THE CARBONIFEROUS FLORA. 



117 



covery has since been extended to all the coal-fields of 

 Europe and America, and it is a perfectly conclusive fact 

 as regards the origin of coal. Each of these "under- 

 clays," as they are called, must, in fact, have been a soil 

 on which grew, in the first instance, Sigillariae and other 

 trees having stigmaria-roots. Thus, the growth of a 

 forest of Sigillarim was the first step toward the accumu- 

 lation of a bed of coal. More than this, in some of the 

 coarser and more impure coals, where there has been 

 sufficient earthy matter to separate and preserve impres- 

 sions of vegetable forms, we can see that the mass of the 

 coal is made up of flattened Sigillarice, mixed with vege- 





FIG. 40. Vegetable tissues from coal, a, Sigillaria and Cordaites. 

 b, Calamodendron. 



table debris of all kinds, including sometimes vast quan- 

 tities of lepidodendroid spores, and the microscopic study 

 of the coal gives similar results (Fig. 40). Further, on 

 the surfaces of many coals, and penetrating the shales or 

 sandstones which form their roofs, we find erect stumps 

 of sigillaria and other trees, showing that the accumula- 

 tion of the coal terminated as it had begun, by a forest- 

 growth. I introduce here a section of a few of the nu- 

 merous beds of coal exposed in the cliffs of the South 

 Joggins, in Nova Scotia, in illustration of these facts. 

 We can thus see how in the slowly subsiding areas of the 

 coal-swamps successive beds of coal were accumulated, 

 alternating with beds of sandstone and shale (Figs. 41, 

 42). For other details of this kind I must refer to 

 papers mentioned in the sequel. 



