206 



LYELL'S ELEMENTS OF GEOLOGY. 



Fossil Forest in Isle of Portland. 



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(Bj\ 



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

 tains the stems of silicified trees laid prostrate. These are partly 

 sunk into the black earth, and partly enveloped by a calcareo- 

 siliceous slate which covers the dirt-bed. The fragments of the 

 prostrate trees are rarely more than three or four feet in length ; 

 but by joining many of them together, trunks have been restored 

 having a length from the ro?»t 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 one foot.ij: Root- 

 shaped cavities were observed by Professor Henslow to descend 

 from the bottom of the dirt-bed into the subjacent Portland stone, 

 so that the uppermost beds of the Portland limestone, though 

 now solid, were in a soft and penetrable state when the trees 

 grew.§ 



The thin layers of calcareous slate (Fig. 196.) were evidently 

 Fig. 196. 



freshwater calcareous 

 slate. 



dirt-bed and ancient 

 forest. 



marine Portland 

 stone. 



De la Beche.) 



deposited tranquilly, and would have been horizontal but for the 

 protrusion of the stumps of the trees, around the top of each of 

 which they form hemispherical concretions. 



The dirt-bed is by no means confined to the island of Port- 

 land, but is seen in the same relative position in a cliff east of 

 Lulworth Cove, in Dorsetshire, where, as the strata have been 

 disturbed, and are now inclined at an angle of 45°, the stumps 

 of the trees are also inclined at the same angle in an opposite 

 direction — a beautiful illustration of a change in the position of 



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

 Dirt-bed. The account here given is drawn from Dr. Buckland and Mr. De la 

 Beche, Geol. Trans., Second Series, vol. iv. p. 1. ; Mantell, Geol. of S. E. of 

 England, p. 336. ; and Dr. Fitton, Geol. Trans., Second Series, vol. iv. p. 220. 



t Fitton, ibid. pp. 220, 221. X Ibid. 



$ Buckland and De la Beche, Geol. Trans., Second Series, vol. iv. p. 16. 



Section in Isle of Portland, Dorset. (Buckland 



