126 



THE GEOLOGIST, 



I ought to have said however, that the bark of the Sigillaria is in 

 general the only part preserved. There was within it only a soft 

 i issue of cells, with a central stem or axis of wood, the latter occupying 

 bat a small part of the cylinder. The soft tissue easily disappeared 

 while fossilizing, or even before the tree fell, for we often find the 

 stump filled with sand, and broken fragments of vegetables mixed 

 within it. In one or two trees of this kind in the sandstone beds 

 of Xova Scotia, Professor Dawson and Sir C. Lyell found a whole 

 colony of centipedes or such like things, with snails and lizards ! 

 We must see how this happened when we come to the mode in which 

 coal was deposited. The clay beneath the coal called an " underclay" 

 just as the roof-shale is called " overclay" — is, as I have said, full of 

 plants. These are the Stigmariae, and our figure above shows what 

 they are like. Now the great importance of Sir William's discovery 

 was this, — that the only fossil found in the clay is, with the rarest 

 exception, the Stigmaria; and it is invariably present. The fire- 

 clay as it is called, is generally a pure sediment.: and close upon it 

 lies the coal, as pure coal as the other is clay. Now if we want to 

 know what plant the coal is made of, we must certainly ask the under- 

 clay where the roots grew ; for there, if anywhere, we shall get an 

 answer. Here Mr. Binney's discovery comes into play, for if Stigmaria 

 is the root of Sigillaria— and is universal in the fire-clay — then, of 

 course, Sigillaria is universal in the coal. 



We have seen, too, that 

 fragments of the Sigillaria 

 trees are among the com- 

 monest in the shale that 

 lies above the coal-bed. In 

 truth the trees were higher 

 than the depth of the coal- 

 seam. Thus we may easily 

 conceive that the roots of a 

 tree may be below the coal — 

 which is seldom above a few 

 feet thick — the lower part 

 of its stump fairly in the 

 coal, and its bole and 

 branches all above. 



Thus it is we find the 

 flattened stems, and thinner 

 branches and leaves, so often 

 in the roof shale. 



There is another tree, Le- 

 piJodcndron, whose roots we 

 do not certainly know, but 

 which appears to have grown in the underclay. It is almost as com- 

 as Sigillaria,) and Dearly as large. Perfect specimens have been 

 found, toi iy feet in length from the soil to the end of the branches. 

 But of coarse it is the rarest of things to meet with such trees. 



Fig. 9.— Pattern of bark (Lepidodendron;. 



