68 THE PRESERVATION OF PLANTS AS FOSSILS. [CH. 



epidermis shows scarcely any indication of alteration since it 

 was deposited in the soft mud of a river's delta. Such fossil 

 leaves are common in the English Tertiary beds, and even in 

 Palaeozoic rocks it is not uncommon to find an impression of a 

 plant on a bed of shale from which the thin brown epidermis 

 may be peeled off the rock, and if microscopically examined it 

 will be found to have retained intact the contours of the 

 cuticularised epidermal cells. A striking example of a similar 

 method of preservation is afforded by the so-called paper-coal 

 of Culm age from the Province of Toula in Russia ^ In the 

 Russian area the Carboniferous or Permian rocks have been 

 subjected to little lateral pressure, and unlike the beds of the 

 same age in Western Europe, they have not been folded and com- 

 pressed by widespread and extensive crust-foldings. Instead of 

 the hard seams of coal there occur beds of a dark brown 

 laminated material, made up very largely of the cuticles of 

 Lepidodendroid plants. 



From such examples we may naturally pass to fossils in 

 which the plant structure has been converted into carbona- 

 ceous matter or even pure coal. This form of preservation is 

 especially common in plant-bearing beds at various geological 

 horizons. In other cases, again, some mineral solution, oxide of 

 iron, talc, and other substances, has replaced the plant tissues. 

 From the Coal-Measures of Switzerland Heer has figured nume- 

 rous specimens of fern fronds and other plants in which the leaf 

 form has been left on the dark coloured rock surface as a thin 

 layer of white talcose material^ In the Buntersandstone of 

 the Yosges and other districts the red imperfectly preserved 

 impressions of plant stems and leaves are familiar fossils^; 

 the carbonaceous substance of the tissues has been replaced by 

 a brown or red oxide of iron. 



Plants frequently occur in the form of incrustations; and 

 in fact incrustations, which may assume a variety of forms, are 

 the commonest kind of fossil. The action of incrusting springs, 

 or as they are often termed petrifying springs, is illustrated at 

 Knaresborough, in Yorkshire, and many other places where 



1 Zeiller (82) and Renault (95). ^ Heer (76). 



3 Schimper and Mougeot (44). 



