1 6 AN INTRODUCTION TO THE STUDY OF FOSSILS 



tained by pressing into it dental wax softened by heating in hot 

 water, forming thus an artificial cast. Soft plaster of paris may 

 likewise be used. If, however, the cavity is partially closed, 

 the mold must be destroyed in extracting the cast. This method 

 of restoring the form of the organism must frequently be used, 

 since plants and animals are often coated with stone while the 

 organic structure is still perfectly retained. Upon the decay 

 of the inclosed organism, which occurs rather quickly, there is 

 left a perfect external mold. In the ancient ash-covered town 

 of Pompeii the use of plaster of paris has enabled the workmen 

 to preserve the external form of several men and dogs buried 

 by the fragmental material which fell during the Mt. Vesuvius 

 eruption of 79 a.d. 



Tracks. — When birds or other animals searching for food 

 walk over the mud fiats of a river or sea, impressions of their 

 feet are left, which when covered with sediment may be pre- 

 served as fossils. Examples of such are the reptile tracks in 

 the Upper Triassic shaly sandstones of the Connecticut Valley 

 (Fig. 153) and the tracks of the Mississippian amphibian, 

 Sauropus primcBVus, in the Mauch Chunk red shales of Penn- 

 sylvania. 



. Trails. — Similar to the preservation of tracks is that of 

 trails, though many of these are made on the soft sediment 

 beneath the surface of the water. Trails are the irregular mark- 

 ings of animals, such as those due to the crawling of a worm or 

 snail, the dragging tentacles of a jelly-fish, the impressions of 

 the fins of fish or the markings left by the movements of crus- 

 taceans or sea urchins. 



Burrows. — When a worm burrows and eats its way through 

 rather compact earth the hole remains for some time and finer 

 material may later be washed into it ; when it burrows its way 

 through very moist sediment, as at the seashore, the finer mate- 

 rial is usually carried into the cavity immediately behind it by 

 the water oozing in from the sides. In both of these cases the 

 resultant is a tube of finer material surrounded by coarser ; as, 



