410 X. H. DARTON PALEOZOIC AND MESOZOIC OF WYOMING 



feet thick and Tinderlain by a somewhat thicker mass of sediments. The 

 top member of the formation is slabby light colored limestone over 100 

 feet thick at most localities, but apjDarently either very thin or absent in 

 Owl Creek canyon. It contains frequent layers and masses of the char- 

 acteristic intraformational conglomerate of flat limestone pebbles more or 

 less intermingled with thin, twisted, and broken layers of limestone in a 

 matrix of shale and fine limestone sand. Many of the pebbles are so 

 thickly covered with grains of glauconite that they appear to be green, 

 but inside they are gray or pinkish, similar to the associated beds. 



Fossils. — Fossils at various horizons in the Deadwood formation are of 

 middle Cambrian age, and Dicelamiis politus and Ptyclioparia oweni are 

 the principal forms. The former is a small oval shell, which occurs in 

 great abundance in the middle sandstones and in the limestone layers in 

 the shales, as well as in the upper limestone series. The Pti/choparia is a 

 trilobite which often abounds in the basal sandstone. In the middle 

 sandstone of the formation, a short distance west of Garfield peak of Bat- 

 tlesnake mountains, the fossils included a new species, which Mr Walcott 

 has designated Obolus (Westonia) dartoni. In the top sandstone west of 

 Lander, Orthis (Plectorthis) wichitaensis was found. 



Oedovician System 

 bighorn limestone 



General relations. — In 1905 I presented to this Society all available 

 facts as to the distribution of the Ordovician in the Northwest.^ In the 

 following summer Mr Woodruff' and I examined the Bighorn limestone in 

 the "Wind Eiver mountains. The formation terminates at the south end 

 of that range and is absent in the Eattlesnake, Casper, Medicine Bow, 

 and Laramie mountains as well as in the southeastern portion of the Big- 

 horn mountains. 



Wind River mountains. — The "Silurian" limestone, first recognized in 

 Wind Eiver mountains by Comstock and also shown on the maps of the 

 Hayden Survey, is the Bighorn limestone. It lies between the Deadwood 

 formation and Lower Carboniferous (Madison) limestone all along the 

 east slope of the range and presents prominent exposures, especially in 

 the canyons. It has the characteristics, which are distinctive in the Big- 

 horn mountains, of a hard, massively bedded, impure dolomite with con- 

 siderable silica streaked through it. It gives rise to a low cliff and on 

 weathering breaks off in huge, rectangular blocks which in places accumu- 



' Bulletin of the Geological Society of America, vol. 17, pp. 541-566. 



