fVood partly petrified hy Carbonate of Lime. 21 1' 



Partially petrified wood from the Vale of the Aire, near Ferrybridge, Yorkshire. 



It is indeed evident that the vessels of the wood have thus acted as conductors to the solution, 

 from the sinuosities in the outline of the petrified portion which follow the irregular direction of 

 the fibres, where they have by knots or other interruptions been forced from a straight line. 

 The calcareous material was probably derived from the neighbouring limestone beds, and might 

 be supposed to diffuse itself through the alluvium ; but in the case of the aqueduct, we cannot 

 imagine that there was so large or so generally diffused a supply of mineral matter. I should 

 mention also, that the examination of thin slices with a microscope shows, that the structure is 

 rather better preserved in the wood at Ferrybridge, than in that from the aqueduct. 



Mr. Brown has also shown me specimens^ in his possession, of petrified 

 wood, in which the greater portion is silicified, but there are some parts still 

 in a soft state, and not at all penetrated by the silex. Some of these are from 

 Lough Neagh, and others from Bonn. 



Although the amount of information obtained by the preceding observa- 

 tions is but small, yet, in establishing the facts, that petrifaction sometimes 

 commences at a number of separate points, and that it is liable to be inter- 

 rupted and again renewed after an interval of time, we have an explanation 

 of the circumstances which have been difficult to understand, in regard to 

 many specimens of fossil wood, in which parts of the ligneous structure are 

 preserved in good condition, while other parts appear to be more or less 

 decayed. 



The same facts appear to occur both in siliceous and calcareous petrifac- 

 tions ; but nothing that I have met with, is calculated to throw any light upon 

 the question, of the means by which the solution of siliceous matter has been 

 accomplished, 



A short description of the different conditions in which fossil wood appears 

 in different specimens may be worthy of attention. 



Sometimes the most minute parts of the structure have been preserved, as 

 is shown under the microscope in the markings of the vessels of Palms, and 

 the well known instances of the circular discs peculiar to the vessels of coni- 



2 E 2 



