NO. 13 DIKELOCEPHALUS AND OTHER GENERA 355 



Jordan formation. — In Wisconsin this is a rather coarse-grained, 

 thick-bedded, compact but soft. sHghtly calcareous, Hght-colored 

 sandstone.^ In Minnesota it is described as white and sihceous and 

 locally forming rather firm layers that break up into angular blocks.^ 



As far as known the Jordan sandstone as limited by Ulrich has not 

 furnished any fossils in situ. There is, however, a fauna collected 

 from sandstones in the vicinity of Devil's Lake, Sauk County, 

 Wisconsin, that may belong at this horizon. It includes from 

 locality 8ib the following species : 



Arenicolitcs woodi Whitfield Saiikia cf. pyrciic Walcott 



Finkelnburgia finkelnburgi (Walcott) Osceolia cf. osccola (Hall) 



Syntrophia barabuensis (A. Winchell) Agraulos ? sp. undt. 



Straparolliis ? {Ophilcta ?) primor- Ptychasp's sp. undt. 



dialis Winchell Platycolpus barabuensis (Whitfield) 



Dikcloccphahis cf. limbatus Hall Plalycolpus cf. catoiti (Whitfield) 



Saukia cf. crassimarginata (Whit- Illccnurus sp. undt. 



field) Conaspis cf. aiiatiiia (Hall). 



The specimens occur in a friable sandstone similar to that of the 

 Jordan formation and unlike the supposedly older, more or less cal- 

 careous St. Lawrence formation. The fauna is of the same general 

 facies as that of the upper portion of the St. Lawrence formation, 

 but the trilobites differ in minor details, and there are also present 

 two trilobites closely simulating Platycolpus barabuensis and P. 

 catoni (Whitfield). The two latter are types suggesting the suc- 

 ceeding Ozarkian period, the typical varieties of the species, being 

 characteristic of the superjacent Mendota dolomite. 



St. Lawrence formation. — The eastern Wisconsin phase of this 

 formation is described as consisting of alternating strata of arena- 

 ceous magnesian limestone, sandy calcareous shales, and shaly and 

 calcareous sandstones.^ 



In Minnesota the St. Lawrence limestone is formed of an upper 

 regularly bedded magnesian limestone from 30 to 50 feet in thickness, 

 and " below these massive layers, which constitute a part of the pre- 

 cipitous bluffs of the county, there is a varying thickness of more 

 fragile indescribable rock, which can best be defined by Dr. Owen's 

 term siliceo-argillaceous dolomite, with occasional layers of an inch 

 or two of crumbling white sand. There is also a slow transition from 

 the crumbling sandstone of the St. Croix to the dolomitic firm rock 



of the St. Lawrence At ten or fifteen feet higher [from the 



base] the rock has assumed that character which is almost indescrib- 



' Chamberlin, Geol. Wisconsin, Vol. 2, 1877, p. 260. 

 'Winchell, N. H., Geol. Minnesota, Vol. i, 1884, p. 253. 

 Tieol. Wisconsin Vol. 2, 1877, p. 261. 



