78 THE PRESERVATION OF PLANTS AS FOSSILS. [CH. 



conceivable that such a cast might be obtained if soft plant 

 fragments were lying on a bed of sand, and were pressed 

 into it by the weight of superincumbent material. The plant 

 fragment would be squeezed into a depression, and its substance 

 might eventually be removed and leave no other trace than the 

 half-relief cast and hollow mould. A twig lying on sand would 

 by its own weight gradually sink a little below the surface ; if 

 it were then blown away or in some manner removed, the 

 depression would show the surface features of the twig. When 

 more sand came to be spread out over the depression, it would 

 find its way into the pattern of the mould, and so produce a 

 cast. If at a later period when the sand had hardened, the 

 upper portion were separated from the lower, from the former 

 there would project a rounded cast of the hollow mould. 

 The preservation of soft algae as half- relief casts has been 

 doubted by Nathorst^ and others as an unlikely occurrence in 

 nature. They prefer to regard such ridges on a rock face as the 

 casts of the trails or burrows of animals. This question of the 

 preservation of the two sides of a mould showing the same impres- 

 sion of a plant has long been a difficult problem ; it is discussed 

 by Parkinson in his Organic Remains. In one of the letters 

 (No. XLVi), he quotes the objection of a sceptical friend, who 

 refuses to believe such a manner of preservation possible, 

 '' until," says Parkinson, " I can inform him if, by involving a 

 guinea in plaster of Paris, I could obtain two impressions of the 

 king's head, without any impression of the reversed" 



It would occupy too much space to attempt even a brief 

 reference to the various materials in which impressions of plants 

 have been preserved. Carbonaceous matter is the most usual 

 substance, and in some cases it occurs in the form of graphite 

 which on dark grey or black rocks has the appearance of a plant 

 drawn in lead pencil. The impressions of plants on the Jurassic 

 (Kimeridgian) slates of Solenhofen=^ in Bavaria, like those on 

 the Triassic sandstones of the Vosges, are usually marked out in 

 red iron oxide. * 



1 Nathorst (86) p. 9. See also Delgado (86). 



2 Parkinson (11) vol. i. p. 431. 



3 The British Museum collection contains many good examples of the Solen- 

 liofen plants. 



