142 THE GEOLOGICAL HISTORY OF PLANTS. 



Sigillaria in the coal-roofs equally testify to the accn- 

 mulation of coal by the growth of successive forests, more 

 especially of Sigillarice. There is, on the other hand, no 

 necessary connection of sporangite-beds with Stigmarian 

 soils. Such beds are more likely to be accumulated in 

 water, and consequently to constitute bituminous shales 

 and cannels. 



6. Lepidodendron and its allies, to which the spore- 

 cases in question appear to belong, are evidently much 

 less important to coal accumulation than Sigillaria, which 

 cannot be affirmed to have produced spore-cases similar 

 to those in question, even though the observation of 

 Goldenberg as to their fruit can be relied on ; the ac- 

 curacy of which, however, I am inclined to doubt. 



On the whole, then, while giving due credit to those 

 who have advocated the spore-theory of coal, for directing 

 attention to this curious and no doubt important constit- 

 uent of mineral fuel, and admitting that I may possibly 

 have given too little attention to it, I must maintain that 

 sporangite-beds are exceptional among coals, and that 

 cortical and woody matters are the most abundant ingre- 

 dients in all the ordinary kinds ; and to this I cannot 

 think that the coals of England constitute an exception. 



It is to be observed, in conclusion, that the spore- 

 cases of plants, in their indestructibility and richly car- 

 bonaceous character, only partake of qualities common to 

 most suberous and epidermal matters, as I have explained 

 in the publications already referred to. Such epidermal 

 and cortical substances are extremely rich in carbon and 

 hydrogen, in this resembling bituminous coal. They are 

 also very little liable to decay, and they resist more than 

 other vegetable matters aqueous infiltration — properties 

 which have caused them to remain unchanged, and to 

 continue free from mineral additions more than other 

 vegetable tissues. These qualities are well seen in the 

 bark of our American white birch. It is no wonder that 



