THE CARBONIFEROUS EPOCH. 



579 



became luxuriant. Tree ferns and all the floral appearance of the tropics 

 grew up and formed dense 

 forests, far thicker than any 

 we know of at the present 

 time. It will readily be 

 understood that the condi- 

 tion of the atmosphere must 

 have been particularly fa- 

 vourable to the growth of 

 plants, and therefore not suit- 

 able for air-breathing animals. 

 Heat and carbonic acid must 

 have been greatly deve- 

 loped. 



We can now perceive 

 how the gradual filling up of 

 the earth for man's recep- 

 tion Was taking place. The Fi S- 6&>--Tree Ferns. 



rain was taking carbonic acid from the air, for the dead plants gave it out 

 in enormous volumes. The carbonate of lime dissolved in the water- 

 springs, etc., was carried to the sea for polypi to build shells from. 

 The trees were absorbing carbonic acid, too, and while purifying the 

 air, were retaining the carbon in their stems and leaves and branches, which 

 (when they decayed) remained untouched, and accumulated in thick layers 

 to sink down, and by pressure be turned into coal. This great effect was 

 carried out several times ; and it is a remarkable fact that we find coal, 

 limestone, and ironstone so near together, all useful to us and to each other 

 in the course of the working of the minerals so we come to the Carboniferous 

 system. 



Coal we have already treated of in Mineralogy, and coal looks at times 

 very different from our preconceived ideas of a sedimentary rock, which we 



know is regularly deposited in layers. 

 But when we split or break the coal we 

 find its cleavage in a certain direction. 

 Coal is wood squeezed and petrified by 

 ages between enormous layers of sedi- 

 mentary rocks, and the coal-seam rests 

 upon the soil in which the plants once 

 grew perhaps more than six hundred 

 thousand years ago ! 



Of course coal, as we burn it, was 

 not all made at once. We can trace it 

 from the swamp as Peat, on to "Lignite," 

 or WO ody coal, through the Tertiary and 

 New Red Sandstone to the coal measures themselves. Even lower down 



