1849.] 



BROWN ON ERECT SIGILLARI^. 



359 



stratum in the ascending order, forms the roof of the coal. In such 

 cases the surface of the peat bog could not have been level when the 

 shale was deposited upon it, some small patches having been still 

 above water ; and as no upright trees are found in the sandstone 

 roof, it may reasonably be inferred that plants would not vegetate 

 upon the bog itself, a layer of soft mud being necessary in the first 

 instance for germinating the seeds ; but when a plant had once taken 

 root in this mud its rootlets penetrated downwards into the peat, 

 and furnished an abundant supply of nutriment for the rapid growth 

 of the tree, from the rich mass of decaying vegetable matter beneath. 



I may here observe that the quality of the upper part of the seam 

 appears invariably to be influenced by the nature of the roof, the coal 

 being highly charged with iron pyrites under a sandstone, but quite 

 free from it under a shale roof*. And since no upright trees are 

 found in the sandstone roofs, is it probable that the luxuriant forest 

 of Sigillariae growing in the mud above the peat bog has taken up 

 the sulphuret of iron, and thus produced such a beneficial eifect upon 

 the quality of the coal ? Both sulphuric acid and the oxide of iron 

 are inorganic constituents of plants, and it has been ascertained that 

 natural and artificial waters that have a sulphureous taste, when em- 

 ployed in irrigating meadows, give birth to a very luxuriant vegeta- 

 tion f, but whether the growth of a forest of Sigillarise upon the sur- 

 face of the peat bog, is an adequate explanation of the absence of iron 

 pyrites from the upper part of the coal seam, I must leave to more 

 skilful botanists to determine. 



Having shown that Sigillaria alternans was provided with roots 

 peculiarly adapted for flourishing upon a soft muddy soil, and thus 

 furnished additional proof that coal seams have been formed from 

 beds of peat or other decaying vegetable matter accumulated on the 

 surface, it only remains for me now to direct attention to the fossil 

 represented in fig. 9, which it will be observed, in its external aspect, 



Fig. 9. 



Stump broken off close to the root, one-tivelfth of natural size. 



* Mr. Buddie states in the Trans, of the Nat. Hist. Society of Newcastle, vol. i. 

 p. 217, that the coal seams in Northumberland are always more or less intermixed 

 with iron pyrites under a sandstone roof. 



t Johnston's Agricultural Chemistry, p. 273. 



