IV] FOSSILS IN HALF-RELIEF. 77 



senting the carbonised leaf or other plant fragment from the 

 rock on which it lies and to mount it whole on a slide. Good 

 examples of plants treated in this way may be seen in the 

 Edinburgh and British Museums, especially Sphenopt&ris fronds 

 from the Carboniferous oil shales of Scotland. In the excellent 

 collection of fossil plants in Stockholm there are still liner 

 examples of such specimens, obtained by Dr Nathorst from 

 some of the Triassic plants of Southern Sweden. In a few in- 

 stances the tissues of a plant have been converted into coal in such 

 a manner as to retain the form of the individual cells, which 

 appear in section as a black framework in a lighter coloured 

 matrix. Examples of such carbonised tissues were figured by 

 some of the older writers, and Solms-Laubach has recently' 

 described sections of Palaeozoic plants preserved in this 

 manner. The section represented in fig. 70 is that of a Calamite 

 stem (8 X 95 cm.) in which the wood has been converted into 

 carbonaceous material, but the more delicate tissues have been 

 almost completely destroyed. The thin and irregular black 

 line a little distance outside the ring of wood, and forming 

 the limit of the drawing, probably represents the cuticle. The 

 whole section is embedded in a homogeneous matrix of cal- 

 careous rock, in which the more resistant tissues of the plant 

 have been left as black patches and faint lines. 



Mention should be made of a special form of preservation 

 which has been described as fossilisation in half-relief. If a 

 stem is imbedded in sand or mud, the matrix receives an im- 

 pression of the plant surface, and if the hollow pith-cavity is 

 filled with the surrounding sediment, the surface of the medul- 

 lary cast will exhibit markings different from those seen on the 

 surface in contact with the outside of the stem. The space 

 separating the pith-cast from the mould bearing the impression 

 of the stem surface may remain empty, or it may be filled with 

 sedimentary material. In half-relief fossils, on the other hand, 

 we have projecting from the under surface of a bed a more 'or 

 less rounded and prominent ridge with certain surface markings, 

 and fitting into a corresponding groove in the underlying rock 

 on which the same markings have been impressed. It is 

 1 Solms-Laubach (952). 



