The Jeffersonville limestone is a white to bluish-gray crystalline fossUiferous flaggy lime- 

 stone, lying between the Silver Creek hydraulic limestone and the underlying Niagara limestone. 

 It has been correlated with the Corniferous of New York. It is exposed at the Falls of the 

 Ohio River, and from it are obtained many fine coral fossils. In this part of the area there 

 was a great coral reef, not greatly unlike many forming at the present time, except the corals 

 are a different «lass from the living ones. 



The irregular contact representing the unconformity between the Devonian and 

 underlying Silurian has been recognized locally. Kindle has noted it at a poi\J?,)0 

 miles south of Indianapolis and in northern Indiana; farther south, accoaiJ^^"^ to 

 Kindle,*'* physical evidence of this unconformity between the DevonLn^'v miH 

 Silurian has not been observed. Along their contact north of Indianapolis later 

 Devonian strata overlap northwestward beyond earlier strata, indicating the 

 gradual submergence of the Wisconsin-Illinois peninsula; and a similar relation is 

 noted on both sides of the Cincinnati arch in Indiana and Ohio. 



The Devonian of Ohio (Columbus limestone to Ohio shale, inclusive) com- 

 prises the equivalents of the New York section from the Onondaga ("Corniferous") 

 limestone to the somewhat indefinite horizon where the Devonian passes into the 

 Carboniferous. 



L 12. SOXJTHWESTEBN MONTANA. 



The type section of the Paleozoic of southwestern Montana is that of Threeforks, 

 near latitude 46°, longitude 111° 30', originally described by Peale. It includes 

 the Jefferson limestone and Threeforks shale, which lie between the recognized 

 Cambrian and lower Carboniferous strata. 



The Jefferson limestone and Threeforks shale were first described by Peale ""- "^^ 

 as representing the Devonian and possibly including some Silurian at the base and 

 some Carboniferous at the top. Peale states the occurrence of pebbly limestones 

 of Upper Cambrian or Lower Ordovician age, and in the folio text says : ^^^'^ 



DEVONO-SILUKIAN . 



The pebbly limestones pass imperceptibly into a series of black magnesian limestones 

 which for several hundred feet are, so far as examined, devoid of fossils. It is possible, as the 

 pebbly limestones have every indication of being on or near the border line between the Cam- 

 brian and the SUurian [Ordovician], that their upper beds and a part of the overlying black 

 limestones may eventually be referred to the SUurian [Ordovician and Silurian]. There is little 

 doubt that the sedimentation was continuous, and we need not be surprised if sometime in the 

 future this now barren interval should somewhere furnish a mingling of Silurian and Devonian 

 forms. 



DEVONIAN. 



The black limestones referred to under the head of Devono-Silurian have been provisionally 

 referred to the Jefferson formation, because the fragments of fossUs found in the middle and 

 upper part, although few and very indistinct, were recognized to be the same as those occurring 

 in the Threeforks shales, which are, without much doubt, Devonian. The total thickness 

 referred to the Devonian is about 870 feet. 



The Jefferson limestone forms the base of the formation, consisting of several hundred feet 

 of black or mud-colored limestone, which is generally crystalline and magnesian from top to 

 bottom. It is well exposed in all parts of the Threeforks region and at most locaUties is non- 

 fossiliferous, only a few forms having been obtained from near the top of the formation, and 

 these only in the region north of the East Gallatin River. 



