908 Mr. Stokes's Notice respecting a Piece of recent 



are more perfect in their form and size, in those parts of the specimen, than in 

 that which remains unchang-ed ; as if the petrification of those portions had 

 taken place, while the wood was yet in sound condition, and the remaining part 

 had since contracted around them, as it gradually decayed. This is shown by 

 the bending of the medullary rays, adjoining the limits of the calcareous portions. 



Dr. Turner has examined one of these portions, and finds it to contain 9 per 

 cent, of ligneous fibre, the rest being carbonate of lime. 



I have not obtained an account of the situation of the wood in the aqueduct, 

 or of any circumstance to explain how it had become exposed to the action of 

 water containing carbonate of lime in solution, but I am unwilling to delay the 

 communication to the Geological Society, of the remarks it has suggested to me. 



Mr. Robert Brown, to whom I had the pleasure of showing this specimen on 

 the day I received it, immediately reminded me, of one of silicified wood from 

 Antigua in my own collection, which we had formerly examined together, and 

 in which the polished horizontal section exhibits a similar appearance of insu- 

 lated portions, that have the structure preserved in a more perfect state than 

 the rest of the specimen, as is represented in the accompanying drawing. 

 (PI. XVII). 



The whole specimen is completely silicified, and takes an excellent polish. 

 The portions best preserved are irregularly circular in form, about -^ of an 

 inch or rather more in diameter, and in them the vessels and medullary rays 

 are distinctly seen in good condition, while in all the other parts (although the 

 general form of the stem is preserved) the vessels are compressed (particularly 

 round the more perfect portions) and the medullary rays are bent and compara- 

 tively indistinct. 



From the different condition in which the different parts of the wood have been preserved, it ap- 

 pears, that the process of petrification had first taken place simultaneously at a number of separate 

 points, and then had stopped, and that the unchanged part had subsequently in some degree decayed. 



If at this period a transverse section of the wood had been made, its condition must have been 

 similar to that of the wood from the Roman aqueduct, having some parts petrified while the greater 

 portion was still in the state of wood, though not in a sound state. After this, the progress of petri- 

 faction was again renewed, and the partially decayed remainder of the wood has been silicified. 



Another instance (which is of calcareous petrification) appears also to have 

 an analogy to the above-mentioned. This is the specimen from Allen Bank in 

 Berwickshire, described and figured by Mr- Witham (On the Structure of Fos- 

 sil Vegetables, PI. VIII. fig. 7), under the name of Anabathra pulcherrima. 



The texture of this plant, whatever it may have been, was much more lax 

 than that of the woods before spoken of. The horizontal section shows some 

 instances, in which the structure is preserved from the centre to the circum- 



