MODE OF PEESERVATION OF FOSSIL PLANTS. 719 



stratified rocks we have in certain instances an identity in species and 

 a considerable number of existing genera. At early epochs the flora 

 appears to have been uniform, to have presented less diversity of 

 forms than at present, and to have been similar in the different 

 quarters of the globe. The vegetation also seems to indicate that the 

 nature of the climate was different from that which characterises the 

 countries in which these early fossil plants are now found. 



Fossil plants are by no means so easily examined as recent species. 

 They are seldom found in a complete state. It is very rare to find 

 any traces of the flowers. The parts of fossil plants are usually 

 separated from each other, and it is very difficult to ascertain what 

 are the portions which should be associated together so as to com- 

 plete a specimen. Specimens are sometimes preserved, so that the 

 anatomical structure of the organs, especially of the stem, can be 

 detected by thin microscopic slices placed under the microscope. 



The mode in which plants are preserved in a fossil state may be 

 referred to four principal classes : — 1 . Oasts of the plants from which 

 aU the original substance and structure have been removed subsequently 

 to the burial of the plants, and the greater or less induration of the rocks 

 in which they are entombed. Such casts are occasionally hollow, but 

 more frequently they are fiUed with the amorphous substance of the 

 rock which has been forced into the cavity, and which exhibits, often 

 with remarkable minuteness, the external aspects of the original speci- 

 men. 2. Carbonisation; in which the original substance of the 

 plant has been chemically altered and converted into coal. All trace 

 of the form of the original plant is generally lost, as is the case with 

 the extensive beds of coal; but frequently, when the organism has 

 been buried in a bed of clay, the external appearance is faithfully 

 preserved, as in the ferns and other foliage found in the shales of the 

 Coal-measures. 3. : Infiltration; in which the vegetable tissues, 

 though carbonised, retain their original form from the infiltration of 

 some mineral in solution, chiefly lime or silex, which has filled the 

 «mpty cells and vessels and preserved their original form. This 

 mode of preservation occurs in the calcareous nodules in coal-beds, in 

 the remarkable ash-beds discovered by Mr. Wtinsch in Arran, and 

 generally in the secondary rocks. 4. Petrifaction; in which the 

 structure is preserved, but the whole of the original substance has 

 been replaced, atom for atom, by an inorganic substance, generally 

 lime, silex, or some ore of iron. This is the condition of the beautiful 

 fossils from Antigua, and of many stems and fruits from rooks of all 

 ages in Britain. Silicified stems have been observed in various parts 

 of the world, with the structure well preserved. 



It is rare to find the organs of reproduction in such a state of 

 preservation as to furnish distinct characters. It is chiefly from the 

 fragments of stems, and the impressions of leaves, and some fruits. 



