8 ANCIENT PLANTS 



thin sheets on whose surfaces the flattened fossils show 

 particularly well. 



It is, however, with the plant fossils that we must 

 concern ourselves, and among them we find great variety 

 of form. Some are more or less complete, and give an 

 immediate idea of the size and appearance of the plant 

 to which they had belonged; but such are rare. One of 

 the best-known examples of this type is the base of a 

 great tree trunk illustrated in the frontispiece. With 

 such a fossil there is no shadow of doubt that it is part 

 of a giant tree, and its spreading roots running so far 

 horizontally along the ground suggest the picture of a 

 large crown of branches. Most fossils, however, are 

 much less illuminating, and it is usually only by the 

 careful piecing together of fragments that we can obtain 

 a mental picture of a fossil plant. 



A fossil such as that illustrated in the frontispiece 

 and on a smaller scale this type of preservation is one 

 of the commonest does not actually consist of the plant 

 body itself. Although from the outside it looks as though 

 it were a stem base covered with bark, the whole of the 

 inner portion is composed of fine hard rock with no 

 trace of woody tissue. In such specimens we have the 

 shape, size, and form of the plant preserved, but none of 

 its actual structure or cells. It is, in fact, a CAST. Fossil 

 casts appear to have been formed by fine sand or mud 

 silting round a submerged stump and enclosing it as 

 completely as if it had been set in plaster of Paris ; then 

 the wood and soft tissue decayed and the hollow was 

 filled up with more fine silt; gradually all the bark also 

 decayed and the mud hardened into stone. Thus the 

 stone mould round the outside of the plant enclosed a 

 stone casting. When, after lying for ages undisturbed, 

 these fossils are unearthed, they are so hard and "set" 

 that the surrounding stone peels away from the inner 

 part, just as a plaster cast comes away from an ob- 

 ject and retains its shape. There are many varieties 

 of casts among fossil plants. Sometimes on breaking 



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