FOSSILISATION. 3 



FOSSILISATION. 



Fossilisation may be applied in a general sense to all the 

 processes through which an organic body passes in order to 

 become a fossil. Here we need only consider the three lead- 

 ing forms in which fossils present themselves. In the first 

 instance, the fossil is to all intents and purposes an actual 

 organic remain, being itself a fragment of an animal or plant. 

 Thus we may meet with fossil bones, shells, or wood, which 

 may have undergone certain changes, such as would be pro- 

 duced by pressure, by the deprivation of organic matter origi- 

 nally present, or by more or less complete infiltration with 

 mineral matter, but which, nevertheless, are practically the real 

 bodies they represent. As a matter of course, it is in the more 

 modern formations that we find fossils least changed from their 

 primitive condition, but all formations almost contain some 

 fossils in which the original structure is more or less com- 

 pletely retained. 



In the second place, we very frequently meet with fossils in 

 the state of " casts " or moulds of the original organic body. 

 What occurs in this case will be readily understood if we ima- 

 gine any common bivalve shell, as an Oyster, or Mussel, or 

 Cockle, embedded in clay or mud. If the clay were sufficiently 

 soft and fluid, the first thing would be that it would gain access 

 to the interior of the shell and would completely fill up the 

 space between the valves. The pressure, also, of the surround- 

 ing matter would insure that the clay would everywhere ad- 

 here closely to the exterior of the shell. If now we suppose 

 the clay to be in any way hardened so as to be converted into 

 stone, and if we were to break 

 up the stone, we should obvi- 

 ously have the following state of 

 parts. The clay which filled the 

 shell would form an accurate 

 cast of the interior of the shell, 

 and the clay outside would give 

 us an exact impression or cast 

 of the exterior of the shell (fig. 



l). We should have, then, tWO Fig- i. Trigonia longa, showing casts 

 j of the exterior and interior of the shell. 



casts, an interior and an exterior, 



and the two would be very dif- 



ferent to one another, since the inside of a shell is very unlike 

 the outside. In the case, in fact, of many univalve shells, the 

 interior cast is so unlike the exterior or unlike the shell itself, 

 that it maybe difficult to determine the true origin of the former. 



