412 



THE CHANGING GENERATIONS 



climate of these regions was evidently mild, humid, and without marked 

 seasons. Luxuriant forests of fast-growing, soft-tissued trees occupied the 

 low wet lands. On the swamp floors partially decayed plant debris — logs, 



twigs, leaves, spores, and seeds- 

 accumulated in thick layers, and from 

 time to time these were buried under 

 sheets of mud during brief incursions 

 of shallow seas. Where the plant 

 layers were merely compacted by the 

 weight of later deposits or were but 

 slightly metamorphosed, they grad- 

 ually altered to soft coal and are 

 rilled with fossils ; but where they were 

 subjected to the pressures that folded 

 up great mountain ranges during the 

 Permian period, they were changed 

 to anthracite, and all fossils were 

 destroyed. 



A remarkable feature of the coal 

 forests is the similarity of their species 

 in all continents. These floras were 

 as nearly cosmopolitan as have ever 

 existed, suggesting wide land con- 

 nections and absence of climatic bar- 

 riers. Most of the great trees were 

 spore bearers descended from Devon- 

 ian horsetails and lycopods. Giant 

 horsetails with fluted columnar stems 

 (Calamites, Fig. A. 14) grew 30 to 

 70 feet high in dense thickets like 

 "canebrakes," and others of the coal- 

 forest trees were even more impos- 

 ing. The scale trees had trunks and 

 branches covered with scalelike leaf 

 scars. Though they were lycopods, 

 related to the small club mosses, or 

 ground pines, of today, they reached 

 heights of 100 to 120 feet and had 

 trunks 4 to 6 feet in diameter at the base. There were two chief kinds— 

 Lepidodendron (Fig. A. 17), with slender trunk and a crown of stubby 

 twigs covered with straplike leaves J^ foot long and with leaf scars 

 running in spirals; and Sigillaria (Fig. A. 16), with thicker trunk, few 



Fig. 26.12. Restoration of Cordaites, 

 with Calamites at right. {Courtesy Amer- 

 ican Museum of Natural History.) 



