STRATIGRAPHIC TAXONOMY 667 



there should be a fair development of Canadian limestones in the Frankr 

 lin Mountains of western Texas. The greater part of the collections is 

 from the Ceratopea zone, which here contains all the common species, be- 

 sides several not observed elsewhere. A second fossiliferous zone, pre- 

 sumably a little lower in the section, compares species for species very 

 closely with a fossiliferous bed holding this relation to the Ceratopea 

 zone in the Wichita mountain section in southwestern Oklahoma. One 

 or two sponges, probably of the genus Calathium, stand out prominently 

 in the collections. 



The Canadian rocks in northern Arkansas and southern Missouri — 

 Yellville formation. — Canadian deposits found on the southern flanks of 

 Ozarkia present some exceedingly intricate stratigraphic and palcogeo- 

 graphic problems. Although much evidence has been collected, satis- 

 factory discussion is not yet possible. The following account, therefore, 

 is confined to only the more important data in liand : 



The Yellville limestone is provisionally regarded as embracing all 

 the rocks in northern Arkansas between the top of the Ozarkian Jeffer- 

 son City dolomite and the base of the Everton limestone.®^ As thus 

 limited the Yellville is bounded both above and below by an important 

 unconformity and comprises all the Canadian deposits on the flanks of 

 Ozarkia. As the formation was laid down only in large or smaller embay- 

 ments of the shore of Ozarkia, it follows that between the emba^anents 

 these unconformities merge into one. The lower unconformity was 

 entirely unknown when I began work in this region nine years ago, and 

 it was not clearly established and its value appreciated till 1905. Obvi- 

 ously, it is desirable to revisit many localities worked prior to the latter 

 date before the relations of certain fossiliferous zones to the Ozarko- 

 Canadian boundary are positively determined. 



As represented in northern Arkansas and southern Missouri, the Yell- 

 ville formation is exceedingly irregular in distribution and variable in 

 composition. In thickness it ranges from to perhaps 250 feet. As 

 a rule, the maximum development in the embayments is less than 150 

 feet. Within the formation occur lines more or less clearly indicating 

 oscillation and interrupted deposition. The member now believed to be 

 the lowest is known only from Lawrence County, Arkansas, at Black 

 Kock and three to five miles northwest of Smithville, where some 40 

 feet of it are exposed. At the latter locality it consists of white, bluisl> 



^ This is in fact a restriction, but as I talie it a permissible one, since Adams, tlie 



author of the term (U. S. Geological Survey, Prof. Paper 24, 1904, p. 18) had a very* 



inadequate and in certain respects an erroneous conception of the series of roclia to 

 which he proposed to apply It. 



