FOSSIL INVERTEBRATED ANIMALS. 473 



changed into sandstone : as to the root, the means of explain- 

 ing this transformation are entirely wanting. 



Thus, having no examples, under our inspection, of the 

 mode in which true petrifaction is effected, we are obliged to 

 form hypotheses respecting its formation. In a field, there- 

 fore, of this kind, where imagination is allowed unbounded 

 latitude, we need not wonder that system after system has been 

 built up, and pulled down, with unparalleled industry and 

 rapidity. 



The hypothesis most usually admitted consists in supposing 

 that the stony matter is substituted for the animal or vegetable 

 in proportion as the latter is decomposed, and (according to 

 the opinion of a celebrated mineralogist, M. Haiiy) because 

 the substitution takes place successively, and, as it were, 

 molecule by molecule, the stony parts arranging themselves in 

 the places left void by the retreat of the ligneous or animal 

 parts, and, moulding themselves in the same cavities, take the 

 impression of the vegetable or animal organization, and copy 

 its forms exactly. According to the same writer, in the pe- 

 trified wood, the organization is destroyed, and the appearance 

 of it alone remains. 



M. Patrin, allowing the ingenuity and plausibility of this 

 theory, states certain facts, which, according to him, render 

 its admission impossible. Among others, he instances the 

 trunks of trees converted into silex, which are found in the 

 midst of mobile sands ; and he is astonished that the fluid 

 which held the stony matter, which has taken the place of the 

 molecules of the wood, has not agglutinated and converted into 

 quartzose sandstone, the sand of which touched this petrified 

 wood — this consequence appearing to him to be inevitable. 

 He denies that the organization is destroyed, because the 

 fibres of the petrified wood, scarcely discernible by the mi- 

 croscope, have completely preserved the form and situation 

 which they had in the most perfect state of the wood, and that, 

 moreover, the colours have not changed. Now, continues this 



