23 



As the clays and sandstones of the Bridger terraces and buttes crumble 

 away, a variety of remains of terrestrial and fresh-water animals are exposed 

 to view. In some of the buttes they are comparatively abundant ; in others, 

 they are rare. The fossils consist of the bones and teeth of vertebrates, and 

 the shells of mollusks. Fragments of silicified wood also occur, though 

 not frequently. Shells of the sandstones are composed of chalcedony; but 

 those imbedded in the indurated clays usually retain their carbonate of lime 



The fossil bones are completely petrified ; that is to say, their more per- 

 ishable constituents have been replaced mainly by siliceous matter. They 

 are frequently as black as ebony ; and the teeth are usually black, with the 

 enamel highly lustrous. Often they are brownish, with a greenish aspect, 

 derived from the greenish matrix in which they were imbedded. They are 

 also found of a yellowish clay color and duller aspect. 



Many of the bones are more or less crushed and distorted, as a result of 

 the pressure of the superincumbent strata. The fragments are generally but 

 slightly dislocated, showing that the crushing occurred while they were 

 imbedded. The stronger bones are often well preserved, especially the rami 

 of lower jaws and teeth, and the smaller bones of the wrist and ankle. 

 Whole skulls are exceedingly rare, and when discovered are much crushed 

 and distorted. Turtle-shells are among the most frequent fossils, but are 

 usually more or less fractured, crushed, and distorted. In searching over 

 the buttes, little piles of bone-fragments are often seen diverging from a 

 prominent point. These, on examination, generally prove to be the remains 

 of a turtle-shell which, after exposure, has fallen to pieces. 



Generally the fossils are sharply preserved ; that is to say, they rarely 

 have a rolled or water-worn appearance, indicating that bones and shells 

 were soon enveloped in mud at the bottom of comparatively quiet water. In 

 the gravelly strata rolled fragments of bones are found. 



Nearly all the fossils collected from the Bridger beds, and described in 

 the succeeding pages, have been collected as loose specimens picked up on 

 the surface of the buttes. No excavations have been made into the latter in 

 search of fossils, except to exhume a partially exposed bone, or some parts 

 of a skeleton supposed to be contiguous to specimens lying in view on the 

 surface. Usually only a few pieces of a skeleton have been found together, 

 and in no instance has a complete one been discovered which has been 

 brought to my notice. Generally, too, there has been no certainty that bones 



