 ligneous texture, is in fact an indication that no true coal will be found in 

 the vicinity, or, at least, in the same formation ; while, on the other hand, 

 wherever the latter mineral is found, the lignites are absent. This latter 

 fact may be readily accounted for, by a more perfect theory ; for since coal 

 is the result of the completion of that process, by the operation of which, to 

 a limited extent, the conversion of vegetable matter into lignite is effected, 

 the inference is obvious, that the latter will not be found, under circum- 

 stances which have been adequate to the production of the former, by the 

 completion of the same process. The fact itself is of such universal pre- 

 valence, in this country at least, that, as Mr. Winch has shown, (Ann. of 

 Phil. vol. xi. p. 68,) while the petrified trunks of trees found in the Whitby 

 Alum-Shale, which is the equivalent to the Lias forming in other localities 

 the lowest member of the Oolitic system, have their bark converted into jet, 

 (the most highly mineralized of all the lignites j) those which occur in the 

 sandstones of the Great Coal-formation, have their bark changed into com- 

 mon or true coal. 



Some part of the coal of Brora and Eastern Yorkshire appears to form 

 the only exception to this rule. The bulk of this coal, though worked in 

 both localities, and in the latter to an extent comparatively great, is a 

 highly mineralized lignite. But "it may be considered," Mr. Murchison 

 observes, in the paper quoted in a former note, " one of the last links be- 

 tween lignites and true coal, approaching very nearly in character to jet, 

 though less tenacious than that mineral ;" and he also states, as might have 

 been expected from these characters, that in some places it is indistinguish- 

 able, either chemically or miner alogically, from ordinary coal. 



This exception, however, like the coal-deposits which afford it, is impor- 

 tant merely in a philosophical point of view ; for it is, as we have seen, a por- 

 tion only of this combustible, the whole of which is insignificant in statis- 

 tical value, that assumes the character of true coal. 



With respect to the British Islands then, the practical rule appears to be 

 valid, that wherever imperfect coal or lignite is found, perfect or true coal 

 does not occur. 



In illustration of this rule may be mentioned the celebrated deposit 

 of lignite, at Bovey Heathfield, in Devonshire, the lower beds of which 

 closely resemble, in appearance, common coal j while the entire aspect of 

 the formation is that of a miniature coal-field. Here the strata have been 

 bored through to the depth of two hundred feet, in search of coal, nothing 

 however but clay having been met with, below the known beds of lignite ; 



