452 
LETTER XXXIII. 
FOSSILS CONSIDERED IN CONNECTION WITH THE STRATA IN WHICH 
THEY ARE CONTAINED. 
I have now arrived at the termination of my proposed attempt, 
having placed before you as correct a sketch as I was able of the fossil 
remains of those organized beings, which existed on this planet pre- 
vious to its possessing its present form. 
Your attention has been hitherto called, chiefly, to the original modes 
of existence of those beings, and to the nature of the changes which 
they have undergone. You have seen, that some of these remains 
have belonged to beings whose living analogues may still be found ; 
whilst others have belonged to beings differing essentially from any 
which are now known to exist, and in those particular characters 
which are employed by naturalists as marking generic difference. 
You have also seen, that the fossil remains of both vegetables and 
animals have undergone the most extraordinary changes. I have en- 
deavoured to prove to you, that most vegetable fossils had undergone 
a process of bituminization, by which their conservation was secured, 
previously to their impregnation with earthy or metallic salts. I have 
also suggested the probability of a correspondent preparatory change 
in many animal substances, previous to their mineralization.* 
* In addition to the instances which have been already adduced in proof of the petrifaction 
of vegetables having been in general affected by the impregnation of previously bituminized 
vegetable matter, with earthy or metallic solutions, and not by substitution, I have met with one 
striking fact. I two years since obtained from the shore at Walton, wood changed into marble, 
capable of receiving a beautiful polish, and which, on being deprived of its carbonate of lime by 
the action of muriatic acid, left the light, inflammable, bituminous wood, possessing a volume very 
little less than that of the marble in which it had been contained. 
