306 GEOLOGY AND MINERALOGY. 
the mass. This conclusion is confirmed by facts derived from the associated beds, 
—as, for instance, the prevalence of Stigmaria in the underclays, and of Sigdlaria 
and Calamites in the roof-shales and erect forests. 
2. The woody matter of the axes of Sigillaric and Calamitew and of Coniferous 
trunks, as wellas the scalariform tissues of the axes of the Lepidodendreew and Ulo- 
dendree, and the woody and vascular bundles of Ferns, appear principally in the 
state of mineral charcoal. The outer cortical envelope of these plants, together with 
such portions of their wood and of herbaceous plants and foliage as were submerged. 
without subaérial decay, occur as compact coal of various degrees of purity; the 
cortical matter, owing to its greater resistance to aqueous infiltration, affording the 
purest coal. The relative amounts of all these substances found in the states of 
mineral charcoal and compact coal depend principally upon the greater or less 
prevalence of subaérial decay, occasioned by greater or less dryness of the swampy 
flats on which the coal accumulated. 
3. The structure of the coal accords with the view that its materials were 
accumulated by growth, without any driftage of materials. The Sigillarie and 
Calamitec, tali and branchless, and clothed only with rigid linear leaves, formed 
dense groves and jungles, in which the stumps and fallen trunks of dead trees 
became resolved by decay into shells of bark and loose fragments of rotten wood, 
which currents would necessarilly have swept away, but which the most gentle 
inundations or even heavy rains could scatter in layers over the surface, where 
they gradually became imbedded in a mass of roots, fallen leaves, and herbaceous 
plants. 
4. The rate of accumulation of coal was very slow. The climate of the period) 
in the northern temperate zone, was of such a character that the true Conifers 
show rings of growth not larger nor much less distinct than many of their modern 
congeners.* The Sigillarie and Calamites were not, as often supposed, composed 
wholly, or even principally, of lax and soft tissues, or necessarily short-lived. 
The former had, it is true, a very thick cellular inner bark; but their dense woody 
axes, their thick and nearly imperishable outer bark, and their scanty and rigid 
foliage would indicate no very rapid growth or decay. In the case of Sigillaria, 
the variations in the leaf-scars in different parts of the trunk, the intercalation of 
new ridges at the surface representing that of new woody wedges in the axis, the 
transverse marks left by the successive stages of upward growth—all indicate that 
at. least several years must have been required for the growth of stems of moderate 
size. The enormous roots of these trees, and the conditions of the coal-swamps, 
must have exempted them from the danger of being overthrown by violence. 
They probably fell, in successive generations, from natura] decay; and, making 
every allowance for other materials, we may safely assert that every foot of thick- 
ness of pure bituminous coal implies the quiet growth and fall of at least fifty genera- 
tions of Szgillarie, and therefore an undisturbed condition of forest-growth enduring 
through many centuries. Further, there is evidence that an immense amount of 
loose parenchymatous tissue, and even of wood, perished by decay; and we do 
not know to what extent even the most durable tissues may have disappeared in 
* Paper on Fosils from Nova Scotia. Quart. Journ. Geol, Soc. 1847. 
