99 



is less rapid, and the buried vegetable remains may be indefi- 

 nitely preserved in the form of carbonaceous films. 



The hard parts of animals are best preserved as fossils. 

 Such are the shells and other external skeletal structures 

 secreted by a variety of animals — as Crustacea, molluscs, 

 echinoderms, corals, and so forth; and the bones, teeth, 

 and other hard structures of the vertebrates. Besides the 

 actual remains of animals and plants, any evidence of their 

 existence which is preserved is commonly included under the 

 name of fossil. Thus, impressions made by living animals 

 and plants in the unconsolidated rock material, and struct- 

 ures built by animals from inorganic material, are fossils if 

 properly buried. Examples of the first are the foot-prints of 

 vertebrates; the tracks and trails of jelly-fish, worms, mol- 

 luscs or Crustacea; the burrows of worms, borings of ani- 

 mals in stones or shells, and the impressions made by sea- 

 weeds in motion. Among the second class are worm tubes, 

 built of sand grains : foraminiferal shells, built of foreign 

 particles; flint implements and other utensils of i)rimitive 

 man: the relics of the Swiss Lake Dwellers; Roman and 

 other ancient coins buried in the peat bogs: and, in fact, all 

 artificial productions of early man or other animals which 

 have become entombed by natural agencies. Thus, three 

 classes of fossils may be recognized, viz : 



1. Organic remains and their impressions. 



2. Trails, tracks, and burrows of organisms. 



3. Artificial structures. 



Mere burial, however, does not in all cases insure preser- 

 vation, even of the hard parts of animals. The fossil must 

 be protected from subsequent destruction. The subsequent 

 destruction of fossils is usually most complete in those beds 

 which have been subjected to alteration by heat during 

 mountain building disturbances, and in such cases the fossils 

 are usually no longer distinguishable. This has been the 

 fate of many organic remains, which have thus been com- 

 pletely destroyed. 



