THE GEOLOGICAL BACKGROUND OF EVOLUTION 



397 



structure. (Occasional; petrified wood; limy hard parts changed to flint, 

 iron ore, pyrite, phosphate, or other substances.) 

 B. With preservation of form alone, details of internal structure lost. 



1. Molds. Formed by percolating ground water dissolving away the original 

 object, leaving a hole in the rock that records its form. In the instance of 

 hollow objects like snail shells there may be an external mold and an internal 

 mold, or core; the latter class of objects is often confused with natural casts. 

 (Common.) 



2. Casts. The interior of a mold may later be refilled with some mineral 

 such as lime or silica, giving rise to a natural cast or pseudomorph (false + 

 form), which has the shape but not the internal structure of the original 



Fig. 25.4. A crane fly imbedded in Baltic amber of Oligocene age. See I:A2c in table on p. 

 396. (Photo courtesy Chicago Natural History Museum.) 



object. A natural cast of a hollow object is formed in the space between 

 the outer mold and the inner mold or core. (Moderately common.) 



3. Imprints. Thin or soft parts may leave impressions of their form on the 

 upper and lower surfaces of the rock layers that enclosed them, and these 

 imprints may remain after the original object has disappeared or has been 

 reduced to a layer of amorphous carbonized material. (Occasional; the 

 commonest sorts of imprints are those of flat objects such as leaves, insect 

 wings, or fish scales.) 



4. Footprints and trails. Made on soft mud or wet sand and preserved by 

 being covered with another layer of sediment. (Occasional.) 



II. Fossils giving evidence of the existence and activities of organisms but not directly 

 recording their form or structure. 

 A. Coprolites. Fossil excrement, sometimes giving direct evidence as to the 



nature of an animal's food and indirect evidence regarding the structure of its 



alimentary canal. (Occasional.) 



