GENERAL PRINCIPLES 5 



mould. A cast may be formed by filling a mould with some sub- 

 stance such as sediment or mineral matter carried by underground 

 water, or by filling a hollow organism like a shell with some solid 

 substance. The cast reproduces the internal form of the shell or 

 organism. Frequently original shell, mould, and cast may be seen 

 in a single specimen, while more commonly the original shell has 

 been dissolved away. Only in rare instances have casts of wholly 

 soft animals, or the soft parts of other animals, such as the Jelly- 

 fishes and Cuttle-fishes, been found in ancient rocks. 



6. Preservation of original form and structure (petrifaction). 

 Here again we have a common kind of fossilization. When a plant 

 or hard part of an animal has been replaced, particle by particle, 

 by mineral matter, we have what is called petrifaction. Often 

 organic matter, such as wood, or inorganic matter, such as a car- 

 bonate-of-lime shell, have been so perfectly replaced that the 

 original minute structures are preserved as in life. Conditions 

 favorable for the petrifaction of flesh seem never to have ob- 

 tained. 



7. Preservation of tracks of animals. Footprints of animals, 

 made in moderately soft mud or sandy mud which soon hardens 

 and becomes covered with more sediment, are especially favorable 

 for preservation. Thousands of examples of tracks of great ex- 

 tinct Reptiles have been found in the red sandstone of the Con- 

 necticut River Valley alone. Tracks or trails of Clams or similar 

 animals, and burrows of Worms, are also not uncommon in the 

 ancient rocks of the earth. 



Rocks in which Fossils occur. — 1. Land deposits. Old soils 

 sometimes contain bones or other organic remains. Peat-bogs are 

 especially favorable for the preservation of fossils, as, for example, 

 the wealth of plants directly associated with the resulting coal 

 seams; remains of animals, such as Frogs, Snakes, etc., which 

 inhabited the swamp or bog; and the bones of other animals which 

 wandered in and became entombed. Cave deposits often cover 

 animal remains, many bones of extinct animals, even including 

 prehistoric Man and the things he used, having been found in such 

 deposits. Wind-blown deposits, like dune-sand, loess, and desert 

 deposits, may contain plant or animal remains. Interglacial de- 

 posits sometimes contain fossils, as, for example, the layers of 

 vegetable matter with occasional bones of animals found in the 

 interglacial deposits of the upper Mississippi Valley. Lavas 



