STRATIGRAPHIC CLASSIFICATION DTASTROPHIC CRITERIA 421 



So far as determined, the oldest 

 member is a light, bluish gray dolo- 

 mite, about 40 feet thick, and seen only 

 in Lawrence County, Arkansas. This 

 member is of unusual interest because 

 it contains the only known occurrence 

 of the Phyllograptus fauna in unfolded 

 interior rocks. 



The second bed is thin, but contains 

 a well-marked fauna and is widely 

 though locally distributed. Tt is ab- 

 sent in the vicinity of Yellville, but is 

 found here and there to the east in 

 Lawrence County and to the west in 

 Christian County, Missouri. The fauna, 

 though probably somewhat older, re- 

 minds greatly of the Fort Cassin facies 

 in the Champlain basin. 



The third bed, also, is thin (0-20 

 feet) and has been observed only in 

 the Yellville embayment. It is a con- 

 glomeratic porous chert, usually very 

 fossiliferous. The fauna consists 

 chiefly of small, slender gastropods, 

 Leperditia, and a brachiopod like T)al- 

 manella electra, and most of the species 

 are confined to this zone. 



Not to enter too much into detail, 

 the fourth zone seems like the first and 

 a following sixth, to be confined in 

 Arkansas to the Lawrence County em- 

 bayment, while the fifth, which makes 

 the bulk or whole of the formation in 

 the Yellville and more western embay- 

 ments, is absent there. 



Post-Ordovician oscillations in North 

 America — General character. — That the 

 foregoing examples of oscillation of 

 seas, due to differential vertical move- 

 ments of comparatively limited but 



^\Su/phur Sprinai 





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Eureka Sprinas 



Berryvi/le 



Yellville- 



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PenlBrs Bluff 



ohhN Black Rock 



