68 VESTIGES OF THE 



tilting the carboniferous group, were formed. It is favour- 

 able to this last view that marine fossils are scarcely found 

 in the body of the coal itself, though abundant in the shale 

 layers above and below it ; also that in several places 

 erect stems of trees are found with their roots still fixed 

 in the shale beds, and crossing the sandstone beds at 

 almost right angles, showing that these, at least, had not 

 been drifted from their original situations. On the other 

 hand, it is not easy to admit such repeated risings and 

 sinkings of surface as would be requii-ed, on this hypo- 

 thesis, to form a series of coal strata. Perhaps we may 

 most safely rest at present with the supposition that coal 

 has been formed under both classes of circumstances, 

 though in the latter only as an exception to the former. 



Upwards of three hundred species of plants have been 

 ascertained to exist in the coal formation ; but it is not 

 necessary to suppose that the whole contained in that 

 system are now, or ever will be, distinguished. Experi- 

 ments show that some great classes of plants become 

 decomposed in water in a much less space of time than 

 others, and it is remarkable that those which decompose 

 soonest are of the classes found most rarely, or not at all, 

 in the coal strata. It is consequently to be inferred that 

 there may have been grasses and mosses at this era, and 

 many species of trees, the remains of which had lost 

 all trace of organic form before their substance sunk into 

 the mass of which coal was formed. In speaking, there- 

 fore, of the vegetation of this period, we must bear in 

 mind that it may have comprehended forms of whicli we 

 have no memorial. 



Supposing, nevertheless, that, in the mnin, the ascer- 

 tained vegetation of the coal system is that which grew 

 at the time of its formation, it is interesting to find that 

 the terrestrial botany of our glol)e begins with classes of 



