12 PRINCIPLES OF PALEONTOLOGY. 



themselves they are inorganic. Under the head of fossils, there- 

 fore, come the footprints of air-breathing vertebrate animals ; 

 the tracks, trails, and barrows of sea-worms, crustaceans, or 

 molluscs; the impressions left on the sand by stranded jelly- 

 fishes; the burrows in stone or wood of certain shell-fish; the 

 " moulds " or " casts " of shells, corals, and other organic re- 

 mains ; and various other bodies of a more or less similar nature. 



FOSSILIZATION. The term " fossilization " is applied to all 

 those processes through which the remains of organized beings 

 may pass in being converted into fossils. These processes are 

 numerous and varied ; but there are three principal modes of 

 fossilization which alone need be considered here. In the first 

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

 tion of the original organized being such as a bone, a shell, or 

 a piece of wood. In some rare instances, as in the case of the 

 body of the Mammoth discovered embedded in ice at the mouth 

 of the Lena in Siberia, the fossil may be preserved almost pre- 

 cisely in its original condition, and even with its soft parts un- 

 injured. More commonly, certain changes have taken place in 

 the fossil, the principal being the more or less removal of the 

 organic matter originally present. Thus bones become light and 

 porous by the removal of their gelantine, so as to cleave to the 

 tongue on being applied to that organ ; whilst shells become 

 fragile, and lose their primitive colors. In other cases, though 

 practically the real body it represents, all the cavities of the 

 fossil, down to its minutest recesses, may have become infiltrated 

 with mineral matter. It need hardly be added, that it is in the 

 more modern rocks that we find the fossils, as a rule, least 

 changed from their former condition ; but the original structure 

 is often more or less completely retained in some of the fossils 

 from even the most ancient formations. 



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 imag- 

 ine 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 acce^ 

 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 obviously 



