Chalk and Flint Formation. 37 



lately seen one large trunk, now lying in one of the 

 quarries, which is twenty- six feet in length, ten feet 

 in girth, and three feet eight inches in its greatest 

 diameter, being the largest specimen yet found there, 

 and well worthy of a place among geological 

 specimens in the magnificent Museum of Natural 

 History at South Kensington. Attention was drawn 

 to these singular remains half a century ago by Mr. 

 Webster, then Secretary of the Geological Society, 6 

 and by Professor Buckland and Mr. De la Beche. 7 

 But these petrifactions, though siliceous, are not of 

 flint : and this is important to be observed in 

 reference to the subject of this paper, if there be a 

 probability of their aqueous origin. The trees found 

 at Portland are said by these authorities to be 

 generally of the dicotyledonous class, or coniferous, 

 from one to two feet in diameter, some three feet : 

 one much larger has been mentioned above. " The 

 woody part," says Mr. Webster, "is siliceous, and 

 the longitudinal vessels are filled by and surrounded 

 with radiated quartz : numerous veins of chalcedony 

 also pass through these stems, but always following 

 the direction of the concentric and radial structure." 

 I should not say that this description exactly fits the 

 specimens which I have, or which I have seen ; but 

 the fibre of the wood in the largest tree appears to be 

 perfectly infiltrated, and converted into silex through 

 and through, when examined with the microscope in 

 a portion taken from the centre of the base or root. 

 In another dissimilar case of a standing stump, I 

 observed it converted into minute crystals. In 



6 Trans. Geol. Soc, 2nd series, vol. ii. p. 38. 



T Trans. Geol. Soc, 2nd series, vol iv. part 1, pp. 14, 15; and 

 Buckland's Bridgwater Treatise, vol. i. p. 498, note, and vol. ii., 

 plates 56, 61. 



