Dr. Dawson on the Vegetable Structures in Coal. 309 



The next chief heading of the paper has reference to structures 

 preserved in the layers of compact coal, which constitutes a far 

 larger proportion of the mass than the mineral charcoal does. The 

 laminae of pitch- or cherry-coal, says Dr. Dawson, when carefully 

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

 outline of flattened trunks. This is also true to a certain extent of 

 the finer varieties of slate-coal ; hut the coarse coal appears to con- 

 sist 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 

 Naggerathia), Lepidodendron, 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 vege- 

 table matter of the coal, these are principally the Sigillariee and 

 Calaviitece, 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 vas- 

 cular 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, aff"ording the purest coal. The 

 relative amounts of all these substances found in the states of mine- 

 ral 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 Sigillaria and Calamitece, tdll and branchless, and 

 clothed only with rigid linear leaves, formed dense groves and jun- 

 gles, 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 cli- 



