Ch. XX.] FOSSIL FOEEST m ISLE OF POETLAND. 297 



Fig. 841. 



Zumia bpiralis. Southern Australia, 



These plants must have become fossil on the spots where they grew. 

 The stumps of the trees stand erect for a height of from 1 to 3 feet, and 

 even in one instance to 6 feet, with their roots attached to the soil at 

 about the same distances from one another as the trees in a modern 

 forest.'* The carbonaceous matter is most abundant immediately around 

 the stumps, and round the remains of fossil Cycade(je.\ 



Besides the upright stumps above mentioned, the dirt-bed contains the 

 stems of siUcified trees laid prostrate. These are partly sunk into the 

 black earth, and partly enveloped by a calcareous slate which covers the 

 dirt-bed. The fragments of the prostrate trees are rarely more than 

 3 or 4 feet in length ; but by joining many of them together, trunks have 

 been I'estored, having a length from the root to the branches of from 

 20 to 23 feet, the stems being undivided for 17 or 20 feet, and then 

 forked. The diameter of these near the roots is about 1 foot. Root- 

 shaped cavities were observed by Professor Henslow to descend from the 

 bottom of the dirt-bed into the subjacent freshwater stone, which, though 

 now solid, must have been in a soft and penetrable state when the 

 trees grew.J 



risr. 342. 



freshwater calcareous slate. 



dirt-bed and ancient forest. 



lowest freshwater beds of the Lower 

 Purbeck. 



Portland stone, marine. 



Section in Isle of Portland, Dorset. (Buckland and De la Beche.) 



* Mr, Webster first noticed the erect position of the trees and described the 

 Dirt-bed. 



\ Fitton, Geol. Trans., Second Series, vol. iv, pp. 220, 221. 



X Buckland and De la Beche, Geol. Trans., Second Series, vol. iv. p. 16. Pro- 

 fessor Forbes has ascertained that the subjacent rock is a freshwater limestone, 

 and not a portion of the Portland oolite, as was previously imagined. 



