628 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



either dolomite or magnesian limestone above. The top of the system 

 also is everywhere in unconformable relations to succeeding deposits. 



The most convincing part of the evidence on which it is claimed that 

 an important stratigraj^hic hiatus separates the upper Cambrian in the 

 southern half of the valley from the overlying Ozarkian lies in the greatly 

 varying age of the beds forming the base of the latter in different locali- 

 ties. Occasionally, as in Eiver Eidge, 3 miles northwest of Morris- 

 town, Tennessee, the Copper Eidge chert rests on the Nolichucky. More 

 commonly the Knox begins with an older division 300 to 700 feet thick, 

 while in the vicinity of Montevallo, Alabama, three still older formationb, 

 aggragating at a maximum something near 2,500 feet of dolomite, inter- 

 vene between the base of the typical Knox and the top of the Conasauga. 

 The hiatus between the two systems, therefore, represents locally in east 

 Tennessee over 3,000 feet of known calcareous deposits laid down else- 

 where in Tennessee and in central Alabama. Even this great thickness of 

 lower Ozarkian deposits does not fully measure the time break, for a gap 

 is still indicated between the Briartield dolomite at the base of the new 

 system and the top of the upper Cambrian. 



On account of the oscillatory nature of the continental surface a com- 

 plete and accessible sedimentary record of any geologic period is an im- 

 possibility ; and the Ozarkian is no exception to the rule. In one section 

 the lower part is well developed, while either the middle or upper, or both, 

 parts may be poorly represented; in another section the basal part is 

 absent, while the middle series may be very fully developed; in a third 

 section an upper series may be present that is wanting in the other ex- 

 posures. The sequence so far as known, then, is made up of more or 

 less disconnected but commonly interlapping parts whose stratigraphic 

 relations are determined according to the faunal and physical criteria 

 and correlation principles discussed in Part II of this work. The typical 

 exposures occur in the Ozark region of Missouri. Though much thinner 

 than the Appalachian record, the Missouri section makes a more satisfac- 

 tory type because here the rocks are more fossiliferous and the strati- 

 graphic sequence, in an epitomized way, more complete. 



Derivation of tlie name and the type section. — For reasons mentioned 

 the term Ozarkian, a modification and restriction of the name "Ozark 

 series," suggested by Broadhead some years ago,^"' seems highly ap- 

 propriate. Broadhead was the first to use the geographic name Ozark 

 as a stratigraphic term; hence it was preoccupied in geological nomen- 



*^ American Geologist, vol. viii, 1891, p. 33. 



