12G 
THE GEOLOGIST, 
I onght to have said however, that the bark of the Sigillaria is in 
general the only part preserved. There was within it only a soft 
tissue of cells, with a central stem or axis of wood, the latter occupjang 
but 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 Nova Scotia, Professor Dawson and Sir C. Lyell found a whole 
colony of centipedes or such like things, with snails and Hzards ! 
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 Stigmarias, and our figure above shows what 
they are like. ISTow 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 any^vhere, 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 fii^e-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 
Hes 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 heloiv 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- 
'pidodendron, whose roots we 
do not certainly know, but 
which appears to have grown in the underclay. It is almost as com- 
mon as Sigillaria, and nearly as large. Perfect specimens have been 
found, forty feet in length from the soil to the end of the branches. 
But of course it is the rarest of things to meet with such trees. 
Fig. 9.— Pattern of bark (Lepidodendi-on;. 
