600 GEOLOGY. 



iferous trees, their ribbed trunks being known to reach six feet in 

 diameter, and 100 feet or more in height. As in lepidodendrons, the 

 stems were densely clothed with erect, rigid, linear leaves, which had 

 two deep furrows on the under side to which the breathing pores were 

 confined. The pores were protected still further by hairs in the fur- 

 rows. The sigillarians, like the lepidodendrons, have secondary wood 

 and a thick cork layer. The degree of development of cork is almost 

 unequalled in modern trees, except in the cork-oak and its allies. 



Stigmaria were the roots, or perhaps stolons, of sigillarians and 

 probably of lepidodendrons, the scars that mark them being the points 

 of attachment of appendages or rootlets. Tap roots are never pres- 

 ent, the main roots, usually four in number at the start, spreading 

 horizontally. 



We have entered thus into detail respecting these remarkable 

 plants, partly because of the interest which they have as the con- 

 stituents of the first-known great forests, and partly because not a 

 few of their features are suggestive of the climatic and other physical 

 conditions by which they were surrounded, a question to be considered 

 presently. 



The lepidodendrons seem to have reached their climax early in the 

 Coal Measure period and to have declined during the later portion, 

 so that they had nearly all disappeared by the close of the period. 

 In the latter part of the period, the sigillarians passed the lepidoden- 

 drons in abundance, but they also were decidedly on the wane at 

 its close. Whether or not this rather sudden decline, followed by an 

 early extinction, was connected with such climatic changes as are 

 indicated by the glaciation of India, Australia, and South Africa, 

 and which seem to have occu ed not far from this time, is a question 

 which naturally arises, but it cannot be answered demonstratively 

 at present. 



The Cordaites. — This was a remarkable extinct family of the gymno- 

 spermous type, having alliances with the cycads, conifers, and ginkgos, 

 and yet many distinctive features of its own. The cordaites were 

 tall, rather slender trees, reaching two or more feet in diameter, and 

 90 feet or more in height. The wood was of the coniferous type, 

 covered, as in so many other plants of the period, by a thick com- 

 plex bark. The trunks had a large pith. The leaves were parallel 

 veined, suggestive of monocotyls of the yucca type, and sometimes 



