Ch. XX.] 



AND LULWORTH COVE. 



391 



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

 the stems of silicified 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 restored, having a leno-th from the root to 



© ' © O 



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 root is 

 about one foot. Root-shaped cavities were observed by Professor 

 Henslow to descend from the bottom of the dirt-bed into the subja- 

 cent freshwater stone, which, though now solid, must have been in a 

 soft and penetrable state when the trees grew.* 



Fis. 378. 



Freshwater calcareous slate. 



Dirt-bed and ancient forest. 



Lowest freshwater beds of the Lower 

 Purbeck. 



Portland stone, marine. 



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



The thin layers of calcareous slate (fig. 378) were evidently de- 

 posited tranquilly, and would have been horizontal but for the pro- 

 trusion 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 Portland, 

 where it has been most carefully studied, but is seen in the same 

 relative position in the cliffs east of Lulworth Cove, in Dorsetshire, 



Fig. 379. 



Freshwater calcareous slate. 

 Dirt-bed, with stools of trees. 



Freshwater. 



Portland stone, marine. 



Section in cliff east of Lulworth Cove. (Buckland and De la Beche.) 



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 



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

 Forbes has ascertained that the subjacent rock is a freshwater limestone, and not a 

 portion of the Portland oolite, as was previously imagined. 



