STRATIGRAPHIC TAXONOMY 669 



Canadian.^^ Commonly, the contact is marked by chert conglomerate 

 secondarily silicified into solid, irregular masses. At other places it is 

 marked by great sand reefs, against which succeeding Yellville deposits 

 lap until finally they are covered. Such contact phenomena are best 

 developed west of Harrison, Arkansas. 



In the vicinity of Yellville the formation which has been named from 

 this town begins with a conglomeratic bed of chert. Locally, as at 

 several points along the railroad from 1 to 3 miles east of North Yell- 

 ville, this basal conglomerate is succeeded by a few (1 to 20) feet of 

 fossiliferous sandy porous chert. The fossils and the kind of chert agree 

 closely with the Yellville in the Wells Creek basin of Tennessee, and with 

 the upper part of the Ceratopea bed in Lawrence County. So far as 

 developed the fauna of this bed (at Yellville) includes two species of 

 Titrritoma, Coelocauhis sp., Hormotoma sp. (cf. Murchisonia argylensis 

 Sard.), HeUcotoma sp. (cf. H. peccatonica Sard.), Protocycloceras 

 lamarcki, Cyrtoceras confertissimum, Isochilina seelyif, and an orthoid 

 ( fValmanella wemplei) resembling Dalmanella subaequata, but having 

 an apically perforated deltidium.®^ 



The bulk of the Yellville, and the part usually seen where the forma- 

 tion is present, succeeds the porous basal cherts described in the preceding 

 paragraph. As a rule this part of the formation consists mainly of 50 to 

 120 feet of noncherty, light colored, very fine grained, argillaceous, often 

 shaly, magnesian limestone or dolomite. The successive layers overlapped 

 the shores of the embajrments so that the upper ones are more widely, 

 distributed than the lower ones. In the mass two layers are usually 

 distinguishable. The upper of the two is the more persistent. It occurs 

 from 35 to nearly 50 feet beneath the sandy base of the Everton or 

 of whatever later formation that happens to rest on it.^^ Usually it is 

 a 2 or 3 foot bed of gray dolomite, containing drusy cavities and rough 

 fossiliferous chert. When it forms the top surface of a hill the whole 



^'' Criiptozoon was observed but a single time in the Yellville. namely, along the White 

 River a few miles west of Mundell, Arkansas. But this occurrence was of a very diflPer- 

 ent species the masses — 2 to 4 feet in diameter — being distinctly mammllated on the 

 surface, the superposition of the V2 to 1 inch waves imparting a columnar appearance 

 to weathered vertical sections contrasting strongly with the simple concentric structure 

 of the Jefferson City species. 



»8 Brachiopods of this type are highly characteristic of the Canadian period. Ap- 

 parently all of the Beekmantown and Levis species that have been In recent years re- 

 ferred to Dalmanella are provided with a deltidium ; hence they are at least generically 

 distinct from that Ordovlcian and later genus. 



9» At many localities the Everton rests on the Yellville. at others it may be the Saint 

 Peter, or the Izard, or the Chattanooga ; or it may be that the first formation to cover 

 it is the Saint Joe. 



