PLANT EECOEDS OF THE ROCKS SEWARD 365 



period; and in some of the beds of ash remarkably well-preserved 

 stems of Lepidodendron have been found, a common tree in the later 

 Paleozoic forests, which is distantly related to the living lycopods 

 or club mosses, though probably not the direct ancestor of any sur- 

 viving members of the great lycopod group. One specimen described 

 several years ago will serve as an illustration : It is a block of stem 

 about 1 foot in diameter encased in a shell of bark surrounding a 

 mass of volcanic ash in which lies the woody axis of the plant. The 

 more delicate tissues between the bark and woody cylinder have 

 decayed, and their place has been taken by the ash. Both the 

 stronger tissues of the bark and wood and the delicate tissue imme- 

 diately surrounding the wood are almost perfectly preserved in sili- 

 ceous material which, one imagines, the heated waters from some 

 neighboring volcanic source deposited in the body of the fallen tree. 

 Specimens of petrified wood, which are often found in beds of sand- 

 stone, have usually been transported a considerable distance from the 

 place where the living trees stood. A piece of stem, which had no 

 doubt been carried as driftwood far from its original home, was 

 found by members of one of the British Antarctic expeditions in 

 a bowlder on one of the moraines of the Beardmore Glacier about 

 1,100 miles from the South Pole ; its anatomical characters show that 

 it was part of the stem of an extinct type of tree known as Rhexoxy- 

 lon^ which was first described from rocks of Triassic age in South 

 Africa. It is clear that the occurrence of driftwood, whether fossil 

 or recent, can not be trusted as evidence of the nature of the vegeta- 

 tion where the wood occurs. 



The majority of fossil plants are not petrified but occur as thin 

 films of carbonaceous matter on beds of shale ; the internal structure 

 is not preserved — only the outlines of cells and strips of the highly 

 resistant superficial skin, or cuticle, which it is often possible to 

 examine microscopically after treating the detached film with cer- 

 tain reagents. Some years ago small pieces of liverworts were dis- 

 covered in England in a bed of carboniferous shale; they had been 

 preserved as delicate mummified scraps from which it was possible 

 to recognize the nature of the plants. These were the first recorded 

 examples of Paleozoic liverworts, and it is noteworthy that in form 

 and in the superficial cellular structure they agree very closely 

 with certain living representatives of the group. Another common 

 type of fossil is that loiown as a cast; in quarrying rocks stumps 

 of trees, spreading at the base into long and forked rootlike branches, 

 are sometimes laid bare. No trace of the original plant structure is 

 left, merely a mass of sandstone or shale identical with the inclos- 

 ing rock and preserving the form, occasionall}^ also the external 

 pattern, of the tree. One can picture broken stumps of forest trees 



