472 THE CARBONIFEROUS SYSTEM. 



into shells of bark and loose fragments of rotten wood, which currents 

 would necessarily have swept away, but which the most gentle inun- 

 dations 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 groAvth not larger or much less 

 distinct than those of many of their modern congeners.* The Sigil- 

 lai^icB 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 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 Sigillarice, 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 stages of upward growth — all indicate that several 

 years must have been required for the growth of stems of mod- 

 erate 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 natural decay ; and, making every allowance for 

 other materials, we may safely assert that every foot of thickness of 

 pure bituminous coal implies the quiet growth and fall of at least fifty 

 generations of Sigillarioe, 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 this way ; so 

 that in many coal-seams we may have only a very small part of the 

 vegetable matter produced. 



" Lastly^ the results stated in this paper refer to coal-beds of the 

 Middle Coal measures. A few facts which I have observed lead me 

 to believe that, in the thin seams of the Lower Coal measures, remains 

 of Cordaites and LejJidodendron are more abundant than in those of 

 the Middle Coal measures. In the upper Coal measures similar 

 modifications may be expected." 



* Paper on Fossils from Nova Scotia, Quart. Journ. Geol. Soc. 1847. 



