FLORA OF THE CARBONIFEROUS EPOCH. 37 



different yarieties of coal which are worked. The occur- 

 rence of punctated tissue indicates the presence of Coni- 

 fera3 in the coal-bed, while scalariform vessels point to ferns, 

 and their allies, such as Sigillaria and Lepidodendron. The 

 anatomical structure of the stems of these plants may have 

 some effect on the microscopic characters of the coal pro- 

 duced from them. In some cannel coals structure resembling 

 that of Acrogens has been observed. A bro^vnish -yellow 

 substance is occasionally present, which seems to yield abund- 

 ance of carburetted hydrogen gas when exposed to heat. 



It appears that in general each bed of coal is accompanied 

 by the remains of a somewdiat limited amount of species. 

 Their number, particularly in the most ancient beds, is scarcely 

 more than eight or ten. In other cases the number is more 

 considerable, but rarely more than thirty or forty. In the 

 same coal-basin each layer often contains several characteristic 

 species which are not met with either in the beds above or 

 below. Thus, there are sometimes small local or temporary 

 floras, each of which has given birth to layers of coal. The 

 quantity of carbonaceous and other matter required to form 

 a bed of coal is immense. Maclaren has calculated that one 

 acre of coal three feet thick is equal to the produce of 1940 

 acres of forest.* The proportion of carbon varies in different 

 kinds of coal. Along with it there is always more or less of 

 earthy matter which constitutes the ashes. When the earthy 

 substances are in such quantity that the coaly deposit will 

 not burn as fuel, then we have what is called a shale. The 

 coal contains plants similar to those of the shales and sand- 

 stones above and below it. Underneath a coal-seam lies 

 the Underclay, containing roots only, and representing the 

 ancient soil ; then comes the Coal, composed of plants whose 

 roots are in the clay, Tvith others which have grown along with 

 and upon them, in a manner precisely similar to the growth of 

 peat at the present day ; while above the coal is the Shale, 



''' Mat'lareu, Geology of Fife and the Lothiaus, p. 11 G. 



