FOSSIL INVERTEBRATED ANIMALS. 433 



to beings once endued with life, and whose remains have been 

 buried, some, after their natural death, by slow depositions — 

 others, by sudden revolutions. But there is one point which, 

 assuredly, we shall never be able to ascertain, and that is, the 

 period of time which was requisite for the deposition and petri- 

 faction of each stratum. Certain substances can be preserved 

 in the earth, and pass into a petrified state ; but all are not 

 alike capable of a similar conservation. Flesh, the bills of birds, 

 claws, horns, soft fruits, or other soft substances, do not appear 

 to have been ever found in the fossil state. Teeth and horns 

 are sometimes petrified ; but, for the most part, they are found 

 preserved — a portion of the gelatinous substance even re- 

 maining in the latter, Avhich are also not unfrequently pene- 

 trated by mineral productions. 



Specimens, which have been found in the coarse limestone, 

 as well as in the superior marine sandstone, and which have 

 been considered as ribs of manati, are changed into a very 

 hard and sonorous calcareous stone, although the strata in 

 which they existed are not petrified. 



It is remarkable enough, that, in many localities, such as 

 Nice, Gibraltar, Cette, Aix, and Corsica, the fossil bones are 

 imbedded in a stony stratum, the colour of which, in all those 

 places, is a brownish red. 



Amber, and the different organic bodies which it contains, 

 are preserved, but never in a state of petrifaction. Wood and 

 ligneous cones are very often changed into silex, and have 

 sometimes altogether disappeared in the petrified strata, leaving 

 nothing but their external mould or impression. 



In general, that body which is found in the completest state 

 of preservation, is the calcareous covering, or shell, of the 

 mollusca. It is often found to be penetrated by different 

 mineral substances ; and our knowledge of certain shells is 

 owing to their having been penetrated by a chalcedonious 

 matter, which has kept them in a state of preservation. 



The study of fossil organized bodies has taught us, that, after 



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