548 



N. H. DARTON FISH REMAINS IN ORDOVICIAN ROCKS 



out before the ending of the massive limestone. In the southward con- 

 tinuation of this increased unconformity it was found that finally in 

 the vicinity of the heads of branches of Buffalo creek west of Houck the 

 Deadwood limestones are eroded off and for a short distance the Madison 

 limestone lies on the Deadwood shales. The supposed relations are shown 

 in figure 2. Farther west in the Bridger range the Ordovician limestone 

 comes in again, presenting relations similar to those in the Bighorn 

 mountains, but without the basal sandstone so far as observed. Probably 

 the Bighorn formation originally extended over the portion of this eroded 

 area now included in the Bighorn Mountain uplift, and its present 

 absence here is entirely due to erosion. The overlap apparently is due to 

 locally increased uplift in post-Ordovician times at the north end of an 

 anticline which involved the southeast corner of the Bighorn mountains 



MADISON < 



Figure 2. — Ideal Section showing stratigraphic Relations in southern Part of Bighorn 



Mountains. 



and probably extended from the Laramie range. The original position 

 of the shoreline of the Bighorn limestone to the south is not known. 



INVERTEBRATE FOSSILS 



The greater part of the Bighorn limestone yields but few fossils. Frag- 

 ments of maclurinas and corals appear occasionally in the lower massive 

 beds, and, as above staed, some beds of limestone near the base of the upper 

 series contain corals in most localities. The principal species is a variety 

 of Holy sites catenulatus (gracilis), or chain coral, which often occurs in 

 large numbers. The locality at which fossils were observed to be most 

 abundant in the lower limestone member is on the top of Medicine- moun- 

 tain, a high peak 5 miles northwest of Bald mountain, in beds about 100 

 feet above the base of the formation. The following forms from this place 

 were determined by Mr E. 0. Ulrich : Streptelasma sp. undet., Protarea 

 n. sp. (massive), Plectorthis plicatella f, Dinorthis pcctinella f, D. 

 subquadrata ?, Rhynchotrema capax f var., Oxy discus sp. undet., Liospira, 

 sp. undet., Trochonema sp. undet. (near T. robbinsi), Holopea excelsa ?, 

 and Huronia sp. undet., a lower Galena-Trenton fauna, as nearly as can 

 be ascertained. 



