98 THE VICTORIAN NATURALIST. 



surface of the silicious threads. In many instances it is apparent 

 that the cellulose partition separating the opposite pairs of pits 

 was ruptured before silicification took place. The cast on the 

 surface of the thread then appears as that of a double pit, the 

 breakage occurring at the neck, which formed the point of 

 juncture between the pit cast and the fibre opposite to that to 

 which the cast remains attached. A group of fibres, whose 

 relative position has not been disturbed, shows round each thread 

 an open space, corresponding to the walls of the tracheides, and 

 portions of the remains of these walls still adhere to the thread in 

 places. 



It is evident from the fibrous condition of the fossil that the 

 process of petrifaction was interrupted prior to the decomposition 

 of the walls of the tracheides. When that stage was reached at 

 which the open vessels had been filled the specimen was 

 evidently buried under the accumulation of silt and gravel, which 

 afterwards formed the rock in which it was found. Subsequent 

 decay removed the organic material of the cell walls, and left the 

 silicious fibres separate and distinct. But it is clear that had the 

 fossil been subjected to similar petrifactive influences after the 

 disappearance of the cell walls, further silicification would have 

 taken place, filling up the spaces formerly occupied by the walls, 

 and converting the fibrous condition of the body into a dense 

 and stony one. 



The question arises as to whether this specimen does not repre- 

 sent the initial stage of a process to which all wood that has 

 undergone petrifaction has been subjected. The theory some- 

 times met with, that in these cases the organic material has been 

 removed atom by atom and replaced by a mineral, has always 

 appeared to me to be inadequate to account for the phenomena. 

 It seems to be more reasonable to suppose that a cast is first taken 

 of the open vessels, and solidity arrived at by the filling in of the 

 interspaces when decomposition of the vegetable matter renders 

 this possible. This explanation will account for that close resem- 

 blance to a woody structure that is frequently to be seen in dense 

 stony masses, wherein are represented more or less distinctly the 

 outlines of the ligneous fibres. It will also account for those cases 

 where the solid condition merges into a fibrous one ; for it is easy 

 to conceive that the extremities of a specimen which had reached 

 the first stage might be so completely solidified to a certain depth, 

 through partial decomposition of the retained woody matter, that 

 the ends might be effectively sealed up, and the interior fibres 

 protected from further petrifactive action. 



