360 CAKBONIFEROUS FLORA. [Ch. XXIV. 



and the whole of them make together a thickness of 2125 feet. They 

 are separated by masses of shale, varying in thickness from 10 to 50 feet. 

 The intercalated coal-beds, sixteen in number, are generally from 1 to 5 

 feet thick, one of them, which has two or three layers of clay interposed, 

 attaining 9 feet.'^ At other points in the same coal-field the shales pre- 

 dominate over the sandstones. The horizontal extent of some seams of 

 coal is much greater than that of others, but they all present one charac- 

 teristic feature, in having, each of them, what is called its underday. 

 These underclays, coextensive with every layer of coal, consist of arena- 

 ceous shale, sometimes called fire-stone, because it can be made into bricks 

 which stand the fire of a furnace. They vary in thickness from 6 inches 

 to more than 10 feet; and Mr. Logan first announced to the scientific 

 world in 1841 that they were regarded by the colliers in South Wales as 

 an essential accompaniment of each of the one hundred seams of coal 

 met with in their coal-field. They are said to form the floor on which 

 the coal rests ; and some of them have a slight admixture of carbonaceous 

 matter, while others are quite blackened by it. 



All of them, as Mr. Logan pointed out, are characterized by inclosing a 

 peculiar species of fossil vegetable called Stigmmia^ to the exclusion of 

 other plants. It was also observed that, while in the overlying shales or 

 " roof" of the coal, ferns and trunks of trees abound without any Stig- 

 marice^ and are flattened and compressed, those singular plants of the 

 underclay very often retain their natural forms, branching freely, and 

 sending out their slender leaf-like rootlets, formerly thought to be leaves, 

 through the mud in all directions. Several species of Stlgmaria had long 

 been known to botanists, and described by them, before their position 

 under each seam of coal was pointed out, and before their true nature as 

 the roots of trees was recognized. It was conjectured that they might 

 be aquatic, perhaps floating plants, which sometimes extended their 

 branches and leaves freely in fluid mud, and which were finally enveloped 

 in the same mud. 



CAKBONIFEROUS FLORA. 



These statements will suffice to convince the reader that we cannot 

 arrive at a satisfactory theory of the origin of coal until we understand the 

 true nature of Stlgmaria ; and in order to explain what is now known of 

 this plant, and of others which have contributed by their decay to pro- 

 duce coal, it will be necessary to offer a brief preliminary sketch of the 

 whole carboniferous flora, an assemblage of fossil plants with which we 

 are better acquainted than with any other vfhich flourished antecedently 

 to the tertiary epoch. It should also be remarked that Goppert has ascer- 

 tained that the remains of every family of plants scattered through the 

 coal-measures are sometimes met with in the pure coal itself, a fact which 

 adds greatly to the geological interest attached to this flora. 



Ferns. — -The number of species of carboniferous plants hitherto de- 

 scribed amounts, according to M. Ad. Brongniart, to about 500. These 



* Memoirs of Geol. Survey, vol. i. p. 1U5. 



