ICHNOLOGY 153 



any silicious, calcareous, or other crystallizable matter that 

 may have been held in solution by water percolating the 

 matrix, and such crystalline deposit may receive and retain 

 some colour from the soft parts of which it thus becomes a cast. 



Evidences of soft-bodied animals, such as Actiniae and 

 Medusae, 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 sandy par- 

 ticles melted in its swift passage through the earth. The 

 hailstone, the ripple wave, the rain-drop, even the wind that 

 bore it along and drove it slanting on the sand, have been 

 registered in casts of the cavities which they originally made 

 on the soft sea-beach ; and the evidence of these and other 

 meteoric actions, as sun-cracks and frost-marks, so written on 

 imperishable stone, have come down to us from times incal- 

 culably remote. Every form of animal life that, writhing, 

 crawling, walking, running, hopping, or leaping, could leave a 

 track, depression, or foot print, behind it, might thereby leave 

 similar lasting evidence of its existence, and also to some 

 extent of its nature. 



The interpretation of such evidences of ancient life has 

 much exercised the sagacity of naturalists since Dr. Duncan, 

 in 1828, first inferred the existence of tortoises at the period 

 of the deposition of certain sandstones in Dumfriesshire, from 

 the impressions loft on those sandstones, and the casts after- 



