138. Some other mineralogists refer one of the ingredients of coal to the vegetable kingdom, but not the other. Unable to resist the conviction which arises from the fibrous structure of parts of strata, and even entire strata of coal, they have supposed, that wood, which had been somehow buried in the earth, or perhaps deposited at the bottom of the sea, had become impregnated with bitumen, which last, however, they consider as of mineral origin. This appears to be the opinion of Lehman; and also of some very late writers. There seems, however, to be hardly less reason for referring the origin of one part of coal to the vegetable or animal kingdom than another. The two last are certainly capable of furnishing both the carbonic and bituminous parts; and therefore, to derive these from different sources, is at least a very unnecessary complication of hypotheses. 139. Another explanation of coal, very different from any of the preceding, has lately been advanced and set up in opposition to the Huttonian Theory. Mr Kirwan,[50] the only mineralogist, I believe, who has attempted to derive both the carbonic and bituminous matter of coal from the mineral kingdom, distinguishes between wood coal and mineral coal, and gives a theory entirely new of the formation of the latter. Wood coal is that in which the ligneous structure is so apparent, as to leave no doubt of its vegetable origin; mineral coal is that in which no such structure can be discovered, and is the same which Dr Hutton derives from the vegetable juices, and other remains, comminuted, dispersed, carried into the sea, and there precipitated, so as to unite with different proportions of earth, and to become afterwards mineralized. [Footnote 50: Geol. Essays, essay vii. p. 290.] These two species of coal, which the Huttonian theory considers as gradations of the same substance, Mr Kirwan regards as perfectly distinct, constituting two minerals, of an origin and formation entirely different. He therefore endeavours to ascertain the distinguishing characters of each, considered geologically.