354 Introduction to Botany. 



flats along the borders of the forests of ferns, Sigillarias, 

 etc. (see Fig. 192). 



The coal-bearing areas of the United States are about 

 300,000 square miles in extent, and about 50,000 square 

 miles of this are workable. The coal seams in the work- 

 able fields average from 4 to 10 feet in thickness, and in 

 some localities they reach a maximum thickness of 60 to 

 100 feet. Some idea of the enormous amount of vegeta- 

 tion embodied in these deposits may be obtained when it is 

 remembered that their thickness is only, roughly speaking, 

 about 7j& of the original depth of the vegetation whkh 

 formed them. During this period there seems to have 

 been a fairly uniform and temperate climate throughout 

 North America, Europe, and Asia, even as far north as 

 Greenland, Scandinavia, and Nova Zembla. 



223. Permian Plants. As time advanced into the 

 Permian period, the giant Lepidodendrons and Sigillarias 

 became rare ; the Calamites were still abundant, and the 

 tree ferns even more so than in the Carboniferous period. 

 The Gymnosperms were represented by the Cordaites, 

 Gingko, and conifers related to the modern yews and 

 spruces. The ferns dominated the landscape, while the 

 Cycads and conifers were not abundant and no Angiosperms 

 had yet appeared. 



As the Paleozoic era drew to its close, the Lepidoden- 

 drons, Sigillarias, and Calamites, which had been the domi- 

 nant land vegetation, became almost extinct, the tree ferns 

 became less abundant, and the Cycads and conifers gained 

 ascendency. 



224. Triassic Plants. In the Triassic period the coni- 

 fers were forming dense forests and the Cycads (Fig. 180) 

 were common ; the tree ferns were becoming more rare, 

 and only a few straggling Sigillarias still persisted. The 



