6 INTRODUCTION. 



cases still covered with their rind of coal, where this has not remained 

 attached to the mould. Of this kind are the so-called Artisiae, casts of the 

 pith-tube of steins of Cordaitae. They may be squeezed quite flat, or they 

 may still show their natural rotundity. As a rule the cast is of the same 

 substance as the enveloping mass ; in some instances the two may be of 

 a different nature. This may be the case if the part of the plant was 

 incrustcd at the moment when there was a change of sediment, and then 

 the envelope will always belong to the older, the cast to the newer layer 

 of material. 



In the casts hitherto spoken of the surface must necessarily show 

 entirely different sculpturings from those of the mould ; the reflection of 

 the latter must be found on the surface of the layer of coal which separates 

 the two, and there it may sometimes be observed in a very delicate form. 

 Such cases it is true can very rarely be seen in our collections, though 

 I have in my possession a Sigillaria the surface of which is wonderfully 

 preserved in the rind of coal, first because the coal as a rule falls to pieces 

 very quickly when exposed to the air, especially if it contains pyrites, and 

 secondly because collectors in their want of understanding often injure their 

 specimens by carefully cleaning them. But, as has been already said, if the 

 organic substance of the inclosed plant has entirely decayed and disappeared, 

 and the resulting cavity, as sometimes happens, is occupied by the enveloping 

 material or by some other substance, the cast thus produced will itself 

 fill the mould and present an exact impression taken in this form of the 

 surface-features of the inclosed plant, the same impression as is shown in 

 the other case on the outside of the rind of coal. The Pliocene and 

 Quaternary tuffs of Meximieux near Lyons and of Cannstadt inclose 

 countless holes and cavities, from which the vegetable substance has 

 entirely disappeared ; these are so many moulds, in which we can restore 

 it bodily with all its external characters by forcing melted wax with the 

 aid of the air-pump into the pieces of tuff, and then dissolving the calcium 

 which surrounds them by dilute hydrochloric acid. Fine specimens of the 

 kind are exhibited in the Paris Museum. In the collection also at the 

 Sorbonne there is a series of such preparations obtained by Munier Chalmas 

 from the lower Eocene tuffs of Sezanne ; the most remarkable of them 

 is a flower of Byttneriaceae with all its parts well preserved. We often 

 find the same process carried out in pyrites, only there it is the work of 

 nature herself; another instance is that of the fern-leaves of the Carboni- 

 ferous beds of the Tarentaise, which glisten white and conspicuous on 

 a dark ground ; in these the substance which has disappeared has been 

 replaced by a cast of magnesium silicate which fills the cleft-like cavity. 



It is evident therefore that in examining specimens of this kind careful 

 attention must be paid to the relations of form between the cast and the 

 mould, and this care will be the more necessary, because circumstances now 



