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been destroyed. If a lump of clay containing a 

 piece of fern frond is heated, the result is an im- 

 pression of the leaf on the hardened matrix and a 

 coaly substance in place of the plant substance. It 

 is occasionally possible by detaching a piece of the 

 black film from a fossil, and heating it with nitric 

 acid and chlorate of potash and then dipping it in 

 ammonia, to obtain a transparent preparation suitable 

 for microscopical examination of the cell-outlines 

 of the superficial layer of the leaf or other plant- 

 fragment. This method of examination, used by 

 several students of fossil plants and with conspicuous 

 success by Professor Nathorst of Stockholm, often 

 affords valuable aids to identification. 



Pieces of plants embedded in sandy sediment, 

 if not preserved by petrifaction, that is by the 

 introduction into the tissues of some siliceous 

 or calcareous solution, gradually decay and their 

 fragmentary remains may be washed away by 

 percolating watei', leaving a hollow^ mould in the 

 gradually hardening sediment, which is afterwards 

 filled with sand or other material. The plant itself 

 is destroyed, but a cast is taken which in the case 

 of fine-grained sediments reproduces the form and 

 surface-pattern of the original specimen. The in- 

 crustation of plants by the falsely named petrifying 

 springs of Knaresborough and other places illustrate 

 another method of fossilisation. 



