18 



least, with respect to their account of these substances, little more 

 than mere copyists of Pliny ; who, although he had affixed names to 

 these substances, descriptive of the forms they possessed, meant not 

 thereby to give any opinion as to their origin. 



In the thirteenth century, Albert le Grand* speaks of the branch 

 of a tree being found, on which was a bird's nest; with birds 

 contained in it, the whole being a mass of stone. The visformatwa 

 of Aristotle was, however, sufficient to account, in the opinion of 

 Albert, for this extraordinary phenomenon. But the opinions, which 

 were entertained at this period, respecting these substances, were 

 exceedingly incorrect. The doctrine of equivocal generation, which 

 had been adopted by the disciples of Aristotle, contributed very 

 much to mislead those who made these substances the subject of 

 their inquiries ; since, by also adopting the aid of certain occult 

 qualities, their origin was supposed to be thus satisfactorily ac- 

 counted for. Certain plastic powers were supposed to employ their 

 influence in the earth, in creating substances, which bore the figure 

 and resemblance of various vegetable and animal substances. To 

 account for their formation, therefore, it was thought sufficient to 

 refer to the hidden powers of the vis plastica, the vis formativa, and 

 the vis lapidificativa. 



In the sixteenth century, about the year 1517, the workmen em- 

 ployed in rebuilding the citadel of St. Felix, at Verona, discovered, 

 that the rock, on which it was built, was full of petrified shells. 

 This discovery excited the attention of the learned, to a very consi- 

 derable degree ; some attributing them to the active influence of 

 the vis formativa ; whilst others perceiving their exact resemblance 

 to real shells, declared, that they must be actual marine bodies thus 

 enveloped in stone, by some accident. We learn, that when Fra- 

 castorius was asked his opinion respecting this phenomenon, he 



* Mineral, tract. I. lib. i. 



