444 PHILOSOPHICAL TRANSACTIONS. [anNO 1794. 



From a succession of such shiftings of the situation of the sea we may have 

 a stratum of marine extraneous fossils, one of earth, mixed probably with vege- 

 tables and bones of land animals, a stratum of terrestrial extraneous fossils, then 

 one of marine productions; but from the sea carrying its inhabitants along with 

 it, wherever there are those of land animals there will also be a mixture of marine 

 ones; and from the sea commonly remaining thousands of years in nearly the 

 same situation, 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 surface of the earth than in it; the air is 

 most probably the great agent in decomposition and combination, and also a 

 certain degree 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 though a piece of wood or bone is dead, when so situated as to be fossilized, 

 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° or 53°, and in the 

 colder regions they are still longer preserved. 



I believe it is generally understood that in extraneous fossils the animal part is 

 destroyed; but I find that this is not the case in any I have met with. Shells, 

 and bones offish, most probably have the least in quantity, having been longest 

 in that state, otherwise they should have the most; for the harder and more com- 

 pact the earth, the better is the animal part preserved; which is an argument in 

 proof of their having been the longest in a fossil state. From experiment and 

 observation, the animal part is not allowed to putrefy, it appears only to be dis- 

 solved into a kind of mucus; and can be discovered by dissolving the earth in an 

 acid; when a shell is treated in this way, the animal substance is not fibrous or 

 laminated, as in the recent shell, but without tenacity, and can be washed ofF 

 like wet dust; in some however it has a slight appearance offtakes. In the 

 shark's tooth, or glosso-petra, the enamel is composed of animal substance and 

 calcareous earth, and is nearly in the same quantity as in the recent; but the 

 central part of the tooth has its animal substance in the state of mucus inter- 

 spersed in the calcareous matter. In the fossil bones of sea animals, as the ver- 

 tebrae of the whale, the animal part is in large quantity, and in 2 states; the 



