ICHNOLOGY. 177 



commencement, of reptilian existence at that period of the 

 earth's history : for, air-breathing ambulatory animals may 

 leave other evidence of their former presence npon earth than 

 their fossilized remains. 



There are several circumstances under which impressions 

 made on a part of the earth's surface, soft enough to admit 

 them, may be preserved after the impressing body has perished. 

 When a shell sinks into sand or mud, which in course of time 

 becomes hardened into stone, and when the shell is removed 

 by any solvent that may have filtered through the matrix, its 

 place may become occupied by crystalline or other mineral 

 matter and the evidence of the shell be thus preserved by a 

 cast, for which the cavity made by the shell has served as a 

 mould. If the shell has sunk with its animal within it, the 

 plastic matrix may enter the dwelling-chamber as far as the 

 retracted soft parts will permit ; and as these slowly melt 

 away, their place may become occupied by deposits of matter 

 that had been held in solution by water percolating the 

 matrix, and such, usually crystalline, deposit may receive and 

 retain some colour from the soft parts of which it thus 

 becomes the substitute. 



Evidences of soft-bodied animals, such as Actinice and 

 Medusce, and of the excremental droppings of higher animals, 

 have been thus preserved. Fossil remains, as they are called, 

 of soft plants, such as sea-weeds, reeds, calamites, and the like, 

 are usually casts in matrix made naturally after the plant 

 itself has wholly perished. 



Even where the impressing force or body has been removed 

 directly or shortly after it has made the pressure, evidence of 

 it may be preserved. A superficial film of clay, tenacious 

 enough to resist the escape of a bubble of gas, may retain, 

 when petrified, the circular trace left by the collapse of the 

 burst vesicle. The lightning flash records its course by the 

 vitrified tube it may have constructed out of the sanely par- 



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