552 N. H. DART0N FISH REMAINS IN ORDOVICIAN ROCKS 



limestone of Lower Carboniferous age lies unconformable upon it, but 

 without any marked erosional features. 



AGE OF BIGHORN FORMATION 



The paleontological evidence, as above presented, indicates that the 

 massive limestone constituting the greater part of the Bighorn limestone 

 is of Trenton age, but it probably represents only the earlier part of the 

 Trenton limestone of other regions. The fish-bearing sandstone is cor- 

 related with the Harding sandstone of Colorado, which, as will be shown 

 later, is believed to represent the Black Eiver limestone, so that it is in 

 practically conformable succession with the massive limestone. The upper 

 limestone member of the Bighorn formation, apparently not everywhere 

 present, is of Eichmond age or separated from the massive limestone by a 

 •hiatus representing later Trenton, Utica, Eden, and Lorraine time, and 

 perhaps also the earliest part of Eichmond time. According to present 

 ideas the later Eichmond represents the last of Ordovician time, so that 

 the hiatus above the Bighorn limestone when the Eichmond representative 

 is present is equivalent to Silurian and Devonian times. The unconform- 

 ity between the Bighorn and Deadwood formations represents the Upper 

 Cambrian and a long period of early Ordovician, comprising Beekman- 

 town, Lower Magnesian, and Saint Peters. 



Ordovician in Owl Creek Mountains 



The Bighorn limestone, which is so conspicuous in the northern portion 

 of the Bighorn uplift, appears extensively in the Owl Creek mountains, 

 but with diminished thickness. Near the canyon of Bighorn river, which 

 is at the east end of the range, the thickness is only 40 feet, but in the 

 vicinity of Phlox mountain, 30 miles west, and Owl Creek canyon it is 

 over 150 feet thick, and in the vicinity' of Crow creek, at the south end of 

 Shoshone mountains, it is about 100 feet. The formation outcrops con- 

 tinuously around the higher central area of the Owl Creek uplift and west- 

 ward along the South fork of Owl creek to a point 3 miles west of longi- 

 tude 109 degrees. It outcrops again on the slopes adjoining the Crow 

 Creek canyon and the upper portion of West fork of Muddy creek and in 

 Bighorn canyon. The most prominent exposures are in the great south 

 and west facing escarpment of Phlox mountain, where its cliffs are nearly 

 150 feet high. 



The formation consists almost entirely of a massive limestone, usually 

 of light buff color, somewhat darker when weathered, filled with a coarse 

 mat or network of irregular silicious masses, mostly from one-half to 1 

 inch in diameter. On weathering, this silicious material stands out a 



