Ch. XXIV.] COAL-MEASURES. 467 



as we see in the deltas of great rivers in warm climates, which are 

 liable to be submerged beneath fresh or salt water should the ground 

 sink vertically a few feet. 



In one section near Swansea, in South Wales, where the total 

 thickness of strata is 3246 feet, we learu from Sir H. De la Beche 

 that there are ten principal masses of sandstone. One of these is 

 500 feet thick, and the whole of them make together a thickness of 

 2125 feet. They are separated by masses of shale, varying in thick- 

 ness from 10 to 50 feet. The intercalated coal-beds, sixteen in num- 

 ber, 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 predominate over the sandstones. 

 The horizontal extent of some seams of coal is much greater than 

 that of others, but they all present one characteristic feature, in hav- 

 ing, each of them, what is called its underclay. These underclays, 

 coextensive with every layer of coal, consist of arenaceous 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 Sir William 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 hun- 

 dred 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 black- 

 ened by it. 



All of them, as Sir William Logan pointed out, are characterized 

 by enclosing a peculiar species of fossil vegetable called Stigmaria, 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 Stigmarice, 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 root- 

 lets, formerly thought to be leaves, through the mud in all directions. 

 Several species of Stigmaria 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. 



CARBONIFEROUS FLORA. 



These statements will suffice to convince the reader that we cannot 

 arrive at a satisfactory theory of the origin of coal until we under- 



* Memoirs of Geol. Survey, vol. i. p. 195. 



