INTRODUCTION. 25 



assumed, a resin from stems of Conifers which lie buried in layers of lignite 

 and hidden beneath the waters of the Baltic, is proved by its occurrence in 

 the form of tears or as filling fissures in wood of that origin. Fossil resins 

 are obtained from other sources, but the best kinds of the copal and dam- 

 mar of commerce are dug out of the earth. Goppert l has attempted the 

 artificial production of amber also, and has succeeded in obtaining a sub- 

 stance to some extent resembling amber and almost insoluble in alcohol 

 from Venetian turpentine digested for some years at a temperature of 80 R. 

 Unfortunately in this case, as in all similar experiments by Goppert, 

 chemical analysis was omitted. 



The insoluble compounds mentioned above, which are said by authors 

 to be agents of petrifaction, cannot of course permeate the substance of 

 plants. They can at most fill the lumina of the cells, being in this case 

 precipitated on the spot from soluble compounds ; the membranes are then 

 preserved in the form of coal, and may eventually disappear by a slow 

 process of oxidation. We may look upon this state of preservation as 

 intermediate between true petrifaction and incrustation, as an incrustation 

 of the separate membranes. Few however of the many supposed cases of 

 the kind, which Blum 2 especially has collected together in his work, can be 

 said certainly to belong to this category. Among true examples, those 

 should first be mentioned in which pyrites is the agent ; yet here every case 

 requires to be carefully examined, because pyrites also occurs not unfre- 

 quently as a homogeneous mass filling internal cavities, as a simple crystal- 

 line cast. In the museum at Jena may be seen fragments of pyritised wood 

 preserved in petroleum, which may easily be separated into the threads of 

 mineral matter which filled the tracheides, and on these, with the help of 

 the microscope in direct light, we may still see the portions of pyrites 

 which filled the bordered pits in the form of rows of lenticular prominences. 

 The substance of the membrane seems quite to have disappeared. Remains 

 of a similar character are also mentioned by Goppert 3 . A second and rare 

 case is that of the remarkable wood of one of the Amygdaleae found not 

 unfrequently in the tuffs of the Kaiserstuhl in Limburg, which has been 

 preserved by deposition of hydrated clay silicates. Here all the membranes 

 have entirely disappeared, and the wood which remains is simply an aggre- 

 gate of exact casts of cells which may be easily isolated ; the form of the 

 cells of the medullary rays, of the tracheides, and of the matter filling the 

 vessels is beautifully preserved, and the septa in the vessels are shown by deep 

 annular indentations in the soft substance that filled them. Unfortunately 

 these remains of wood can only be preserved by being saturated with gum, 

 and even then with difficulty. 



1 Goppert (15). * Blum (1). 3 Goppert (17), p. 731. 



