668 



THE WONDERS OF GEOLOGY. Lect. VII. 



(2.) Coal. A carbonized mass, in which the external 

 forms of the plants and trees composing it are obliterated, 

 but the internal structure remains. Large trunks, stems, 

 and leaves, are rarely distinguishable in it. 



(3.) The Roof, or upper bed. This generally consists 

 of slaty clay, abounding in leaves, trunks, branches, fruit, 

 &c. ; and includes layers and nodules of iron-stone, inclos- 

 ing leaves, insects, crustaceans, &c. In some localities beds 

 of fresh-water shells, and in others of marine shells, are 

 intercalated, and interstratified with the shale, finely 

 laminated clay, micaceous sand, grit and pebbles of lime- 

 stone, sandstone, and other rocks, often occur. The prin- 

 cipal illustrative specimens of the leaves, fruits, &c. of the 

 carboniferous flora, are found in this bed, which appears to 

 be an accumulation of water-worn detritus of other rocks, 

 promiscuously intermingled with the dense foliage and 

 stems of a prostrate forest, the whole drifted from a distance 

 by a strong current, or flood. 



7. Mode of deposition of the coal. — Thus we have, 

 in the first place, spread uniformly over the bottom, and 

 constituting the foundation on which the coal reposes, a 

 stratum of fine pulverulent clay, several feet thick, which, 

 possibly, may have once constituted the soil of a vast plain 

 or savannah. The only fossil remains found in it, except 

 in a few localities, are the roots of the large trees of which 

 the coal is in a great measure composed ; for such the 

 Stigmarim now prove to be, and not aquatic plants, as was 

 formerly supposed.* 



* Medals of Creation, vol. i. p. 143. The invariable occurrence 

 of the fossil roots, termed Stigmaria), in the under-clay, and their 

 rarity in the coal and shale, was noticed by Mr. Martin (Petri/. 

 Derbiensia), Dr. Macculloch, and other observers : but the importance 

 of this fact was not duly appreciated till Mr. Logan drew attention to it. 

 In the Welsh coal-field, in a depth of twelve thousand feet, there are 

 sixty beds of coal, each lying on a stratum of clay abounding in stig- 



