NATURE OF THE GEOLOGIC RECORD 129 



cavity is called a mold, it may be filled by natural deposits, forming a natu- 

 ral cast of the form of the original object. Or if a natural cast does not form, 

 the mold when discovered may be filled with plaster of Paris or some 

 plastic compound to produce an artificial cast. Perhaps the best-known 

 examples of the use of this technique are the casts produced of people who 

 perished in the eruption of Vesuvius which buried Pompeii under many 

 feet of volcanic ash in the year a.d. 79. Such molds and casts reveal the 

 shape but not the internal structure of the original object. Natural and arti- 

 ficial casts of the interior of the brain cavities of extinct animals form the 

 only material available for study of the brains of these animals. Casts of 

 such soft-bodied creatures as jellyfishes form almost our only source of 

 information concerning members of these groups. Molds and casts of the 

 burrows made by prehistoric animals frequently reveal something of the 

 nature of the latter. 



Similar to molds are impressions sometimes left by vanished objects or 

 parts of the body upon the surrounding material. The impression is made 

 while the latter is soft — much as one leaves an impression on softened 

 sealing wax with a signet ring. Thanks to such impressions we know some- 

 thing of the shapes and venations of prehistoric leaves, of the feathers of 

 extinct birds, of the wing membranes of flying reptiles, of the skin surface 

 of dinosaurs, and so on. Footprints of extinct animals are also impressions 

 affording much valuable information about the animals which made them. 



Occasionally the disintegrating soft parts of a body leave behind a thin 

 film of carbon. Because of this we know, for example, the exact body out- 

 lines of the extinct swimming reptile. Ichthyosaurus (Fig. 3.5, p. 30). 



Among the most perfect fossils known are the insects preserved in 

 amber. When a biologist wishes to preserve an insect permanently in suita- 

 ble condition for detailed study with a microscope he embeds it in balsam 

 on a glass slide. Balsam is the pitch or resin from coniferous trees. Millions 

 of years ago insects became entangled and entombed in soft, sticky resin 

 exuding from pine trees, just as their modern descendants may be ob- 

 served to do today. The resin hardened and eventually changed to amber, 

 preserving the minutest details of structure of the contained insects. 



A few extinct animals are known from frozen specimens in which the 

 flesh as well as the bones has been preserved in remarkably fresh condi- 

 tion for thousands of years. This type of specimen has occurred principally 

 in northern Siberia where the ground remains permanently frozen. The 

 woolly mammoth is the animal best known from such specimens. 



There are other types of fossils but the ones mentioned are most com- 

 mon and most generally useful. Usually fossils are more or less distorted or 



