INTRODUCTION. 3 



anatomy, for without this it is impossible to determine the value of the 

 points of anatomical comparison which have been observed. 



In all cases in which neither of these two criteria could be obtained, 

 palaeophytologists have preferred to make use of conclusions drawn from 

 the frequent or constant occurrence of the parts together in their places of 

 deposit. Conclusions of this kind have been largely employed by Brong- 

 niart andGoldenberg, and also by Schimper. It cannot be denied that they 

 may be resorted to in certain cases, but only where they are accompanied 

 by various indications of probability. In all other cases this method of 

 proof is not only dangerous, but is absolutely to be rejected. To see this, 

 we have only to reflect that in this way we might distinctly conclude from 

 the detached fossil remains of a forest composed of Podccarpus and larch, 

 that the leaves of the one and the cones of the other genus belonged to one 

 another, because being the more durable parts they lay there in large 

 quantities, while the needles of the larch had rotted away and disappeared, 

 and the seeds of Podocarpus if present at all had been crushed into shape- 

 lessness. It has been particularly unfortunate that authors in adopting 

 this method have very commonly neglected to state the proofs on which 

 they have founded their identifications, so that we remain in many cases 

 of the kind in great doubt as to their value, and are often compelled to 

 have recourse to tedious critical investigations. 



The fullest accounts of the mode of preservation of fossil remains of 

 plants will be found in Unger 1 , in Goppert in many places, and in 

 Schimper 2 . Two essentially different cases must be distinguished ; these 

 have been known since Goppert 's 3 time as true petrifaction and as incrusta- 

 tion. To certain other cases intermediate between these two we shall recur 

 in a future page. 



The characteristic mark of true petrifaction is that the remains of the 

 plant are thoroughly permeated by the petrifying substance. This may be 

 amorphous or distinctly crystalline, and in the latter case the plant is simply 

 traversed by the cleavage-surfaces. The effect is usually very beautiful, 

 especially in petrifactions in the carbonates. If the petrifying substance is 

 removed by suitable solvents, there remains behind an organic body usually 

 of small size, which originates in the inclosed fossil and shows its form, and 

 is mostly brown or black, but in a few cases displays an approach to the 

 original colour. Goppert 3 made many experiments in the dissolution of 

 fossils, and even believes that he obtained cellulose-reaction in residua of 

 this kind. 



Very various bodies occur in nature as agents of petrifaction. Among 

 these silicic acid has the first claim to mention here in the form both of opal 



1 Unger (6). 2 Schimper (1), Introd. 3 Goppert (1), Jntrod. 



B 2 



