72 THE PKESERVATION OF PLANTS AS FOSSILS. [CH. 



leaf-cushions and leaf-scars of a Lepidodendron. The stem 

 was embedded in soft sand, and as the latter became hard and 

 set, an impression was obtained of the external markings of 

 the Lepidodendron. Decay subsequently removed the substance 

 of the plant. 



Fig. 11. Equisetites columnaris Brongn. Prom a specimen in the Woodwardian 

 Museum, Cambridge. ^ nat. size. 



In fig. 11 some upright stems of a fossil Horse-tail (Equi- 

 setites columnaris) from the Lower Oolite rocks near Scarborough, 

 are seen in a vertical position in sandstone. On the surface of 

 the fossils there is a thin film of carbonaceous matter, which is 

 all that remains of the original plant substance ; the stems were 

 probably floated into their present position and embedded ver- 

 tically in an arenaceous matrix. The hollow pith-cavity was 

 filled with sand, and as the tissues decayed they became in part 

 converted into a thin coaly layer. The vertical position of 

 such stems as those in fig. 11 naturally suggests their pre- 

 servation in situ, but in this as in many other cases the erect 

 manner of occurrence is due to the settling down of the drifted 

 plants in this particular position. 



An example of Stigmaria drawn in fig. 12 further illustrates 



