76 THE WONDERS OF GEOLOGY. Lect.I. 



and nests with eggs, from Derbyshire ; a bird, from Knares- 

 borough, in Yorkshire ; and twigs and branches partially 

 inerusted, from Ireland, Russia, &c. 



Lign. 8. — A Twig incrtjsted with Tufa. 

 a, a, a, Extremities of the twig unchanged; b, b, b, the tufaceous crust. 



I need scarcely observe, that on breaking such specimens, 

 we find the inclosed substances to have undergone no 

 change but that of decay. In this incrustation (Lign. 8) 

 the twig, which is exposed in several places, is not per- 

 meated by stony matter, but is withered, and brittle. 

 Now, a true petrifaction is altogether of a different 

 nature, the substance being more or less completely 

 mineralized ; if we break it, we find that every part of 

 its structure has undergone a change. Wood, for in- 

 stance, is often entirely transmuted into flint or chalce- 

 dony, and may be cut so thin, that with a powerful 

 lens the ramifications of the vessels and the structure of 

 the tissues may be seen, and from their form, and dispo- 

 sition, we may determine the particular kind of tree to 

 which the specimen belonged, although it may have been 

 enshrined in the stone for ages. When bone is petrified, 

 the same phenomena are observable ; the most delicate 

 parts of the internal structure are preserved, and all the 

 cells are filled up with calcareous spar, which is oftentimes 

 of a different colour from that of the walls of the cells, and 

 thus a natural anatomical preparation of great beauty and 

 interest is formed. 



