But though Peat may thus, as it should seem, pass directly into pure Coal, there are many cases in which it first assumes a more imperfect form, known under the name of Lignite. This substance is described as of a brownish color, “soft and mellow in consistence when freshly quarried, but becoming brittle by exposure, the fracture following the direction of the fibre of the wood.”[66] It clearly occupies an intermediate position between Peat and Coal. Like the former, it still exhibits the stems and woody fibre of the plants from which it is derived, very little altered in their structure; while on the other hand it is already beginning to acquire some of the consistency and density of Coal; to which also it approaches much more closely in its chemical composition. It should be remembered, moreover, that Lignite does not designate a substance of a fixed, invariable character. On the contrary, under the one general name are comprised a definite number of varieties, leading from one extreme to the other by a series of almost insensible gradations; the extreme variety on one side being scarcely distinguishable from Peat, while the extreme variety on the other is practically identical with ordinary Coal. It can hardly be doubted, therefore, that Coal must have the same origin as Lignite, while it is at least equally certain that Lignite has been derived from Peat; and we have already seen what overwhelming evidence may be adduced to show that the origin of Peat is to be sought for in the sunken swamps and forests of a long past age. Lastly, when we come to examine the texture of Coal itself, we find much to confirm the conclusion at which we have thus arrived. In beds of pure Coal the remains of many species of plants have been detected, and sometimes in such abundance as to constitute visibly the bulk of the Coal. Even large trees are sometimes found standing erect in the Coal fields, with their bark actually converted into this mineral. The annexed Figure represents a portion of the stem, together with the roots of a tall forest tree, Sigillaria, discovered not long ago in a Coal mine at Saint Helens, near Liverpool. The stem, which was nine feet high, was found erect in the seam of Coal, while the roots, ten in number, stretched away into the vegetable soil beneath. [Illustration: Fig. 11.--Stem and roots of a Forest Tree, Sigillaria. From a Coal-mine, near Liverpool. _a_, The trunk traversing a bed of Coal. _b_, The roots spreading out in the underclay.]