THE CENOZOIC AGE. 215 



Animal L/Je of the Green River Group. — As already stated, fresh 

 water shells were exceedingly abundant. Fish seemed to be enor- 

 mously abundant, at least in individuals, though the number of gen- 

 era represented are comparatively few. One of the commonest of 

 genera is Clupea, of which some half a dozen of species have been 

 clearly distinguished. Our existing, shad and herring belong to 

 this group. Several other genera, each represented by from one 

 to seven species, have been described. Turtles were abundant in 

 this old lake. But with the exception of a crocodile described by 

 Leidv, about the size of the one living in the Nile, the higher rep- 

 tiles of the period were not preserved. In mammallian life it does 

 not compare with the preceding or the next group. 



Fort Bridger Group. — The areas of the Bridger beds are sur- 

 rounded by strata of the Green River formation, which pass under 

 them, exhibiting slight nonconformity. — (King.) They are 2,^oo 

 feet thick. " The materials are largely made up of almost uniform 

 buff and gray marls, and clays, interrupted at several horizons by 

 beds of peculiar green earth." " Fine sand and clay predominate, 

 arranged in varying proportions, and occasionally changed by cal- 

 careous admixtures. It is a sand and clay formation, while the 

 preceding Green River group is highly calcareous." The Bridger 

 beds are intricately eroded into all kinds of fantastical architectural 

 forms. At and near Cherokee Ridge a line of bold escarpments 

 extend northeast for fourteen miles. Here the Bridger beds rise 

 300 feet above the level valley, and present many abrupt, nearly ver- 

 tical faces, worn into innumerable architectural forms. Often out- 

 liers stand detached in bold, isolated blocks, which have been sculp- 

 tured by the winds into many singular forms. Sometimes enor- 

 mous masses project from the main wall, the stratification lines be- 

 ing traced by the creamy, gray and green sands and marls, which 

 resemble courses of gigantic masonry. Narrow galleries often pro- 

 ject far into these labyrinths. The whole appearance is like a line 

 of Egyptian structures. Among the most interesting forms are the 

 isolated blocks, often over 100 feet in height, which are the last rel- 

 icts of the b^ds which once covered this region. The plains skirt- 

 ing these " Bad Lands" are quite level, there seldom being any 

 talus at the bottoms or base of the cliffs. The excessive fineness of 

 the materials is leveled by the water agencies that have for so long 

 a period been producing erosion. — (King.) 



