THE VEGETABLE WORLD. 283 



twenty-seventh part of its thickness, an approximative 

 conception may be formed of the duration of that period ; 

 since the super-imposed layers of Coal, in England for 

 instance, often have a collective thickness of forty-four feet, 

 and correspond therefore to a period of time almost 

 equalling 100,000 years. The character of the vegetable 

 world of the Carboniferous period, expressed in the pre- 

 dominance of the great arborescent Cryptogamia, especially 

 the Ferns, reminds us most of the Flora of the tropical 

 South Sea Islands, and the vegetation of these organisms 

 seems to be particularly dependant on a hot atmosphere, 

 saturated with moisture, such as we are compelled to 

 assume to have existed in that epoch of the earth's 

 history. 



In the succeeding period of the Secondary rocks, the 

 previously existing islands, with their Flora, appear to have 

 become partly submerged again, while other, more extended 

 tracts of land rose up, the soil of which consisted prin- 

 cipally of the lime and sandstone of the Coal period. 

 However, these tracts of land partly united with the islands 

 already formed, and thus particular forms of plants of the 

 previous epoch were preserved and continued over into the 

 new order of things, while the most peculiar races partly 

 sank with their soil, and partly died out, in consequence 

 of the gradual essential alteration of the physical condi- 

 tions. The arborescent Ferns and the Calamites still 

 existed but became more rare, while the Cycadacecs and 

 Conifers developed in great abundance, and in numerous 

 peculiar forms, forming dense forests on the borders of the 

 great lakes, in which great Sedge and Rush-like plants 

 vegetated. Grand forms of Liliacece rising into trees, the 

 Bucklandias and Clathrarias, perhaps constituted peculiar 

 groups on the more elevated grounds. Among these 



