414 The late Mr. Hunter’s Observations 
will also be a mixture of marine ones ; and from the sea com- 
monly remaining thousands of years in nearly the same situa- 
tion, we have marine fossils unmixed with any others. 
All operations respecting the growth or decomposition of 
animal and vegetable substances go on more readily on the sur- 
face of the earth than in it ; the air is most probably the great 
agent in decomposition and combination, and also a certain de- 
gree of heat. Thus the deeper we go into the earth, we find 
the fewer changes going on ; and there is probably a certain 
depth where no change of any kind can possibly take place. 
The operation of vegetation will not go on at a certain depth, 
but at this very depth a decomposition can take place, for the 
seed dies, and in time decays ; but at a still greater depth, the 
seed retains its life for ages, and when brought near enough to 
the surface for vegetation, it grows. Something similar to this 
takes place with respect to extraneous fossils ; for although a 
piece of wood or bone is dead, when so situated as to be fos- 
silized, yet they are sound and free from decomposition, and 
the depth, joined with the matter in which they are often 
found, as stone, clay, &c. preserves them from putrefaction, and 
their dissolution requires thousands of years to complete it; 
probably they may be under the same circumstances as in a 
vacuum; the heat in such situations is uniform, probably in 
common about 52 0 or 53 0 , and in the colder regions they are 
still longer preserved. 
I believe it is generally understood that in extraneous fos- 
sils the animal part is destroyed ; but I find that this is not the 
case in any I have met with. 
Shells, and bones of fish, most probably have the least in 
quantity, having been longest in that state, otherwise they 
