168 



THE GEOLOGIST. 



when carefully traced over the surfaces of accumulation, are found to present the 

 outline of flattened tranks. This is also true to a certain extent of the finer 

 varieties of slate-coal ; but the coarse coal appears to consist of extensive laminae 

 of disintegrated vegetable matter mixed with mud. When the coal (especially 

 the more shaly varieties) is held obliquely under a strong light, in the manner 

 recommended by Goeppert, the surfaces of the laminae of coal present the forms of 

 many well-known coal-plants, as Sigillaria, Stigmaria, Poacites (or Nceggerathia). 

 .Lejjidodendron, Ulodendron, and rough bark, perhaps of Conifers. When the coal 

 is traced upward into the roof-shales, we often find the laminae of compact coal 

 represented by flattened coaly trunks and leaves, now rendered distinct by being 

 separated by clay. 



The relation of erect trees to the mass of the coal, and the state of preservation 

 in which the wood and bark of these trees occur,— the microscopic appearances of 

 coal, — the abundance of cortical tissue in the coal, associated with remains of 

 herbaceous plants, leaves, &c., are next treated of 



The author offers the following general conclusions 



(1.) With respect to the plants which have contributed the vegetable matter of 

 the coal, these are principally the Sigillarice and Calamitece, but especially the 

 former. 



(2. ) The woody matter of the axes of Sigillarice and Calamitece and of coniferous 

 trunks, as well as the scalariform tissues of the axes of the Lepidodendrece and 

 Ulodendrece, 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 subaerial 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 subaerial 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 Sigillarice and 

 Calamitece, tall 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 must 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, or much less distinct than those of many of their 

 northern congeners.* The Sigillarice and Calamites were not, as often supposed, i 

 succulent plants. The former had, it is true, a very thick cellular inner bark ; 

 but their dense woody axes, their thick and nearly imperishable outer bark, their I 

 scanty and rigid foliage, Avould indicate no very rapid growth. In the case of | 

 Sigillarice, the variations in the leaf-scars in different parts of the tnmk, the inter- j 

 calation of new ridges at the surface representing that of new woody wedges in i 

 the axis, the transverse marks left by the successive stages of upward growth, all 

 indicate that at least several years must have been rec| lued 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 over- 

 thrown 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 bitummous coal implies the quiet growth and fall 

 of at least fifty generations of Sigillarice, and therefore an undisturbed condition 

 of forest-f^rowth enduring through many centuries. Further, there is evidence 

 that an nnmense amount of loose parenchymatous tissue, and even of wood, 

 perished by decay ; and we do not know to what extent even the most durable 



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



