21 



climate : the succulent plants of Italy differ materially in 

 size from those of South America and of other regions in 

 the warmer climates. 



Proceeding now to the examination of vegetable fossils, 

 in which the introduction of earthy or metallic particles 

 has taken place, it may be necessary again to observe, that 

 wood or other vegetable matter is not likely to undergo 

 this kind of impregnation whilst in a state of soundness 

 and integrity ; but that the states most favourable for this 

 kind of change are that spongeous state which accompanies 

 bituminization, and that kind of decay which has been pro- 

 duced by the abstraction of almost all the constituents 

 except the ligneous fibre, leaving that substance to which 

 is generally applied the term rotten wood. The admission 

 of water in the former state, strongly impregnated with 

 earths or metals, must, as the deposition and consequent 

 earthy or metallic change proceeds, stay the further pro- 

 gress of the bituminizating process, and, giving solidity to 

 the mass, secure the conservation of the form and even 

 structure which the mass possessed at the period when 

 this impregnation commenced : and here it may be ob- 

 served, that specimens of silicified wood exist, in which it 

 may be seen that petrifaction has arrested the bituminizating 

 process in almost all its stages. Specimens are frequently 

 found which seem to prove the wood to have existed in a 

 decayed state, as rotten wood, previous to its mineralization. 

 I. Calcareous Vegetable Fossils. — Lime is not very 

 frequently the mineralizing matter of vegetable fossils ; it 

 is however sometimes found introduced into the remains of 

 wood in the form of spar ; and sometimes it becomes, in the 

 form of limestone, the internal substance of fossil reeds 

 and of various succulent plants. 



1. Calcareous spathose wood, previously decayed. — Colour 

 light brown, surface rough and dull, but susceptible of 

 polish; fracture dull, uneven, and rather spicular; inter- 



