364 BUFFALO LAND. 



concretions and septaria are abundant. In some 

 places the latter are of great size and, being em- 

 bedded in the stratum, have suffered denudation of 

 their contents, and, the septa standing out, form a 

 huge honey-comb. This region and the neighbor- 

 hood of Eagle Tail, Colorado, are noted for the 

 beauty of their gypsum-crystals, the first abundantly 

 found in the cretaceous formation. These are hex- 

 agonal-radiate, each division being a pinnate or 

 feather-shaped lamina of twin rows of crystals. The 

 clearness of the mineral, and the regular leaf and 

 feather forms of the crystals give them much beauty. 

 The bones of vertebrate fossils preserved in this bed 

 are often much injured by the gypsum formation 

 which covers their surface and often penetrates them 

 in every direction. 



The yellow bed of the Mobrara group disappears 

 to the south-west, west, and north-west of Fort Wal- 

 lace, beneath a sandy conglomerate of uncertain age. 

 Its color is light, sometimes white, and the component 

 pebbles are small and mostly of white quartz. The 

 rock wears irregularly into holes and fissures, and 

 the soil covering it generally thin and poor. It is 

 readily detached in large masses, which roll down 

 the bluffs. No traces of life were observed in it, but 

 it is probably the eastern margin of the southern ex- 

 tension of the White River Miocene Tertiary stratum. 

 This is at least indicated by Dr. Hayden, in his geo- 

 logical preface to Leidy's extinct mammals of Da- 

 kota and Nebraska. 



Commercially, the beds of the Mobrara formation 

 possess little value, except when burned for manure. 



